Determination of the maximum reverse voltage of diodes. Rectification current

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I. CALCULATION OF PARAMETERS OF SEMICONDUCTOR DIODES

Rectifier diodes are designed to rectify low frequency alternating current (typically less than 50 kHz). As rectifiers, planar diodes are used, which, due to the large contact area, allow a large rectified current. The current-voltage characteristic of a diode expresses the dependence of the current flowing through the diode on the value and polarity of the voltage applied to it (Figure 1.1). The branch located in the first quadrant corresponds to the forward (throughput) direction of the current, and located in the third quadrant to the opposite direction of the current.

The steeper and closer to the vertical axis the direct branch, and closer to the horizontal return branch, the better the rectifying properties of the diode. With a sufficiently large reverse voltage, a breakdown occurs at the diode, i.e. the reverse current rises sharply. Normal operation of a diode as an element with one-sided conductivity is possible only in modes when the reverse voltage does not exceed the breakdown voltage.

The diode currents are temperature dependent (see Figure 1.1). If a direct current flows through the diode, then with a change in temperature, the voltage drop across the diode changes by approximately 2 mV / ° C. With an increase in temperature, the reverse current doubles for germanium and 2.5 times for silicon diodes for every 10 ° C. The breakdown voltage decreases with increasing temperature.

High-frequency diodes are universal devices: for rectifying currents in a wide frequency range (up to several hundred MHz), for modulation, detection and other nonlinear transformations. Dot diodes are mainly used as high-frequency ones. High-frequency diodes have the same properties as rectifier ones, but their operating frequency range is much wider.

Main settings:

Unp- constant forward voltage at a given constant forward current;

Uobr- constant reverse voltage applied to the diode in the opposite direction;

Ipp- direct direct current flowing through the diode in the forward direction;

Iobr- constant reverse current flowing through the diode in the opposite direction at a given reverse voltage;

Unp.obr- the value of the reverse voltage causing the breakdown of the diode junction;

Inp.cp- average forward current, average value of forward diode current over the period;

Ivp sr- average rectifier current, the average value of the rectified current flowing through the diode over the period (taking into account the reverse current);

Iobr.cp- the average reverse current, the average value of the reverse current over the period;

Rpr- forward dissipated power, the value of the power dissipated by the diode when the forward current flows;

Pcr is the average power dissipated by the diode, the average over the period of the power dissipated by the diode when the forward and reverse current flows;

Rdif- differential resistance of the diode, the ratio of a small increment in the voltage of the diode to a small increment in the current on it for a given mode

(1.1)

Rnp.d... - forward resistance of the diode for direct current, the value of the resistance of the diode, obtained as a quotient of dividing the direct forward voltage on the diode and the corresponding forward current

Robr.d- reverse resistance of the diode; diode resistance value obtained as the quotient of dividing the constant reverse voltage across the diode and the corresponding constant reverse current

(1.3)

The maximum allowable parameters determine the limits of the operating conditions at which the diode can operate with a given probability during a specified service life. These include: maximum permissible DC reverse voltage Uobr.max; maximum permissible forward current Ipr.max, maximum permissible average forward current Wed Wed.max, maximum permissible average rectified current Iv.e. av.max, the maximum allowable average power dissipation of the diode Rcr.max.

The specified parameters are given in the reference literature. In addition, they can be determined experimentally and by volt-ampere characteristics.

We find the differential resistance as the cotangent of the angle of inclination of the tangent drawn to the straight branch of the I – V characteristic at the point Ipr= 12 mA ( Rdiff ~ ctg Θ ~)

(1.4)

The forward resistance of the diode is found as the ratio of the constant voltage across the diode Upr= 0.6V to the corresponding DC current Ipr= 12mA on the direct branch of the I - V characteristic.

(1.5)

We see that Rdif < Rpr.d... In addition, note that the values of these parameters depend on the specified mode. For example, for the same diode at Ipp= 4mA

(1.6) , (1.7)

Calculate Robr.d for diode GD107 at Uobr= 20 V and compare with the calculated value Rpr.d... On the reverse branch of the I - V characteristic of GD107 (see Fig. 1.2) we find: Iobr= 75μA at Uobr= 20V. Hence,

(1.8)

We see that Robr>>Rpr.d, which indicates the one-sided conductivity of the diode. The conclusion about one-sided conductivity can be made directly from the analysis of the I - V characteristic: forward current Ipp~ mA at Upr <1B, в то время как Iobp~ tens of μA at Uobr ~ tens volts, i.e. the forward current exceeds the reverse one by hundreds or thousands of times

(1.9)

Zener diodes and stabilizers are designed to stabilize the voltage level when the current flowing through the diode changes. For zener diodes, the working section is the electrical breakdown of the current-voltage characteristic in the region of reverse voltages (Fig. 1.3).

In this section, the voltage across the diode remains practically constant with a significant change in the current flowing through the diode. A similar characteristic is possessed by alloy diodes with a base made of a low-resistance (high-alloyed) material. In this case, a narrow p-n-junction is formed, which creates conditions for the occurrence of electrical breakdown at relatively low reverse voltages (units - tens of volts). Namely, these voltages are needed to power many transistor devices. In germanium diodes, electrical breakdown quickly turns into thermal, therefore, silicon diodes are used as zener diodes, which are more resistant to thermal breakdown. For stabilizers, a straight section of the current-voltage characteristic serves as a worker (Fig. 1.4). Double-sided (double-anode) zener diodes have two counter-connected p-n junctions, each of which is the main one for the opposite polarity.

