Design and Magnetic Optimization of Dual Active Bridge Converters for Energy Storage Application

Dual active bridge (DAB) dc-dc converters are widely used in energy storage systems with low voltage and high current ratings. As the converter may operate at light load operation for extended amount of time, the converter efficiency throughout entire operating range is of great importance. Having an accurate magnetic design and an optimum modulation over entire power range are two of the main factors for any industrial DAB converter. In this paper, a new interleaved transformer winding is proposed for high current applications with parallel turns to achieve equal current sharing. In addition, the dc-bias in transformer caused by asymmetrical enhanced phase-shift (AEPS) modulation is analyzed and a closed-loop control is proposed to mitigate any dc voltage across the low voltage (LV) transformer winding. Using a dc-block capacitor on high voltage (HV) side required for asymmetric modulation and applying the proposed closed-loop control on the LV side, achieving zero dc-flux in transformer is guaranteed. Based on this analysis, a 5kW DAB converter is implemented and full-power efficiency of 98% is achieved at nominal operating point
$(\mathrm{V}-\text{LV}=50\mathrm{V}, \mathrm{V}- \text{HV}=400\mathrm{V}$
. The high frequency transformer and current sharing enhancement are verified using finite element analysis and dc-bias elimination is validated in simulation.