Please explain hard switching and soft switching using IGBTs.

Hard switching and soft switching are switching technologies used in power conversion devices such as inverters and converters, and switching power supplies. They are classified based on the relationship between current and voltage when switching on and off. Soft switching is a technology developed to improve the problems associated with hard switching, such as switching loss and noise (including EMI) on power lines.

Fig. 1. Hard switching waveform and operation trajectory : Waveforms and operating locus during hard switching. Shows how current and voltage overlap during on/off transitions, resulting in switching losses.
Fig. 1. Hard switching waveform and operation trajectory

Hard switching
Hard switching is a method of simply turning a switching device on and off (a method of forcibly cutting off current using the device's own cutting-off capability). The switching operation waveform and operation trajectory are shown in Fig. 1. During on/off switching (the switching transition period), both voltage and current are applied to the device, and it changes abruptly from on to off or off to on. This causes surge voltages, switching noise, and large switching losses. This method is applied to simple switch applications, inverter devices for motor drives, switching power supplies, etc.

Fig. 2. Soft switching waveform and operation trajectory : Even during on/off transitions, the current and voltage barely overlap, showing that switching losses are extremely small.
Fig. 2. Soft switching waveform and operation trajectory

Soft switching
Soft switching uses an LC resonant circuit to switch the device on and off when the current or voltage is zero. Typical operating waveforms and operating locus are shown in Fig. 2. Representative methods include ZCS (Zero Current Switching), which uses a series LC circuit to perform switching when the current is zero, and ZVS (Zero Voltage Switching), which uses a parallel LC circuit to perform switching when the voltage is zero. Because the element turns on and off when the voltage or current is close to zero, noise and loss associated with switching are small. Representative soft switching applications of IGBTs include induction rice cookers, cooking appliances, and microwave ovens. Also, as can be seen from the fig, the soft switching method places a smaller load on the device in terms of the safe operating area (SOA) compared to hard switching, improving reliability.

Fig. 3. Hard switching waveform with superimposed switching noise and operating trajectory : If switching noise caused by surges is superimposed, it may exceed the SOA.
Fig. 3. Hard switching waveform with superimposed switching noise and operating trajectory

In both hard switching and soft switching, switching noise caused by surges and other factors occurs during the on/off transition (Fig. 3).
- Turn on: Switching noise E is superimposed on the current IC, and the operating locus is
A → E → B.
- Turn off: Switching noise D is superimposed on the voltage VCE, and the operating locus is
B → D → A.
This may result in the maximum ratings or safe operating area (SOA) being exceeded, so check is required.

Soft switching is also explained in the following FAQ:

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