Synchronous rectifiers drain voltage overshoot reduction in PSFB converters

Abstract

The requirements for the blocking voltage of the rectification devices on the secondary side of phase shift full bridge (PSFB) dc-dc converter topology are, by nature, higher than for other quasiresonant or fully resonant topologies (DAB, LLC). This is especially aggravated in wide range operation converters and further increased by the rectifiers' drain voltage overshoot. Unlike other resonant topologies, the inductor at the output of the PSFB effectively decouples the capacitor bank from the rectification stage, which otherwise acts as a strong lossless snubber. Higher blocking voltage requirements for the rectification devices worsen their Figure of Merit, increasing their related losses and decreasing the overall efficiency of the converter. In this article, the main causes of the rectifiers drain voltage overshoots in PSFB are analyzed. Design guidelines for the mitigation of the different causes are introduced, as well as a novel modulation scheme for the overshoot reduction, while the output filter operates in discontinuous conduction mode, without penalties in performance, complexity, or cost. A prototype of PSFB DC-DC converter of 3300 W, with 400-V input to 54.5-V output nominal voltages, was designed and built to test the proposed solutions achieving a peak efficiency of 98.12% at 50% of load.