Rectifier diodes are used in control circuits, switching circuits, limiting and decoupling circuits, in power supplies for converting (rectifying) alternating voltage into direct voltage, in voltage multiplying circuits and direct voltage converters, where high demands are not placed on the frequency and time parameters of signals. Depending on the value of the maximum rectified current, there are low power rectifier diodes(\(I_(pr max) \le (0.3 A)\)), medium power(\((0.3 A)< I_{пр max} \le {10 А}\)) и high power(\(I_(pr max) > (10 A)\)). Low-power diodes can dissipate the heat generated on them by their housing; medium and high-power diodes must be located on special heat sinks, which is provided, among other things. and the corresponding design of their bodies.
Typically, the permissible current density passing through the \(p\)-\(n\) junction does not exceed 2 A/mm2, therefore, to obtain the above values of the average rectified current in rectifier diodes, planar \(p\)-\ (n\)-transitions. Such junctions have a significant capacitance, which limits the maximum permissible operating frequency (\(f_р\)) of rectifier diodes.
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 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. Therefore, diodes have one-way conductivity, which allows them to be used as rectifier elements. The current-voltage characteristics (CV) of germanium and silicon diodes are different. In Fig. For comparison, Figure 2.3-1 shows typical current-voltage characteristics for germanium and silicon rectifier diodes at different ambient temperatures.
Rice. 2.3-1. Current-voltage characteristics of rectifier diodes at different ambient temperatures
From the given current-voltage characteristics it is clear that the reverse current of silicon diodes is significantly less than the reverse current of germanium diodes. In addition, the reverse branch of the current-voltage characteristic of silicon diodes does not have a clearly defined saturation region, which is due to the generation of charge carriers in the \(p\)-\(n\) junction and leakage currents along the surface of the crystal. When a reverse voltage is applied that exceeds a certain threshold level, a sharp increase in the reverse current occurs, which can lead to breakdown of the \(p\)-\(n\) junction. In germanium diodes, due to the large reverse current, the breakdown is thermal in nature. Silicon diodes have a low probability of thermal breakdown; electrical breakdown predominates in them. The breakdown of silicon diodes has an avalanche nature, therefore, unlike germanium diodes, the breakdown voltage increases with increasing temperature. The permissible reverse voltage of silicon diodes (up to 1600 V) significantly exceeds that of germanium diodes.
Reverse currents are highly dependent on the junction temperature. The figure shows that with increasing temperature the reverse current increases. For an approximate estimate, we can assume that with an increase in temperature by 10 °C, the reverse current of germanium diodes increases by 2 times, and that of silicon diodes by 2.5 times. The upper limit of the operating temperature range for germanium diodes is 75...80 °C, and for silicon diodes - 125 °C. A significant disadvantage of germanium diodes is their high sensitivity to short-term pulse overloads.
Due to the lower reverse current of the silicon diode, its forward current, equal to the current of the germanium diode, is achieved at a higher forward voltage. Therefore, the power dissipated at the same currents in germanium diodes is less than in silicon diodes. Forward voltage at low forward currents, when the voltage drop across the junction predominates, decreases with increasing temperature. At high currents, when the voltage drop across the resistance of the neutral regions of the semiconductor predominates, the dependence of the forward voltage on temperature becomes positive. The point at which there is no dependence of forward voltage on temperature (i.e. this dependence changes sign) is called inversion point. For most low- and medium-power diodes, the permissible forward current, as a rule, does not exceed the inversion point, and for high-power diodes, the permissible current can be higher than this point.
Introduction........................................................ ........................................................ ................... 3
§1. Rectifier diodes................................................... ....................................... 4
§2. Zener diodes........................................................ ........................................................ .... 9
§3. Varicaps........................................................ ........................................................ ......... 12
§4. LEDs........................................................ ........................................................ ...... 15
§5. Photodiodes........................................................ ........................................................ ....... 18
Bibliography................................................ ........................................................ 22
A diode (from ancient Greek δι - two and -od from the word electrode) is a two-electrode electronic device that has different conductivity depending on the direction of the electric current. The diode electrode connected to the positive pole of the current source when the diode is open (that is, has low resistance) is called the anode, connected to the negative pole - the cathode.
The development of diodes began in the third quarter of the 19th century in two directions at once: in 1873, the British scientist Frederick Guthrie discovered the principle of operation of thermionic (directly heated vacuum tube) diodes, in 1874, the German scientist Karl Ferdinand Braun discovered the principle of operation of crystalline (solid-state) diodes.
