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Determination of the maximum reverse voltage of diodes. Current rectification

Hello dear readers of the site sesaga.ru. In the first part of the article, we figured out what a semiconductor is and how current arises in it. Today we will continue the topic we started and talk about the operating principle of semiconductor diodes.

A diode is a semiconductor device with a single pn junction, having two terminals (anode and cathode), and designed for rectification, detection, stabilization, modulation, limiting and conversion electrical signals.

In my own way functional purpose diodes are divided into rectifier, universal, pulse, microwave diodes, zener diodes, varicaps, switching, tunnel diodes, etc.

Theoretically, we know that a diode passes current in one direction and not in the other. But how and in what way he does this is not known and understood by many.

Schematically, a diode can be represented as a crystal consisting of two semiconductors (regions). One region of the crystal has p-type conductivity, and the other has n-type conductivity.

In the figure, holes that predominate in the p-type region are conventionally depicted as red circles, and electrons that predominate in the n-type region are shown in blue. These two areas are the diode's electrodes anode and cathode:

Anode is the positive electrode of a diode, in which the main charge carriers are holes.

The cathode is the negative electrode of the diode, in which the main charge carriers are electrons.

Contact metal layers are applied to the outer surfaces of the areas, to which the wire leads of the diode electrodes are soldered. Such a device can only be in one of two states:

1. Open - when it conducts current well; 2. Closed - when it conducts current poorly.

Direct connection of the diode. Direct current.

If you connect a constant voltage source to the electrodes of the diode: to the “plus” terminal of the anode and to the “minus” terminal of the cathode, then the diode will be in the open state and a current will flow through it, the magnitude of which will depend on the applied voltage and the properties of the diode.

With such a polarity of connection, electrons from the n-type region will rush towards the holes in the p-type region, and holes from the p-type region will move towards the electrons in the n-type region. At the interface between the regions, called an electron-hole or p-n junction, they will meet, where their mutual absorption or recombination occurs.

For example. The majority charge carriers in the n-type region, electrons, overcoming the p-n junction, enter the p-type hole region, in which they become minority. Having become minority electrons, they will be absorbed by majority carriers in the hole region - holes. In the same way, holes entering the n-type electronic region become minority charge carriers in this region, and will also be absorbed by the majority carriers - electrons.

A diode contact connected to the negative pole of a constant voltage source will give up an almost unlimited number of electrons to the n-type region, replenishing the decrease in electrons in this region. And the contact connected to the positive pole of the voltage source is able to accept the same number of electrons from the p-type region, due to which the concentration of holes in the p-type region is restored. Thus, conductivity p-n junction will become large and the resistance to the current will be small, which means that a current will flow through the diode, called the forward current of the diode Ipr.

Reverse connection of the diode. Reverse current.

Let's change the polarity of the constant voltage source - the diode will be in the closed state.

In this case, electrons in the n-type region will move towards the positive pole of the power source, moving away from the p-n junction, and holes in the p-type region will also move away from the p-n junction, moving towards the negative pole of the power source. As a result, the boundary of the regions will seem to expand, which creates a zone depleted of holes and electrons, which will provide great resistance to the current.

But, since minority charge carriers are present in each region of the diode, a small exchange of electrons and holes between the regions will still occur. Therefore, a current many times less than the forward current will flow through the diode, and such a current is called the reverse current of the diode (Irev). As a rule, in practice, the reverse current of the p-n junction is neglected, and from this we conclude that the p-n junction has only one-way conductivity.

Forward and reverse diode voltage.

The voltage at which the diode opens and forward current flows through it is called forward (Upr), and the reverse polarity voltage at which the diode closes and reverse current flows through it is called reverse (Urev).

At forward voltage (Upr), the resistance of the diode does not exceed several tens of ohms, but at reverse voltage (Urev), the resistance increases to several tens, hundreds and even thousands of kiloohms. This is not difficult to verify if you measure the reverse resistance of the diode with an ohmmeter.

The resistance of the p-n junction of the diode is not constant and depends on the forward voltage (Upr) that is supplied to the diode. The greater this voltage, the less resistance the p-n junction has, the greater the forward current Ipr flows through the diode. In the closed state, almost all the voltage drops across the diode, therefore, the reverse current passing through it is small, and the resistance of the p-n junction is high.

For example. If you connect a diode to the circuit alternating current, then it will open at positive half-cycles at the anode, freely passing forward current (Ipr), and close at negative half-cycles at the anode, almost without passing current in the opposite direction - reverse current (Ibr). These properties of diodes are used to convert alternating current into direct current, and such diodes are called rectifying diodes.

Current-voltage characteristic of a semiconductor diode.

The dependence of the current passing through the pn junction on the magnitude and polarity of the voltage applied to it is depicted in the form of a curve called the current-voltage characteristic of the diode.

The graph below shows such a curve. Along the vertical axis in the upper part the values of the forward current (Ipr) are indicated, and in the lower part - the reverse current (Irev). The horizontal axis on the right side indicates the values of the forward voltage Upr, and on the left - the reverse voltage (Urev).

The current-voltage characteristic consists of two branches: the forward branch, in the upper right part, corresponds to the forward (pass) current through the diode, and the reverse branch, in the lower left part, corresponding to the reverse (closed) current through the diode.

The forward branch goes steeply upward, pressing against the vertical axis, and characterizes the rapid growth of the forward current through the diode with an increase in the forward voltage. The reverse branch runs almost parallel to the horizontal axis and characterizes the slow growth of the reverse current. The steeper the forward branch is to the vertical axis and the closer the reverse branch is to the horizontal, the better the rectifying properties of the diode. The presence of a small reverse current is a disadvantage of diodes. From the current-voltage characteristic curve it is clear that the forward current of the diode (Ipr) is hundreds of times greater than the reverse current (Irev).

As the forward voltage across the pn junction increases, the current initially increases slowly, and then a section of rapid current growth begins. This is explained by the fact that a germanium diode opens and begins to conduct current at a forward voltage of 0.1 - 0.2V, and a silicon diode at 0.5 - 0.6V.

For example. At a forward voltage Upr = 0.5V, the forward current Ipr is equal to 50mA (point “a” on the graph), and already at a voltage Upr = 1V the current increases to 150mA (point “b” on the graph).

But such an increase in current leads to heating of the semiconductor molecule. And if the amount of heat generated is greater than that removed from the crystal naturally, or with the help special devices cooling (radiators), then irreversible changes can occur in the conductor molecule, up to the destruction of the crystal lattice. Therefore, the forward current of the p-n junction is limited to a level that prevents overheating of the semiconductor structure. To do this, use a limiting resistor connected in series with the diode.

For semiconductor diodes, the forward voltage Upr at all operating currents does not exceed: for germanium diodes - 1V; for silicon diodes - 1.5V.

With an increase in the reverse voltage (Urev) applied to the p-n junction, the current increases slightly, as evidenced by the reverse branch of the current-voltage characteristic. For example. Let's take a diode with the parameters: Urev max = 100V, Irev max = 0.5 mA, where:

Urev max – maximum constant reverse voltage, V; Irev max – maximum reverse current, µA.

With a gradual increase in the reverse voltage to a value of 100V, you can see how slightly the reverse current increases (point “c” on the graph). But with a further increase in voltage, above the maximum for which the p-n junction of the diode is designed, there is a sharp increase in the reverse current (dashed line), heating of the semiconductor crystal and, as a result, breakdown of the p-n junction occurs.

Breakdowns of the p-n junction.

Breakdown of a pn junction is the phenomenon of a sharp increase in reverse current when the reverse voltage reaches a certain critical value. There are electrical and thermal breakdowns of the p-n junction. In turn, the electrical breakdown is divided into tunnel and avalanche breakdowns.

Electrical breakdown.

Electrical breakdown occurs as a result of exposure to a strong electric field in a pn junction. Such a breakdown is reversible, that is, it does not damage the junction, and when the reverse voltage decreases, the properties of the diode are preserved. For example. Zener diodes - diodes designed to stabilize voltage - operate in this mode.

Tunnel breakdown.

Tunnel breakdown occurs as a result of the phenomenon of the tunnel effect, which manifests itself in the fact that with a strong electric field strength acting in a p-n junction of small thickness, some electrons penetrate (leak) through the transition from the p-type region to the n-type region without changing their energy . Thin p-n transitions are possible only at a high concentration of impurities in the semiconductor molecule.

Depending on the power and purpose of the diode, the thickness of the electron-hole junction can range from 100 nm (nanometers) to 1 micrometer (micrometer).

Tunnel breakdown is characterized by a sharp increase in reverse current at an insignificant reverse voltage - usually several volts. Tunnel diodes operate based on this effect.

Due to their properties, tunnel diodes are used in amplifiers, generators of sinusoidal relaxation oscillations and switching devices at frequencies of up to hundreds and thousands of megahertz.

Avalanche breakdown.

Avalanche breakdown is that under the influence of a strong electric field, minority charge carriers under the influence of heat in a p-n junction are accelerated so much that they are able to knock out one of its valence electrons from the atom and throw it into the conduction band, thereby forming an electron-hole pair. The resulting charge carriers will also begin to accelerate and collide with other atoms, forming the following electron-hole pairs. The process takes on an avalanche-like character, which leads to a sharp increase in reverse current at a practically constant voltage.

