Thyristor is a semiconductor device with a four layer structure, which is widely used in the field of power electronics to control high voltage and large current. The commutation technology of thyristor refers to turning on the thyristor into the cut-off state by some means, which is of great significance for the application of AC/DC conversion, frequency conversion control and power regulation. This paper will introduce the principle of thyristor commutation technology in detail.
Basic structure and working principle of thyristor
The basic structure of thyristor is composed of PNPN four layer semiconductor material, with three electrodes: anode, cathode and gate. Under normal conditions, when positive voltage is applied to the anode relative to the cathode, the thyristor is in a blocking state. However, when a positive trigger current is applied between the control electrode and the cathode, FM25CL64-GTR Thyristors will conduct, that is, a low resistance channel will be formed between anode and cathode, allowing large current to pass through. The conduction state will continue until the anode current drops below the sustaining current.
Reversing principle
The commutation technology of thyristor can be divided into natural commutation and forced commutation.
Natural commutation
Natural commutation is generally used in AC circuits. The alternating current periodically passes through the zero point. When the current naturally drops to zero and the voltage reverses, the thyristor will automatically turn off. This commutation mode does not need additional circuits, and depends on the natural cycle characteristics of the alternating current.
1. Sine wave zero crossing: in the AC power supply, the current will pass through zero once every half cycle. When the current naturally drops to zero, the thyristor will automatically turn off.
2. Auxiliary function of inductance and capacitance: In some circuits, inductance and capacitance can store energy and release it at an appropriate time, making the current temporarily zero, thus realizing natural commutation.
Forced commutation
Forced commutation is used in DC circuits or some special AC circuits, and additional circuits are needed to turn off the thyristor. Common forced commutation methods include capacitive commutation, inductive commutation and auxiliary thyristor commutation.
1. Capacitor commutation: use the charging and discharging characteristics of the capacitor in the circuit to generate a reverse voltage to turn off the thyristor by charging or discharging the capacitor at an appropriate time.
● Capacitor charging: during thyristor conduction, the capacitor is charged through another path.
● Capacitor discharge: when it is necessary to turn off the thyristor, the capacitor will discharge rapidly through the conduction path, generating a reverse voltage to make the anode voltage of the thyristor drop to zero or negative, thus realizing the turn off.
2. Inductive commutation: use inductance to store magnetic energy in the circuit and release it at an appropriate time to generate reverse current to turn off the thyristor.
● Inductive energy storage: During thyristor conduction, the inductance stores magnetic energy through the circuit.
● Inductance release: when the thyristor needs to be turned off, the inductance quickly releases the stored energy through another path to generate a reverse current, which reduces the anode current of the thyristor to zero, thus realizing the turn off.
3. Auxiliary thyristor commutation: turn off the main thyristor by introducing auxiliary thyristor or other semiconductor switching elements.
● Auxiliary thyristor conduction: during the conduction of the main thyristor, the auxiliary thyristor remains closed.
● Trigger of auxiliary thyristor: when it is necessary to turn off the main thyristor, the auxiliary thyristor is triggered to turn on, forming a new current path, rapidly reducing the current of the main thyristor and turning it off.
Application examples
1. AC motor control: In AC motor control, thyristor commutation technology can be used to adjust the speed and torque of the motor. By precisely controlling the turn-on and turn off time of the thyristor, the input voltage and current of the motor can be adjusted to control the running state of the motor.
2. DC power conversion: in DC power conversion, thyristor commutation technology can be used to achieve efficient DC to DC conversion. For example, in switching power supply, the precise control of DC voltage can be achieved by using the fast on and off characteristics of thyristor.
3. Power regulation: In power regulation, thyristor commutation technology can be used to achieve stable control of power system. For example, in the HVDC system, the thyristor commutation technology can be used to achieve accurate regulation of the voltage and current of the transmission line, thus improving the transmission efficiency and system stability.
Precautions for commutation circuit design
1. Circuit protection: when designing the thyristor commutation circuit, it is necessary to consider the circuit protection measures. For example, fuse, overvoltage protector, overcurrent protector and other elements are used to prevent damage to thyristor and other elements in case of circuit failure.
2. Heat dissipation design: thyristor will generate certain heat in the process of conduction and commutation. It is necessary to design a suitable heat dissipation system to ensure that the thyristor is kept within a safe temperature range during operation.
3. Electromagnetic interference (EMI) suppression: the fast turn-on and turn off of the thyristor will produce electromagnetic interference (EMI), so appropriate filters and shielding measures need to be designed to reduce the interference to other electronic equipment.
conclusion
Thyristor commutation technology is a key technology in the field of power electronics. Through natural commutation and forced commutation methods, it can achieve efficient switching control of thyristor. Different application scenarios and circuit requirements determine the specific commutation method and circuit design. In practical application, circuit protection, heat dissipation design and electromagnetic interference suppression should be comprehensively considered to ensure the reliability and stability of thyristor commutation circuit. Through continuous optimization and innovation, thyristor commutation technology will play an increasingly important role in the field of power electronics.