The operating principles of the thermionic diode were rediscovered on February 13, 1880 by Thomas Edison, and then patented in 1883 (US Patent No. 307031). However, the idea was not further developed in Edison's works. In 1899, German scientist Karl Ferdinand Braun patented a rectifier on a crystal. Jadish Chandra Bose further developed Brown's discovery into a device applicable to radio detection. Around 1900, Greenleaf Picard created the first crystal diode radio. The first thermionic diode was patented in Britain by John Ambrose Fleming (a scientific advisor to the Marconi company and a former employee of Edison in 1904 in November sixteenth (US patent No. 803684 of November 1905). In 1906 in November twentieth Picard patented a silicon crystal detector (US patent No. 836531).
At the end of the 19th century, devices of this kind were known as rectifiers, and only in 1919 William Henry Ickles coined the word “diode,” derived from the Greek roots “di” - two, and “odos” - path.
Electric current rectifier is a mechanical, electrovacuum, semiconductor or other device designed to convert an alternating input electrical current into a direct output electrical current.
A diode rectifier or diode bridge (that is, 4 diodes for a single-phase circuit (6 for a three-phase half-bridge circuit or 12 for a three-phase full-bridge circuit), interconnected in a circuit) is the main component of power supplies for almost all electronic devices.
A diode bridge is an electronic circuit designed to convert (“rectify”) alternating current into pulsating direct current. This type of rectification is called full-wave rectification.
Let us highlight two options for connecting bridge circuits: single-phase and three-phase.
Single-phase bridge circuit:
An alternating voltage is supplied to the input of the circuit (for simplicity, we will consider a sinusoidal one); in each half-cycle, the current passes through two diodes, the other two diodes are closed (Fig. 1 a, b).
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Figure 1 a) Rectification of the positive half-wave b) Rectification of the negative half-wave
As a result of this conversion, the output of the bridge circuit produces a pulsating voltage twice the frequency of the input voltage (Fig. 2 a, b, c)
Figure 2. a) initial voltage (input voltage), b) half-wave rectification, c) full-wave rectification
Three-phase bridge circuit:
In a three-phase rectifier bridge circuit, the result is an output voltage with less ripple than in a single-phase rectifier (Fig. 3).
Figure 3. Three-phase rectifier output voltage
Diode rectifiers are also widely used to rectify three-phase voltages. Very common rectifier circuits are based on half-bridge diode rectifiers (Fig. 4.
Figure 4. Three-phase half-bridge rectifier circuit
As a rule, to smooth out the pulsating voltage at the output of the rectifier, a filter in the form of a capacitor or inductor is used, and a zener diode is installed to stabilize the output voltage (Fig. 5.
Figure 5. Diode rectifier circuit with filter
Design, advantages
Figure 6. Diode bridge using discrete elements
The design of diode bridges can be made of individual diodes, or in the form of a monolithic structure (diode assembly). Monolithic construction, as a rule, is preferable - it is cheaper and smaller in volume. The diodes in it are selected at the manufacturer’s factory and the parameters are as similar as possible to each other, unlike individual diodes, where the parameters may differ from each other, moreover, in operating condition, the diodes in the diode assembly operate in the same thermal regime, which reduces the likelihood of failure element. Another advantage of the diode assembly is its ease of mounting on the board. The main disadvantage of a monolithic design is that it is not possible to replace one diode that has failed with another; in this case, it is necessary to change the entire assembly, but this happens extremely rarely if the operating modes of the diode bridge are selected correctly.
Figure 7. Diode assembly
The scope of application of rectifier bridges is extensive, for example:
Lighting devices (fluorescent lamps, electronic ballasts, solar battery modules);
Electricity meters;
Power supplies and control units for household appliances (TVs, mixers, washing machines, vacuum cleaners, set-top-boxes, computers, refrigerators, power tools, etc.), chargers for mobile phones and laptops, AC/DC-DC/DC converters;
Industrial (power supplies, chargers, electric motor control units, power regulators, etc.), automotive rectifiers.