Diodes that use the avalanche breakdown effect are used in powerful rectifier units used in the metallurgical and chemical industries, railway transport and other electrical products in which a reverse voltage higher than permissible may occur.

Thermal breakdown.

Thermal breakdown occurs as a result overheating p-n transition at the moment a large current flows through it and with insufficient heat removal, which does not ensure the stability of the thermal regime of the transition.

As the reverse voltage (Urev) applied to the p-n junction increases, the power dissipation at the junction increases. This leads to an increase in the temperature of the transition and the adjacent regions of the semiconductor, the vibrations of the crystal atoms increase, and the bond of valence electrons with them weakens. There is a possibility of electrons moving into the conduction band and the formation of additional electron-hole pairs. Under poor conditions for heat transfer from the pn junction, an avalanche-like increase in temperature occurs, which leads to destruction of the junction.

Let's finish here, and in the next part we will look at the design and operation of rectifier diodes and a diode bridge. Good luck!

Source:

1. Borisov V.G - Young radio amateur. 19852. Goryunov N.N. Nosov Yu.R - Semiconductor diodes. Parameters, measurement methods. 1968

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Basic parameters of diodes, forward diode current, reverse diode voltage

The main parameters of diodes are the forward diode current (Ipr) and the maximum reverse diode voltage (Urev). These are the ones you need to know if the task is to develop a new rectifier for a power source.

Forward diode current

The forward diode current can be easily calculated if the total current that the load of the new power supply will draw is known. Then, to ensure reliability, it is necessary to slightly increase this value and you will get a current for which you need to select a diode for the rectifier. For example, the power supply must withstand a current of 800 mA. Therefore, we choose a diode whose forward diode current is 1A.

Diode Reverse Voltage

The maximum reverse voltage of a diode is a parameter that depends not only on the value of the AC input voltage, but also on the type of rectifier. To explain this statement, consider the following figures. They show all the basic rectifier circuits.

Rice. 1

As we said earlier, the voltage at the output of the rectifier (at the capacitor) is equal to the effective voltage of the secondary winding of the transformer multiplied by √2. In a half-wave rectifier (Fig. 1), when the voltage at the diode anode is at a positive potential relative to ground, the filter capacitor is charged to a voltage that is 1.4 times the effective voltage at the rectifier input. During the next half-cycle, the voltage at the anode of the diode is negative relative to ground and reaches the amplitude value, and at the cathode it is positive relative to ground and has the same value. During this half-cycle, a reverse voltage is applied to the diode, which is obtained due to the series connection of the transformer winding and the charged filter capacitor. Those. The reverse voltage of the diode must be no less than double the amplitude voltage of the transformer secondary or 2.8 times higher than its effective value. When calculating such rectifiers, it is necessary to select diodes with a maximum reverse voltage 3 times higher than the effective value of the alternating voltage.

Rice. 2

Figure 2 shows a full-wave rectifier with a midpoint output. In it, as in the previous one, diodes must be selected with a reverse voltage 3 times higher than the effective input value.

Rice. 3

The situation is different in the case of a full-wave bridge rectifier. As can be seen in Fig. 3, in each half-cycle, double the voltage is applied to two non-conducting diodes connected in series.

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Operating principle and purpose of diodes

A diode is one of the types of devices designed on a semiconductor basis. It has one p-n junction, as well as anode and cathode terminals. In most cases, it is designed for modulation, rectification, conversion and other actions with incoming electrical signals.

Principle of operation:

An electric current acts on the cathode, the heater begins to glow, and the electrode begins to emit electrons.
An electric field is formed between the two electrodes.
If the anode has a positive potential, then it begins to attract electrons to itself, and the resulting field is a catalyst this process. In this case, an emission current is generated.
Between the electrodes, a negative spatial charge is formed, which can interfere with the movement of electrons. This happens if the anode potential is too weak. In this case, some of the electrons are unable to overcome the influence of the negative charge, and they begin to move in the opposite direction, returning to the cathode again.
All electrons that reach the anode and do not return to the cathode determine the parameters of the cathode current. Therefore, this indicator directly depends on the positive anode potential.
The flow of all electrons that were able to get to the anode is called the anode current, the indicators of which in the diode always correspond to the parameters of the cathode current. Sometimes both indicators can be zero; this happens in situations where the anode has a negative charge. In this case, the field that arises between the electrodes does not accelerate the particles, but, on the contrary, slows them down and returns them to the cathode. The diode in this case remains in a locked state, which leads to an open circuit.

Device

Below is detailed description diode devices, studying this information is necessary for further understanding of the principles of operation of these elements:

The housing is a vacuum cylinder that can be made of glass, metal or durable ceramic varieties of material.
There are 2 electrodes inside the cylinder. The first is a heated cathode, which is designed to ensure the process of electron emission. The simplest cathode in design is a filament with a small diameter, which heats up during operation, but today indirectly heated electrodes are more common. They are cylinders made of metal and have a special active layer capable of emitting electrons.
Inside the indirectly heated cathode there is a specific element - a wire, which glows under the influence of electric current, it's called a heater.
The second electrode is the anode, it is needed to receive the electrons that were released by the cathode. To do this, it must have a potential that is positive relative to the second electrode. In most cases, the anode is also cylindrical.
Both electrodes of vacuum devices are completely identical to the emitter and base of the semiconductor variety of elements.
Silicon or germanium is most often used to make a diode crystal. One of its parts is p-type electrically conductive and has a deficiency of electrons, which is formed by an artificial method. The opposite side of the crystal also has conductivity, but it is n-type and has an excess of electrons. There is a boundary between the two regions, which is called a p-n junction.

Such features internal device diodes are endowed with their main property - the ability to conduct electric current in only one direction.

Purpose

Below are the main areas of application of diodes, from which their main purpose becomes clear:

Diode bridges are 4, 6 or 12 diodes connected to each other, their number depends on the type of circuit, which can be single-phase, three-phase half-bridge or three-phase full-bridge. They perform the functions of rectifiers; this option is most often used in automobile generators, since the introduction of such bridges, as well as the use of brush-collector units with them, has made it possible to significantly reduce the size of this device and increase its reliability. If the connection is made in series and in one direction, this increases the minimum voltage required to unlock the entire diode bridge.
Diode detectors are obtained by combining these devices with capacitors. This is necessary so that it is possible to isolate low-frequency modulation from various modulated signals, including the amplitude-modulated variety of the radio signal. Such detectors are part of the design of many household appliances, such as televisions or radios.
Ensuring protection of consumers from incorrect polarity when switching on circuit inputs from occurring overloads or switches from breakdown by electromotive force that occurs during self-induction, which occurs when the inductive load is turned off. To ensure the safety of circuits from overloads that occur, a chain is used consisting of several diodes connected to the supply buses in the reverse direction. In this case, the input to which protection is provided must be connected to the middle of this chain. During normal operation of the circuit, all diodes are in a closed state, but if they have detected that the input potential has gone beyond the permissible voltage limits, one of the protective elements is activated. Due to this, this permissible potential is limited within the permissible supply voltage in combination with a direct drop in the voltage on the protective device.
Diode-based switches are used to switch high-frequency signals. Such a system is controlled using direct electric current, high-frequency separation and the supply of a control signal, which occurs due to inductance and capacitors.
Creation of diode spark protection. Shunt-diode barriers are used, which provide safety by limiting the voltage in the corresponding electrical circuit. In combination with them, current-limiting resistors are used, which are necessary to limit the electric current passing through the network and increase the degree of protection.

The use of diodes in electronics today is very widespread, since virtually no modern type of electronic equipment can do without these elements.

Direct diode connection

The p-n junction of the diode can be affected by voltage supplied from external sources. Indicators such as magnitude and polarity will affect its behavior and the electrical current conducted through it.

Below we consider in detail the option in which the positive pole is connected to the p-type region, and the negative pole to the n-type region. In this case, direct switching will occur:

Under the influence of voltage from an external source, an electric field will be formed in the p-n junction, and its direction will be opposite to the internal diffusion field.
The field voltage will decrease significantly, which will cause a sharp narrowing of the blocking layer.
Under the influence of these processes, a significant number of electrons will be able to freely move from the p-region to the n-region, as well as in the opposite direction.
The drift current indicators during this process remain the same, since they directly depend only on the number of minority charged carriers located in the region of the pn junction.
Electrons have an increased level of diffusion, which leads to the injection of minority carriers. In other words, in the n-region there will be an increase in the number of holes, and in the p-region an increased concentration of electrons will be recorded.
The lack of equilibrium and an increased number of minority carriers causes them to go deep into the semiconductor and mix with its structure, which ultimately leads to the destruction of its electrical neutrality properties.
In this case, the semiconductor is able to restore its neutral state, this occurs due to the receipt of charges from a connected external source, which contributes to the appearance of direct current in the external electrical circuit.