Main settings:

Ust- stabilization voltage, voltage across the zener diode when the rated current is flowing;

∆Ust.nom- the spread of the nominal stabilization voltage, the deviation of the voltage on the Zener diode from the nominal value;

Rdif.st- differential resistance of the zener diode, the ratio of the stabilization voltage increment on the zener diode to the small current increment that caused it in a given frequency range;

α CT is the temperature coefficient of the stabilization voltage, the ratio of the relative change in the stabilization voltage to the absolute change in the ambient temperature at a constant stabilization current.

Maximum allowed parameters. These include: maximum Ist.max, minimum Ist.min stabilization currents, maximum permissible forward current Imax, the maximum allowable power dissipation Pmax.

The principle of operation of the simplest semiconductor voltage regulator (Fig. 1.5) is based on the use of the nonlinearity of the current-voltage characteristics of the zener diodes (see Fig. 1.3). The simplest semiconductor stabilizer is a voltage divider consisting of a limiting resistor Rogr and a silicon Zener diode VD. Load Rn is connected to a zener diode,

In this case, the voltage across the load is equal to the voltage across the zener diode

U R N = U VD = U ST(1.10)

and the input voltage is shared between Rogr and VD

U IN = U R OGR + U ST(1.11)

Current through Rogr according to the first Kirchhoff's law is equal to the sum of the load currents and the zener diode

I R OGR = I ST + I N (1.12)

The quantity Rogr is selected so that the current through the zener diode is equal to the nominal, i.e. corresponded to the middle of the working area.

I ST.NOM = (I ST.MIN + I ST.MAX) / 2 (1.13)

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

A diode is a semiconductor device with one pn junction, which has two leads (anode and cathode), and is designed for rectifying, detecting, stabilizing, modulating, limiting and converting electrical signals.

According to their 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 does not in the other. But how and how he does it, not many people know and understand.

A diode can be schematically 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 prevailing in the p-type region are conventionally depicted in red circles, and electrons prevailing in the n-type region are shown in blue. These two areas are the anode and cathode electrodes of the diode:

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

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

Contact metal layers are applied to the outer surfaces of the regions, 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 does not conduct current well.

Direct connection of the diode. Direct current.

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

With this 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 the electron-hole or p-n junction, they will meet, where their mutual absorption or recombination takes place.

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

The diode contact connected to the negative pole of the constant voltage source will give off the n-type region an almost unlimited number of electrons, 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, the conductivity of the 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 switching on 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 to the negative pole of the power source. As a result, the boundary of the regions will, as it were, expand, which forms a zone depleted in holes and electrons, which will provide high resistance to the current.

But, since minority charge carriers are present in each of the diode regions, a small exchange of electrons and holes between the regions will still occur. Therefore, a current that is many times smaller than the forward current will flow through the diode, and this current is called the reverse current of the diode (Iobr). As a rule, in practice, the reverse current of the pn junction is neglected, and from this it is concluded that the pn junction has only one-sided conductivity.

Forward and reverse voltage of the diode.

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

With a forward voltage (Upr), the resistance of the diode does not exceed several tens of Ohms, but with a reverse voltage (Urev), the resistance increases to several tens, hundreds, and even thousands of kilo-ohms. 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), which is applied 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 large.

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

Current-voltage characteristic of a semiconductor diode.

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

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

The current-voltage characteristic consists, as it were, of two branches: the forward branch, in the upper right part, corresponds to the forward (throughput) 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 to the horizontal return branch, the better the rectifying properties of the diode. The presence of a small reverse current is a disadvantage of diodes. It can be seen from the current-voltage characteristic curve that the forward current of the diode (Ipr) is hundreds of times higher than the reverse current (Irev).

With an increase in the forward voltage across the p-n junction, the current first increases slowly, and then a section of rapid current rise begins. This is due to the fact that the germanium diode opens and begins to conduct current at a forward voltage of 0.1 - 0.2V, and the silicon diode at 0.5 - 0.6V.

For example. With a forward voltage Upr = 0.5V, the forward current Ipr is 50mA (point "a" on the graph), and already with a voltage of 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 released is greater than that removed from the crystal naturally, or with the help of special cooling devices (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 excludes overheating of the semiconductor structure. To do this, use a limiting resistor in series with the diode.

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

With an increase in the reverse voltage (Urev) applied to the p-n junction, the current increases insignificantly, as evidenced by the reverse branch of the current-voltage characteristic. Let's take a diode with parameters: Urev max = 100V, Iobr 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 100V, it can be seen how insignificantly the reverse current grows (point "b" on the graph). But with a further increase in the voltage, above the maximum for which the p-n junction of the diode is designed, a sharp increase in the reverse current (dashed line) occurs, the semiconductor crystal is heated and, as a result, a breakdown of the p-n junction occurs.

Breakdowns of the p-n junction.

Breakdown of the p-n junction is the phenomenon of a sharp increase in the reverse current when the reverse voltage reaches a certain critical value. Distinguish between electrical and thermal breakdown of the pn 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 the pn junction. Such 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. In this mode, zener diodes work - diodes designed to stabilize the voltage.

