Читать книгу Soft-Switching Technology for Three-phase Power Electronics Converters - Rui Li - Страница 18

A power device undergoes a switching transient process during its turn‐on as shown in Figure 1.8. From time t₁ to t₂, both voltage u_S1 across the device S₁ and current i_S1 through it have high values during turn‐on transient process. The voltage and current overlap on the device occurs during switch turn‐on process, which results in turn‐on loss. The turn‐on loss E_on is equal to integral of multiplication of the voltage u_S1 across the device S₁ and the current i_S1 through it in the turn‐on transient duration. Similarly, when the device turns off, it also undergoes a turn‐off transient process. The voltage and current overlap on the device occurs in the switch turn‐off process, which results in turn‐off loss E_off.

To reduce the turn‐on and turn‐off loss of the power device, soft‐switching techniques occur. The soft‐switching technique is a way to shape voltage and current of the power device during switching transient process by changing converter topology and/or introducing a unique control. Thus the overlapping of voltage and current on the power device during switching commutations is reduced. It not only reduces switching loss but also suppresses voltage stress on the power devices and electromagnetic interference (EMI) noise.

Figure 1.8 Typical switching waveforms of a power device.