The Impact of Periodic Variable Switching Frequency Modulation on Power Transmission in Dual-Active-Bridge Converter and an Improved Scheme

Variable switching frequency modulation (VSFM) is an effective means to suppress electromagnetic interference (EMI). It disperses the energy of narrow-band harmonic to a wider spectrum range, so that the EMI spikes can be attenuated. However, the current application of VSFM on dual-active-bridge (DAB) converters has compatibility issues, and the variation of switching frequency may cause extra fluctuations of power transmission. This article takes the DAB converter as the research object, and derives the spectrum mathematical representation of the active bridge's output voltage with periodic VSFM theoretically. VSFM introduces extra periodic components to the active bridge's output voltage, which can impact power transmission and EMI mitigation. Subsequently, an improved phase-shifted VSFM scheme is proposed, which controls the power delivery to match the variation of switching frequency by establishing a functional relationship between the degree of freedom of the phase shift and the switching frequency. On one hand, the conducted EMI spikes are mitigated through VSFM; on the other hand, as the by-products of the variation of switching frequency, the secondary dc voltage and power fluctuations are also effectively suppressed. Simulation and experiments are provided to prove the feasibility and validity of the proposed method in a DAB converter.