Quasi-Resonant Flyback Converter as Auxiliary Power-Supply of an 800 V Inductive-Charging System for Electric Vehicles

Abstract

This paper presents evaluation of quasi-resonant flyback (QRF) dc-dc converter 57W with valley-switching in an emerging application. The QRF was supplied from an 800V variable dc-link and was used as the auxiliary power-supply of a wireless inductive-charging system (ICS). Comparison of state-of-the-art QRF control ICs is presented and suggestions for their improvements are given. Notes on the power-supply architecture, design items specific for the ICS application, over-power protection, and key-component choice are provided. During experiments several original and novel results are generated. The QRF efficiency graphs in ICS power transfer, ICS stand-by, and constant-load operation are analyzed. The maximum efficiency of 87.1% was reached at 620V and rated load. Moreover, the unique analysis of QRF losses at no-load, showed their quadratic dependency vs. input-voltage. The measured 'switching frequency vs. load' graph is presented. It was changeable with load and input-voltage as expected. From Bode plots the bandwidth, phase-margin, and gain-margin are extracted and plotted versus input-power - for the first time. They were changeable with input-voltage and load as expected. Comparison of simulated and measured Bode plots showed that, even when they were not matched, one can still design a Type-2 compensator that ensures stable operation. Evaluation of cross-regulation, when output with 24.1% of total power was regulated, showed that such approach - contrary to the more common of regulating the biggest one - is feasible too. It is discovered that, for a QRF with variable switching frequency, choice of compensator's zero or the regulated output has influence on its efficiency. The power-thresholds, to ensure valley-switching operation, represented as 'input power vs. input voltage' are shown for the first time. Comparison of bandwidth, phase margin, and gain margin vs. input power, between an active-clamped flyback (ACF) and the QRF converters, were discussed. Conclusion is that QRF, for the same specification, cannot have the same compensator as an ACF or conventional flyback dc-dc converter. The difference must be at least in a placement of a zero.