Zener diode (Zener diode) is a semiconductor diode designed to stabilize voltage in power supplies. Compared to conventional diodes, it has a fairly low regulated breakdown voltage (when turned on in reverse) and can maintain this voltage at a constant level even with a significant change in the reverse current strength. The materials used to create the p-n junction of zener diodes have a high concentration of impurities. Therefore, at relatively small reverse voltages, a strong electric field arises in the junction, causing its electrical breakdown, which in this case is reversible (if thermal breakdown does not occur due to too much current).
Figure 8. Zener diode designation on circuit diagrams
Figure 9. Designation of a two-anode zener diode on circuit diagrams
The operation of the zener diode is based on two mechanisms:
· Avalanche breakdown of p-n junction
· Tunnel breakdown of p-n junction (Zener effect in English literature)
Despite the similar results of action, these mechanisms are different, although they are present together in any zener diode, but only one of them predominates. For zener diodes, up to a voltage of 5.6 volts, tunnel breakdown with a negative temperature coefficient predominates; above 5.6 volts, avalanche breakdown with a positive temperature coefficient becomes dominant. At a voltage of 5.6 volts, both effects are balanced, so choosing this voltage is the optimal solution for devices with a wide temperature range of application.
The breakdown mode is not associated with the injection of minority charge carriers. Therefore, in a zener diode, injection phenomena associated with the accumulation and resorption of charge carriers during the transition from the breakdown region to the blocking region and back are practically absent. This allows them to be used in pulse circuits as level clamps and limiters.
Types of zener diodes:
Precision - have increased stability of the stabilization voltage, for them additional standards are introduced for temporary voltage instability and temperature coefficient of voltage (for example: 2S191, KS211, KS520);
Two-node - provide stabilization and limitation of bipolar voltages, for them the absolute value of the stabilization voltage asymmetry is additionally normalized (for example: 2S170A, 2S182A);
High-speed - have a reduced barrier capacitance (tens of pF) and a short duration of the transient process (units ns), which allows you to stabilize and limit short-term voltage pulses (for example: 2S175E, KS182E, 2S211E).
Semiconductor diodes and their characteristics
A diode is a semiconductor device that consists of one - transition and has two terminals: anode and cathode. Semiconductor diodes are very numerous, and one of the main classification features is their purpose, which is associated with the use of a certain phenomenon in - transition.
Diodes designed to convert alternating current to direct current are called rectifying. D For them, the main one is the valve effect (a large ratio of forward current to reverse), but there are no strict requirements for time and frequency characteristics. They are designed for significant currents and have a large area - transition. In real diodes, as a rule, asymmetrical diodes are used. - transitions. In such transitions, one of the regions of the crystal (the region with a higher concentration of majority carriers) is usually quite low-resistivity, and the other is high-resistivity. The low-resistance region is the dominant source of mobile charge carriers, and the current through the diode when the junction is directly turned on is almost completely determined by the flow of its majority carriers. Therefore, the low-resistance region of the diode semiconductor crystal is called the emitter. The difference in the concentration of the main charge carriers also affects the location - transition at the boundary of regions with different types of electrical conductivity. Due to the higher carrier concentration in the low-resistivity region (as noted above), the width - there is less transition in it than in the high-resistivity one. If the difference in the concentration of the main carriers is large, then - The transition will be located almost entirely in the high-resistivity region, which is called the base.
Current-voltage characteristics of real diodes and - transitions are close to each other, but not identical (Figure 1.6). Differences are observed both on the forward and reverse branches. This is explained by the fact that when analyzing processes in a junction, neither the dimensions of the crystal and the junction, nor the resistance of the semiconductor layers adjacent to the junction are taken into account. The presence in the semiconductor crystal of a high-resistance base region, which is characterized by resistance, leads to an additional voltage drop, as a result of which the direct branch of the diode passes lower than in the junction. The reverse branch of the diode's current-voltage characteristic is lower than that of an ideal junction, because The leakage current along the surface of the crystal is added to the saturation current.
Figure 1.6 - Symbol for diode (a);
current-voltage characteristics (v):
1 - ideal - transition, 2 – real diode
Diodes can be made from germanium or silicon; their current-voltage characteristics have significant differences (Figure 1.7)
Figure 1.7 - Current-voltage characteristics of germanium (1),
silicon (2) diodes
The shift of the forward branch of the characteristic to the left is due to the difference in the magnitude of the potential barrier, and the position of the reverse branch is determined by the difference in the concentrations of minority carriers, which depend on the band gap of the semiconductor.
The type of current-voltage characteristic depends on the temperature of the semiconductor crystal (Figure 1.8).
