Analysis and Design of Light-Load Control of Bidirectional DC-DC Converter for Electric Vehicle Applications

To prevent inrush current from the high voltage battery that charges aluminum capacitors located at the traction inverter of electric vehicles, a bidirectional dc-dc series resonant converter (SRC) was recently proposed. By utilizing energy from the low voltage battery, the aluminum capacitors are charged by the bidirectional dc-dc SRC before the high voltage battery is connected. However, operation of the SRC at light loads suffers due to losing zero voltage switching (ZVS) of primary-side switches and due to operation at high switching frequencies. In this paper, a detailed analysis and design procedure of the control method for the bidirectional SRC to achieve both ZVS and operation in a limited switching frequency range at light loads is provided. First, the circuit of a series resonant converter for bidirectional dc-dc converter is described. Then, the operation at light loads with lead-time and delay-time control is described in both voltage step-down and voltage step-up modes. Key equations for the calculation of lead-time and delay-time vs. output power at light loads are also provided. Finally, experimental waveforms and measured efficiencies of a 2.5kW bidirectional dc-dc converter prototype at light-load operation are provided. At nominal input voltage of 330 V, efficiencies of 92.5% and 89% are achieved at approximately 10% and 5% of full load, respectively.