Passive Lossless Snubber Technique In A Current-fed Full-bridge Single-stage PFC Converter

The input side of power electronics mostly adopts diode rectifiers which willcause harmonic pollution to the power grid. Active power factor correction is a neffective way to improve the power factor of electrical equipments and suppressharmonic pollution. Single-stage PFC integrates the functions of PFC and DC/DCconversion in a single controller and has the advantage of simple structure and highefficiency. Single-stage isolated power factor correction converter based on the full-bridge structure is attractive in applications of medium or high power. However,there is a voltage spike across the bridge leg when switches’ states convert, since theprimary side of the transformer leakage inductor, reducing the reliability of thecircuit.This article mainly researches the passive lossless snubber technique in acurrent-fed full-bridge single-stage PFC converter. We bring up two kinds of passivelossless snubbers(dual passive LC resonant snubber and single passive LC resonantsnubber) which are composed of capacitors, inductors and rectifiers. Both of themare simple and reliable. Without control, they can suppress the voltage spike of theprimary side of transformer and transfer energy of passive snubber capacitors to theoutput side by the resonance of capacitors and inductors.In the Current-fed full-bridge single-stage PFC with dual passive LC resonantsnubber, capacitors in series are connected on bridge arm ends through a diode. Thereare two LC resonant circuits when it operates. This article analyzes in detail theworking process and influence mechanism of the key parameters on the voltage andcurrent stress of each switch in the PFC converter, it also gives the design method ofparameters.Parameter errors in the dual passive LC resonant snubber will cause voltage andcurrent oscillation problem. Adopting the coupled inductor instead of the twoseparated inductors can suppress the asynchronous resonance. Based on the coupledinductor structure, we build the equivalent model with leakage inductors,magnetizing inductors and an ideal transformer. Realization mechanism ofsynchronization resonance in each resonance loop is analyzed using coupled inductor.On the basis, design considerations of coupling inductor are summarized. For dual passive LC resonant snubber, parameter limit is very strict. Weproposed a class of improved single passive LC resonant snubber. In the singlepassive LC resonant snubber, capacitors are directly connected in parallel with theswitches on the upper arms (or lower legs) through diodes. There is only one LCresonant circuit when it operates. Based on the analysis of working process in thesingle passive LC resonant circuit, we analyze the improvements mechanism inaspects of voltage and current stress on the switch, voltage spike suppression effect,parameter selection ranges and reliability compared to the dual passive LC resonantsnubber.On the basis of theoretical analysis and simulation results, we build anexperimental platform and carried out a set of detailed experimental study for the twokinds of passive snubbers. The experimental results verify the validity of thetheoretical analysis.