Diode reverse connection

Now we will consider another method of switching on, during which the polarity of the external source from which the voltage is transmitted changes:

The main difference from direct connection is that the created electric field will have a direction that completely coincides with the direction of the internal diffusion field. Accordingly, the barrier layer will no longer narrow, but, on the contrary, expand.
The field located in the pn junction will have an accelerating effect on a number of minority charge carriers, for this reason, the drift current indicators will remain unchanged. It will determine the parameters of the resulting current that passes through the pn junction.
As the reverse voltage increases, the electric current flowing through the junction will tend to reach its maximum. It has a special name - saturation current.
In accordance with the exponential law, with a gradual increase in temperature, the saturation current indicators will also increase.

Forward and reverse voltage

The voltage that affects the diode is divided according to two criteria:

Direct voltage is the one at which the diode opens and direct current begins to flow through it, while the resistance of the device is extremely low.
Reverse voltage is one that has reverse polarity and ensures that the diode closes with reverse current passing through it. At the same time, the resistance indicators of the device begin to increase sharply and significantly.

The resistance of a pn junction is a constantly changing indicator, primarily influenced by the forward voltage applied directly to the diode. If the voltage increases, then the junction resistance will decrease proportionally.

This leads to an increase in the parameters of the forward current passing through the diode. When this device is closed, virtually the entire voltage is applied to it, for this reason the reverse current passing through the diode is insignificant, and the transition resistance reaches peak parameters.

Diode operation and its current-voltage characteristics

The current-voltage characteristic of these devices is understood as a curved line that shows the dependence of the electric current flowing through the p-n junction on the volume and polarity of the voltage acting on it.

Such a graph can be described as follows:

The axis is located vertically: the upper area corresponds to the forward current values, the lower area to the reverse current parameters.
Horizontal axis: The area on the right is for forward voltage values; area on the left for reverse voltage parameters.
The direct branch of the current-voltage characteristic reflects the passing electric current through the diode. It is directed upward and runs in close proximity to the vertical axis, since it represents the increase in forward electric current that occurs when the corresponding voltage increases.
The second (reverse) branch corresponds to and displays the state of the closed electric current, which also passes through the device. Its position is such that it runs virtually parallel to the horizontal axis. The steeper this branch approaches the vertical, the higher the rectifying capabilities of a particular diode.
According to the graph, it can be observed that after an increase in the forward voltage flowing through the p-n junction, a slow increase in the electric current occurs. However, gradually, the curve reaches an area in which a jump is noticeable, after which an accelerated increase in its indicators occurs. This is due to the diode opening and conducting current at forward voltage. For devices made of germanium, this occurs at a voltage of 0.1V to 0.2V (maximum value 1V), and for silicon elements a higher value is required from 0.5V to 0.6V (maximum value 1.5V).
The indicated increase in current readings can lead to overheating of semiconductor molecules. If the heat removal that occurs due to natural processes and the operation of radiators is less than the level of its release, then the structure of the molecules can be destroyed, and this process will be irreversible. For this reason, it is necessary to limit the forward current parameters to prevent overheating of the semiconductor material. To do this, special resistors are added to the circuit, connected in series with the diodes.
By examining the reverse branch, you can notice that if the reverse voltage applied to the pn junction begins to increase, then the increase in current parameters is virtually unnoticeable. However, in cases where the voltage reaches parameters exceeding the permissible norms, a sudden jump in the reverse current may occur, which will overheat the semiconductor and contribute to the subsequent breakdown of the p-n junction.

Basic diode faults

Sometimes devices of this type fail, this may occur due to natural depreciation and aging of these elements or for other reasons.

In total, there are 3 main types of common faults:

Breakdown of the junction leads to the fact that the diode, instead of a semiconductor device, becomes essentially a very ordinary conductor. In this state, it loses its basic properties and begins to pass electric current in absolutely any direction. Such a breakdown is easily detected using a standard multimeter, which starts beeping and shows a low resistance level in the diode.
When a break occurs, the reverse process occurs - the device generally stops passing electric current in any direction, that is, it essentially becomes an insulator. To accurately determine a break, it is necessary to use testers with high-quality and serviceable probes, otherwise they can sometimes falsely diagnose this malfunction. In alloy semiconductor varieties, such a breakdown is extremely rare.
A leak during which the seal of the device body is broken, as a result of which it cannot function properly.

Breakdown of p-n junction

Such breakdowns occur in situations where the reverse electric current begins to suddenly and sharply increase, this happens due to the fact that the voltage of the corresponding type reaches unacceptable high values.

There are usually several types:

Thermal breakdowns, which are caused by a sharp increase in temperature and subsequent overheating.
Electrical breakdowns that occur under the influence of current on the junction.

The graph of the current-voltage characteristic allows you to visually study these processes and the difference between them.

Electrical breakdown

The consequences caused by electrical breakdowns are not irreversible, since they do not destroy the crystal itself. Therefore, with a gradual decrease in voltage, it is possible to restore all the properties and operating parameters of the diode.

At the same time, breakdowns of this type are divided into two types:

Tunneling breakdowns occur when high voltage passes through narrow junctions, which allows individual electrons to escape through it. They usually occur if semiconductor molecules contain a large number of different impurities. During such a breakdown, the reverse current begins to increase sharply and rapidly, and the corresponding voltage is at a low level.
Avalanche types of breakdowns are possible due to the influence of strong fields that can accelerate charge carriers to the maximum level, due to which they knock out a number of valence electrons from atoms, which then fly into the conductive region. This phenomenon is of an avalanche nature, due to which this type breakdowns and received this name.

Thermal breakdown

The occurrence of such a breakdown can occur for two main reasons: insufficient heat removal and overheating of the p-n junction, which occurs due to the flow of electric current through it at too high rates.

An increase in temperature in the transition and neighboring areas causes the following consequences:

The growth of vibrations of the atoms that make up the crystal.
Electrons entering the conductive band.
A sharp increase in temperature.
Destruction and deformation of the crystal structure.
Complete failure and breakdown of the entire radio component.

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Rectifier diode | Volt-info

Figure 1. Current-voltage characteristic of a rectifying diode.

Current-voltage characteristic of a rectifying diode

In the figure, the first quadrant contains the forward branch, and the third - the reverse branch of the diode characteristic. The direct branch of the characteristic is removed under the action of forward voltage, the reverse branch, respectively, when the reverse voltage is applied to the diode. The forward voltage on the diode is the voltage at which a higher electric potential is formed at the cathode relative to the anode, and if we speak in sign language - at the cathode minus (-), at the anode plus (+), as shown in Figure 2.

Figure 2. Circuit for studying the current-voltage characteristics of a diode when connected directly.

Figure 1 shows the following symbols:

Iр – operating current of the diode;

Ud – voltage drop across the diode;

Uо – diode reverse voltage;

Upr – breakdown voltage;

Iу – leakage current, or reverse current of the diode.

Concepts and designations of characteristics

The operating current of the diode (Ip) is a direct electric current passing through the diode for a long time, during which the device is not subject to irreversible temperature destruction, and its characteristics do not undergo significant qualitative changes. In reference books it may be indicated as direct maximum current. Voltage drop across the diode (Ud) is the voltage at the diode terminals that occurs when a direct operating current passes through it. In reference books it can be designated as forward voltage on the diode.

Direct current flows when the diode is connected directly.

Diode reverse voltage (Uо) is the permissible reverse voltage on the diode, applied to it for a long time, at which irreversible destruction of its p-n junction does not occur. In reference literature it may be called maximum reverse voltage.

Breakdown voltage (Upr) is the reverse voltage on the diode, at which irreversible electrical breakdown of the p-n junction occurs, and, as a result, failure of the device.

Diode reverse current, or leakage current (Iу) is a reverse current that does not cause irreversible destruction (breakdown) of the p-n junction of the diode for a long time.

When choosing rectifier diodes, they are usually guided by the above characteristics.

Diode operation

Subtleties work p-n transition, a topic for a separate article. Let's simplify the problem and consider the operation of the diode from the perspective of one-way conductivity. And so, the diode works as a conductor when connected forward, and as a dielectric (insulator) when connected in reverse. Consider the two circuits in Figure 3.

Figure 3. Reverse (a) and forward (b) connection of the diode.

The figure shows two versions of the same circuit. In Figure 3 (a), the position of switches S1 and S2 ensures electrical contact of the diode anode with the minus of the power source, and the cathode through the HL1 light bulb with the plus. As we have already decided, this is the reverse connection of the diode. In this mode, the diode will behave as an electrically insulating element, the electrical circuit will be practically open, and the lamp will not light.

When changing the position of contacts S1 and S2, Figure 3 (b), electrical contact is provided between the anode of the diode VD1 and the plus of the power source, and the cathode through the light bulb with the minus. In this case, the condition for direct switching of the diode is met, it “opens” and the load (lamp) current flows through it, like through a conductor.

If you have just begun to study electronics, you may be a little confused by the complexity of the switches in Figure 3. Draw an analogy based on the given description, based on the simplified diagrams in Figure 4. This exercise will allow you to understand and orient yourself a little regarding the principle of construction and reading electrical diagrams.

Figure 4. Diagram of reverse and direct connection of a diode (simplified).

In Figure 4, the change in polarity at the diode terminals is ensured by changing the position of the diode (by turning it over).

