Enhancing Inherent Flux Balancing in a Dual-Active Bridge Using Adaptive Modulation

The Dual-Active Bridge DC-DC converter (DAB) is capable of bidirectional power transfer and soft switching (ZVS) can be achieved over a wide operating area. Because of these attributes the DAB gains further attention. However a highly efficient converter design is sensitive to small mismatches in the PWM signals, which lead to large DC offsets in the transformer currents. These offsets can cause disadvantages as saturation of the transformer core and asymmetric switching currents, which will worsen further as switching frequencies are expected to increase further in the future. While conventionally DC offsets were prevented by additional circuitry or active flux control, this paper now enhances the inherent flux balancing of ZVS with a modified modulation strategy that strictly limits the turnoff current magnitude. Thereby the DC offset can be reduced without any additional components. The strategy is analyzed and experimentally verified for a wide operating range.