Tunnel breakdown.

Tunneling breakdown occurs as a result of the phenomenon of the tunneling effect, which manifests itself in the fact that with a strong electric field acting in a pn 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 junctions 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 micron (micrometer).

Tunneling breakdown is characterized by a sharp rise in the reverse current at an insignificant reverse voltage - usually several volts. Tunnel diodes work on the basis of this effect.

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

Avalanche breakdown.

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

Diodes that use the effect of avalanche breakdown 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 the allowable voltage may occur.

Thermal breakdown.

Thermal breakdown occurs as a result of overheating of the p-n junction 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 junction.

With an increase in the reverse voltage (Urev) applied to the p-n junction, the dissipated power at the junction increases. This leads to an increase in the temperature of the transition and adjacent regions of the semiconductor, the vibrations of the atoms of the crystal increase, and the bond of valence electrons with them weakens. The probability arises of the transition of electrons to the conduction band and the formation of additional electron-hole pairs. Under poor conditions of heat transfer from the p-n junction, an avalanche-like increase in temperature occurs, which leads to the destruction of the junction.

Let's finish this, and in the next part we will consider the structure and operation of rectifier diodes, diode bridge.

A source:

1. Borisov VG - Young radio amateur. 1985 2. Goryunov N.N. Nosov Yu.R. - Semiconductor diodes. Parameters, measurement methods. 1968

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

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

Diode forward current

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

Diode reverse voltage

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

Fig. one

As we said earlier, the voltage at the output of the rectifier (across 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 anode of the diode has a positive potential with respect to ground, the filter capacitor is charged to a voltage that exceeds the effective voltage at the input of the rectifier by 1.4 times. During the next half-cycle, the voltage at the anode of the diode is negative with respect to the ground and reaches an amplitude value, and at the cathode it is positive with respect to the 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 not less than the double amplitude voltage of the secondary of the transformer or 2.8 times higher than its effective value. When calculating such rectifiers, it is necessary to choose diodes with a maximum reverse voltage 3 times higher than the effective value of the alternating voltage.

Fig. 2

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

Fig. 3

The situation is different in the case of a full-wave bridge rectifier. As you can see in fig. 3, in each of the half-cycles, twice the voltage is applied to two non-conducting, series-connected diodes.

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The principle of operation and purpose of diodes

A diode is a type of semiconductor-based device. It has one p-n junction, as well as anode and cathode terminals. In most cases, it is intended 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 emits electrons.
An electric field is generated 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 for this process. In this case, the formation of an emission current occurs.
A negative space charge is formed between the electrodes, which can interfere with the movement of electrons. This happens if the potential of the anode is too weak. In this case, parts of the electrons cannot overcome the effect of the negative charge, and they begin to move in the opposite direction, again returning to the cathode.
All electrons that have reached the anode and have not returned 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 could 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 generated between the electrodes does not accelerate the particles, but, on the contrary, decelerates them and returns them to the cathode. The diode in this case remains in the closed state, which leads to the opening of the circuit.

Device

Below is a detailed description of the diode device, the study of this information is necessary for a further understanding of the principles of operation of these elements:

The body is a vacuum cylinder that can be made of glass, metal or durable ceramic materials.
There are 2 electrodes inside the balloon. The first is a heated cathode, which is designed to provide the process of electron emission. The cathode, which is the simplest in design, is a filament with a small diameter, which is heated 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 that is heated under the influence of an electric current, it is called a heater.
The second electrode is the anode and is needed to receive electrons that have been released by the cathode. For this, it must have a positive potential with respect 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 type of elements.
For the manufacture of a diode crystal, silicon or germanium is most often used. One of its parts is electrically conductive in the p-type and has a lack of electrons, which is formed by an artificial method. The opposite side of the crystal also has n-type conductivity and has an excess of electrons. There is a border between the two areas, which is called a p-n junction.

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

Appointment

Below are the main areas of application of diodes, by the example of 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 assemblies together with them, made it possible to significantly reduce the size of this device and increase its degree of reliability. If the connection is made in series and in one direction, then this increases the minimum voltage indicators that will be required to unlock the entire diode bridge.
Diode detectors are obtained by combined use of these devices with capacitors. This is necessary in order to be able to isolate the low-frequency modulation from various modulated signals, including the amplitude-modulated type of radio signal. Such detectors are part of the design of many household consumers, such as televisions or radios.
Ensuring protection of consumers from wrong polarity when switching on the circuit inputs from arising overloads or switches from breakdown by the electromotive force arising from self-induction, which occurs when the inductive load is disconnected. To ensure the safety of the circuits from the resulting overloads, a chain is used, consisting of several diodes that are connected to the supply buses in the opposite direction. In this case, the input to which the protection is provided must be connected to the middle of this chain. During the 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. As a result, this permissible potential is limited within the permissible supply voltage in addition to the direct voltage drop across the protective device.
Diode switches are used to switch high frequency signals. The control of such a system is carried out using direct electric current, high frequency separation and supply of a control signal, which occurs thanks to inductors and capacitors.
Creation of diode spark protection. Shunt diode barriers are used that provide safety by limiting the voltage in the appropriate electrical circuit. Together with them, current-limiting resistors are used, which are necessary to limit the indicators of electric current passing through the network and increase the degree of protection.

