Quasi-Resonant Flyback Converter in an 800 V Inductive Charging System: Abnormal Operation and Control Aspects

This paper studies quasi-resonant flyback (QRF) dc-dc converter 57 W with valley-switching mode (VSM) in an emerging application. The QRF was supplied from variable 800 V input and was used as the auxiliary power supply of an inductive charging system. When choosing switch rated voltage it was shown that one shall consider maximum voltage during short-circuits too. Influence of high temperatures on transformer magnetizing inductance were analyzed and showed why this shall be considered when evaluating different vendors and designs. Operational waveforms are simulated and measured showing that key quantities were matched. Comprehensive overview of QRF abnormal operations is presented for the first time covering start-up, short-circuit, and over-power cases, including experiments. It was demonstrated that when solving noise-related start-up problem, by increasing filtering capacitance, one got positive influence on QRF operation (e.g. switching-frequency range was reduced, efficiency increased, and control loop bandwidth, phase margin and gain margin got improved). The short-circuits were evaluated through nine test-cases. When QRF operated in VSM the short-circuit protection worked as expected. But, if short-circuit happened at non-regulated output, at no load, it may go undetected—which is dangerous. Hence several mitigation strategies were proposed to prevent damage of converter. Also, for converters with variable switching frequency one shall state whether efficiency measurements were done when load was increasing or decreasing thus considering influence of controller’s hysteresis. The transition thresholds, which ensure VSM operation, presented as “input power vs. input voltage”, showed non-linear dependence.