Snubber circuits for power electronic devices

Snubber circuits for power electronic devices

In a control system composed of power electronic devices, in addition to the protection circuit of the power electronic device described above, its buffer circuit is a very important auxiliary circuit.

Snubber Circuit is also known as Snubber Circuit. Its function is to suppress the internal overvoltage and du/dt or overcurrent and di/dt of the power electronic device, and reduce the switching loss of the device. In power electronic circuits, it is used to improve the voltage and current waveforms that power electronic devices are subjected to when they are turned on and off.

Usually, the power electronic devices in the power electronic device work in the switching state, and the turn-on and turn-off of the devices are not instantaneously completed. When the device is just turned on, the equivalent impedance of the device is large. If the device current rises quickly, it will cause a large turn-on loss; similarly, when the device is almost completely turned off, the current of the device is still relatively large, and if the voltage that the device is subjected to rises rapidly, it will also cause a large turn-off loss.

Switching losses can cause heating or even damage to the device, and for power transistors (GTRs), it can also lead to secondary breakdown of the device. In actual power electronic circuits, the turn-on current of power electronic devices is often increased due to the reverse recovery current of diodes and thyristors. Due to inductive loads or distributed inductance of wires, the devices are subjected to high induced voltages when they are turned off. Using a snubber circuit can improve the switching conditions of power electronic devices.

The basic working principle of the snubber circuit is to suppress the current rise rate of the device by using the characteristic that the inductor current cannot abruptly change, and to suppress the voltage rise rate of the device using the characteristic that the capacitor voltage cannot change abruptly. Figure 1a is a simple snubber circuit using IGBT as an example. Among them, Li is connected in series with the IGBT to suppress the current rise rate di/dt when the IGBT is turned on, and Ri and VDi form a freewheeling loop; capacitor CS and diodes VDS and RS form a charge-discharge RCD snubber circuit to suppress the rise rate du/dt of the terminal voltage when the IGBT is turned off. The resistor RS provides a discharge path for the capacitor CS.

Snubber circuits for power electronic devices
Figure 1 – Buffer circuit and waveforms

Figure 1b is the IGBT collector current and collector-emitter voltage waveforms during the switching process, and the dashed line represents the waveform when there is no snubber circuit. It can be seen from the waveform that the current rises rapidly when it is turned on, and the di/dt is very large. When it is turned off, the current of the external circuit will decrease sharply, and the inductance (including the wiring stray inductance) in the main circuit will generate a large induced electromotive force. This results in a high overshoot voltage peak at V during turn-off and a large du/dt. The solid line waveform in the figure represents the waveform when there is a snubber circuit. When V is turned on, CS first discharges to V through RS, so that the current ic has a small sudden change. Under the action of Li, the current ic rises slowly. When V is turned off, the load current charges CS through VDS, which shunts V. At the same time, since the capacitor voltage cannot be abruptly changed, du/dt and overvoltage are suppressed, and the peak overvoltage energy released by the inductor is absorbed.

The reason why the snubber circuit can reduce the switching loss is to transfer the switching loss from the device itself to the snubber circuit. According to the direction of the transferred energy, the snubber circuit can be divided into energy-consuming and energy-feeding snubber circuits: the energy-consuming snubber circuit consumes the absorbed energy on the resistor; the energy-feeding snubber circuit feeds the absorbed energy back to the load or power supply. This type of circuit has high efficiency, but the circuit is complex and is rarely used in practice.

Snubber circuits come in many forms to suit different devices and different circuits. The snubber circuit shown in Figure 1a is called a charge-discharge RCD snubber circuit, which is suitable for medium-capacity applications. The RC snubber circuit in Figure 2a is mainly used for small-capacity devices, and the discharge-preventing RCD snubber circuit in Figure 2b is used for medium and large-capacity devices.

Snubber circuits for power electronic devices
Figure 2 – Commonly used snubber circuit

The values of CS and RS in the buffer circuit can be determined experimentally or refer to relevant engineering manuals. VDS must use a fast recovery diode, and its rated current should not be less than 1/10 of the rated current of the main circuit device.

In actual use, the thyristor generally only bears commutation overvoltage, there is no turn-off overvoltage problem, and there is no large du/dt when it is turned off. Generally, an RC absorption circuit is used.

For large-capacity devices with relatively low operating frequencies such as thyristors and GTRs, it is often necessary to turn on the snubber circuit; for those high-frequency devices, the stray inductance of the line can play the role of turning on the snubber, and it is usually not necessary to turn on the snubber circuit.