The use of diodes in electronics today is very wide, since in fact no modern type of electronic equipment is complete without these elements.

Direct diode switching

The pn junction of the diode can be influenced by voltage supplied from external sources. Indicators such as magnitude and polarity will affect its behavior and the electric current carried through it.

Below we consider in detail the option in which the plus is connected to the p-type region, and the negative pole to the n-type region. In this case, there will be a direct connection:

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

Reverse switching on of the diode

Now we will consider another way 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 generated electric field will have a direction that completely coincides with the direction of the internal diffusion field. Accordingly, the blocking 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 rises, the electric current flowing through the junction will tend to reach its maximum performance. It has a special name - saturation current.
In accordance with the exponential law, with a gradual increase in temperature, the saturation current will also increase.

Forward and reverse voltage

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

Forward voltage is the one at which the diode opens and the forward current begins to pass through it, while the resistance indicators of the device are extremely low.
Reverse voltage is one that has reverse polarity and ensures that the diode turns off and reverse current flows through it. At the same time, the resistance indicators of the device begin to grow sharply and significantly.

The resistance of the p-n-junction is a constantly changing indicator, first of all, it is influenced by the forward voltage applied directly to the diode. If the voltage increases, then the junction resistance indicators will decrease proportionally.

This leads to an increase in the parameters of the forward current passing through the diode. When this device is closed, then virtually all the voltage acts on it, for this reason, the indicators of the reverse current passing through the diode are insignificant, and the junction resistance at the same time reaches its peak parameters.

Diode operation and its current-voltage characteristic

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.

A similar schedule can be described as follows:

Vertical axis: the upper area corresponds to the forward current values, the lower area corresponds to the reverse current parameters.
Horizontal axis: the area on the right is for forward voltage values; left area for reverse voltage parameters.
The forward branch of the current-voltage characteristic reflects the electric current passing through the diode. It is directed upwards and runs in close proximity to the vertical axis, since it reflects the increase in forward electric current that occurs when the corresponding voltage increases.
The second (reverse) branch corresponds and displays the state of a closed electric current that also flows 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 indicators occurs. However, gradually, the curve reaches the area in which a jump is noticeable, after which there is an accelerated increase in its indicators. This is due to the opening of the diode and the conduction of current with forward voltage. For devices made from germanium, this occurs at a voltage of 0.1V to 0.2V (maximum value 1V), and for silicon cells, a higher value is required from 0.5V to 0.6V (maximum value 1.5V).
The shown increase in current values can lead to overheating of semiconductor molecules. If the removal of heat, which 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 already be irreversible. For this reason, it is necessary to limit the parameters of the forward current in order to prevent overheating of the semiconductor material. For this, special resistors are added to the circuit, which are connected in series with diodes.
Exploring the reverse branch, you can see that if the reverse voltage, which is applied to the p-n junction, begins to increase, then the increase in the current parameters is practically imperceptible. However, in cases where the voltage reaches parameters exceeding the permissible limits, a sudden jump in the reverse current may occur, which will overheat the semiconductor and contribute to the subsequent breakdown of the pn junction.

Basic diode malfunctions

Sometimes devices of this type fail, this may be 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 inherently the most common conductor. In this state, it loses its basic properties and begins to pass an electric current in absolutely any direction. Such a breakdown is easily detected using a standard multimeter, which starts to beep and show a low resistance level in the diode.
In the event of a break, the opposite process occurs - the device generally ceases to transmit electric current in any direction, that is, it becomes, in essence, an insulator. For the accuracy of determining the break, it is necessary to use testers with high-quality and serviceable probes, otherwise, they can sometimes falsely diagnose this malfunction. In alloyed semiconductor varieties, such a breakdown is extremely rare.
Leak, during which the tightness of the body of the device is broken, as a result of which it cannot function properly.

Breakdown of the pn junction

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

Several types are usually distinguished:

Thermal breakdowns, which are caused by a sharp rise in temperature and subsequent overheating.
Electrical breakdowns arising 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 cause the destruction of the crystal itself. Therefore, with a gradual decrease in voltage, it is possible to restore the entire properties and operating parameters of the diode.

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

Tunnel breakdowns occur when high voltage passes through narrow junctions, which allows individual electrons to slip through it. They usually occur if semiconductor molecules contain a large number of different impurities. During such a breakdown, the reverse current begins to rise sharply and rapidly, and the corresponding voltage is at a low level.
Avalanche breakdowns are possible due to the action of strong fields capable of accelerating charge carriers to the limiting level, due to which they knock out a number of valence electrons from atoms, which then fly out into the conductive region. This phenomenon is of an avalanche-like nature, due to which this type of breakdown has received such a name.

Thermal breakdown

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

An increase in the temperature regime in the junction and adjacent areas has the following consequences:

The growth of vibrations of the atoms that make up the crystal.
The hit of electrons in the conductive zone.
A sharp rise 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 rectifier diode.

Volt-ampere characteristic of a rectifier diode

In the figure, in the first quadrant there is a direct, in the third - a reverse branch of the diode characteristic. The forward branch of the characteristic is removed under the action of the forward voltage, the reverse, respectively, of the reverse voltage on the diode. The forward voltage across the diode is the voltage at which a higher electric potential is formed at the cathode in relation to the anode, and speaking in the language of signs - at the cathode minus (-), at the anode plus (+), as shown in Figure 2.