Unidirectional diode conduction

Figure 5. Voltage diagrams before and after the rectifier diode.

Let us assume conditionally that the electric potential of switch S2 is always equal to 0. Then the voltage difference –US1-S2 and +US1-S2 will be applied to the anode of the diode, depending on the position of switches S1 and S2. A diagram of such a rectangular alternating voltage is shown in Figure 5 (top diagram). When the voltage difference at the anode of the diode is negative, it is locked (works as an insulating element), while no current flows through the HL1 lamp and it does not burn, and the voltage across the lamp is almost zero. When the voltage difference is positive, the diode is turned on (acts as an electrical conductor) and current flows through the diode-lamp series circuit. The lamp voltage increases to UHL1. This voltage is slightly less than the power supply voltage because part of the voltage drops across the diode. For this reason, voltage differences are sometimes called "voltage drops" in electronics and electrical engineering. Those. in this case, if the lamp is considered as a load, then there will be a load voltage across it, and a voltage drop across the diode.

Thus, periods of a negative voltage difference are, as it were, ignored by the diode, cut off, and current flows through the load only during periods of a positive voltage difference. This conversion of alternating voltage into unipolar (pulsating or direct) is called rectification.

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1. Semiconductor diodes, operating principle, characteristics:

SEMICONDUCTOR DIODE - a semiconductor device with two electrodes that has one-way conductivity. Semiconductor diodes include a wide group of devices with p-n junctions, metal-semiconductor contacts, etc. The most common are electrical-converting semiconductor diodes. Serve to transform and generate electrical vibrations. One of the main modern electronic devices. Principle of operation of a semiconductor diode: The principle of operation of a semiconductor diode is based on the properties of the electron-hole junction, in particular, the strong asymmetry of the current-voltage characteristic relative to zero. In this way, a distinction is made between direct and reverse connection. When connected directly, the diode has low electrical resistance and conducts electricity well. In the opposite way - at a voltage less than the breakdown voltage, the resistance is very high and the current is blocked. Characteristics:

2. Semiconductor diodes, direct and reverse connection, voltage:

Direct and reverse connection:

When a p-n junction is directly connected, an external voltage creates a field in the junction that is opposite in direction to the internal diffusion field. The strength of the resulting field decreases, which is accompanied by a narrowing of the blocking layer. As a result, a large number of majority charge carriers are able to diffusely move into the neighboring region (the drift current does not change, since it depends on the number of minority carriers appearing at the boundaries of the transition), i.e. a resulting current will flow through the junction, determined mainly by the diffusion component. The diffusion current depends on the height of the potential barrier and increases exponentially as it decreases.

Increased diffusion of charge carriers through the junction leads to an increase in the concentration of holes in the n-type region and electrons in the p-type region. This increase in minority carrier concentration due to the influence of an external voltage applied to the junction is called minority carrier injection. Nonequilibrium minority carriers diffuse deep into the semiconductor and disrupt its electrical neutrality. The restoration of the neutral state of the semiconductor occurs due to the supply of charge carriers from an external source. This is the reason for the occurrence of current in the external circuit, called direct.

When a pn junction is turned on in the reverse direction, an external reverse voltage creates an electric field that coincides in direction with the diffusion field, which leads to an increase in the potential barrier and an increase in the width of the blocking layer. All this reduces the diffusion currents of the majority carriers. For minority carriers, the field in the pn junction remains accelerating, and therefore the drift current does not change.

Thus, a resulting current will flow through the junction, determined mainly by the minority carrier drift current. Since the number of drifting minority carriers does not depend on the applied voltage (it only affects their speed), then as the reverse voltage increases, the current through the junction tends to the limiting value IS, which is called the saturation current. The higher the concentration of donor and acceptor impurities, the lower the saturation current, and with increasing temperature the saturation current grows exponentially.

The graph shows the current-voltage characteristics for forward and reverse connection of the diode. They also say the forward and reverse branches of the current-voltage characteristic. The direct branch (Ipr and Upr) displays the characteristics of the diode when connected directly (that is, when “plus” is applied to the anode). The reverse branch (Irev and Urev) displays the characteristics of the diode when turned on in reverse (that is, when “minus” is applied to the anode).

The blue thick line is the characteristic of a germanium (Ge) diode, and the black thin line is the characteristic of a silicon (Si) diode. The figure does not show units of measurement for the current and voltage axes, since they depend on the specific brand of diode.

To begin with, let us define, as for any flat coordinate system, four coordinate angles (quadrants). Let me remind you that the first quadrant is considered to be the one located at the top right (that is, where we have the letters Ge and Si). Next, the quadrants are counted counterclockwise.

So, our II and IV quadrants are empty. This is because we can only turn on the diode in two ways - forward or reverse. A situation is impossible when, for example, a reverse current flows through a diode and at the same time it is switched on in the forward direction, or, in other words, it is impossible to simultaneously apply both “plus” and “minus” to one output. More precisely, it is possible, but then it will be a short circuit. There are only two cases left to consider: direct connection of a diode and reverse connection of a diode.

The direct connection graph is drawn in the first quadrant. This shows that the greater the voltage, the greater the current. Moreover, up to a certain point, the voltage increases faster than the current. But then a turning point occurs, and the voltage remains almost unchanged, but the current begins to increase. For most diodes, this turning point occurs in the range of 0.5...1 V. It is this voltage that is said to “drop” across the diode. This 0.5...1 V is the voltage drop across the diode. A slow increase in current to a voltage of 0.5...1V means that in this section there is practically no current flowing through the diode, even in the forward direction.

The reverse switching graph is drawn in the third quadrant. From this it can be seen that over a significant area the current remains almost unchanged, and then increases like an avalanche. If you increase the voltage, for example, to several hundred volts, then this high voltage will “break through” the diode, and current will flow through the diode. But “breakdown” is an irreversible process (for diodes). That is, such a “breakdown” will lead to burnout of the diode and it will either completely stop passing current in any direction, or vice versa – it will pass current in all directions.

The characteristics of specific diodes always indicate the maximum reverse voltage - that is, the voltage that the diode can withstand without “breakdown” when turned on in the reverse direction. This must be taken into account when developing devices that use diodes.

Comparing the characteristics of silicon and germanium diodes, we can conclude that in the p-n junctions of a silicon diode, the forward and reverse currents are less than in a germanium diode (at the same voltage values at the terminals). This is due to the fact that silicon has a larger band gap and for electrons to move from the valence band to the conduction band, they need to be given more additional energy.

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The maximum reverse voltage on the diodes is determined by the formula

Urev. max = 1.045Uav.

In a number of practical applications, thyristor converters are used to rectify alternating current and smoothly control the power transmitted to the load. At the same time, small control currents make it possible to control large load currents.

An example of the simplest power-controlled thyristor rectifier is shown in Fig. 7.10.

Rice. 7.10. Thyristor rectifier circuit

In Fig. Figure 7.11 shows timing diagrams that explain the principle of regulating the average value of the rectified voltage.

Rice. 7.11. Timing diagrams of thyristor rectifier operation

In this circuit, it is assumed that the input voltage Uin for an adjustable thyristor is generated, for example, by a full-wave rectifier. If control pulses Uу of sufficient amplitude are supplied at the beginning of each half-cycle ( area o-a on the Uout diagram), the output voltage will repeat the voltage of the full-wave rectifier. If you shift the control pulses to the middle of each half-cycle, then the output pulses will have a duration equal to a quarter of the half-cycle (section b-c). Further displacement of the control pulses will lead to a further decrease in the average amplitude of the output pulses (section d – e).

Thus, by applying control pulses to the thyristor that are phase-shifted relative to the input voltage, you can turn a sinusoidal voltage (current) into a sequence of pulses of any duration, amplitude and polarity, that is, you can change the effective value of the voltage (current) within a wide range.

7.3 Anti-aliasing filters

The considered rectification circuits make it possible to obtain a unipolar pulsating voltage, which is not always applicable for powering complex electronic devices, since, due to large pulsations, they lead to instability of their operation.

To significantly reduce ripple, smoothing filters are used. The most important parameter of the smoothing filter is the smoothing coefficient S, determined by the formula S=1/2, where 1 and 2 are the ripple coefficients at the input and output of the filter, respectively. The ripple factor shows how many times the filter reduces ripple. In practical circuits, the ripple factor at the filter output can reach values of 0.00003.

The main elements of filters are reactive elements - capacitance and inductance (chokes). Let us first consider the principle of operation of the simplest anti-aliasing filter, the diagram of which is shown in Fig. 7.12.

Rice. 7.12. Circuit of the simplest smoothing filter with a half-wave rectifier

In this circuit, smoothing of the voltage across the load after a half-wave diode rectifier VD is carried out using a capacitor C connected in parallel with the load Rн.

Timing diagrams explaining the operation of such a filter are shown in Fig. 7.13. In the t1 – t2 section, the input voltage opens the diode and charges the capacitor. When the input voltage begins to decrease, the diode closes with the voltage accumulated on the capacitor Uc (section t1 - t2). During this interval, the input voltage source is disconnected from the capacitor and the load, and the capacitor is discharged through the load resistance Rн.

