Please explain the operation of current-resonant soft switching of the IGBT.

Figure (a) shows the schematic of a current-resonant induction cooktop as an application example of soft switching. Figure (b) shows its operation and waveforms. The heating coil (L₁) and the capacitor (C₁) go into resonance, causing load current to be sinusoidal. (The voltage applied to the IGBT has a square waveform.) IGBT₁ turns on, causing the current generated by the resonance between L₁ and C₁ to flow. When IGBT₁ turns off thereafter, resonance current flows through the closed L₁ーC₁ーFWD₂ loop. When resonance current reverses its direction, IGBT₂ turns on, causing resonance current to flow through the closed C₁ーL₁ーIGBT₂ loop. When IGBT₂ turns off, resonance current freewheels through the L₁ーFWD₂ーC₂ーC₁ loop. When it drops to zero, IGBT₁ turns on again. This sequence is repeated.

A current-resonant circuit requires more components than a voltage-resonant circuit. The current-resonant circuit is used for high-power stationary cookers and aluminum pan (all-metal) cookers requiring high-frequency switching. A DC power supply from a PFC circuit is desirable for stable operation. In practice, however, current resonance provides a high power factor without PFC. C₂ can be a small capacitor having enough capacitance to provide a stable switching operation for one cycle. The AC power supply is designed to provide a sinusoidal load current.