Figure 2. Scheme for studying the I - V characteristic of a diode with direct connection.

Figure 1 shows the following conventions:

Iр is the operating current of the diode;

Uд - voltage drop across the diode;

Uо - reverse voltage of the diode;

Upr - breakdown voltage;

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

Concepts and designations of characteristics

The operating current of the diode (Iр) is a direct electric current passing through the diode for a long time, at which the device does not undergo irreversible thermal destruction, and its characteristics do not undergo significant qualitative changes. In directories, it can be indicated as a direct maximum current. The voltage drop across the diode (Uд) is the voltage at the terminals of the diode that occurs when a direct operating current passes through it. In reference books, it can be designated as the forward voltage across the diode.

Forward current flows when the diode is turned on directly.

Reverse voltage of the diode (Uo) - the permissible reverse voltage on the diode, applied to it for a long time, at which there is no irreversible destruction of its p-n junction. May be referred to as maximum reverse voltage in reference literature.

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

Reverse current of the diode, or leakage current (Iу) - reverse current, which for a long time does not cause irreversible destruction (breakdown) of the pn junction of the diode.

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

Diode operation

The subtleties of the p-n transition, a topic for a separate article. Let's simplify the task and consider the operation of the diode from the position of one-sided conduction. And so, the diode works as a conductor when it is forward, and as a dielectric (insulator) when it is turned back on. Consider two circuits in Figure 3.

Figure 3. Reverse (a) and direct (b) switching on of the diode.

The figure shows two versions of the same circuit. In Figure 3 (a), the positions of the switches S1 and S2 provide electrical contact of the anode of the diode with a minus of the power supply, and the cathode through the HL1 lamp with a plus. As we have already determined, this is the reverse switching on of the diode. In this mode, the diode will behave like an electrically insulating element, the electrical circuit will be practically open, the lamp will not burn.

When changing the position of contacts S1 and S2, Figure 3 (b), an electrical contact of the anode of the VD1 diode with the plus of the power supply is provided, and the cathode through the light bulb - with a minus. In this case, the condition of direct switching on of the diode is fulfilled, it "opens" and the load current (lamp) flows through it, as through a conductor.

If you have just started to study electronics, you may be a little confused by the complexity of the switches in Figure 3. Draw an analogy according to the above description, based on the simplified diagrams of Figure 4. This exercise will allow you to understand and orientate a little about the principle of building and reading electrical circuits.

Figure 4. Scheme of reverse and forward switching of a diode (simplified).

In Figure 4, the change in polarity at the terminals of the diode is provided by changing the position of the diode (inversion).

Unidirectional diode conductance

Figure 5. Diagrams of voltages 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 supplied to the diode anode, depending on the position of switches S1 and S2. A diagram of such a rectangular AC voltage is shown in Figure 5 (top diagram). With a negative voltage difference at the anode of the diode, it is locked (works as an insulating element), while the current does not flow through the HL1 lamp and it does not burn, and the voltage on the lamp is practically zero. With a positive voltage difference, the diode is unlocked (acts as an electrical conductor) and current flows through the diode-lamp series circuit. The voltage across the lamp rises to UHL1. This voltage is slightly less than the supply voltage because some of the voltage drops across the diode. For this reason, the voltage difference in electronics and electrical engineering is sometimes referred to as "voltage drop". 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, the periods of negative voltage difference are, as it were, ignored by the diode, cut off, and current flows through the load only during periods of positive voltage difference. This conversion of alternating voltage to unipolar (pulsating or constant) was called rectification.

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1.Semiconductor diodes, principle of operation, characteristics:

SEMICONDUCTOR DIODE - a semiconductor device with two electrodes, which has one-sided conductivity. Semiconductor diodes include an extensive group of devices with pn-junction, metal-semiconductor contact, etc. The most common are electroconverting semiconductor diodes. Serve for the transformation and generation of electrical vibrations. One of the main modern electronic devices. The principle of operation of a semiconductor diode: The principle of operation of a semiconductor diode is based on the properties of an electron-hole junction, in particular, a strong asymmetry of the current-voltage characteristic relative to zero. Thus, direct and reverse inclusion are distinguished. In direct connection, the diode has low electrical resistance and conducts electric current well. In the opposite case, when the voltage is less than the breakdown voltage, the resistance is very high and the current is blocked. Characteristics:

2.Semiconductor diodes, forward and reverse switching, wax:

Direct and reverse inclusion:

When a pn junction is switched on directly, an external voltage creates a field in the junction that is opposite in direction to the internal diffusion field. The resulting field strength decreases, which is accompanied by a narrowing of the blocking layer. As a result, a large number of major charge carriers are able to diffusely transfer to the neighboring region (the drift current does not change in this case, since it depends on the number of minority carriers appearing at the transition boundaries), i.e. the resulting current will flow through the junction, which is mainly determined 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 transition leads to an increase in the concentration of holes in the n-type region and electrons in the p-type region. This increase in the minority carrier concentration due to the effect of an external voltage applied to the junction is called minority carrier injection. Non-equilibrium minority carriers diffuse deep into the semiconductor and violate its electroneutrality. 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 a current in the external circuit, called direct.

