DC-DC converters currently have two different start-up operation modes, soft switching and hard switching, while MOS and IGBT have different switching losses under the premise of hard switching. Let's start with two cases of turn-on and turn-off under the premise of hard switching, and take a look at the loss comparison of MOS and IGBT.

First, let's look at the difference in turn-on losses between MOS and IGBT when the DC-DC converter is under hard switching. Because the output capacitance of the MOSFET is large, when the device is in the off state, the input voltage is added to the output capacitance, and the output capacitance stores a large amount of energy. When successively turned on, all of this energy is consumed in the device, and the turn-on loss is large. The turn-on loss of the device is proportional to the output capacitance, proportional to the frequency, and proportional to the square of the input voltage. The output capacitance of IGBT is much smaller than that of MOSFET, and the energy stored in the capacitor is smaller when off, so the turn-on loss is smaller.

Next, let's take a look at the losses of MOS and IGBT switches in terms of turn-off losses. Under this condition, the MOSFET is a unipolar device. By applying the gate reverse bias voltage, the charge on the input capacitor can be quickly drawn off to accelerate the turn-off, so that the current will quickly drop to zero when the MOSFET is turned off. There is a tail current, so the turn-off loss is small. The tail current of the IGBT is inevitable and has a long duration (up to several microseconds), so the turn-off loss is large.

A comprehensive analysis of the losses in the case of MOS and IGBT turn-on and turn-off can be concluded that under the premise that the DC-DC converter is hard-switched, the switching loss of the MOSFET is mainly caused by the turn-on loss, while the IGBT is mainly caused by the turn-off Breaking loss. Therefore, the circuit using MOSFET as the main switching device should make the converter work under ZVS conditions, so that before the device is turned on, the voltage between the drain and the source drops to zero first, and the stored energy on the output capacitor is very small, which can greatly Reduce the turn-on loss of MOSFET. The circuit using IGBT as the main switching device should make the converter work under ZCS conditions, so that before the device is turned off, the current flowing through the device is first reduced to zero, which can greatly reduce the turn-off loss caused by the tail current.