Rice. 7.13. Timing diagrams of filter operation with a half-wave rectifier

If the capacitance is large enough, the discharge of the capacitance through Rн will occur with a large time constant =RнС, and therefore, the decrease in voltage on the capacitor will be small, and the smoothing effect will be significant. On the other hand, the larger the capacitance, the shorter the segment t1 - t2 during which the diode is open and current i flows through it, increasing (for a given average load current) as the difference t2 - t1 decreases. This mode of operation can lead to failure of the rectifier diode, and, in addition, is quite heavy for the transformer.

When using full-wave rectifiers, the amount of ripple at the output of the capacitive filter decreases, since the capacitor is smaller during the time between the appearance of pulses, which is well illustrated in Fig. 7.14.

Rice. 7.14. Full-Wave Rectifier Ripple Smoothing

To calculate the magnitude of ripple at the output of a capacitive filter, we will approximate the ripple of the output voltage using a sawtooth current curve, as shown in Fig. 7.15.

Rice. 7.15. Ripple voltage approximation

The change in charge on the capacitor is given by the expression

∆Q=∆UC=I nT1,

where T1 is the pulsation period, In is the average value of the load current. Taking into account the fact that Iн = Иср/ Rн, we obtain

From Fig. 7.15 it follows that

in this case, the double amplitude of the pulsations is determined by the expression

Inductive filters also have smoothing properties, and the best smoothing properties are found in filters containing inductance and capacitance connected as shown in Fig. 7.16.

Rice. 7.16. Anti-aliasing filter with inductance and capacitance

In this circuit, the capacitance of the capacitor is selected so that its reactance is significantly less than the load resistance. The advantage of such a filter is that it reduces the value of the input ripple ∆U to a value where ω is the ripple frequency.

In practice, various types of F-shaped and U-shaped filters have become widespread, the construction options of which are presented in Fig. 7.17.

At low load currents, the F-shaped rectifier shown in Fig. works well. 7.16.

Rice. 7.17. Filter construction options

In the most critical schemes, multi-link filtering circuits are used (Fig. 7.17 d).

Often the inductor is replaced with resistors, which somewhat reduces the quality of filtration, but significantly reduces the cost of filters (Fig. 7.17 b, c).

The main external characteristic of rectifiers with a filter is the dependence of the average value of the output voltage Uav (load voltage) on the average value of the output current.

In the considered circuits, an increase in the output current leads to a decrease in Uav due to an increase in the voltage drop across the transformer windings, diodes, lead wires, and filter elements.

The slope of the external characteristic at a given average current is determined through the output resistance Rout, determined by the formula:

Icр – set. The smaller the value of Rout, the less the output voltage depends on the output current, the better the rectifier circuit with a filter. In Fig. Figure 7.18 shows typical dependences of Uav on Iav for various filtration options.

Rice. 7.18. Typical dependences of Uav on Iav for various filtering schemes

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What is reverse voltage? - Interior renovation construction

Reverse voltage

Reverse voltage is a type of energy signal created when the polarity of an electrical current is reversed. This voltage often occurs when reverse polarity is applied to a diode, causing the diode to react by operating in the opposite direction. This reverse function can also create a breakdown voltage within the diode, as this often breaks the circuit to which the voltage is applied.

Reverse voltage occurs when the power signal connection source to a circuit is applied in an inverted manner. This means that the positive lead source is connected to the ground or negative conductor of the circuit and vice versa. This voltage transfer is often not intended, as most electrical circuits are not capable of handling voltages.

When minimum voltage is applied to a circuit or diode, it may cause the circuit or diode to operate in reverse. This may cause a reaction such as the box fan motor turning incorrectly. The element will continue to function in such cases.

When the amount of voltage applied to a circuit is too large, the signal for the receiving circuit, however, is called breakdown voltage. If the input signal that has been reversed exceeds the allowable voltage for the circuit to maintain, the circuit may be damaged beyond the rest of the usable. The point at which the circuit is damaged refers to the breakdown voltage value. This breakdown voltage has a couple of other names, reverse peak voltage or reverse breakdown voltage.

Reverse voltage can cause breakdown voltage, which also affects the operation of other circuit components. Beyond the damaging diodes and reverse voltage circuit functions, it can also become a reverse voltage peak. In such cases, the circuit cannot contain the amount of input power from the signal that has been reversed, and may create a breakdown voltage between the insulators.

This breakdown voltage, which can occur across circuit components, can cause breakdown of components or wire insulators. This can turn them into signal conductors and damage the circuit by conducting voltage to different parts of the circuit that should not receive it, causing instability throughout the circuit. This can cause voltage arcs from component to component, which can also be powerful enough to ignite various circuit components and cause a fire.

TT system in electrical installations with voltage up to 1000V

Published Date: 12/23/2017

Do you know what reverse voltage is?

Reverse voltage

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Characteristics and parameters of rectifier and universal diodes

Rectifier diodes are used to rectify low frequency alternating current. The rectifying properties of these diodes are based on the principle of one-way conductivity of electron-hole p-and-junctions.

Universal diodes are used in various electronic equipment as high and low frequency AC rectifiers, multipliers and frequency converters, detectors of large and small signals, etc. The range of operating currents and voltages of rectifier and universal diodes is very wide, therefore, they are produced with both point and planar pn junctions in the semiconductor structure with areas from tenths of a square millimeter to several square centimeters. Typically, universal diodes use junctions with small areas and capacitances, but with relatively high values of forward currents and reverse voltages. These requirements are met by point, microalloy planar and mesaplanar diodes. The characteristics and parameters of universal diodes are the same as those of rectifier diodes.

Volt-ampere characteristics(volt-voltage characteristic) of rectifier diodes expresses the dependence of the current passing through the diode on the value and polarity of the direct voltage applied to it. The direct branch of the characteristic shows the dependence of the current through the diode with the direct through polarity of the applied voltage. The strength of the forward current depends exponentially on the forward voltage applied to the diode and can reach large values with a small (about 0.3 - 1 V) voltage drop across the diode.

The reverse branch of the characteristic corresponds to the non-conducting direction of current through the diode with reverse polarity of the voltage applied to the diode. The reverse current (section OD) slightly depends on the applied reverse voltage. At a relatively high reverse voltage (point B on the characteristic), electrical breakdown of the p-n junction occurs, at which the reverse current quickly increases, which can lead to thermal breakdown and damage to the diode. As the temperature rises, the thermal current and the generation current of charge carriers in the junction will increase, which will lead to an increase in forward and reverse currents and a shift in the characteristics of the diode.

The properties and interchangeability of diodes are assessed by their parameters. The main parameters include currents and voltages associated with the current-voltage characteristic Diodes are used in both AC and DC circuits. Therefore, to evaluate the properties of diodes, along with the parameters, differential parameters are used that characterize their operation on alternating current.

Rectified (direct) current Ipr is the current (average value per period) passing through the diode, which ensures its reliable and long-term operation. The strength of this current is limited by heating or maximum power Pmax. Exceeding the forward current leads to thermal breakdown and damage to the diode.

Forward voltage drop UPr.Av - average value over a period on the diode when the permissible forward current passes through it.
Allowable reverse voltage U0br is the average value over the period at which reliable and long-term operation of the diode is ensured. Exceeding the reverse voltage leads to breakdown and failure of the diodes. As the temperature increases, the reverse voltage and forward current values decrease.
Reverse current Irev - average value for the period of reverse current at an acceptable Urev. The lower the reverse current, the better

You are the rectifying properties of the diode. An increase in temperature for every 10 °C leads to an increase in the reverse current for germanium and silicon diodes by 1.5 - 2 times or more.

Maximum constant, or the average power Pmax dissipated by a diode over a period, at which the diode can operate for a long time without changing its parameters. This power is the sum of the products of currents and voltages at the forward and reverse biases of the junction, i.e., for the positive and negative half-cycles of the alternating current. For high-power devices operating with good heat dissipation, Pmax = (Tp.max - Tk)/Rpk. For low power devices operating without a heat sink,

Pmax = (Tp.max - T s) / Rp.s.

Maximum junction temperature Gp.max depends on the material (band gap) of the semiconductor and the degree of its doping, i.e., on the resistivity of the p-n junction region - the base. The Gp.max range for germanium lies within 80 - 110 °C, and for silicon 150 - 220 °C.