When the pn junction is turned on in the opposite direction, the external reverse voltage creates an electric field that coincides in direction with the diffusion one, 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, the resulting current will flow through the junction, which is mainly determined by the drift current of minority carriers. Since the number of drifting minority carriers does not depend on the applied voltage (it affects only 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 an increase in temperature, the saturation current grows exponentially.

The graph shows the I - V characteristics for the forward and reverse switching on of the diode. They also say the forward and reverse branch of the current-voltage characteristic. The direct branch (Ipr and Upr) displays the characteristics of the diode during direct connection (that is, when a "plus" is applied to the anode). The reverse branch (Iobr and Uobr) displays the characteristics of the diode when it is turned back on (that is, when a "minus" is applied to the anode).

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

To begin with, we define, as for any plane coordinate system, four coordinate angles (quadrants). Let me remind you that the first is the quadrant, which is located on the upper right (that is, where we have the letters Ge and Si). Further, the quadrants are counted counterclockwise.

So, the II-nd and IV-th 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 the diode and at the same time it is turned on in the forward direction, or, in other words, it is impossible to simultaneously apply both "plus" and "minus" to one terminal. More precisely, it is possible, but then it will be a short circuit. It remains to consider only two cases - direct switching on of the diode and reverse switching on of the diode.

The direct inclusion graph is drawn in the first quadrant. From this it can be seen that the greater the voltage, the greater the current. Moreover, up to a certain point, the voltage grows faster than the current. But then a break occurs, and the voltage hardly changes, and the current begins to rise. For most diodes, this break occurs in the range of 0.5 ... 1 V. It is this voltage, as they say, "drops" across the diode. These 0.5 ... 1 V is the voltage drop across the diode. The slow growth of the current to a voltage of 0.5 ... 1V means that in this section the current through the diode practically does not go even in the forward direction.

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

In the characteristics of specific diodes, the maximum reverse voltage is always indicated - that is, the voltage that the diode can withstand without "breakdown" when turned on in the opposite direction. This must be taken into account when developing devices where diodes are used.

Comparing the characteristics of silicon and germanium diodes, we can conclude that the forward and reverse currents in pn junctions of a silicon diode are less than in a germanium diode (with the same voltage values at the terminals). This is due to the fact that silicon has a wider bandgap and for the transition of electrons from the valence band to the conduction band, they need to be imparted a large additional energy.

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The maximum reverse voltage across diodes is determined by the formula

Urev. max = 1.045Uav.

In a number of practical applications, thyristor converters are used for AC rectification and modulating control of 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.

Fig. 7.10. Thyristor rectifier circuit

In fig. 7.11 shows timing diagrams explaining the principle of regulation of the average value of the rectified voltage.

Fig. 7.11. Timing diagrams of thyristor rectifier operation

In this circuit, it is assumed that the input voltage Uin for an adjustable thyristor is formed, for example, by a full-wave rectifier. If control pulses Uy of sufficient amplitude are applied at the beginning of each half-cycle (section o-a on the Uout diagram), the output voltage will repeat the voltage of the full-wave rectifier. If we shift the control pulses to the middle of each half-cycle, then the pulses at the output will have a duration equal to a quarter of a 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 supplying control pulses to the thyristor that are phase-shifted relative to the input voltage, it is possible to transform the sinusoidal voltage (current) into a sequence of pulses of any duration, amplitude and polarity, that is, the effective voltage (current) value can be changed over a wide range.

7.3 Smoothing filters

The considered rectification circuits allow obtaining a unipolar ripple voltage, which is not always applicable to power complex electronic devices, since, due to large ripples, they lead to instability of their operation.

Smoothing filters are used to significantly reduce ripple. 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 the filters are reactive elements - capacitance and inductance (chokes). Let us first consider the principle of operation of the simplest smoothing filter, the diagram of which is shown in Fig. 7.12.

Fig. 7.12. Scheme of the simplest smoothing filter with a half-wave rectifier

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

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

Fig. 7.13. Timing diagrams of the filter with a half-wave rectifier

If the capacitance is large enough, the discharge of the capacitance through Rn will occur with a large time constant  = RnC, and therefore, the decrease in the voltage across 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 the current i flows through it, increasing (at a given average load current) as the difference t2 - t1 decreases. This mode of operation can lead to failure of the rectifier diode, and, moreover, is quite difficult for the transformer.

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

Fig. 7.14. Full-wave rectifier ripple smoothing

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

Fig. 7.15. Ripple voltage approximation

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

∆Q = ∆UC = I nT1,

where T1 is the ripple period, In is the average value of the load current. Taking into account that In = Isr / Rn, we obtain

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 possessed by filters containing inductance and capacitance, connected as shown in Fig. 7.16.

Fig. 7.16. Smoothing filter with inductance and capacitance

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

In practice, various types of F-shaped and U-shaped filters are widely used, the construction options of which are shown in Fig. 7.17.

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

Fig. 7.17. Filter options

In the most critical schemes, multi-tier filtration schemes are used (Fig. 7.17 d).

Often the choke 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 (voltage across the load) 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, supply wires, filter elements.

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

Icr - 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. 7.18 shows typical dependences of Uav on Iav for various filtration options.

