The working principle and conduction conditions of unidirectional thyristors

The working principle and conduction conditions of unidirectional thyristors

Unidirectional thyristor (SCR) is a semiconductor device and the earliest type of thyristor. It consists of four semiconductor regions with three electrodes: anode, cathode, and control electrode. Unidirectional thyristors have unidirectional conductivity and can only flow from the anode to the cathode, with very little reverse current. Meanwhile, unidirectional thyristors have controllability and can control their conduction state through signals from the control electrode, making them widely used in power electronic devices.

The main characteristics of unidirectional thyristors include low voltage drop, high current carrying capacity, high temperature carrying capacity, and high reliability. It can withstand currents of several hundred amperes, work normally in high temperature environments, have a long lifespan, and can operate normally in harsh working environments. Therefore, unidirectional thyristors are widely used in power electronic devices, such as power supplies, motor control, welding machines, lighting equipment, and other fields.

Working principle of unidirectional thyristor

The working principle of a unidirectional thyristor can be divided into two stages: triggering and conduction.

Trigger phase: When a positive trigger pulse is applied to the control electrode and a positive voltage is applied to the anode, the thyristor is in an off state. At this point, when the amplitude of the positive trigger pulse applied by the control electrode reaches a certain value, the p-n junction of the thyristor will break down, forming a current channel, and current will begin to flow through the thyristor. This process is called triggering.

Conduction stage: Once the thyristor is triggered, it will enter a conducting state, and current will flow from the anode to the cathode. At this point, the anode current of the thyristor will rapidly increase, and the trigger pulse applied to the control electrode can be stopped. The thyristor will still remain conductive until the anode current drops to zero or the reverse voltage exceeds its rated value before stopping conducting. This process is called conduction.

It should be noted that the conduction state of a unidirectional thyristor is unidirectional and can only flow from the anode to the cathode, with very little reverse current. Therefore, in practical applications, it is necessary to select the appropriate type of thyristor according to specific circuit requirements and design the circuit reasonably to ensure that the thyristor can work properly.

There are two main working modes of unidirectional thyristors: AC trigger mode and DC trigger mode.

Communication trigger mode: In communication trigger mode, the control electrode of the thyristor is triggered by an AC signal. When the amplitude of the communication signal reaches a certain value, the thyristor will be triggered and enter a conducting state. This working mode is suitable for the field of AC power control and regulation.

DC trigger mode: In DC trigger mode, the control electrode of the thyristor is triggered by a DC signal. When the amplitude of the DC signal reaches a certain value, the thyristor will be triggered and enter a conducting state. This working mode is suitable for the field of DC power control and regulation.

It should be noted that the conduction state of a unidirectional thyristor is unidirectional and can only flow from the anode to the cathode, with very little reverse current. Therefore, in practical applications, it is necessary to select the appropriate type of thyristor according to specific circuit requirements and design the circuit reasonably to ensure that the thyristor can work properly.

Unidirectional thyristor conduction conditions

The condition for a unidirectional thyristor to conduct is that a positive trigger pulse is applied to the control electrode, and a positive voltage is applied to the anode. When the amplitude of the positive trigger pulse applied to the control electrode reaches a certain value, the thyristor will enter a conducting state. At this point, the anode current of the thyristor will rapidly increase, and the trigger pulse applied to the control electrode can be stopped. The thyristor will still remain conductive until the anode current drops to zero or the reverse voltage exceeds its rated value before stopping conducting.

It should be noted that the conduction state of a unidirectional thyristor is unidirectional and can only flow from the anode to the cathode, with very little reverse current. Therefore, in practical applications, it is necessary to select the appropriate type of thyristor according to specific circuit requirements and design the circuit reasonably to ensure that the thyristor can work properly.