Elimination of High Frequency Oscillation in Dual Active Bridge Converters by dv/dt Optimization

With the increased switching speed, the high-frequency oscillation (HFO) problem caused by the stray capacitances of the transformer and the high dv/dt of the dual active bridge (DAB) converter becomes more serious. In this paper, a frequency-domain model considering the circuit parasitic parameters is derived to figure out the mechanism of HFO. It shows that HFO could be mitigated by reducing the dv/dt of the H-bridge output voltage. However, HFO will become apparent again, as the dv/dt continues to decrease. Based on the mathematical model, the HFO problem could be eliminated theoretically by placing the zero crossing points (ZCP) of the DAB dv/dt excitation at the oscillation frequency. The DAB dv/dt is adjusted by adding parallel capacitors to the power devices. A calculation method for the required capacitance is given to obtain an optimal dv/dt. Not only the HFO problem is eliminated, but also the efficiency could be improved due to the better soft-switching condition and less oscillation loss in the transformer. The simulations and experiments on a 6.6-kW DAB prototype are carried out to validate the proposed method. The HFO voltage spike amplitude is reduced by about 95% with the proposed method, which accords well with the theoretical analysis.