Thermal resistance Rp.k between the transition and the housing is determined by the temperature difference between the junction Tpi housing Tk and the average power Ra released in the transition and is 1 - 3 ° C / W: Ra.K = (Ta - TK) / Pa. The thermal resistance Rn c between the junction and the environment depends on the temperature difference between the junction Tp and the environment Tc. Since practically RPK

The limiting mode of diode use is characterized by the maximum permissible reverse voltage URev max, the maximum rectifier current IPr max and the maximum junction temperature TPmax. With an increase in the frequency of the alternating voltage supplied to the diode, its rectifying properties deteriorate. Therefore, to determine the properties of rectifier diodes, the operating frequency range Df or the maximum rectification frequency fmax is usually specified. At frequencies greater than fmax, the minority charge carriers accumulated during the forward half-cycle in the base do not have time to compensate, therefore, during the reverse half-cycle of the rectified voltage, the transition remains forward-biased for some time (that is, it loses its rectifying properties). This property is manifested more significantly, the larger the forward current pulse or the higher the frequency of the supplied alternating voltage. In addition, on high frequencies the shunting effect of the barrier and diffusion capacitances of the p-n junction begins to appear, reducing its rectifying properties

When calculating the rectifier mode, the static resistance to direct current and the differential resistance of diodes to alternating current are used

Differential alternating current resistance rdiff=dU/dI or rDiff=ДU/ДI determines the change in current through the diode when the voltage changes near the selected operating point on the diode characteristic. When the voltage is directly switched on, rdif Pr = 0.026/ /IPr and the current Ipr > 10 mA, it amounts to several ohms. When connecting the reverse voltage, rdif pr is large (from tens of kilo-ohms to several mega-ohms).
Static diode resistance to direct current rprd = Upr/Ipr, rrev d = Urev/Irev V In the region of forward currents rFor d>rdiff pr, and in the region of reverse currents r0br d

Diode capacitances have a significant impact on their performance at high frequencies and in pulsed modes. The passport data of diodes usually gives the total capacitance of the diode CD, which, in addition to the barrier and diffusion capacitance, includes the capacitance of the device body. This capacitance is measured between the external current leads of the diode at a given reverse bias voltage and current frequency

Semiconductor diode - This is a semiconductor device with one p-n junction and two electrodes. The principle of operation of a semiconductor diode is based on the phenomenon of p-n junction, so for further study of any semiconductor devices you need to know how it works.

Rectifier diode (also called a valve) is a type of semiconductor diode that serves to convert alternating current to direct current.

The diode has two terminals (electrodes) anode and cathode. The anode is connected to the p layer, the cathode to the n layer. When a plus is applied to the anode and a minus to the anode (direct connection of the diode), the diode passes current. If a minus is applied to the anode and a plus to the cathode (reverse connection of the diode), there will be no current through the diode, this can be seen from the volt-ampere characteristics of the diode. Therefore, when an alternating voltage is supplied to the input of the rectifier diode, only one half-wave passes through it.

Current-voltage characteristic (volt-ampere characteristic) of the diode.

The current-voltage characteristic of the diode is shown in Fig. I. 2. The first quadrant shows the direct branch of the characteristic, which describes the state of high conductivity of the diode with a forward voltage applied to it, which is linearized by a piecewise linear function

u = U 0 +R D i

where: u is the voltage on the valve when current i passes; U 0 - threshold voltage; R d - dynamic resistance.

In the third quadrant there is a reverse branch of the current-voltage characteristic, which describes the state of low conductivity when a reverse voltage is applied to the diode. In a state of low conductivity, practically no current flows through the semiconductor structure. However, this is only true up to a certain reverse voltage value. With reverse voltage, when the electric field strength in the pn junction reaches about 10 s V/cm, this field can impart to mobile charge carriers - electrons and holes, constantly appearing throughout the entire volume of the semiconductor structure as a result of thermal generation - kinetic energy sufficient for ionization neutral silicon atoms. The resulting holes and conduction electrons, in turn, are accelerated by electrical p-n field transition and also ionize neutral silicon atoms. In this case, an avalanche-like increase in the reverse current occurs, i.e. e. avalanche breakdown.

The voltage at which a sharp increase in reverse current occurs is called breakdown voltage U 3 .

TOPIC 3. SEMICONDUCTOR DIODES

A semiconductor diode is an electrical converting semiconductor device with one electrical junction and two terminals, which uses the properties pn junction A.

Semiconductor diodes are classified:

1) by purpose: rectifier, high-frequency and ultra-high-frequency (HF and microwave diodes), pulse, semiconductor zener diodes (reference diodes), tunnel diodes, reverse diodes, varicaps, etc.;

2) according to design and technological features: planar and point;

3) by type of source material: germanium, silicon, arsenide-gallium, etc.

Figure 3.1 – Design of point diodes

A point diode uses a germanium or silicon plate with n-type electrical conductivity (Fig. 3.1), 0.1...0.6 mm thick and 0.5...1.5 mm2 in area; A sharpened wire (needle) with an impurity deposited on it comes into contact with the plate. In this case, impurities diffuse from the needle into the main semiconductor, which create an area with a different type of electrical conductivity. Thus, a miniature hemispherical pn junction is formed near the needle.

To make germanium point diodes, a tungsten wire coated with indium is welded to a germanium plate. Indium is an acceptor for germanium. The resulting region of p-type germanium is emitter.

Silicon point diodes are made using n-type silicon and a wire coated with aluminum, which serves as an acceptor for the silicon.

In planar diodes, a pn junction is formed by two semiconductors with different types of electrical conductivity, and the junction area of different types of diodes ranges from hundredths of a square millimeter to several tens of square centimeters (power diodes).

Planar diodes are manufactured by fusion (fusing) or diffusion methods (Fig. 3.2).

Figure 3.2 – Design of planar diodes manufactured by the alloy (a) and diffusion method (b)

A drop of indium is fused into a plate of n-type germanium at a temperature of about 500°C (Fig. 3.2, a), which, fused with germanium, forms a layer of p-type germanium. The region with p-type electrical conductivity has a higher impurity concentration than the main plate and is therefore an emitter. Lead wires, usually made of nickel, are soldered to the main germanium plate and to the indium plate. If p-type germanium is taken as the starting material, then antimony is smelted into it and then an n-type emitter region is obtained.

The diffusion method of manufacturing a p-n junction is based on the fact that impurity atoms diffuse into the main semiconductor (Fig. 3.2, b). To create a p-layer, the diffusion of an acceptor element (boron or aluminum for silicon, indium for germanium) through the surface of the source material is used.

3.1 Rectifier diodes

A rectifying semiconductor diode is a semiconductor diode designed to convert alternating current into direct current.

Rectifier diodes are made on the basis of a pn junction and have two regions, one of them is lower resistance (contains a higher impurity concentration), and is called the emitter. The other area, the base, is more highly resistant (contains a lower concentration of impurities).

The operation of rectifier diodes is based on the property of one-way conductivity of the p-n junction, which lies in the fact that the latter conducts current well (has low resistance) when connected directly and practically does not conduct current (has a very high resistance) when connected in reverse.

As is known, the forward current of the diode is created by the main ones, and the reverse current is created by non-primary charge carriers. The concentration of majority charge carriers is several orders of magnitude higher than the concentration of non-majority carriers, which determines the valve properties of the diode.

The main parameters of rectifying semiconductor diodes are:

· forward current of the diode Ipr, which is normalized at a certain forward voltage (usually Upr = 1...2V);

· maximum permissible forward current Ipr max diode;

· the maximum permissible reverse voltage of the diode Urev max, at which the diode can still operate normally for a long time;

· constant reverse current Irev flowing through the diode at a reverse voltage equal to Urev max;

· average rectified current Ivp.sr, which can pass through the diode for a long time at an acceptable temperature of its heating;

· maximum permissible power Pmax dissipated by the diode, at which the specified reliability of the diode is ensured.

According to the maximum permissible value of the average rectified current, diodes are divided into low-power (Ivp.av £ 0.3A), medium-power (0.3A 10A).

To maintain the performance of a germanium diode, its temperature should not exceed +85°C. Silicon diodes can operate at temperatures up to +150°C.

Figure 3.3 – Change in the volt-ampere characteristics of a semiconductor diode depending on temperature: a – for a germanium diode; b – for a silicon diode

The voltage drop when passing direct current for germanium diodes is DUpr = 0.3...0.6V, for silicon diodes - DUpr = 0.8...1.2V. Large voltage drops when direct current passes through silicon diodes compared to direct voltage drops on germanium diodes are associated with a higher potential barrier height of p-n junctions formed in silicon.

With increasing temperature, the forward voltage drop decreases, which is associated with a decrease in the height of the potential barrier.

When a reverse voltage is applied to a semiconductor diode, a slight reverse current arises in it, due to the movement of minority charge carriers through the pn junction.

As the temperature of the pn junction increases, the number of minority charge carriers increases due to the transition of some electrons from the valence band to the conduction band and the formation of electron-hole charge carrier pairs. Therefore, the reverse current of the diode increases.

When a reverse voltage of several hundred volts is applied to the diode, the external electric field in the blocking layer becomes so strong that it can pull electrons from the valence band into the conduction band (Zener effect). In this case, the reverse current increases sharply, which causes heating of the diode, a further increase in the current and, finally, thermal breakdown (destruction) of the p-n junction. Most diodes can operate reliably at reverse voltages not exceeding (0.7...0.8) Uprob.

The permissible reverse voltage of germanium diodes reaches - 100...400V, and for silicon diodes - 1000...1500V.

In a number of powerful converter installations, the requirements for the average value of the forward current and reverse voltage exceed the nominal value of the parameters of existing diodes. In these cases, the problem is solved by parallel or series connection of diodes.

Parallel connection of diodes is used when it is necessary to obtain a forward current greater than the limiting current of one diode. But if diodes of the same type are simply connected in parallel, then, due to the mismatch of the direct branches of the current-voltage characteristic, they will be differently loaded and, in some, the forward current will be greater than the limiting one.

