Design of Input and Output Filter Capacitors of a DC-DC Dual Active Bridge Converter with Time-domain Analysis of Voltage Ripple

Ahstract-A dual active bridge (DAB) converter is widely used in isolated DC-to-DC topology due to its attractive features, such as bidirectional power flow capability and the ability to attain soft-switching transitions over a wide range. The design of input and output differential-mode filter capacitors is critical for high power-density and low-cost applications. Peak-to-peak allowable voltage ripple and the ripple current RMS flowing through the capacitor are two important factors that determine the value and size of the capacitors. Conventionally, the peak-to-peak voltage ripple is analyzed in the frequency domain after considering the total ripple current RMS is concentrated in one frequency, leading to the over-design of the capacitors. This article analyses the voltage ripple of one of the operating modes of the DAB converter in the time domain after considering the actual ripple current waveforms in the input and output DC rails. It also provides analytical closed-from expressions of the capacitance value and the RMS current for given specifications of input and output voltages, power, and allowable voltage ripples. The theoretical design is validated through simulation and experiments performed on hardware at 425 V input, 40 V and 55 A at the output, and switching frequency of 250 kHz.