PWM Control of Dual Active Bridge Converters

Background: A dual active bridge (DAB) DC-DC converter is ideally suited for high-power galvanically isolated DC-DC conversion. The DAB DC-DC converter has advantages of high power density Zero Voltage Switching (ZVS) bidirectional power transfer capability a modular and symmetric structure and simple control requirements. The DAB DC-DC converter can also be used for multi-port operation which is a feature that is useful in interfacing several DC sources and loads using a single converter. Notwithstanding all of the advantages of the conventional DAB converter for applications requiring wide voltage variations such as an interface for energy storage fuel cells or photovoltaics the DAB converter has limited ZVS range and high circulating currents at low loads. The high circulating currents at low loads results in poor efficiency when the DAB converter is under a low load condition. Thus there is a need for an improved DAB converter that provides an increased ZVS range and/or increased efficiency particularly at low load conditions. Technology Description: To address these issues researchers at Arizona State University have developed novel control schemes for bidirectional dc-dc dual active bridge converters. The proposed control schemes combine the traditional method of phase shift control with Pulse Width Modulation (PWM) of one single H-bridge and two converter bridges simultaneously in a composite control scheme that depends on the input to output voltage ratio and the load condition. One key element is that the scheme automatically transitions between dual PWM single PWM and only phase shift control by utilizing directly measured input and output voltages and using the load information implicit in the required phase shift. Applications: 1) Uninterruptible Power Supplies (UPS) 2) Grid Tie Renewable Resources (Photovoltaic Energy) 3) Fuel Cells