Figure 1.8 - Dependence of the type of current-voltage characteristic of the diode on temperature
As the temperature increases, the forward voltage drop across the diode decreases at a constant forward current. The forward voltage changes by 2.1 mV with a 1ºC temperature change.
The reverse current increases with increasing temperature by two times when the temperature changes by 10ºC for germanium diodes and three times for silicon diodes, however, it should be taken into account that the reverse current of silicon diodes is three orders of magnitude less than that of germanium diodes.
Currently, silicon rectifier diodes are most widely used, which have the following advantages:
Many times smaller (compared to germanium) reverse currents at the same voltage; high value of permissible reverse voltage, which reaches 1000...1500 V, while for germanium diodes it is in the range of 100...400 W;
The performance of silicon diodes is maintained at temperatures from -60 to +150 °С, germanium - only from -60 to +85 °C (at temperatures above 85 °C, thermal generation in germanium increases sharply, which increases the reverse current and can lead to the diode losing its valve properties).
However, in rectifier devices of low voltages and high currents, it is more profitable to use germanium diodes, since their resistance in the forward direction is 1.5...2 times less than that of silicon diodes at the same load current, which reduces the power dissipated inside the diode.
Main parameters of rectifier diodes:
maximum permissible reverse voltage diode - the value of voltage applied in the reverse direction that the diode can withstand for a long time without affecting its performance;
average rectified current diode - the average value of the rectified current flowing through the diode over the period;
pulsed direct current diode - peak value of the current pulse at a given maximum duration, duty cycle and pulse shape;
average reverse current diode - the average value of the reverse current over the period;
average forward voltage diode at a given average value of forward current;
average power dissipation diode - average power over a period dissipated by a diode when current flows in the forward and reverse directions;
differential resistance diode - the ratio of the forward voltage increment on the diode to the small current increment that caused it.
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 p-n junction phenomenon, 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 is used 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 the electric field of the pn junction 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 .
The main purpose of rectifier diodes is voltage conversion. But this is not the only area of application for these semiconductor elements. They are installed in switching and control circuits, used in cascade generators, etc. Beginning radio amateurs will be interested in learning how these semiconductor elements are structured, as well as their operating principle. Let's start with the general characteristics.
Device and design features
The main structural element is a semiconductor. This is a wafer of silicon or germanium crystal, which has two regions of p and n conductivity. Because of this design feature, it is called planar.
When manufacturing a semiconductor, the crystal is processed as follows: to obtain a p-type surface, it is treated with molten phosphorus, and for a p-type surface, it is treated with boron, indium or aluminum. During heat treatment, diffusion of these materials and the crystal occurs. As a result, a region with a p-n junction is formed between two surfaces with different electrical conductivities. The semiconductor obtained in this way is installed in the housing. This protects the crystal from external influences and promotes heat dissipation.
Designations:
- A – cathode output.
- B – crystal holder (welded to the body).
- C – n-type crystal.
- D – p-type crystal.
- E – wire leading to the anode terminal.
- F – insulator.
- G – body.
- H – anode output.
As already mentioned, silicon or germanium crystals are used as the basis for the p-n junction. The former are used much more often, this is due to the fact that in germanium elements the reverse currents are much higher, which significantly limits the permissible reverse voltage (it does not exceed 400 V). While for silicon semiconductors this characteristic can reach up to 1500 V.
In addition, germanium elements have a much narrower operating temperature range, it varies from -60°C to 85°C. When the upper temperature threshold is exceeded, the reverse current sharply increases, which negatively affects the efficiency of the device. For silicon semiconductors, the upper threshold is about 125°C-150°C.
Power classification
The power of the elements is determined by the maximum permissible direct current. In accordance with this characteristic, the following classification has been adopted:
List of main characteristics
Below is a table describing the main parameters of rectifier diodes. These characteristics can be obtained from the datasheet (technical description of the element). As a rule, most radio amateurs turn to this information in cases where the element indicated in the diagram is not available, which requires finding a suitable analogue for it.
Note that in most cases, if you need to find an analogue of a particular diode, the first five parameters from the table will be quite sufficient. In this case, it is advisable to take into account the operating temperature range of the element and frequency.
Principle of operation
The easiest way to explain the principle of operation of rectifier diodes is with an example. To do this, we simulate the circuit of a simple half-wave rectifier (see 1 in Fig. 6), in which power comes from an alternating current source with voltage U IN (graph 2) and goes through VD to the load R.