Fig. 7.18. Typical dependences of Uav on Iav for various filtration schemes

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What is Reverse Voltage? - Renovation interior construction

Reverse voltage

Reverse voltage is a type of energy signal created when the polarity of an electric current is reversed. This voltage often occurs when reverse polarity is applied to the diode, causing the diode to react by operating in the opposite direction. This inverse 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 source connecting the energy signal to the circuit is applied in an inverted manner. This means that the positive lead source is connected to the grounded or negative conductor of the circuit and vice versa. This voltage transmission is often not designed as most electrical circuits are not capable of handling voltages.

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

When the magnitude of the voltage applied to the circuit is too large, the signal to the received circuit is however called breakdown voltage. If the input signal, which was reversed, exceeds the allowable voltage for the circuit to maintain, the circuit may be damaged outside of the rest of its use. The point at which the circuit is damaged refers to the value of the breakdown voltage. This breakdown voltage has a couple of other names, Peak Reverse Voltage or Reverse Breakdown Voltage.

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

This breakdown voltage, which can occur across circuit components, can cause breakdown of components or insulator wires. This can turn them into signal conductors and damage the circuit by applying voltage to different parts of the circuit that should not receive it, resulting in 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 up to 1000v

U arr. m ax = 1.045U avg.

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

Fig. 7.10. Thyristor rectifier circuit

In fig. 7.11 shows timing diagrams explaining the principle of regulation of the average value of the rectified voltage.

Fig. 7.11. Timing diagrams of thyristor rectifier operation

In this circuit, it is assumed that the input voltage U in for an adjustable thyristor is formed, for example, by a full-wave rectifier. If control pulses U with sufficient amplitude are applied at the beginning of each half-cycle (section o-a on the U out diagram), the output voltage will repeat the voltage of the full-wave rectifier. If we shift the control pulses to the middle of each half-cycle, then the pulses at the output will have a duration equal to a quarter of a 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).

7.3 Smoothing filters

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

Smoothing filters are used to significantly reduce ripple. 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.

Fig. 7.12. Scheme of the simplest smoothing filter with a half-wave rectifier

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

Timing diagrams explaining the operation of such a filter are shown in Fig. 7.13. In the section t1 - t2, the diode opens with the input voltage, and the capacitor is charged. When the input voltage begins to decrease, the diode is closed by the voltage accumulated on the capacitor U c (section t1 - t2). In this interval, the input voltage source is disconnected from the capacitor and the load, and the capacitor is discharged through the load resistance R n.

Fig. 7.13. Timing diagrams of the filter with a half-wave rectifier

If the capacitance is large enough, the discharge of the capacitance through R n will occur with a large time constant  = R n C, and therefore, the decrease in the voltage across 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 the current i  flows through it, increasing (at a given average load current) with a decrease in the difference t2 - t1. This mode of operation can lead to failure of the rectifier diode, and, moreover, is quite difficult for the transformer.

Fig. 7.14. Full-wave rectifier ripple smoothing

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

Fig. 7.15. Ripple voltage approximation

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

∆Q = ∆UC = I n T 1,

where T 1 is the ripple period, I n is the average value of the load current. Taking into account that I n = And av / R n, we obtain

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 possessed by filters containing inductance and capacitance, connected as shown in Fig. 7.16.

Fig. 7.16. Smoothing filter with inductance and capacitance

In practice, various types of F-shaped and U-shaped filters are widely used, the construction options of which are shown in Fig. 7.17.

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

Fig. 7.17. Filter options

In the most critical schemes, multi-tier filtration schemes are used (Fig. 7.17 d).

Often the choke 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 U cf (voltage across the load) on the average value of the output current.

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

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

I cр - given. The smaller the value of R out, the less the output voltage depends on the output current, the better the rectifier circuit with the filter. In fig. 7.18 shows typical dependences of U cf. on I cf for various filtration options.

Fig. 7.18. Typical dependences of U cf. on I cf for various filtration schemes

A diode is a semiconductor device with one p-n junction, which has two outputs (cathode and anode), it is designed to stabilize, rectify, modulate, detect, convert and limit electrical signals reverse current.

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

In theory, we know that a diode only passes current in one direction. However, not many people know and understand exactly how he does it. Schematically, a diode can be imagined as a crystal consisting of 2 regions (semiconductors). One of these regions of the crystal has n-type conductivity, and the other has p-type conductivity.

The figure shows holes that predominate in the n-type region, which are shown in blue circles, and electrons that predominate in the p-type region are shown in red. These two areas are the cathode and anode diode electrodes:

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

The anode is the positive electrode of a diode, the main charge carriers of which are holes.

On the outer surfaces of the regions, contact metal layers are applied, to which the wire leads of the diode electrodes are soldered. A device of this kind can only be in one of two states:

1. Closed - this is when it conducts current poorly;

2. Open is when it conducts current well.

The diode will be in the off state if the polarity of the constant voltage source is applied.

In this case, electrons from the n-type region will begin to move to 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 to the negative pole. In the end, the boundary of the regions will expand, which forms a zone united by electrons and holes, which will offer tremendous resistance to the current.

However, minority charge carriers are present in each of the diode regions, and a small exchange of electrons and holes between the regions will still occur. Therefore, many times less current will flow through the diode than the direct current, and this current is called reverse current diode... In practice, as a rule, the reverse current of the p-n junction is neglected, and hence it turns out that the p-n junction has only one-sided conductivity.

