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AN-6921

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

Fairchild Semiconductor 

AN-6921

Once Lm is determined, the maximum peak current and

RMS current of the MOSFET in normal operation are

obtained as:

I PK

DS

= VO.PFC.L ⋅ Dmax

L f min

m S .QR

(24)

I RMS

DS

=

I PK

DS

Dmax

3

(25)

The MOSFET non-conduction time at heavy load and low

line is obtained as:

tOFF .L

=

(1 − Dmax )

f min

S .QR

(26)

The MOSFET non-conduction time at heavy load and high

line is obtained as:

tOFF .H

= t ⋅ VV OFF.L

O . PFC . L

O . PFC . H

⋅ VO.PFC.H + VRO

VO.PFC.L + VRO

(27)

To guarantee the first valley switching at high line and

heavy-load condition, tOFF.H should be larger than 8µs.

Figure 12. Switching Timing of QR Flyback Converter

When designing the transformer, the maximum flux density

swing in normal operation (B) as well as the maximum flux

density in transient (Bmax) should be considered. The

maximum flux density swing in normal operation is related

to the hysteresis loss in the core, while the maximum flux

density in transient is related to the core saturation.

The minimum number of turns for the transformer primary

side to avoid the over temperature in the core is given by:

N min

P

=

Lm I DS PK

AeΔB

(28)

where B is the maximum flux density swing in Tesla. If

there is no reference data, use B =0.25~0.30T.

APPLICATION NOTE

Once the minimum number of turns for the primary side is

determined, calculate the proper integer for NS so that the

resulting NP is larger than Npmin as:

N P = n ⋅ NS > N Pmin

(29)

The number of turns of the auxiliary winding for VDD is

given as:

N AUX

=

V nom

DD

+ VFA

(VO +VF )

⋅ NS

(30)

where VDDnom is the nominal VDD voltage, which is typically

18V and VFA is forward voltage drop of VDD diode.

Once the number of turns of the primary winding is

determined, the maximum flux density when the drain

current reaches its pulse-by-pulse current limit level should

be checked to guarantee the transformer is not saturated

during transient or fault condition.

The maximum flux density (Bmax) when drain current

reaches ILIM is given as:

Bmax

=

Lm I LIM

Ae NP

< Bsat

(31)

Bmax should be smaller than the saturation flux density. If

there is no reference data, use Bsat =0.35~0.40T.

(Design Example) Setting the minimum frequency

is 52kHz and the falling time is 0.8µs:

Dmax

=

VRO

VRO

+ VO.PFC.L

⋅ (1 −

f min

S .QR

⋅tF )

= 130 ⋅ (1− 52×103 ⋅ 0.8×10−6 ) = 0.319

130 + 260

Lm

=

ηQR

⋅ (VO.PFC.L ⋅ Dmax )2

2⋅

f P min

S.QR O

= 0.95⋅(260⋅ 0.319)2 = 700μH

2 ⋅52×103 ⋅90

I PK

DS

=

260⋅ 0.319

700×10−6 ⋅52×103

=

2.28A

tOFF .L

=

(1− Dmax )

f min

S .DD

=

1− 0.319

52 ×103

= 13μs

tOFF . H

= t ⋅ VV OFF.L

O.PFC.L

O.PFC.H

⋅ VO.PFC.H + VRO

VO.PFC.L + VRO

= 13μs ⋅ 260 ⋅ 400 +130 = 11.48μs > 8μs

400 260 +130

Assuming POT3319 (Ae=159mm2) core is used and

the flux swing is 0.26T

© 2010 Fairchild Semiconductor Corporation

Rev. 1.0.1 • 8/24/10

9

www.fairchildsemi.com

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