Active Mitigation of Transformer Saturation in a Dual-Active-Bridge Converter for Electric Vehicle Chargers

Transformer saturation presents a constant challenge in isolated converter designs, especially when aiming for high power density. This paper presents a non-linear active transformer saturation mitigation technique suitable for Dual-Active-Bridge dc-dc converters, commonly used in on-board Electric Vehicle (EV) battery chargers. The proposed technique detects the variation in the current slope near the boundary of saturation and requires only one low-cost current sensor. The control scheme is demonstrated experimentally on a 450 V, 6.6 kW on-board EV charger, where the DAB transformer is intentionally operated near the limit of saturation and the controller performs the necessary corrective actions. In comparison with the existing active transformer protection methods that sense and control the average current from each DAB full-bridge, the proposed control provides a fast and simple way to detect transformer saturation and take corrective action through duty-cycle control from only one full-bridge. The detection and reaction speed of the proposed controller allows for a reduced safety margin to be used in the transformer design, ultimately leading to an increase in core utilization and converter density.