D iodine- the simplest device in the glorious family of semiconductor devices. If you take a semiconductor plate, for example, germanium, and introduce an acceptor impurity into its left half, and into the right donor one, then on one side you will get a semiconductor of type P, respectively, on the other side of type N. P-N transition as shown in Figure 1.

The same figure shows the conventional graphic designation of the diode in the diagrams: the output of the cathode (negative electrode) is very similar to the "-" sign. It's easier to remember that way.

In total, such a crystal has two zones with different conductivity, from which two leads come out, therefore the resulting device was named diode because the prefix "di" means two.

In this case, the diode turned out to be a semiconductor, but similar devices were known before: for example, in the era of vacuum tubes there was a tube diode called the kenotron. Now such diodes have gone down in history, although the adherents of the "tube" sound believe that in a tube amplifier even an anode voltage rectifier should be a tube!

Figure 1. The structure of the diode and the designation of the diode in the diagram

At the junction of semiconductors with P and N conductivities, we obtain P-N junction, which is the basis of all semiconductor devices. But unlike a diode, which has only one transition, they have two P-N transitions, and, for example, consist of four transitions at once.

P-N transition at rest

Even if the P-N junction, in this case a diode, is not connected anywhere, interesting physical processes still occur inside it, which are shown in Figure 2.

Figure 2. Diode at rest

There is an excess of electrons in the N region, it carries a negative charge, and in the P region, the charge is positive. Together, these charges form an electric field. Since opposite charges tend to attract, electrons from the N zone penetrate into the positively charged P zone, filling some holes. As a result of this movement inside the semiconductor, a current, albeit very small (units of nanoamperes), arises.

As a result of this movement, the density of the substance on the P side increases, but up to a certain limit. Particles usually tend to spread evenly throughout the entire volume of a substance, just like the smell of perfume spreads throughout a room (diffusion), therefore, sooner or later, electrons return back to the N zone.

If for most consumers of electricity, the direction of the current does not matter - the light is on, the tile is warming up, then for the diode the direction of the current plays a huge role. The main function of a diode is to conduct current in one direction. It is this property that is provided by the P-N transition.

Turning on the diode in the opposite direction

If you connect a power supply to the semiconductor diode, as shown in Figure 3, then the current through the P-N junction will not pass.

Figure 3. Reverse connection of the diode

As you can see in the figure, the positive pole of the power supply is connected to the N area, and the negative pole to the P area. As a result, electrons from the N region rush to the positive pole of the source. In turn, positive charges (holes) in the P region are attracted by the negative pole of the power source. Therefore, in the region of the P-N junction, as can be seen in the figure, a void is formed, there is simply nothing to conduct the current, there are no charge carriers.

With an increase in the voltage of the power source, electrons and holes are more and more attracted by the electric field of the battery, while in the region of the P-N transition there are less and less charge carriers. Therefore, in the reverse connection, the current does not flow through the diode. In such cases, it is customary to say that the semiconductor diode is reverse voltage locked.

An increase in the density of the substance near the poles of the battery leads to the occurrence of diffusion, - the desire for a uniform distribution of matter throughout the volume. This is what happens when the battery is disconnected.

Semiconductor diode reverse current

This is where the time has come to recall the minor media that were conventionally forgotten. The fact is that even in the closed state, an insignificant current, called reverse current, passes through the diode. This reverse current and is created by minor carriers, which can move in exactly the same way as the major ones, only in the opposite direction. Naturally, such a movement occurs with reverse voltage. The reverse current is generally low due to the small amount of minority carriers.

As the crystal temperature rises, the number of minority carriers increases, which leads to an increase in the reverse current, which can lead to the destruction of the P-N junction. Therefore, the operating temperatures for semiconductor devices - diodes, transistors, microcircuits are limited. In order to prevent overheating, powerful diodes and transistors are installed on heat sinks - radiators.

Forward diode turn-on

Shown in Figure 4.

Figure 4. Direct connection of a diode

Now let's change the polarity of switching on the source: connect the minus to the N region (cathode), and the plus to the P region (anode). With this inclusion in the N region, the electrons will be repelled from the minus of the battery, and move towards the P-N junction. In the P region, the positively charged holes will be repulsed from the positive terminal of the battery. Electrons and holes rush towards each other.

Charged particles with different polarities gather around the P-N junction, and an electric field arises between them. Therefore, the electrons overcome the P-N transition and continue to move through the P zone. In this case, some of them recombine with holes, but most of them rush to the battery plus, the current Id went through the diode.

This current is called direct current... It is limited by the technical data of the diode, some maximum value. If this value is exceeded, there is a danger of diode failure. However, it should be noted that the direction of the forward current in the figure coincides with the generally accepted, reverse motion of electrons.

It can also be said that in the forward direction of switching on, the electrical resistance of the diode is relatively small. With the reverse connection, this resistance will be many times greater, the current does not flow through the semiconductor diode (a slight reverse current is not taken into account here). From the foregoing, we can conclude that the diode behaves like an ordinary mechanical valve: turned in one direction - water flows, turned in the other - the flow stopped. For this property, the diode was named semiconductor valve.

To understand in detail all the abilities and properties of a semiconductor diode, you should get acquainted with its volt - ampere characteristic... It is also a good idea to learn about the various diode designs and frequency properties, the advantages and disadvantages. This will be discussed in the next article.