Dual Active Bridge Converter With Variable Transformer for Wide Voltage and Wide Load Range Operation

This study explores the use of a magnetically controlled transformer within a dual active bridge (DAB) converter. This novel approach introduces a new degree of freedom to the control of the topology through the variation of the equivalent turns ratio of its high-frequency transformer. By adjusting the turns ratio, the converter maintains a primary referred DC voltage gain of one, regardless of varying input/output voltages. This theoretical approach represents the simplest solution to achieve minimum effective currents and enables zero voltage switching (ZVS) under light-load conditions, particularly for unmatched voltage scenarios. This paper provides a comprehensive analysis of the converter’s currents, output power, efficiency, and their relationship with the magnetic flux density conditions in the transformer. Furthermore, the trade-off between the total turns ratio variation and the circulating currents in the converter is described in detail. Additionally, considering the specific characteristics of this controllable magnetic device, a straightforward modulation strategy to minimize the effective currents under wide output voltage variations is introduced. An experimental prototype with an input voltage of 100 V and a rated power of 1.2 kW presents a minimum efficiency of 90% under light-load conditions (5% of the maximum load), considering output voltage variations ranging from 130 V to 180 V.