AN-6921

APPLICATION NOTE

© 2010 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.1 • 8/24/10

3

Figure 4. Frequency Variation of BCM PFC

Since the design of line filter and inductor for a BCM PFC

converter with variable switching frequency should be done

at minimum frequency condition, it is worthwhile to

examine how the minimum frequency of BCM PFC

converter changes with operating conditions.

Figure 5 shows the minimum switching frequency, which

occurs at the peak of line voltage, as a function of the RMS

line voltage for different output voltage settings. For

universal line application, the minimum switching

voltage is lower than about 405V.

Figure 5. Minimum Switching Frequency vs.

3. Operation Principle of Quasi-

Resonant Flyback Converter

QR flyback converter topology can be derived from a

conventional square wave, pulse-width modulated (PWM),

flyback converter without adding additional components.

Figure 6 and Figure 7 show the simplified circuit diagram

of a quasi-resonant flyback converter and its typical

waveforms. The basic operation principles are:

drawn from the input and stored in the inductor.

When the MOSFET is turned off, the energy stored in

the inductor forces the rectifier diode (D) to turn on.

is applied across the secondary-side inductor and the

the inductor has been delivered to the output. During

this period, the output voltage is reflected to the

When the diode current reaches zero, the drain-to-

7. Quasi-resonant switching is achieved by turning on

This reduces the MOSFET turn-on switching loss

caused by the capacitance loading between the drain

and source of the MOSFET.

Figure 6. Schematic of QR Flyback Converter