A Full-Range ZVS Comprehensive Control Strategy for Dual-Port LLLC Induction Heating System

The full-range Zero Voltage Switching (ZVS) comprehensive control strategy is proposed for a high-power dual-port LLLC induction heating system which allows the system to operate efficiently across the entire power range. Under heavy load, a novel dual-phase-shift (DPS) control strategy is proposed in this paper. Using time-domain analysis, the minimum circulating current curve and the ZVS range under DPS conditions are derived. The optimal phase-shift angle relationship is determined by combining the minimum circulating current and the full-range ZVS region. However, under DPS control, the minimum output power is one-quarter of its full load capacity due to the lack of internal phase-shift in inverter II. To overcome this limitation, a Frequency-doubling Pulse Frequency Modulation (FD-PFM) control strategy is proposed under light load. Frequency-doubling control is implemented through time-division multiplexing between the two inverters, which enables the resonance circuit frequency to be doubled without altering the switching frequency of each inverter. The FD-PFM approach facilitates a faster response to power variation with frequency, enabling the system to adjust power in a narrower frequency range. This enables smooth, wide-range power adjustment across the entire system and ensures ZVS operation across the full power range. A 14 kW prototype was developed, and the experimental results align closely with the theoretical analysis.