Efficiency optimization of an automotive multi-phase bi-directional DC-DC converter

The paper proposes methods to improve the efficiency of a bi-directional, multi-phase buck+boost DC-DC converter for application in Hybrid Electrical Vehicles (HEV) or Fuel Cell Vehicles (FCV). Thereto, the modulation strategy for a highly-compact, 30 kW/Liter, constant-frequency soft-switching converter is optimized based on a converter loss model that includes the losses in the power semiconductors and the buck+boost inductor. An algorithm for numerical calculation of the optimum switching times is given, whereas the values for the loss-optimized operation of the converter are stored in a lookup-table that is accessed by the digital controller. In addition, a novel method and control concept to ensure a Zero Voltage Switching (ZVS) of all semiconductor switches by determination of a zero voltage across the MOSFET switches with analog comparators is proposed that results in the lowest inductor RMS currents for ZVS operation at the same time. Furthermore, at low output power an absolute efficiency gain of over 2.8% is achieved by partial operation of the six interleaved converter phases. A detailed description on the control concept that determines the optimum number of activated phases for the current operating point of the converter is given and verified by experimental results. The measurements prove the capability to instantaneously switch the number of active phases during operation without a overshoot or drop in the converter output voltage.