A Secondary‐Side Modulation Strategy for Wide‐Range ZVS in Strongly Coupled Bidirectional Wireless Power Transfer Systems

ABSTRACT Strong magnetic coupling is usually adopted to increase the power density of bidirectional wireless power transfer (BWPT) systems. However, a large coupling coefficient distorts the resonant currents and complicates zero‐voltage‐switching (ZVS) operation, especially when the load and voltage gain vary. This paper proposes a secondary‐side pulse‐width phase‐shift modulation strategy for strongly coupled series–series BWPT (SC‐SS‐BWPT) systems. The strategy combines PWM with phase‐shift control on the secondary bridge, whereas the primary bridge remains at a fixed 50% duty cycle. An S‐domain model is developed to derive the output power and the inductor currents at the switching instants. The analysis shows that the primary‐side switching current is independent of both the secondary duty cycle and the voltage gain. Therefore, primary‐side ZVS can be preserved when the dead‐time current requirement is satisfied. The secondary‐side switches can also achieve ZVS over a wide operating range. Experiments on an SC‐SS‐BWPT prototype verify bilateral ZVS under forward and reverse power transfer, different voltage gains, and different load powers, whereas the primary‐side switching current remains nearly constant. The maximum power transfer efficiency of the system reaches 97.9%.