Please explain the operation of voltage-resonant soft switching of an IGBT.

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

Figure (a) shows the schematic of a voltage-resonant induction cooktop as an application example of soft switching. Figure (b) shows its operation and waveforms.

In the circuit of Figure (a), when the IGBT turns on, current flows through the heating coil (L1). When the IGBT turns off, L1 and C1 go into resonance, causing sinusoidal voltage to be applied to the IGBT. The direction of resonance between L1 and C1 reverses, causing the C1 voltage to offset the C2 voltage. When the C1 voltage exceeds the C2 voltage, current begins to flow through the C1-C2-FWD-C1 loop. During this period, the collector-emitter voltage of the IGBT is equal to the forward voltage (VF) of the freewheeling diode (FWD), which is almost zero. At this time, the IGBT turns back on. As a result, current flows through the heating coil (L1) from the input side again. This sequence is repeated.

A voltage-resonant circuit is inexpensive because it does not require many components. However, when a system needs a high power capacity, an IGBT with very high withstand voltage is required so as to handle high resonance voltage. Therefore, a voltage-resonant circuit is used in many induction home appliances with a capacity of up to 1.5 kW at 100 VAC and up to 3 kW at 200 VAC. The smoothing capacitor (C2) on the input side has low capacitance because it receives electric power during a single pulse period. The voltage across C2 has a full-sine waveform, leading to a high power factor on the input side. Therefore, a voltage-resonant circuit eliminates the need for a power factor correction (PFC) circuit.

(b) Example of operating waveforms
(b) Example of operating waveforms
  1. The IGBT turns on.
    Collector current flows via L1.
  2.  The IGBT turns off.
    L1 and C1 go into resonance, causing voltage to increase.
  3. The FWD conducts.
    The energy stored in C1 flows back to the power supply via the FWD. An “on” signal is applied to the IGBT when the resonance voltage drops below the reference voltage.

Steps 1 to 3 are repeated.

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