SCR module

　　The thyristor module is also called a thyristor (Silicon Controlled Rectifier, SCR). Since its inception in the 1950s, it has developed into a large family. Its main members are unidirectional thyristors, bidirectional thyristors, light-controlled thyristors, reverse-conducting thyristors, turn-off thyristors, fast thyristors, and so on. Today everyone uses a unidirectional thyristor, which is what people often call an ordinary thyristor. It is composed of four layers of semiconductor materials, with three PN junctions and three external electrodes: 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 the control electrode G, and the electrode drawn from the fourth layer of N-type semiconductor is called cathode K. It can be seen from the circuit symbol of the thyristor that it is a unidirectional conductive device like a diode, and the key is that it has an additional control electrode G, which makes it have completely different working characteristics from the diode.

　　The three electrodes of the thyristor can be distinguished with a multimeter

　　The three electrodes of ordinary thyristors can be measured with the R×100 gear of the multimeter. As we all know, there is a PN junction between the thyristors G and K (Figure 2(a)), which is equivalent to a diode, G is the positive pole, and K is the negative pole. Therefore, according to the method of testing the diode, find out two of the three poles. One pole, measure its forward and reverse resistance, the resistance is small, the black pen of the multimeter is connected to the control pole G, the red pen is connected to the cathode K, and the remaining one is the anode A. To test whether the thyristor is good or bad, you can use the teaching board circuit just demonstrated (Figure 3). Turn on the power switch S, press the button switch SB once, the light bulb is good if it is luminous, and it is bad if it is not luminous.

　　The main use of thyristors in circuits

　　The most basic use of ordinary thyristors is controlled rectification. The familiar diode rectification circuit belongs to the uncontrollable rectification circuit. If the diode is replaced with a thyristor, a controllable rectification circuit, inverter, motor speed regulation, motor excitation, non-contact switch and automatic control can be formed. Now I draw the simplest single-phase half-wave controllable rectification circuit [Figure 4(a)]. During the positive half cycle of the sinusoidal AC voltage U2, if there is no trigger pulse Ug input to the control pole 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 pole, the thyristor is triggered to conduct. Now, draw its waveform diagram [Figure 4(c) and (d)], it can be seen that only when the trigger pulse Ug arrives, there is a voltage UL output on the load RL (the shaded part on the waveform diagram). If Ug arrives early, the thyristor will turn on early; if Ug arrives late, the thyristor will turn on later. By changing the arrival time of the trigger pulse Ug on the control pole, the average value UL of the output voltage on the load (the area of the shaded part) can be adjusted. In electrotechnical technology, the half cycle of alternating current is often set as 180°, which is called electrical angle. In this way, in each positive half cycle of U2, the electrical angle experienced from the zero value to the moment when the trigger pulse arrives is called the control angle α; the electrical angle at which the thyristor is turned on in each positive half cycle is called the conduction angle θ. Obviously, both α and θ are used to represent the turn-on or block range of the thyristor in the half cycle of the forward voltage. By changing the control angle α or conduction angle θ, the average value UL of the pulse DC voltage on the load is changed, and the controllable rectification is realized.

　　How to identify the three poles of silicon controlled rectifier

　　　The method of identifying the three poles of the thyristor is very simple. According to the principle of the PN junction, just use a multimeter to measure the resistance value between the three poles.

　　The forward and reverse resistance between the anode and the cathode is more than a few hundred thousand ohms, and the forward and reverse resistance between the anode and the control electrode is more than a few hundred thousand ohms (there are two PN junctions between them, and the direction On the contrary, so the positive and negative directions of the anode and the control pole are not connected).

　　There is a PN junction between the control electrode and the cathode, so its forward resistance is in the range of several ohms to hundreds of ohms, and the reverse resistance is larger than the forward resistance. However, the characteristics of the control pole diode are not ideal. The reverse direction is not completely blocked, and a relatively large current can pass through. Therefore, sometimes the measured control pole reverse resistance is relatively small, which does not mean that the control pole characteristics are not good. . In addition, when measuring the forward and reverse resistance of the control pole, the multimeter should be placed in the R*10 or R*1 block to prevent the reverse breakdown of the control pole when the voltage is too high.

