A three-phase soft-transition inverter with a novel control strategy for zero-current and near zero-voltage switching

This paper proposes a new soft-transition control strategy for a three-phas e zero-current-transition (ZCT) inverter circuit. Each phase leg of the inv erter circuit consists of an LC resonant tank, two main switches, and two a uxiliary switches. The proposed strategy is realized by planning the switch ing patterns and timings of these four switches based on the load current i nformation. It enables all the main switches and auxiliary switches to be t urned on and turned off under zero-current conditions, and achieves a near zero-voltage turn-on for the main switches. Compared with existing ZCT stra tegies, the diode reverse recovery current and switching turn-on loss are s ubstantially reduced, the current and thermal stresses in the auxiliary dev ices are evenly distributed over every switching cycle, and the resonant ca pacitor voltage stress is reduced from twice the dc bus voltage to 1.3-1.4 times the dc bus voltage. Meanwhile, the soft transition for each phase is executed independently from the main controller. Consequently, any PWM sche me developed for hard-switching inverters is applicable, and there is no co mpromise in order to use any well-proven control techniques. The proposed s trategy is also suitable for three-phase power-factor-correction (PFC) rect ifier applications. The operation principles, including a detailed analysis based on the state-plane technique, and a design rule are described in thi s paper. The circuit operation is first verified by a computer simulation, and is then tested with a 50-kW three-phase inverter to the full power leve l together with a three-phase induction motor in a closed-loop speed/torque control. Significant reductions in switching losses and voltage/current st resses over existing techniques have been experimentally demonstrated.