Improvement of Switching Frequency and Input Voltage Range in Single-Stage PFC Flyback Converter for LED Driver Applications

The boundary conduction mode (BCM) flyback power factor correction (PFC) converter is well-suited for low to medium power-level applications. It isolates input and output voltages while improving the power factor (PF) by modifying the transformer turns ratio. However, the converter switching frequency varies throughout the line cycle, and the frequency range depends on the input voltage and load conditions. Consequently, the electromagnetic interference (EMI) spectra show significant variations, making the EMI filter design more challenging. This paper presents a constant on-time (COT) control strategy that employs a variable inductor (VI) to limit the range of switching frequency fluctuations across the universal input voltage range ($85-264 \mathrm{~V}_{\mathrm{rms}}$).