Dual active bridge (DAB) converters have been gaining increasing popularity in the context of high-frequency solid state transformers. However, modeling of a DAB converter remains a challenge to retain the ripple information for the design of high performance stable digital control under uniform sampling. This paper proposes a discrete-time framework using approximate discrete-time models considering various practical parasitics. These models are used to derive various discrete-time small-signal transfer functions under phase shift modulation using both voltage mode and current mode control techniques. The accuracy of the proposed models is verified through SIMPLIS simulation as well as experimentation in time-domain and in the frequency domain using SIMPLIS simulation. Finally, a design case-study using digital voltage-mode control is considered for a prototype DAB converter under phase-shift modulation with a power rating of 50 W and switching frequency of 500 kHz. The digital controller is implemented using an FPGA device, and the test results are demonstrated. The proposed framework can be extended to different modulation techniques as well as other isolated DC-DC converter topologies to design high frequency digital control.
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