Current Self-Balancing Mechanism in ZVS Full-Bridge Converters

Full-bridge converters are widely used in isolated dc-dc conversion and wireless power transfer systems. A full-bridge converter can operate in zero-voltage-switching (ZVS) mode with the assistance of an inductive branch that connects the two switch nodes in parallel with the load. The inductive branch provides a ZVS current and has a very low resistance. A tiny volt-second imbalance between the switch nodes may cause a large current bias on the branch. However, it can be demonstrated experimentally that the current on the branch can be almost self-balanced without any dc blocking capacitor or balancing control. This paper quantitatively studies the current self-balancing mechanism and shows that the dead-time switch-node voltage can give a strong negative feedback effect that pushes the current bias toward zero. Experimental verification was carried out on a gallium-nitride device-based converter.