Bidirectional Three-Phase LLC Resonant Converter

Resonant circuit has been widely implemented to achieve high conversion efficiency by means of soft switching for the power transistor. In bidirectional converter application, the most commonly implemented resonant converter is series resonant converter due to its natural capability for bidirectional power flow operation, simple operation, and simple design. However, a series resonant converter operating gain range is limited to maximum at unity, resulting in difficulty maintaining output voltage for bidirectional operation. To improve the output voltage regulation for bidirectional operation, LLC resonant tank was selected as interesting alternative to SRC resonant tank. Using LLC resonant tank in bidirectional converter application requires two different operation modes depending on the direction of power flow. For forward power flow operation, the converter operates with switching frequency lower than the resonant frequency to achieve boost mode operation (Gain>1). For reverse power flow operation, the converter will operate at switching frequency higher than its resonant frequency to achieve buck mode operation (Gain<1). To increase the power handling capability, three resonant tanks are utilized together to form a three-phase resonant converter. In the end, a 400 V-IN/400 V-OUT 8 kW working prototype was built to verify design feasibility. From the experimental measurement, peak efficiency of over 96% for both power flow operation was achieved.