Thyristor

definition:

Thyristor is a semiconductor device used to control electrical energy at high current and high or medium voltage levels. It is mainly made of silicon and contains at least three PN junctions and four layers of alternating P-type and N-type materials. As a switching device, the thyristor has unidirectional conduction characteristics similar to that of a diode, but it is controllable and only conducts after the gate receives a trigger pulse.

Common types of thyristors:

SCR (Silicon Controlled Rectifier):
The most basic type of thyristor, mainly used for switching control of DC or AC circuits.

Reverse Conducting Thyristor (RCT):
Integrated reverse damping diode, suitable for applications requiring fast shutdown.

Gate Turn-Off Thyristor (GTO, Gate Turn-Off Thyristor):
Can be turned off by the gate, providing better control capabilities.

Thyristor (MCT, MOS Controlled Thyristor):
It is controlled by MOSFET gate and has fast switching capability.

Triac, Triode for Alternating Current:
Can conduct current in both directions and is often used for AC power control.

Photothyristor:
The trigger is controlled through light signals and is used in isolation and remote control situations.

There are many ways to classify thyristors:

(1) Classification according to turn-off, conduction and control modes: Thyristors can be divided into ordinary thyristors, bidirectional thyristors, reverse conduction thyristors, gate turn-off thyristors (GTO), BTG thyristors according to their turn-off, conduction and control modes. There are many kinds of temperature-controlled thyristors and light-controlled thyristors.

(2) Classification by pins and polarity: Thyristors can be divided into diode thyristors, triode thyristors and quadrupole thyristors according to their pins and polarity.

(3) Classification by packaging form: Thyristors can be divided into three types according to their packaging form: metal packaged thyristors, plastic packaged thyristors and ceramic packaged thyristors. Among them, metal-encapsulated thyristors are divided into bolt-shaped, flat-plate, round-shell, etc.; plastic-encapsulated thyristors are divided into two types: with heat sink type and without heat sink type.

(4) Classification by current capacity: Thyristors can be divided into three types: high-power thyristors, medium-power thyristors and low-power thyristors according to current capacity. Usually, high-power thyristors are mostly packaged in metal shells, while medium and small-power thyristors are mostly packaged in plastic or ceramic packages.

(5) Classification by turn-off speed: Fast thyristors Thyristors can be divided into ordinary thyristors and high-frequency (fast) thyristors according to their turn-off speed.

working principle:

When the control terminal (gate) of the thyristor receives a forward pulse signal, the internal PN structure forms a conductive path, causing the thyristor to enter the conducting state from the off state. The thyristor now allows current to flow from the cathode to the anode. In an AC circuit, each AC cycle needs to be retriggered to maintain the conduction state, so the effective value of the current can be controlled by adjusting the triggering moment.

The main parameters:

Rated current (IT(AV)): The average current that the thyristor can withstand when it is continuously conducting.
Peak reverse repetition voltage (VRRM): The highest reverse voltage that a thyristor can withstand without triggering the gate.
Gate trigger current (IGT): The minimum gate current required to trigger the thyristor to conduct.
Holding current (IH): Once the thyristor is triggered and conducts, the minimum current required to maintain the conductive state.
Switching frequency: The maximum switching frequency at which the thyristor can operate safely.
Turn-off time (tq): The time required for the thyristor to return from the on-state to the off-state.

application:

Power control: used for motor control, light dimming, electric furnace temperature control, etc.
AC/DC conversion: acts as a rectifying component in rectifiers and frequency converters.
Inverter: Converts DC power to AC power.
Non-contact switch: used in situations where high-speed switching and long life are required.
Overvoltage protection: With appropriate control circuits, equipment can be protected from voltage surges.

Precautions:

Heat dissipation: The thyristor generates heat when it is turned on, and appropriate heat dissipation measures are required to prevent overheating.
Overcurrent protection: Appropriate protection circuitry must be designed to prevent unexpected high currents from damaging the thyristors.
Trigger signal: It is necessary to ensure that the amplitude and duration of the gate trigger signal are sufficient to trigger the thyristor stably.
Anti-static: Thyristors may be damaged by electrostatic discharge and appropriate anti-static measures should be taken.

Main uses in circuits:

Control element: As a key control element in the circuit, thyristors can perform precise current control.
Rectifier: In power supplies and frequency converters, converts alternating current into direct current.
Phase Controller: In AC dimmers and motor speed regulators, the output voltage is controlled by adjusting the firing angle of the thyristor.
Pulse circuit: Converts a continuous electrical signal into a pulse signal in a pulse former.
Thyristor is a very important device in the field of power electronics. Its controllable conduction characteristics make it very popular in applications for controlling large currents. When using it, you also need to pay attention to the circuit design to ensure that the thyristor can operate in a safe working condition.