Thyristor is also called thyristor. Since it came out in 1950s, it has developed into a big family, and its main members are unidirectional thyristors, bidirectional thyristors, light-controlled thyristors, reverse thyristors, turn-off thyristors, fast thyristors and so on. Today, people use unidirectional thyristors, which are commonly called ordinary thyristors. They are composed of four layers of semiconductor materials, with three PN junctions and three external electrodes (Figure 2 (a)): the electrode drawn from the first layer of P-type semiconductor is called anode A, the electrode drawn from the third layer of P-type semiconductor is called control electrode G, and the electrode drawn from the fourth layer of N-type semiconductor is called cathode K. As can be seen from the circuit symbol of thyristor [Figure 2 (b)], it is a unidirectional conducting device similar to a diode, and the key point is to add a diode.
Figure 2
Second, the main working characteristics of thyristor
In order to intuitively understand the working characteristics of thyristor, let's take a look at this teaching board (Figure 3). Thyristor VS is connected in series with small bulb EL and connected to DC power supply through switch S. Note that anode A is the positive power supply, cathode K is the negative power supply, and control electrode G is connected to the positive power supply of 3V DC through button switch SB (KP5 thyristor is used here, and if KP 1 is used, it should be connected to the positive power supply of 1.5V DC). This connection between thyristor and power supply is called forward connection, that is, DC voltage is applied to the anode and control electrode of thyristor. Now we turn on the power switch S, and the small light bulb does not light up, indicating that the thyristor is not conductive; Press the button switch SB again, input a trigger voltage to the control electrode, and the small light bulb lights up, indicating that the thyristor is on. What does this demonstration experiment give us?
Figure 3
This experiment tells us that to turn on the thyristor, one is to apply a DC voltage between its anode A and cathode K, and the other is to input a forward trigger voltage between its control electrode G and cathode K. After the thyristor is turned on, release the button switch, remove the trigger voltage and keep it on.
Thyristors are characterized by "hair trigger". However, if a reverse voltage is applied to the anode or the control electrode, the thyristor cannot be turned on. The function of the control electrode is to turn on the thyristor by applying a positive trigger pulse, but it cannot turn off the thyristor. So, what method can be used to turn off the conducting thyristor? Turning off the conducting thyristor can disconnect the anode power supply (switch S in Figure 3) or make the anode current less than the minimum value of maintaining conduction (called holding current). If AC voltage or pulsating DC voltage is applied between the anode and cathode of the thyristor, the thyristor will automatically turn off when the voltage crosses zero.
Third, can you distinguish the three electrodes of the thyristor with a multimeter? How to check the quality of thyristor?
The three electrodes of common thyristor can be measured by the ohm range R× 100 of multimeter. As we all know, there is a PN junction between thyristors G and K [Figure 2 (a)], which is equivalent to a diode, with G as the positive electrode and K as the negative electrode. So according to the method of testing diodes, find two of the three poles and measure their positive and negative resistances. When the resistance is small, the black probe of the multimeter is connected to the control electrode G, the red probe is connected to the cathode K, and the remaining one is the anode A. To test the quality of the thyristor, you can use the teaching board circuit just demonstrated (Figure 3). Turn on the power switch S and press the button switch SB, the light bulb is good when it is on, and it is bad when it is not on.
4. What is the main use of thyristor in the circuit?
The most basic use of ordinary thyristor is controllable rectification. The common diode rectifier circuit belongs to uncontrollable rectifier circuit. If thyristor is used instead of diode, a controllable rectifier circuit can be formed. Now I draw a simplest single-phase half-wave controllable rectifier circuit [Figure 4 (a)]. During the positive half cycle of sinusoidal AC voltage U2, if the trigger pulse Ug is not input to the control electrode of VS, VS still cannot be turned on. Only when U2 is in the positive half cycle and the trigger pulse Ug is applied to the control electrode, the thyristor is triggered to conduct. Now draw its waveform diagram [Figure 4(c), (d)], it can be seen that only when the trigger pulse Ug comes, the voltage UL (the shaded part on the waveform diagram) is output on the load RL. Ug came early, and the thyristor opened early; If Ug comes late, the thyristor will turn on later. By changing the arrival time of the trigger pulse Ug on the control electrode, the average value UL (the area of the shaded part) of the output voltage on the load can be adjusted. In electrical technology, the half cycle of alternating current is always 180, which is called electrical angle. In this way, in every positive half cycle of U2, the electrical angle from zero to the arrival time of the trigger pulse is called the control angle α; The electrical angle at which the thyristor conducts in every positive half cycle is called the conduction angle θ. Obviously, both α and θ are used to indicate the turn-on or turn-off range of the thyristor in the half cycle of DC voltage. By changing the control angle α or conduction angle θ, the average value UL of pulse DC voltage on the load is changed to realize controllable rectification.
