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AN-6920MR

Integrated Critical-Mode PFC / Quasi-Resonant Current-Mode PWM Controller FAN6920

Fairchild Semiconductor 

AN-6920

Figure 6. Schematic of Dual-Switch Flyback Converter

Ids (MOSFET Drain-to-Source Current)

Ipk

ID (Diode Current)

Ipk Np/Ns

APPLICATION NOTE

and low cost. The bootstrap circuit is useful in a high-

voltage gate driver and operates as follows. When the high-

side VS goes below the IC supply voltage VDD or is pulled

down to ground (the low-side switch is turned on and the

high-side switch is turned off), the bootstrap capacitor,

CBOOT, charges through the transformer primary-side, from

the VDD power supply, as shown in Figure 8. This is

provided by VBS when high-side VS is pulled to a higher

voltage by the high-side switch. The VBS supply floats and

the bootstrap diode reverses bias and blocks the rail voltage

(the low-side switch is turned off and high-side switch is

turned on) from the IC supply voltage, VDD. However, the

dual-switch flyback high-side and low-side MOSFET turn

on and off at the same time. Therefore, once the high-side

MOSFET turns on, high-side VS equals PFC VO, the VDD

can’t charge the CBOOT, even though the high-side VS is

pulled down to ground at leakage energy recycle period, but

the period is too short to charge CBOOT.

Figure 8 shows the high-side gate-driver circuit with the

auxiliary power supply. If VCBOOT is less than the HV IC

under-voltage threshold, the high-side gate output (VHO)

maintains turned-off state, then the low-side MOSFET turns

on and charges the CBOOT for one cycle, high-side driver

restarts at the next PWM cycle. Finally, the voltage of

auxiliary power supply follows the output voltage rise and

continues to supply energy to the high-side circuit.

Vds

VIN

VIN /2

Vo/2 Np/Ns

Vo/2 Np/Ns

tON

tD

tS

Figure 7. Typical Waveforms of Dual-Switch QR

Flyback Converter

4. High-Side Gate-Drive Circuit

Figure 8 and Figure 9 show the high/low-side gate driver

circuit. The high-side gate drive IC achieves high-

performance, is simple and inexpensive, but has a limitation

for dual-switch flyback application.

One of the most widely used methods to supply power to

the high-side gate driver circuitry of the high-voltage gate-

drive IC is the bootstrap power supply. This bootstrap

power supply technique has the advantage of being simple

Figure 8. High-Side Driver Circuit and Start Waveform

© 2010 Fairchild Semiconductor Corporation

Rev. 1.0.0 • March 10, 2011

4

www.fairchildsemi.com

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