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.