SCR working principle

SCR working principle

　A P1N1P2N2 four-layer three-terminal device with silicon single crystal as the basic material was created in 1957. Because its characteristics are similar to vacuum thyristors, it is commonly called silicon thyristors in the world, or SCR T for short. Since the silicon controlled rectifier was originally used in the controllable rectification, it is also called the silicon controlled rectifier element, referred to as the silicon controlled rectifier SCR.

　　In terms of performance, thyristor not only has unidirectional conductivity, but also has more valuable controllability than silicon rectifier elements (commonly known as "dead silicon"). It has only two states of on and off.

　　Thyristors can control high-power electromechanical equipment with milliampere currents. If the frequency exceeds this frequency, the switching loss of the components will increase significantly, and the average current allowed to pass will decrease. At this time, the nominal current should be degraded for use.

　　Thyristor has many advantages, such as: controlling high power with low power, the power magnification is as high as hundreds of thousands of times; the response is extremely fast, turning on and off within microseconds; non-contact operation, no spark, no noise; High efficiency, low cost and so on.

　　Weaknesses of silicon controlled rectifiers: poor static and dynamic overload capability; easy to be misconducted by interference.

　　Thyristors are mainly classified from the shape: bolt shape, flat plate shape and flat bottom shape.

1. The structure of the thyristor element

　　Regardless of the shape of the thyristor, their dies are four-layer P1N1P2N2 structures composed of P-type silicon and N-type silicon. see picture 1. It has three PN junctions (J1, J2, J3), the anode A is drawn from the P1 layer of the J1 structure, the cathode K is drawn from the N2 layer, and the control electrode G is drawn from the P2 layer, so it is a four-layer three-terminal semiconductor device .

2. Working principle

　　The thyristor is a P1N1P2N2 four-layer three-terminal structural element, with three PN junctions in total. When analyzing the principle, it can be regarded as composed of a PNP tube and an NPN tube. The equivalent diagram is shown in Figure 1.

　　When the positive voltage is applied to the anode A, both BG1 and BG2 tubes are in the amplified state. At this time, if a positive trigger signal is input from the control pole G, BG2 will have a base current ib2 flowing through it, amplified by BG2, and its collector current ic2=β2ib2. Because the collector of BG2 is directly connected to the base of BG1, ib1=ic2. At this time, the current ic2 is amplified by BG1 again, so the collector current ic1 of BG1=β1ib1=β1β2ib2. This current flows back to the base of BG2, forming a positive feedback, which makes ib2 continuously increase. As a result of such a positive feedback loop, the current of the two tubes increases sharply, and the SCR is saturated and turned on.

　　Due to the positive feedback formed by BG1 and BG2, once the thyristor is turned on, even if the current of the gate G disappears, the thyristor can still maintain the conduction state, because the trigger signal only acts as a trigger and does not turn off function, so this thyristor cannot be turned off.

　　Since the thyristor has only two working states of on and off, it has switching characteristics. This characteristic needs certain conditions to be converted. See Table 1 for this condition.

The basic volt-ampere characteristics of thyristor are shown in Figure 2

Figure 2 Basic volt-ampere characteristics of thyristor

(1) Reverse characteristics

　　When the control pole is open and the anode is applied with a reverse voltage (see Figure 3), the J2 junction is forward-biased, but the J1 and J2 junctions are reverse-biased. At this time, only a small reverse saturation current can flow. When the voltage is further increased to the avalanche breakdown voltage of the J1 junction, the junction J3 also breaks down, and the current increases rapidly. The characteristics in Figure 3 begin to bend, such as the characteristic OR As shown in the paragraph, the voltage URO at the bend is called "reverse turning voltage". At this point, the SCR will undergo permanent reverse

(2) Positive characteristics

　　When the control pole is open and a forward voltage is applied to the anode (see Figure 4), the J1 and J3 junctions are forward-biased, but the J2 junction is reverse-biased, which is similar to the reverse characteristics of ordinary PN junctions, and only a small flow can flow. Current, this is called the forward blocking state, when the voltage increases, the characteristics of Figure 3 bend, as shown in the characteristic OA section, the bend is UBO called: forward turning voltage

Figure 4 Anode plus forward voltage

　　After the voltage rises to the avalanche breakdown voltage of the J2 junction, the avalanche multiplication effect occurs at the J2 junction, and a large number of electrons and holes are generated in the junction region. The electrons enter the N1 region and the holes enter the P2 region. The electrons entering the N1 region recombine with the holes injected into the N1 region from the P1 region through the J1 junction. Similarly, the holes entering the P2 region recombine with the electrons injected into the P2 region from the N2 region through the J3 junction, avalanche breakdown, and the electrons entering the N1 region The electrons and the holes entering the P2 region cannot all recombine. In this way, there will be electron accumulation in the N1 region, and there will be hole accumulation in the P2 region. As a result, the potential of the P2 region will increase, and the potential of the N1 region will decrease, and the J2 junction will When it becomes forward-biased, as long as the current increases slightly, the voltage drops rapidly, and the so-called negative resistance characteristic appears, as shown in the dotted line AB in Figure 3.

