Modified Topology and PWM Modulation for Bidirectional LLC-DCX Converter With Center-Tapped Transformer

In low-voltage (LV) high-current applications, LLC dc-dc transformer (LLC-DCX) converter with center-tapped circuit is attractive in volume, cost, and efficiency. However, under parameter fluctuation of resonant components, if synchronous rectification (SR) is not accurately achieved, the switches connecting with center-tapped circuit may suffer from high-voltage (HV) spike due to force interruption of the current circulating path. To deal with this, lots of SR technologies have been studied with hardware detection for unidirectional applications. However, in bidirectional applications, SR with hardware detection becomes a challenge, since SR bridge should be exchanged under different power directions. This article proposes a topology modification and pulsewidth modulation (PWM) for bidirectional LLC-DCX converter with a center-tapped transformer. The active clamp circuits, composed of capacitors and auxiliary switches, are added in parallel with the LV-side power switches to avoid turn-off voltage spike. Under the proposed modulation strategy, both the power switches in the HV side and LV side as well as auxiliary switches can turn on with zero voltage switching (ZVS), which can reduce electromagnetic interference (EMI) and switching loss. Meanwhile, since the driving signals are consistent in bidirectional transmission, both SR detection and logic exchanging are avoided and the converter can achieve naturally bidirectional power flow changing. The experimental results verify the correctness of theoretical analysis.