Figure 3.4 – Parallel connection of rectifier diodes

To equalize the currents, diodes with a small difference in the direct branches of the current-voltage characteristic are used (they are selected) or equalizing resistors with a resistance of units of Ohms are connected in series with the diodes. Sometimes additional resistors are included (Fig. 3.4, c) with a resistance several times greater than the direct resistance of the diodes, so that the current in each diode is determined mainly by the resistance Rd, i.e. Rd>>rpr vd. The value of Rd is hundreds of ohms.

Series connection of diodes is used to increase the total permissible reverse voltage. When exposed to reverse voltage, the same reverse current Irev flows through diodes connected in series. however, due to the difference in the reverse branches of the current-voltage characteristic, the total voltage will be distributed unevenly across the diodes. A diode whose reverse branch of the current-voltage characteristic is higher will have a greater voltage applied to it. It may be higher than the limit, which will lead to breakdown of the diodes.

Figure 3.5 – Series connection of rectifier diodes

To ensure that the reverse voltage is distributed evenly between the diodes regardless of their reverse resistance, diodes are shunted with resistors. The resistances Rsh of the resistors must be the same and significantly less than the smallest reverse resistance of the diodes Rsh 3.2 Zener diodes

A semiconductor zener diode is a semiconductor diode, the voltage on which in the region of electrical breakdown weakly depends on the current and which is used to stabilize the voltage.

Semiconductor zener diodes use the property of a slight change in the reverse voltage at the p-n junction during an electrical (avalanche or tunnel) breakdown. This is due to the fact that a small increase in the voltage at the pn junction in the electrical breakdown mode causes a more intense generation of charge carriers and a significant increase in the reverse current.

Low-voltage zener diodes are made on the basis of heavily alloyed (low-resistance) material. In this case, a narrow planar junction is formed, in which a tunneling electrical breakdown occurs at relatively low reverse voltages (less than 6V). High-voltage zener diodes are made on the basis of lightly alloyed (high-resistance) material. Therefore, their principle of operation is associated with avalanche electrical breakdown.

Main parameters of zener diodes:

· stabilization voltage Ust (Ust = 1…1000V);

· minimum Ist mіn and maximum Ist max stabilization currents (Ist mіn" 1.0...10 mA, Ist max "0.05...2.0A);

· maximum permissible power dissipation Рmax;

· differential resistance in the stabilization section rd = DUst/DIst, (rd" 0.5...200 Ohm);

temperature coefficient of voltage in the stabilization section:

TKU of a zener diode shows by what percentage the stabilizing voltage will change when the temperature of the semiconductor changes by 1°C

(TKU= −0.5…+0.2%/°С).

Figure 3.6 – Volt-ampere characteristic of the zener diode and its symbolic graphic designation

Zener diodes are used to stabilize the voltages of power supplies, as well as to fix voltage levels in various circuits.

Low-voltage voltage stabilization within 0.3...1V can be achieved by using the direct branch of the I-V characteristic of silicon diodes. A diode in which the direct branch of the current-voltage characteristic is used to stabilize the voltage is called a stabistor. There are also double-sided (symmetrical) zener diodes that have a symmetrical current-voltage characteristic relative to the origin.

Zener diodes can be connected in series, with the resulting stabilizing voltage equal to the sum of the zener diode voltages:

Ust = Ust1 + Ust2 +…

Parallel connection of zener diodes is unacceptable, because due to the scatter of characteristics and parameters of all parallel-connected zener diodes, current will arise only in one, which has the lowest stabilizing voltage Ust, which will cause overheating of the zener diode.

3.3 Tunnel and reverse diodes

A tunnel diode is a semiconductor diode based on a degenerate semiconductor, in which the tunnel effect leads to the appearance of a negative differential resistance section on the current-voltage characteristic at forward voltage.

The tunnel diode is made of germanium or gallium arsenide with a very high concentration of impurities, i.e. with very low resistivity. Such semiconductors with low resistance are called degenerate. This makes it possible to obtain a very narrow pn junction. In such transitions, conditions arise for relatively free tunneling of electrons through a potential barrier (tunnel effect). The tunnel effect leads to the appearance of a section with negative differential resistance on the direct branch of the diode’s current-voltage characteristic. The tunnel effect is that at a sufficiently low height of the potential barrier, electrons can penetrate through the barrier without changing their energy.

Main parameters of tunnel diodes:

· peak current Iп – forward current at the maximum point of the current-voltage characteristic;

· valley current Iв – forward current at the minimum point of the current-voltage characteristic;

· ratio of tunnel diode currents Iп/Iв;

· peak voltage Uп – forward voltage corresponding to the peak current;

· valley voltage Uв – forward voltage corresponding to the valley current;

· solution voltage Uрр.

Tunnel diodes are used to generate and amplify electromagnetic oscillations, as well as in high-speed switching and pulse circuits.

Figure 3.7 – Current-voltage characteristic of a tunnel diode

A reverse diode is a diode based on a semiconductor with a critical concentration of impurities, in which the conductivity at reverse voltage due to the tunneling effect is significantly greater than at forward voltage.

The principle of operation of a reverse diode is based on the use of the tunnel effect. But in reverse diodes the concentration of impurities is lower than in conventional tunnel diodes. Therefore, the contact potential difference for reversed diodes is smaller, and the thickness of the pn junction is greater. This leads to the fact that under the influence of direct voltage, a direct tunnel current is not created. The forward current in reversed diodes is created by the injection of non-majority charge carriers through the p-n junction, i.e. direct current is diffusion. When the voltage is reversed, a significant tunneling current flows through the junction, created by the movement of electrons through the potential barrier from the p-region to the n-region. The working section of the current-voltage characteristic of a reversed diode is the reverse branch.

Thus, reversed diodes have a rectifying effect, but their passing (conducting) direction corresponds to reverse connection, and the blocking (non-conducting) direction corresponds to direct connection.

Figure 3.8 – Volt-ampere characteristic of a reversed diode

Reversed diodes are used in pulse devices, and also as signal converters (mixers and detectors) in radio engineering devices.

3.4 Varicaps

A varicap is a semiconductor diode that uses the dependence of capacitance on the magnitude of the reverse voltage and is intended for use as an element with electrically controlled capacitance.

The semiconductor material for the manufacture of varicaps is silicon.

Basic parameters of varicaps:

· nominal capacitance Sv – capacitance at a given reverse voltage (Sv = 10...500 pF);

Capacity overlap coefficient; (Ks = 5...20) – the ratio of the varicap capacitances at two given values of reverse voltages.

Varicaps are widely used in various circuits for automatic frequency adjustment and in parametric amplifiers.

Figure 3.9 – Capacitance-voltage characteristic of a varicap

3.5 Calculation of electrical circuits with semiconductor diodes.

In practical circuits, some load, for example a resistor, is connected to the diode circuit (Fig. 3.10, a). Direct current flows when the anode has a positive potential relative to the cathode.

The mode of the diode with a load is called the operating mode. If the diode had linear resistance, then calculating the current in such a circuit would not be difficult, since the total resistance of the circuit is equal to the sum of the diode's resistance to direct current Ro and the resistance of the load resistor Rн. But the diode has a nonlinear resistance, and its Ro value changes as the current changes. Therefore, the current calculation is done graphically. The task is as follows: the values of E, Rn and the characteristics of the diode are known; it is necessary to determine the current in the circuit I and the voltage on the diode Ud.

Figure 3.10

The diode characteristic should be considered as a graph of some equation connecting the quantities I and U. And for resistance Rн, a similar equation is Ohm’s law:

(3.1)

So, there are two equations with two unknowns I and U, and one of the equations is given graphically. To solve such a system of equations, you need to construct a graph of the second equation and find the coordinates of the point of intersection of the two graphs.

The equation for resistance Rн is an equation of the first degree with respect to I and U. Its graph is a straight line called the load line. It is constructed using two points on the coordinate axes. For I= 0, from equation (3.1) we obtain: E − U= 0 or U= E, which corresponds to point A in Fig. 3.10, b. And if U= 0, then I= E/Rн. we plot this current on the ordinate axis (point B). We draw a straight line through points A and B, which is the load line. The coordinates of point D give the solution to the problem.

It should be noted that a graphical calculation of the diode operating mode can be omitted if Rн >> Ro. In this case, it is permissible to neglect the resistance of the diode and determine the current approximately: I»E/Rн.

The considered method for calculating direct voltage can be applied to amplitude or instantaneous values if the source provides alternating voltage.

Since semiconductor diodes conduct current well in the forward direction and poorly in the reverse direction, most semiconductor diodes are used to rectify alternating current.

The simplest circuit for rectifying alternating current is shown in Fig. 3.11. It is connected in series with a source of alternating emf - e, a diode VD and a load resistor Rн. This circuit is called half-wave.

The simplest rectifier works as follows. During one half-cycle, the voltage for the diode is direct and a current passes, creating a voltage drop UR across the resistor Rн. During the next half-cycle, the voltage is reversed, there is practically no current and UR = 0. Thus, a pulsating current passes through the diode and load resistor in the form of pulses lasting half a cycle. This current is called rectified current. It creates a rectified voltage across the resistor Rн. Graphs in Fig. 3.11, b illustrate the processes in the rectifier.

