Flyback converter is commonly used for off-line power supplies. Besides switching at a fixed frequency for flyback converters,quasi-resonant mode is widely used due to its lower switch-on losses and better EMI performance.One challenge of the quasi-resonant flyback converter is the variable switching frequency, which increases with decreasing output power. This offsets the efficiency improvement through the quasi-resonant operation, especially at medium and Iow load condition. To overcome this problem,strategies for switching frequency modulation are developed. In this paper, some strategies are compared and system performance based on the probably best strategy is given.
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