Thyristor, also known as silicon controlled rectifier (SCR), can control its conduction through a signal, but cannot control its turn-off, so it is called a semi-controlled device. The name thyristor often refers specifically to a basic type of thyristor, but broadly speaking, thyristors also include many derivative devices, such as Tri-Electrode AC switch (TRIAC), Fast Switching Thyristor (FST), Reverse-Conducting Thyristor (RCT) and Light-Triggered Thyristor (LTT).

 Catalog

I. Structure

At present, the commonly used appearance structures of high-power thyristors are bolt type and plate type. It has a four-layer structure with three PN junctions. Its appearance, structure, and graphic symbols are shown in the figure.

Inside the thyristor is a PNPN four-layer semiconductor structure, named P1, N1, P2, and N2. The two electrodes drawn from the outermost P layer and N layer are anode A and cathode K, respectively, and the electrode drawn from the middle P2 layer is the gate electrode G (also called the control electrode). The four regions form three PN junctions, J1, J2, and J3. Therefore, a thyristor can be equivalent to three PN junctions in series. The international name of the thyristor is Thyristor, abbreviated as VT.

A thyristor is a power electronic device, which generates heat due to loss during operation, so a radiator must be installed. For the bolt type package, the bolt is usually the anode, and the bolt shape is made to be tightly connected to the radiator and easy to install. The anode (bolt) is screwed on the aluminum radiator for natural cooling; the flat thyristor consists of two A mutually insulated radiator clamps the thyristor and is cooled by cold air.

The plate type has a good heat dissipation effect on both sides, and thyristors with rated current greater than 200A adopt the plate type structure. In addition, it can be cooled by cooling methods such as water cooling and oil cooling.

II. Working Principle

The thyristor is a semi-controlled power electronic device, and its working conditions are as follows:

1. When a thyristor bears the reverse anode voltage, no matter what voltage the gate bears, the thyristor is in the reverse blocking state.

2. When the thyristor is subjected to a positive anode voltage, the thyristor will only be turned on when the gate is subjected to a positive voltage. At this time, the thyristor is in the forward conduction state, which is the thyristor's thyristor characteristic, that is, the controllable characteristic.

3. When the thyristor is turned on, as long as there is a certain positive anode voltage, regardless of the gate voltage, the thyristor remains on, that is, after the thyristor is turned on, the gate loses its function. The gate only serves as a trigger.

4. When the thyristor is turned on, when the voltage (or current) of the main circuit decreases to close to zero, the thyristor turns off.

Do a simple experiment through the circuit shown in the figure below, to illustrate the working principle of the thyristor. The main circuit of the thyristor is composed of the power supply Us, the incandescent lamp, and the anode and cathode of the thyristor. The power supply Uc, the switch S, the gate of the thyristor and the cathode constitute a control circuit, also called a trigger circuit.

When the anode A of the thyristor is connected to the positive terminal of the power supply Us. The cathode K is connected to the negative terminal of the power supply through the incandescent lamp, and the thyristor is subjected to a positive voltage at this time. When the switch S in the control circuit is off, the incandescent lamp does not light up, indicating that the thyristor is not conducting.

When the anode and cathode of the thyristor bear a positive voltage, the switch S in the control circuit is closed, so that the control electrode also applies a forward voltage (the voltage between the control electrode and the cathode). At this time, the incandescent lamp is on, indicating that the thyristor is turned on.

When QT is turned on, remove the voltage on the control pole (about to turn off the switch), and the incandescent lamp is still on. It shows that once the thyristor is turned on, the control pole loses control.

When a reverse voltage is applied between the anode and cathode of QT, regardless of whether the voltage is applied to the control pole, the light will not turn on, and the thyristor will be cut off at this time. If a reverse voltage is applied to the control pole, the thyristor will not conduct regardless of whether the main circuit of the QT is applied with a forward voltage or a reverse voltage.

The following conclusions were drawn from the above experimental results.

(1) To make the thyristor turn on, two conditions must be met at the same time: A forward voltage is added between the anode A and cathode K of the thyristor, and an appropriate forward voltage and current are also added between the gate G and cathode K of the thyristor.

(2) Once the thyristor is turned on, the gate loses its control function, so the thyristor is a semi-controlled device.

