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AN-6921

Integrated Critical Mode PFC / Quasi-Resonant Current Mode PWM Controller FAN6921

Fairchild Semiconductor 

AN-6921

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

frequency occurs at high line (265VAC) as long as the output

voltage is lower than about 405V.

APPLICATION NOTE

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:

ƒ During the MOSFET on time (tON), input voltage (VIN)

is applied across the primary-side inductor (Lm).

MOSFET current (IDS) increases linearly from zero to

the peak value (Ipk). During this time, the energy is

drawn from the input and stored in the inductor.

ƒ When the MOSFET is turned off, the energy stored in

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

During the diode ON time (tD), the output voltage (Vo)

is applied across the secondary-side inductor and the

diode current (ID) decreases linearly from the peak

value to zero. At the end of tD, all the energy stored in

the inductor has been delivered to the output. During

this period, the output voltage is reflected to the

primary side as Vo× Np/Ns. Then, the sum of input

voltage (VIN) and the reflected output voltage (Vo×

Np/Ns) is imposed across the MOSFET.

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

source voltage (VDS) begins to oscillate by the

resonance between the primary-side inductor (Lm) and

the MOSFET output capacitor (Coss) with an amplitude

of Vo× Np/Ns on the offset of VIN, as depicted in Figure

7. Quasi-resonant switching is achieved by turning on

the MOSFET when VDS reaches its minimum value.

This reduces the MOSFET turn-on switching loss

caused by the capacitance loading between the drain

and source of the MOSFET.

Figure 5. Minimum Switching Frequency vs.

RMS Line Voltage (L = 780µH, POUT = 100W)

Figure 6. Schematic of QR Flyback Converter

© 2010 Fairchild Semiconductor Corporation

Rev. 1.0.1 • 8/24/10

3

www.fairchildsemi.com

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