Figure 3.11

The amplitude of the positive half-waves on the diode is very small. This is explained by the fact that when direct current passes, most of the source voltage drops across the load resistor Rн, the resistance of which significantly exceeds the resistance of the diode. In this case

For conventional semiconductor diodes, the forward voltage is no more than 1...2V. For example, let the source have an effective voltage E = 200V and . If Up max = 2V, then URmax = 278V.

With a negative half-wave of the supplied voltage, there is practically no current and the voltage drop across the resistor Rн is zero. The entire source voltage is applied to the diode and is the reverse voltage for it. Thus, the maximum value of the reverse voltage is equal to the amplitude of the source emf.

The simplest diagram of using a zener diode is shown in Fig. 3.12, a. The load (consumer) is connected in parallel with the zener diode. Therefore, in stabilization mode, when the voltage on the zener diode is almost constant, the same voltage will be on the load. Usually Rogr is calculated for the midpoint T of the zener diode characteristics.

Let's consider the case when E = const, and Rн varies from Rн min to Rн max..

The value of Rolim can be found using the following formula:

(3.3)

where Iav = 0.5(Ist min+Ist max) – average zener diode current;

Iн = Ust/Rн – load current (at Rн = const);

In.av = 0.5(In min+In max), (with Rn = var),

and And .

Figure 3.12

The operation of the circuit in this mode can be explained as follows. Since Rogr is constant and the voltage drop across it, equal to (E − Ust), is also constant, then the current in Rogr, equal to (Ist + In.sr), must be constant. But the latter is only possible if the zener diode current I and the load current Iн change to the same extent, but in opposite directions. For example, if In increases, then the current I decreases by the same amount, and their sum remains unchanged.

Let's consider the principle of operation of a zener diode using the example of a circuit consisting of a series-connected source of variable EMF - e, a zener diode VD and a resistor R (Fig. 3.13, a).

During the positive half-cycle, a reverse voltage is applied to the zener diode, and up to the breakdown voltage of the zener diode, all voltage is applied to the zener diode, since the current in the circuit is zero. After electrical breakdown of the zener diode, the voltage on the zener diode VD remains unchanged and the entire remaining voltage of the EMF source will be applied to resistor R. During the negative half-cycle, the zener diode is switched on in the conducting direction, the voltage drop across it is about 1V, and the remaining voltage of the EMF source is applied to resistor R.

A semiconductor diode is a semiconductor device with one electrical junction and two terminals, which uses one or another property of the electrical junction. The electrical junction can be an electron-hole junction, a metal-semiconductor junction, or a heterojunction.

The region of the diode semiconductor crystal that has a higher concentration of impurities (and therefore the majority charge carriers) is called the emitter, and the other, with a lower concentration, is called the base. The side of the diode to which the negative pole of the power source is connected when connected directly is often called the cathode, and the other is called the anode.

According to their purpose, diodes are divided into:

1. rectifiers (power), designed to convert alternating voltage from industrial frequency power supplies to direct voltage;

2. Zener diodes (reference diodes) designed to stabilize voltages , having on the reverse branch of the current-voltage characteristic a section with a weak dependence of voltage on the flowing current:

3. varicaps intended for use as a capacitance controlled by electrical voltage;

4. pulse, designed to work in high-speed pulse circuits;

5. tunnel and reverse, designed to amplify, generate and switch high-frequency oscillations;

6. ultra-high-frequency, designed for conversion, switching, and generation of ultra-high-frequency oscillations;

7. LEDs designed to convert an electrical signal into light energy;

8. photodiodes, designed to convert light energy into an electrical signal.

The system and list of parameters included in the technical descriptions and characterizing the properties of semiconductor diodes are selected taking into account their physical and technological features and scope of application. In most cases, information about their static, dynamic and limit parameters is important.

Static parameters characterize the behavior of devices at direct current, dynamic parameters characterize their time-frequency properties, limit parameters determine the area of stable and reliable operation.

1.5. Current-voltage characteristic of the diode

The current-voltage characteristic (volt-ampere characteristic) of the diode is similar to the current-voltage characteristic p-n-transition and has two branches – forward and reverse.

The diode's current-voltage characteristic is shown in Figure 5.

If the diode is turned on in the forward direction ("+" - to the area R, and “-” – to the area n), then when the threshold voltage is reached U Then the diode opens and direct current flows through it. When turned back on ("-" to the area R, and “+” – to the area n) an insignificant reverse current flows through the diode, that is, the diode is actually closed. Therefore, we can consider that the diode passes current in only one direction, which allows it to be used as a rectifier element.

The values of the forward and reverse currents differ by several orders of magnitude, and the forward voltage drop does not exceed a few volts compared to the reverse voltage, which can be hundreds or more volts. The rectifying properties of diodes are better, the lower the reverse current at a given reverse voltage and the lower the voltage drop at a given forward current.

The parameters of the current-voltage characteristic are: dynamic (differential) resistance of the diode to alternating current and static resistance to direct current.

The static resistance of the diode to direct current in the forward and reverse directions is expressed by the relation:

, (2)

Where U And I specify specific points on the diode’s current-voltage characteristic at which the resistance is calculated.

Dynamic AC resistance determines the change in current through a diode with a change in voltage near a selected operating point on the diode characteristic:

. (3)

Since a typical I-V characteristic of a diode has sections with increased linearity (one on the forward branch, one on the reverse branch), r d is calculated as the ratio of a small voltage increment across the diode to a small current increment through it at a given mode:

. (4)

To derive an expression for r d, it is more convenient to take current as an argument I, and consider the voltage as a function and, taking the logarithm of equation (1), bring it to the form:

. (5)

. (6)

It follows that with increasing forward current r d decreases quickly, since when the diode is turned on directly I>>I S .

In the linear section of the current-voltage characteristic when the diode is connected directly, the static resistance is always greater than the dynamic resistance: R st > r d. When turning the diode back on R st r d.

Thus, the diode's electrical resistance in the forward direction is much less than in the reverse direction. Therefore, the diode has one-way conductivity and is used to rectify alternating current.

Diodes are often referred to as "forward" and "reverse". What is this connected with? What is the difference between a “forward” diode and a “reverse” diode?

What is a "forward" diode?

A diode is a semiconductor that has 2 terminals, namely the anode and the cathode. It is used for processing different ways electrical signals. For example, for the purpose of straightening, stabilizing, transforming them.

The peculiarity of a diode is that it passes current only in one direction. In the opposite direction - no. This is possible due to the fact that the diode structure contains 2 types of semiconductor regions that differ in conductivity. The first conditionally corresponds to the anode, which has a positive charge, the carriers of which are so-called holes. The second is the cathode, which has a negative charge; its carriers are electrons.

The diode can operate in two modes:

open;
closed

In the first case, current flows well through the diode. In the second mode - with difficulty.

You can open the diode by direct connection. To do this, you need to connect the positive wire from the current source to the anode, and the negative wire to the cathode.

Direct voltage can also be called diode voltage. Unofficially, the semiconductor device itself. Thus, it is not it that is “direct”, but the connection to it or the voltage. But for ease of understanding, in electrical engineering the diode itself is often referred to as “direct”.

What is a "flyback" diode?

The semiconductor is closed by, in turn, applying reverse voltage. To do this, you need to change the polarity of the wires from the current source. As in the case of a forward diode, a reverse voltage is generated. By analogy with the previous scenario, the diode itself is also called “reverse”.

Comparison

The main difference between a “forward” diode and a “reverse” diode is in the method of supplying current to the semiconductor. If it is applied to open the diode, then the semiconductor becomes “straight”. If the polarity of the wires from the current source changes, then the semiconductor closes and becomes “reverse”.

Having considered the difference between a “forward” diode and a “reverse” diode, we will reflect the main conclusions in the table.

Published Date: 12/23/2017

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Determination of the maximum reverse voltage of diodes. Current rectification

Direct connection of the diode. Direct current.

Reverse connection of the diode. Reverse current.

Forward and reverse diode voltage.

Current-voltage characteristic of a semiconductor diode.

Breakdowns of the p-n junction.

Electrical breakdown.

Tunnel breakdown.

Avalanche breakdown.

Thermal breakdown.

Basic parameters of diodes, forward diode current, reverse diode voltage

Forward diode current

Diode Reverse Voltage

Operating principle and purpose of diodes

Device

Purpose

Direct diode connection

Diode reverse connection

Forward and reverse voltage

Diode operation and its current-voltage characteristics

Basic diode faults

Breakdown of p-n junction

Electrical breakdown

Thermal breakdown

Rectifier diode | Volt-info

Current-voltage characteristic of a rectifying diode

Concepts and designations of characteristics

Diode operation

Unidirectional diode conduction

1. Semiconductor diodes, operating principle, characteristics:

2. Semiconductor diodes, direct and reverse connection, voltage:

The maximum reverse voltage on the diodes is determined by the formula

7.3 Anti-aliasing filters

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1.5. Current-voltage characteristic of the diode

What is a "forward" diode?

What is a "flyback" diode?

Comparison

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