A Bidirectional LLC Converter Enabled by Common-Mode and Differential-Mode Operation

Higher switching frequencies facilitate miniaturization and higher performance in power electronics, and the LLC resonant converter can be an advantageous topology at increased frequencies. The LLC’s frequency-dependent gain curve provides a wide range of voltage conversion ratios for only a narrow frequency range, but in conventional LLC converters, this steep gain curve is only present for one (forward) power flow direction. In this work, we demonstrate a converter topology that achieves the advantageous LLC gain characteristic for both forward and reverse power flow. This topology splits the traditional LLC topology into two equal halves connected at their resonant tanks, and it uses common-mode and differential-mode operating schemes to selectively utilize the resonant components to replicate LLC gain behavior for both power flow directions. We experimentally demonstrate these techniques in a 1 kW prototype that converts power bidirectionally between a 250-500 V bus and a 10.5-15 V bus at switching frequencies of up to 2 MHz. The bidirectional LLC gain characteristic proves to be vital for meeting all forward direction (step-down) and reverse-direction (step-up) operating points. Such bidirectional, wide gain power conversion is valuable for inter-bus converters in electric vehicles and other energy storage applications.