The inverter uses parallel resonant load structure. The basic circuit is shown in Figure 2: Figure IG1-IG4 shows the composition of full-bridge inverter circuit. Because the IGBT cannot withstand the reverse blocking capability, the series fast recovery diode (D1- D4) of each bridge arm withstands the back pressure. L and r refer to the equivalent inductance and resistance of the inductive load. C refers to the compensation capacitor. The parallel resonant circuit consists of L, C and r.
IGBT is a full-controlled device, which can realize the bridge arm current commutation through control of the turn-off gate pole, so that the inverter can work either in the capacitive status or under the inductive load.
When the inverter works in a capacitive status, the diode reverse recovery current of the bridge arm is turned off in a reversal manner, and is loaded to the IGBT of commutation bridge arm. In this case, commutation peak current exists during IGBT turn-on. In order to reduce the peak current, the PD1 -4 should use the fast recovery diode. When the inverter works in capacitive near-resonance status, the load also has the best power factor.
The load parameters (L, r) of the induction heating power supply vary with the size of the work, the productivity and the temperature of the work piece. Therefore, the resonant frequency of the load also changes accordingly. In order to keep the inverter in the best working condition, the trigger mode of the inverter must be able to achieve the automatic tracking of the load frequency. The power supply adopts phase-locked tracking mode to realize the automatic tracking of the load frequency. It is called ZVS control technology.
