Products >> Discrete semiconductor product >> Thyristor

Thyristor is a four - layer semiconductor device with three PN junctions. It is a crucial component in power electronics circuits.

Structure and Symbol
- Structurally, it consists of P - type and N - type semiconductor materials arranged in an alternating pattern of P - N - P - N. The most common type is the silicon - controlled rectifier (SCR), whose symbol has an anode (A), a cathode (K), and a gate (G). The anode is the positive terminal, the cathode is the negative terminal, and the gate is used to control the conduction of the device.
Working Principle
- The thyristor has two stable states: off and on. In the off state, the device blocks the current flow between the anode and the cathode. When a positive voltage is applied to the anode with respect to the cathode and a proper positive trigger pulse is applied to the gate, the thyristor turns on. Once it is turned on, it will continue to conduct current even if the gate signal is removed, as long as the anode - cathode current is above a certain minimum value called the holding current. The conduction of the thyristor is based on the injection of carriers (holes or electrons) through the gate junction, which reduces the potential barrier of the middle junction and allows a large current to flow through the device.
Characteristics
- High - power handling capacity: Thyristors can handle high voltages and currents. They are widely used in high - power applications such as power supplies, motor control, and high - voltage direct - current (HVDC) transmission systems. For example, in an HVDC transmission system, thyristors are used to convert alternating current to direct current and vice versa, enabling efficient long - distance power transmission.
- Controllability: The ability to control the turn - on time of the thyristor through the gate signal provides a means of controlling the amount of power delivered to a load. In applications such as light dimmers and variable - speed motor drives, the conduction angle of the thyristor can be adjusted to vary the power output.
- Low - on - state voltage drop: When the thyristor is in the conducting state, it has a relatively low voltage drop across it. This characteristic ensures that the power loss due to the device itself is minimized during conduction, which is beneficial for high - power applications.
Applications
- Power Rectification: Thyristors are commonly used in rectifier circuits to convert alternating current (AC) to direct current (DC). In industrial power supplies, battery chargers, and electrochemical processes, they play a vital role in providing a stable DC output. For example, in a three - phase rectifier circuit using thyristors, a more efficient conversion of AC to DC can be achieved compared to traditional diode rectifiers, especially when variable DC output voltages are required.
- AC Power Control: In applications such as electric heaters, light dimmers, and induction heating systems, thyristors are used to control the amount of AC power delivered to the load. By adjusting the conduction angle of the thyristor, the effective RMS value of the AC voltage applied to the load can be controlled. This allows for energy - saving and precise power control in various industrial and domestic applications.
- Inverter and Motor Control: Thyristors are also used in inverter circuits to convert DC power to AC power. In variable - frequency drive (VFD) systems for motors, thyristors can be used to control the speed and torque of the motor by adjusting the frequency and voltage of the output AC power. This is widely applied in industrial automation, elevators, and electric vehicles to improve energy efficiency and motor performance.