How Does Thyristor Rectifier Achieve Rectification And Voltage Regulation?

May 31, 2020
How Does Thyristor Rectifier Achieve Rectification And Voltage Regulation?

The rectifier is a device that converts AC power to DC power. The main application of the rectifier is to convert AC power to DC power. Since many electrical equipment needs to use DC, but the power supply of the power company is AC, so unless the battery is used, otherwise Rectifiers are indispensable inside the power supply.

As a commonly used rectifier, silicon controlled rectifiers are widely used in ion-exchange membrane caustic soda electrolysis, non-ferrous metal electrolysis, electrolysis of water to produce hydrogen, electrolysis of rare earth, electrolysis of hydrogen fluoride to produce chlorine, electrolysis of seawater to produce sodium hypochlorite, electrolysis of brine to produce sodium hypochlorite, electroplating, anodization Electrophoresis, electropolishing, charging, DC arc furnace heating, crystal heating, nuclear power experiment and other fields are widely used in various industries of the national economy.

So how does the silicon controlled rectifier achieve AC to DC, that is, rectification and output voltage regulation?

1. Diode rectifier circuit           

Rectification (rectification circuit) is a process that uses the unidirectional conductivity of a diode to convert alternating current into pulsating direct current. Common rectifier circuits have half-wave, full-wave, bridge and voltage doubler rectification.

Let us first use the diode as an example to describe the principle of rectification:

Bridge rectifier (bridge rectifier): Four diodes are connected to form a bridge, so it is called bridge rectifier.

In the positive half of the voltage on the secondary side of the transformer, D1 and D3 are turned on, D2 and D4 are turned off, and the direction of current i1 is shown by the red arrow. During the negative half cycle of the voltage, the direction of the current i2 is shown by the blue arrow.


In one cycle of AC voltage u2, diodes D1, D3 and D2, D4 are turned on and off in turn, and single direction of full-wave pulsating voltage and current is obtained on load RL, this process realizes AC to DC, that is rectification.

2. What is SCR?

SCR (silicon control rectifier) is also called thyristor, is a controllable unidirectional conductive switch, can be used as a high-power semiconductor device controlled by strong electricity, because it can be reliable under the effect of weak current signal it controls the various circuits of the strong electric system, so the application of semiconductor electronic technology is expanded from the weak electric field to the strong electric field. Thyristors can pass large currents, and have the advantages of high pressure resistance, fast response, good control characteristics, small size, light weight, convenient use and maintenance, etc., so they are widely used in various scientific research fields such as power, electronics and control, and are increasingly widely used. In the control system of medical X-ray machine. However, it also has shortcomings such as low overload capacity and poor anti-interference ability, which need to be taken to overcome in practical applications. 

The thyristor adds gate on the basis of the diode. In the rectifier circuit, the gate trigger signal needs to be provided to the gate to control the conduction time of the thyristor to achieve rectification and adjust the DC output.

3. The structure and working principle of thyristor

(1) Thyristor structure

The thyristor consists of two layers of P-type and two layers of N-type semiconductor alternately. Its three electrodes are anode A, cathode K and control electrode G. In the middle of this PNPN device, three PN junctions J1, J2, J3 are formed, which is equivalent to three diodes connected in series in the forward and reverse phases. If only a voltage is applied between the anode A and the cathode K, regardless of the polarity of the applied voltage, at least one of the three diodes is in reverse bias, so it will not turn on, and the device is in the off state.


(2) The working principle of thyristor

In order to explain the working principle of thyristor, we regard it as being formed by connecting two crystal transistors of PNP and NPN type, the base of each transistor is connected to the collector of another transistor, as shown in the figure. The anode A corresponds to the emitter of the PNP transistor T1, and the cathode K corresponds to the emitter of the NPN transistor T2.


After turning on, the voltage drop is very small, the power supply voltage is almost all added to the load, and the load current flows through the thyristor.After the thyristor is turned on, its conduction state is completely maintained by the positive feedback effect of the tube itself. Even if the control electrode current disappears, the thyristor is still in the conduction state. Therefore, the role of the control pole is only to trigger the thyristor to turn on, and after turning on, the control pole loses control. To turn off the thyristor, the anode current must be reduced so that it cannot maintain the positive feedback process, or the anode power supply can be disconnected or a reverse voltage is added between the anode and cathode of the thyristor.

Therefore, the thyristor is a controllable unidirectional conductive switch. Compared with the diode, the difference is that the SCR forward conduction is controlled by the current of the control pole; compared with the triode, the difference is that the SCR has no amplification effect on the current of the control pole.

4. Thyristor rectification

The process of applying thyristor to change the alternating current into a DC output voltage with adjustable size is called controlled rectification. The more commonly used controlled rectifier circuit is a semi-controlled bridge rectifier circuit, which is similar to a single-phase uncontrolled bridge rectifier circuit, except that the diodes in the two arms are replaced by thyristors.



At time t1, T1 is turned on, and at T / 2 + t1, T2 is turned on, and the circuit waveform is as shown in the figure. Assume



α=0,Uo=0.9U2,The output voltage is the highest, which is equivalent to the single-phase bridge rectification voltage of the uncontrollable diode;  α=180°,Uo=0,The thyristor is completely turned off. The average value of the rectified current in the load resistance RL is:    


It can be seen that when U2 is fixed, changing the control angle α, that is, changing the time of adding the trigger pulse, can change the average value of the DC output voltage, and the purpose of controllable rectification is achieved.

In practical applications, the rectification form is more complicated, with three-phase bridge fully controlled rectification, double anti-star with balanced reactor rectifier circuit, 6 pulse, 12 pulse, 24 pulse and other multi-pulse rectification, in-phase anti-parallel Non-in-phase anti-parallel structure and so on.

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