5. In the bridge rectifier circuit, is it controllable to replace all diodes with thyristors?
In the bridge rectifier circuit, a full-wave controllable rectifier circuit can be formed only by replacing two diodes with thyristors. Now draw the circuit diagram and waveform diagram (Figure 5), and you can see it clearly.
6. How is the trigger pulse required for the thyristor control electrode generated?
There are many kinds of thyristor trigger circuits, such as resistance-capacitance phase-shifting bridge trigger circuit, single-junction transistor trigger circuit, transistor trigger circuit, trigger circuit using small thyristor to trigger large thyristor and so on. Today, everyone's voltage regulator uses a single-junction transistor trigger circuit.
7. What is a single junction transistor? What are its special properties?
A single junction transistor, also known as a double-base diode, is a semiconductor device consisting of a PN junction and three electrodes (Figure 6). Let's draw a schematic diagram of its structure first [Figure 7 (a)]. Two electrodes, namely a first base B 1 and a second base B2, are fabricated at both ends of the N-type silicon wafer; On the other side of the silicon wafer, near B2, a PN junction is made, which is equivalent to a diode. The electrode drawn from the P region is called the emitter E. For the convenience of analysis, the N-type region between B 1 and B2 can be equivalent to a pure resistor RBB, which is called the base resistor, and can be regarded as the series connection of two resistors RB2 and RB 1 [Figure 7 (b)]. It is worth noting that the resistance of RB 1 will change with the change of emitter current IE, which has the characteristics of variable resistance. If a DC voltage UBB is applied between two bases B2 and B 1, the voltage UA at point A is: If the emitter voltage UE
Eight, how to use single junction transistor to form thyristor trigger circuit?
The trigger pulse generation circuit composed of single-junction transistors has been applied to the voltage regulators made by everyone today. In order to illustrate its working principle, we draw the circuit of the single-junction transistor relaxation oscillator separately (Figure 8). It consists of a single junction transistor and an RC charge-discharge circuit. After the power switch S is turned on, the power supply UBB charges the capacitor C through the potentiometer RP, and the voltage UC on the capacitor rises exponentially. When UC rises to the peak voltage UP of the single-junction transistor, the single-junction transistor suddenly turns on, the base resistance RB 1 drops sharply, and the capacitor C rapidly discharges to the resistance R 1 through the PN junction, so that the voltage Ug at both ends of R 1 jumps forward, forming a steep pulse front [Figure 8 (b)]. With the discharge of capacitor C, UE decreases exponentially until the single junction transistor turns off below the valley voltage UV. In this way, the peak trigger pulse is output at both ends of R 1. At this time, the power supply UBB starts charging the capacitor C again, and enters the second charging and discharging process. In this way, the circuit oscillates periodically. Adjusting RP can change the oscillation period.
9. It is found in the waveform diagram of the controllable rectifier circuit that the time for the thyristor to send out the first trigger pulse is the same in every half DC voltage cycle, that is, the control angle α and the conduction angle θ are equal. Then, how can the single-junction transistor relaxation oscillator accurately cooperate with AC power supply to achieve effective control?
In order to realize the "controllable" output voltage of the rectifier circuit, it is necessary to make the trigger circuit send out the first trigger pulse at the same time every half cycle when the thyristor bears DC voltage. This cooperative working mode is called trigger pulse synchronization with power supply.
How can we achieve synchronization? Let's take a look at the circuit diagram of the voltage regulator (figure 1). Please note that the power supply of the single-junction transistor relaxation oscillator here is the full-wave pulse DC voltage output by the bridge rectifier circuit. When the thyristor is not conductive, the capacitor C of the relaxation oscillator is charged by the power supply. When the UC index rises to the peak voltage UP, the single junction transistor VT turns on. During the conduction of VS, there is AC voltage and current on the load RL. At the same time, the voltage drop across the turned-on VS is very small, forcing the relaxation oscillator to stop working. When the AC voltage crosses zero, the thyristor VS is forcibly turned off, the relaxation oscillator is powered on, and the capacitor C is recharged, and the above process is repeated. In this way, every time the AC voltage crosses zero, the moment when the relaxation oscillator sends out the first trigger pulse is the same, which depends on the resistance of RP and the capacitance of C. By adjusting the resistance of RP, the charging time of the capacitance of C can be changed, which also changes the time when the first Ug is sent out, and accordingly changes the control angle of the thyristor, so that the average value of the output voltage on the load RL changes, thus achieving the purpose of voltage regulation.
T 1 and T2 of bidirectional thyristor are not interchangeable. Otherwise, the pipeline and related control circuits will be damaged.