Maximum-Power-Per-Ampere Variable Frequency Modulation for Dual Active Bridge Converters in Battery-Balancing Application

Varying the operating frequency helps dual-active bridge topologies increase the system efficiency since the soft-switching is regained at low-load condition. This article proposes a conduction-loss-based variable frequency modulation (VFM) that decouples the phase-shift from the frequency control that conventional VFM applies. The power losses for the shared-bus battery-balancing topology are elaborated as switching frequency varies. The results show that increasing the operating frequency within the reasonable boundaries can benefit not only the switching loss but also the conduction losses. The switching loss is nearly constant, whereas the conduction losses decrease significantly. A factor of output power and transformer current, namely power-per-ampere, is derived as a function of operating conditions and phase-shift independent of switching frequency. The operating setpoints are selected to minimize the factor using both online and offline optimizations. The proposed control strategy outperforms constant frequency modulation by up to 30% below 30% rated power. Furthermore, compared with conventional VFM, the proposed method improves the efficiency of the system under test by 1.5% below 50% normalized power, with significantly less computational resources needed.