At this time, the three junctions of J1, J2, and J3 are all in the forward bias, and the thyristor enters the forward conductive state---on state. At this time, its characteristics are similar to the forward characteristics of ordinary PN junctions, as shown in Figure 2. BC segment

2. Trigger conduction

Figure 5. Both the anode and the control pole are applied with forward voltage

Figure 1. Schematic diagram and symbol diagram of thyristor structure

3. What is the main purpose of the thyristor in the circuit?

　　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 silicon controlled rectifier, a controlled rectifier circuit 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 SCR conduction time will be early; if Ug arrives late, the SCR conduction time will be late. 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.

4. In the bridge rectifier circuit, does it become a controlled rectifier circuit if all the diodes are replaced with silicon controlled rectifiers?

　　In the bridge rectifier circuit, only two diodes need to be replaced with thyristors to form a full-wave controllable rectifier circuit. Now draw the circuit diagram and waveform diagram (Figure 5), you can understand

5. How is the trigger pulse required for the thyristor control pole generated?

There are many forms of thyristor trigger circuits, commonly used are resistance-capacitance phase-shift bridge trigger circuits, unijunction transistor trigger circuits, transistor triode trigger circuits, trigger circuits that use small thyristors to trigger large thyristors, and so on.

6. What is a unijunction transistor and what are its special properties?

　　A unijunction transistor, also known as a double-base diode, is a semiconductor device composed of a PN junction and three electrodes (Figure 6). We first draw a schematic diagram of its structure [Figure 7(a)]. At both ends of an N-type silicon chip, two electrodes are made, which are called the first base B1 and the second base B2; a PN junction is made on the other side of the silicon chip near B2, which is equivalent to a diode. The electrode drawn from the P area is called the emitter E. For the convenience of analysis, the N-type region between B1 and B2 can be equivalent to a pure resistance RBB, called the base resistance, and can be regarded as a series connection of two resistances RB2 and RB1 (Figure 7(b)). It is worth noting that the resistance value of RB1 will change with the change of the emitter current IE, which has the characteristic of variable resistance. If a DC voltage UBB is applied between the two bases B2 and B1, the voltage UA at point A is: if the emitter voltage UE<UABR>

7. How to use a unijunction transistor to form a thyristor trigger circuit?

　　We draw alone the circuit for the unijunction transistor relaxation oscillator (Figure 8). It is composed of unijunction transistor and RC charge and discharge circuit. After closing the power switch S, 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 unijunction transistor, the unijunction transistor is suddenly turned on, the base resistance RB1 decreases sharply, and the capacitor C discharges rapidly to the resistor R1 through the PN junction, causing a positive jump in the voltage Ug across R1 change, forming a steep pulse front [Figure 8(b)]. With the discharge of capacitor C, UE decreases exponentially until the unijunction transistor is cut off when it is lower than the valley point voltage UV. In this way, the peak trigger pulse is output at both ends of R1. At this time, the power supply UBB starts to charge the capacitor C again, and enters the second charging and discharging process. This goes round and round, and the circuit oscillates periodically. Adjusting RP can change the oscillation period

8. In the waveform diagram of the controllable rectifier circuit, it is found that in every half cycle of the thyristor receiving the forward voltage, the moment when the first trigger pulse is issued is the same, that is, the control angle α and the conduction angle θ are equal , so how can a unijunction transistor relaxation oscillator be precisely matched to an AC power source for effective control?

　　In order to realize the "controllable" output voltage of the rectifier circuit, it is necessary to make the thyristor withstand the forward voltage in every half cycle, and the moment when the trigger circuit sends out the first trigger pulse is the same. This kind of cooperative working method is called The trigger pulse is synchronized with the power supply. How can we achieve synchronization? Let's look at the circuit diagram of the voltage regulator (Figure 1). Note that the power supply for the unijunction transistor relaxation oscillator here is the full-wave pulsed DC voltage from the output of the bridge rectifier circuit. When the thyristor is not turned on, the capacitor C of the relaxation oscillator is charged by the power supply, and when UC rises exponentially to the peak voltage UP, the unijunction transistor VT is turned on, and during the turn-on period of VS, there is an AC voltage on the load RL and current, meanwhile, the voltage drop across the turned-on VS is small, forcing the relaxation oscillator to stop. When the AC voltage crosses zero instantly, the thyristor VS is forced to turn off, the relaxation oscillator is energized, and starts to charge the capacitor C again, repeating the above process. 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, and this moment depends on the resistance value of RP and the capacitance of C. Adjusting the resistance value of RP can change the charging time of capacitor C, which also changes the moment when the first Ug is emitted, and correspondingly changes the control angle of the thyristor, so that the average value of the output voltage on the load RL changes. To achieve the purpose of pressure regulation.

T1 and T2 of triacs are not interchangeable. Otherwise it will damage the tube and related control circuit.