What is a thyristor?

SCR is the abbreviation of silicon controlled rectifier, abbreviated as SCR, also known as thyristor.

Since its introduction in the 1950s, it has developed into a large family. Its main members include unidirectional thyristors, bidirectional thyristors, light-controlled thyristors, reverse-conducting thyristors, turn-off thyristors, fast thyristors, etc. Today, we use unidirectional thyristors, which are commonly known as ordinary thyristors. They are composed of four layers of semiconductor materials, have three PN junctions, and have three electrodes facing the outside (Figure 2 (a)): the electrode leading out of the first layer of P-type semiconductor is called anode A, the electrode leading out of the third layer of P-type semiconductor is called control electrode G, and the electrode leading out of the fourth layer of N-type semiconductor is called cathode K. From the circuit symbol of the thyristor (Figure 2 (b)), we can see that it is a unidirectional conductive device like a diode, but the key is that it has an additional control electrode G, which makes it have completely different working characteristics from a diode.

To turn on the thyristor, first, a forward voltage is applied between its anode A and cathode K, and second, a forward trigger voltage is input between its control electrode G and cathode K. After the thyristor is turned on, the trigger voltage is removed by releasing the button switch, and the thyristor still maintains the on state.

Characteristics of thyristors : They are "trigger-ready". However, if a reverse voltage is applied to the anode or 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 it off. So, what method can be used to turn off the thyristor that is turned on? To turn off the thyristor that is turned on, you can disconnect the anode power supply (switch S in Figure 3) or make the anode current less than the minimum value to maintain conduction (called holding current). If an AC voltage or a pulsating DC voltage is applied between the anode and cathode of the thyristor, then when the voltage passes through zero, the thyristor will turn off by itself.

The structure of the thyristor : It consists of four overlapping PN regions, three PN junctions, and three electrodes, namely anode A, cathode K, and control electrode G. The three PN junctions actually form two interconnected triodes. One is PNP and the other is NPN. The collector of each tube is connected to the base of the other tube to form positive feedback.

Thyristors can be divided according to their current capacity: those above 50A are high-power tubes, those below 5A are low-power tubes, and those in between are medium-power tubes.

The main parameters of thyristor are defined as follows:

The rated average forward current of the thyristor is the average value of the 50HZ positive half-wave current that can continuously pass between the anode and the cathode under specified conditions.

Holding current of thyristor: the minimum forward current that keeps the thyristor conducting under specified conditions.

Drain-source saturation current of thyristor: the drain-source current when the gate-source is short-circuited and the drain-source voltage is large enough.

Forward transconductance of a thyristor: In a common source circuit, the change in drain-source output current caused by a 1V increase in gate-source input voltage.

The maximum gate-source voltage of the thyristor: Because the gate of the MOS tube has a very high insulation resistance, the tube is easily damaged when the gate is open. When storing, be sure to short-circuit the three pins.

Example: N-channel junction field effect transistor: 3DJ6D

Saturation drain-source current: <0.35ma Pinch-off voltage: <4V(-4V) Gate-source insulation resistance: 10(8)ohm Forward transconductance: >1000 Input capacitance: <5pf Capacitor feedback: <2pf Low frequency noise: <5db High frequency power gain: >10db Maximum oscillation frequency: >30mhz Maximum drain-source voltage: 20V Maximum gate-source voltage: 20V Maximum dissipated power: 100mw Maximum drain-source current: 15ma

Forward (reverse) blocking voltage of thyristor: It is defined as the voltage value after subtracting 100V from the turning voltage. The forward voltage is not allowed to exceed this value when in use.

The control electrode trigger voltage and current of the thyristor: the minimum control electrode DC voltage and current value required to turn on the thyristor under specified conditions. Generally, the control electrode voltage does not exceed 10V and the current does not exceed 1A.

The conduction time of the thyristor is the time required from the addition of the signal to the control electrode to the anode current rising to 90% of the final value. It includes the delay time, which is the time for the anode current to rise to 10%, and the rise time, which is the time required from 10% to 90%.

The turn-off time of a thyristor is the time required from cutting off the forward current to the time when the control electrode regains control capability.

Example: 3CT061 (small current unidirectional tube) Current: 1A Trigger voltage: <2V

Trigger current of thyristor: 0.01-30ma, conduction time <80us

Thyristor turn-off time: <2.5us Instantaneous current: 9.5A

3CTS5 (small current bidirectional tube) Current: 5A Trigger voltage: <3V

Trigger current: 50ma Instantaneous current: 42A

Thyristors are mainly used in contactless switches, speed regulation, dimming, voltage stabilization, frequency conversion, etc.

Thyristors are widely used in automatic control, electromechanical fields, industrial electrical and household appliances. Thyristors are active switching elements that usually remain in a non-conducting state until they are triggered or "ignited" by a small control signal to make them conductive. Once ignited, they remain in a conductive state even if the trigger signal is removed. To cut them off, a reverse voltage can be applied between the anode and cathode or the current flowing through the thyristor diode can be reduced to below a certain value.

The thyristor diode can be simulated by two transistors of different polarities (PNP and NPN), as shown in Figure G1. When the gate of the thyristor is suspended, BG1 and BG2 are both in the cut-off state. At this time, there is basically no current flowing through the load resistor RL in the circuit. When a positive pulse voltage is input to the gate, BG2 is turned on, causing the base potential of BG1 to drop, and BG1 begins to turn on. The turn-on of BG1 further increases the base potential of BG2, and the base potential of BG1 further decreases. After this positive feedback process, BG1 and BG2 enter the saturated turn-on state. The circuit quickly enters the turn-on state from the cut-off state. At this time, even if there is no trigger pulse on the gate, the circuit will remain in the turn-on state due to the effect of positive feedback. If a reverse voltage is applied to the anode and cathode at this time, the circuit will be quickly turned off because BG1 and BG2 are both in a reverse bias state. In addition, if the resistance of the load resistor RL is increased to reduce the circuit current, the base current of BG1 and BG2 will also decrease. When it is reduced to a certain value, due to the positive feedback of the circuit, the circuit will quickly flip from the on state to the off state. We call this current the holding current. In practical applications, we can use a switch to short-circuit the anode and cathode of the thyristor to achieve the turn-off of the thyristor.

Application Examples

In practical applications, the most diverse circuit of thyristor is its gate trigger circuit, which can be summarized into DC trigger circuit, AC trigger circuit, phase trigger circuit and so on.

1. DC trigger circuit: As shown in Figure G2, this is a commonly used overvoltage protection circuit for televisions. When the voltage of E+ is too high, the voltage at point A also becomes high. When it is higher than the voltage value of the voltage regulator DZ, DZ is turned on, and the thyristor D is triggered and turned on to short-circuit E+, causing the fuse RJ to melt, thereby playing the role of overvoltage protection.

2. Phase trigger circuit: Phase trigger circuit is actually a kind of AC trigger circuit, as shown in Figure G3. The method of this circuit is to use the RC loop to control the phase of the trigger signal. When the R value is small, the RC time constant is small, and the phase shift A1 of the trigger signal is small, so the load obtains a larger electric power; when the R value is large, the RC time constant is large, and the phase shift A2 of the trigger signal is large, so the load obtains less electric power. This typical electric power stepless adjustment circuit is used in many electrical products in daily life .