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

(a) Example of a current-resonant circuit
(a) Example of a current-resonant circuit

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.

Symbol and measuring point
Symbol Measuring point
VGE1 IGBT1 Gate - Emitter Voltage
VCE1 IGBT1 Gate - Emitter Voltage
VGE2 IGBT2 Gate - Emitter Voltage
VCE2 IGBT2 Gate - Emitter Voltage
(b) Example of operating waveforms
(b) Example of operating waveforms
  1. IGBT1 is on.
    Resonance current flows through C2ーIGBT1ーL1ーC1ーC2.
  2. IGBT1 turns off.
    Resonance current flows through the closed L1ーC1ーFWD2ーL1 loop.
  3. IGBT2 turns on.
    When the current flowing through L1 reverses its direction, IGBT2 turns on, causing current to flow through the closed C1ーL1ーIGBT2ーC1 loop.
  4. IGBT2 turns off.
    Current flows through the closed L1ーFWD1ーC2ーC1ーL1 loop.

Steps 1 to 4 are repeated.