Rice. 6. Operating principle of a single-diode rectifier During the positive half-cycle, the diode is in the open position and passes current through it to the load. When the turn of the negative half-cycle comes, the device is locked and no power is supplied to the load. That is, there is a kind of cutting off of the negative half-wave (in fact, this is not entirely true, since during this process there is always a reverse current, its value is determined by the I arr. characteristic).
As a result, as can be seen from graph (3), at the output we receive pulses consisting of positive half-cycles, that is, direct current. This is the principle of operation of rectifying semiconductor elements.
Note that the pulse voltage at the output of such a rectifier is only suitable for powering low-noise loads, an example would be a charger for a flashlight acid battery. In practice, this scheme is used only by Chinese manufacturers in order to reduce the cost of their products as much as possible. Actually, the simplicity of the design is its only pole.
The disadvantages of a single-diode rectifier include:
- Low level of efficiency, since negative half-cycles are cut off, the efficiency of the device does not exceed 50%.
- The output voltage is approximately half that of the input.
- High noise level, which manifests itself in the form of a characteristic hum at the frequency of the supply network. Its reason is asymmetrical demagnetization of the step-down transformer (in fact, this is why for such circuits it is better to use a damping capacitor, which also has its negative sides).
Note that these disadvantages can be somewhat reduced; to do this, it is enough to make a simple filter based on a high-capacity electrolyte (1 in Fig. 7).
Rice. 7. Even a simple filter can significantly reduce ripple The operating principle of such a filter is quite simple. The electrolyte is charged during the positive half-cycle and discharged when the negative half-cycle occurs. The capacitance must be sufficient to maintain the voltage across the load. In this case, the pulses will be somewhat smoothed out, approximately as shown in graph (2).
The above solution will improve the situation somewhat, but not much; if you power, for example, active computer speakers from such a half-wave rectifier, a characteristic background will be heard in them. To fix the problem, a more radical solution will be required, namely a diode bridge. Let's look at the operating principle of this circuit.
Design and principle of operation of a diode bridge
The significant difference between such a circuit (from a half-wave circuit) is that voltage is supplied to the load in each half-cycle. The circuit diagram for connecting semiconductor rectifier elements is shown below.
As can be seen from the above figure, the circuit uses four semiconductor rectifier elements, which are connected in such a way that only two of them operate during each half-cycle. Let us describe in detail how the process occurs:
- The circuit receives an alternating voltage Uin (2 in Fig. 8). During the positive half-cycle, the following circuit is formed: VD4 – R – VD2. Accordingly, VD1 and VD3 are in the locked position.
- When the sequence of the negative half-cycle occurs, due to the fact that the polarity changes, a circuit is formed: VD1 – R – VD3. At this time, VD4 and VD2 are locked.
- The next period the cycle repeats.
As can be seen from the result (graph 3), both half-cycles are involved in the process and no matter how the input voltage changes, it flows through the load in one direction. This principle of operation of a rectifier is called full-wave. Its advantages are obvious, we list them:
- Since both half-cycles are involved in the work, the efficiency increases significantly (almost twice).
- Ripple at the output of the bridge circuit also doubles the frequency (compared to a half-wave solution).
- As can be seen from graph (3), the level of dips decreases between pulses, so it will be much easier for the filter to smooth them out.
- The voltage at the rectifier output is approximately the same as at the input.
Interference from the bridge circuit is negligible, and becomes even less when using a filter electrolytic capacitance. Thanks to this, this solution can be used in power supplies for almost any amateur radio design, including those that use sensitive electronics.
Note that it is not at all necessary to use four rectifier semiconductor elements; it is enough to take a ready-made assembly in a plastic case.
This case has four pins, two for the input and the same number for the output. The legs to which AC voltage is connected are marked with a “~” sign or the letters “AC”. At the output, the positive leg is marked with the symbol “+”, respectively, the negative leg is marked with “-”.
On a schematic diagram, such an assembly is usually denoted in the form of a diamond, with a graphic display of a diode located inside.
The question of whether it is better to use an assembly or individual diodes cannot be answered unambiguously. There is no difference in functionality between them. But the assembly is more compact. On the other hand, if it fails, only a complete replacement will help. If in this case individual elements are used, it is enough to replace the failed rectifier diode.