　　If it is measured that the cathode and anode of the component have been short-circuited, or the anode and the control pole are short-circuited, or the control pole and the cathode are short-circuited in reverse, or the control pole and the cathode are open-circuited, it means that the component is damaged.

　　Thyristor is the abbreviation of silicon controlled rectifier element, which is a high-power semiconductor device with a four-layer structure of three PN junctions. In fact, the function of the thyristor is not only rectification, it can also be used as a non-contact switch to quickly switch on or off the circuit, realize the inversion of direct current into alternating current, and change the alternating current of one frequency into another frequency of alternating current, and so on. SCRs, like other semiconductor devices, have the advantages of small size, high efficiency, good stability, and reliable operation. Its appearance has brought semiconductor technology from the field of weak electricity to the field of strong electricity, and has become a component that is eagerly used in industry, agriculture, transportation, military scientific research, as well as commercial and civilian electrical appliances.

　　1. The structure and characteristics of thyristor

　　■Thyristors are mainly divided into three types in terms of appearance: spiral type, flat type and flat type (see chart-25). There are many spiral applications.

　　■ SCR has three electrodes ---- anode (A), cathode (C) and control pole (G). It has a die with a four-layer structure composed of overlapping P-type conductors and N-type conductors, and there are three PN junctions in total. Its structural diagram and symbols are shown in Chart-26.

　　■As can be seen from Chart-26, the structure of the thyristor and the silicon rectifier diode with only one PN junction is quite different. The four-layer structure of the thyristor and the reference of the control pole have laid the foundation for its excellent control characteristics of "controlling the big with the small". When using a silicon controlled rectifier, as long as a small current or voltage is applied to the control pole, a large anode current or voltage can be controlled. At present, thyristor elements with a current capacity of several hundred amperes or even thousands of amperes have been manufactured. Generally, the thyristor below 5 amperes is called low-power thyristor, and the thyristor above 50 amperes is called high-power thyristor.

　　■ Why does the thyristor have the controllability of "controlling the big with the small"? Below we use Chart-27 to briefly analyze the working principle of the thyristor.

　　■First of all, we can see that the first, second, and third layers from the cathode are an NPN type transistor, while the second, third, and fourth layers form another PNP type transistor. Among them, the second and third layers are shared by two overlapping tubes. In this way, the equivalent circuit diagram of Chart-27(C) can be drawn for analysis. When a forward voltage Ea is applied between the anode and the cathode, and a positive trigger signal is input between the control electrode G and the cathode C (equivalent to the base-emitter of BG1), BG1 will generate a base current Ib1, through Amplified, BG1 will have a collector current IC1 amplified by β1. Since the collector of BG1 is connected to the base of BG2, IC1 is the base current Ib2 of BG2. BG2 amplifies the collector current IC2 by β2 than Ib2 (Ib1) and sends it back to the base of BG1 for amplification. In this way, the loop is amplified until BG1 and BG2 are completely turned on. In fact, this process is a "trigger-on-the-fly" process. For the thyristor, the trigger signal is added to the control electrode, and the thyristor is turned on immediately. The conduction time is mainly determined by the performance of the thyristor.

　　■After the thyristor is triggered and turned on, due to the loop feedback, the current flowing into the base of BG1 is not only the initial Ib1, but the current amplified by BG1 and BG2 (β1 *β2 *Ib1). This current is huge For Ib1, it is enough to keep BG1 continuously turned on. At this time, even if the trigger signal disappears, the thyristor remains on. Only when the power supply Ea is cut off or Ea is lowered so that the collector current in BG1 and BG2 is less than the minimum value for maintaining conduction, the thyristor can be turned off. Of course, if the polarity of Ea is reversed, BG1 and BG2 will be in the cut-off state due to the reverse voltage. At this time, even if the trigger signal is input, the thyristor cannot work. Conversely, Ea is connected positively, while the trigger signal is negative, and the thyristor cannot be turned on. In addition, if the trigger signal is not added, and the positive anode voltage exceeds a certain value, the thyristor will also be turned on, but this is already an abnormal working condition.

　　■The controllable characteristic of silicon controlled rectifiers, which is controlled by a trigger signal (small trigger current) to turn on (a large current passes through the silicon controlled silicon), is an important feature that distinguishes it from ordinary silicon rectifier diodes.