A Novel Passive Regenerative Snubber for the Phase-Shifted Full-Bridge Converter: Analysis, Design and Experimental Verification
The development of Wide Bandgap (WBG) devices has enabled power electronic converters to operate at much higher frequencies, voltages and high power. Working at a higher switching frequency minimises the size of magnetics but results in significant switching losses and electromagnetic interference (EMI) noise. Thus, it necessitates the use of soft-switching techniques to reduce these losses. Phase-Shifted Full-Bridge (PSFB) Converter is the most widely used soft-switching topology in the high-voltage and high-power, unidirectional, DC-DC conversion. The phase shift PWM control utilises the converter parasitics to achieve zero voltage switching (ZVS) turn ON. The gating technique allows the magnetic energy stored in the leakage inductance of the isolation transformer to charge and discharge the output capacitances of the inverter leg. However, the converter suffers from severe voltage overshoots across the rectifier bridge during the zero to the active state transition. The resonant circuit formed between the transformer leakage inductance and the parasitic diode capacitance of the rectifier is responsible for the high-voltage ringing. Many passive and active snubbers are presented in the literature to mitigate the high-voltage overshoots across the diode bridge. While passive snubbers are relatively simple to implement than active snubbers, they are lossy. On the other hand, the active snubbers require additional gate driver circuitry and complex control. The first part of the thesis proposes a novel passive regenerative snubber to overcome the mentioned drawbacks of the existing snubbers. The proposed snubber is ideally lossless with no control complexity. The work covers a detailed analysis of the PSFB operation with the proposed snubber while obtaining closed-form expressions for the converter state variables at the end of each topological stage. The study considers all the major converter parasitics, such as transformer leakage and magnetising inductances, and parasitic capacitances of the converter. Given the new snubber, the thesis also lays out a step-by-step PSFB design procedure utilising the analysis carried out in the first part of the work. The design aimed to develop a 100 kHz PSFB for an input voltage of 360-440 V in the output power range of 0.5-1.5 kW at a fixed output voltage of 48 V. The design approach focuses on two design objectives - All inverter switches must achieve ZVS turn ON and the desired converter gain for all possible operating conditions. A hardware prototype is built and tested. The experimental results validate the effectiveness of the snubber in reducing the voltage overshoot. Further, the analysis and design accuracy is verified using the measured state variables. The work, at last, presents the overall converter efficiency and the loss distribution among the converter components.