(3) In order to turn off the thyristor, the anode A current must be reduced to zero or below a certain value. This can be done by reducing the anode A voltage to zero or applying a reverse voltage to the anode.

III. Types

1. Gate Turn-Off Thyristor - KG

An ordinary thyristor, after the control electrode G plus the positive trigger signal, the ordinary thyristor is turned on, the control electrode G will lose its function. To turn off the ordinary thyristor, the positive voltage of anode A and cathode K must be zero or plus Apply negative voltage. The turn-off thyristor can be turned off effectively by adding a sufficiently large and wide negative trigger current to the control electrode G.

Usage: Generally used in chopper speed regulation, variable frequency speed regulation, inverter power supply, and DC control circuits, such as power on and off of DC loads (relay coils, solenoid valves, electromagnetic clutches, DC motors), automobile ignition, etc.

2. Bi-directional Controlled Rectifier - KS

Ordinary thyristor, if a negative voltage is applied to the anode A and cathode K, even if a positive trigger signal is applied to the control electrode G, the ordinary thyristor will not turn on. The Tri-Electrode AC switch can be turned on. To turn off the bidirectional thyristor, the voltage between anode A and the cathode K must be zero.

Usage: Generally used in AC/DC control circuits, such as voltage regulators; inverters; AC/DC load switches, etc.

3. Reverse conducting thyristor - KN

In some applications (such as inverters, choppers, etc.), the anode A and cathode K of the thyristor are required to be connected in parallel with the reverse diode. The reverse conducting thyristor is based on this requirement. On a silicon wafer material, the thyristor and the reverse diode are integrated. Therefore, the reverse-conducting thyristor has the characteristics of controllable forward conduction, natural conduction in reverse, and uncontrollable flow of large current; high-voltage resistance, high-temperature resistance, short turn-off time, low on-state voltage, and other excellent properties.

Usage: Generally used for switching power supplies; UPS uninterruptible power supplies; high-voltage, high-power loads. Such as the power supply of rail transit.

4. Fast Switching Thyristor - KK

Because in some application fields, the thyristor is required to be able to respond quickly and have the on-off function, and the fast thyristor is based on the ordinary thyristor, and the manufacturing process is changed, which greatly reduces the turn-on and turn-off time of the thyristor. The turn-on time is about 8μs; The turn-off time is about 50μs. The main circuit commutation time is less than 60ms, and the operating frequency is also increased to several thousand Hz.

Usage: often used in UPS uninterruptible power supply; three-phase variable frequency power supply; intermediate frequency power supply; ultrasonic power supply; inverter, chopper; fast switching, pulse width modulator, multivibrator, etc.

5. Light-Triggered Thyristor

Light-Triggered Thyristor is a thyristor that can be turned on by a light source; it can also be turned on by a trigger signal from a gate G as an ordinary thyristor. It has two ends (no gate G) and three ends in structure. Optical control is that its main circuit and control circuit are electrically insulated.

Uses: generally made into photocouplers; peripheral terminals in automatic control systems (such as monitoring, logic control input/output, etc.); high-voltage direct current transmission circuits, etc.

IV. Applications

There are many types of thyristors, which can be used in different fields according to the specific requirements of the application circuit.  

Ordinary unidirectional thyristors can be used for AC and DC voltage control, controllable rectification, AC voltage regulation, inverter power supply, switching power supply protection circuit, etc.

Bi-directional Controlled Rectifiers can be used in circuits such as AC switches, AC voltage regulation, AC motor linear speed regulation, lamp linear dimming, solid-state relays, and solid-state contactors.

Gate-Turn-Off Thyristor can be used for AC motor variable frequency speed regulation, chopper, inverter power supply, and various electronic switch circuits, etc.

BTG thyristors can be used in sawtooth generators, long-time delays, overvoltage protectors, and high-power transistor trigger circuits.

Reverse-Conducting Thyristor can be used in induction cookers, electronic ballasts, ultrasonic circuits, superconducting magnetic energy storage systems and switching power supplies.

Light-Triggered thyristors can be used for photoelectric couplers, light detectors, light alarms, light counters, photoelectric logic circuits and operation monitoring circuits of automatic production lines.