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AN-6920MR

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

Fairchild Semiconductor 

AN-6920

Non-Conduction Time of the MOSFET (tOFF)

FAN6920 has a minimum non-conduction time of MOSFET

(5µs), during which turning on the MOSFET is prohibited.

To maximize the efficiency, it is necessary to turn on the

MOSFET at the first valley of MOSFET drain-to-source

voltage at heavy-load condition. Therefore, the MOSFET

non-conduction time at heavy load condition should be

larger than 5µs.

After determining fS.QRmin and tF, the maximum duty cycle is

calculated as:

Dmax

=

VRO

VRO

+ VO.PFC.L

⋅ (1−

f min

S .QR

⋅tF )

(20)

Then, the primary-side inductance is obtained as:

Lm

=

ηQR ⋅

2

(VO . PFC . L

⋅

f min

S .QR

⋅ Dmax

POUT

)

2

(21)

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

(22)

I RMS

DS

=

I PK

DS

Dmax

3

(23)

The MOSFET non-conduction time at heavy load and low

line is obtained as:

tOFF .L

=

(1− Dmax )

f min

S .QR

(24)

The MOSFET non-conduction time at heavy load and

higher voltage of PFC output (VO.PFC.H) is obtained as:

tOFF .H

= t ⋅ VV OFF.L

O . PFC . L

O. PFC . H

⋅ VO.PFC.H + VRO

VO.PFC.L + VRO

(25)

To guarantee the first valley switching at high line and

heavy-load condition, tOFF.H should be larger than 5µs.

APPLICATION NOTE

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 over temperature in the core is given by:

N min

P

=

Lm I DS PK

Ae∆B

(26)

where B is the maximum flux density swing in Tesla.

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

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:

NP

= n⋅ NS

>

N min

P

(27)

The number of turns of the auxiliary winding for VDD is

given as:

N AUX

=

V nom

DD

+

VFA

(VO + VF )

⋅ NS

(28)

where VDDnom is the nominal VDD voltage, the range about

12~20V, and the VFA is forward-voltage drop of VDD diode,

about 1V.

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 ILIM

Ae NP

< Bsat

(29)

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

65kHz and the falling time is 1µs, and assuming

VO.PFC.L=300V:

D max

=

VRO

VRO

+ VO.PFC.L

⋅ (1−

f min

S .QR

⋅tF

)

= 240 ⋅ (1− 70 ×103 ⋅1×10−6 ) = 0.413

240 + 300

Figure 14. Switching Timing of QR Flyback Converter

© 2010 Fairchild Semiconductor Corporation

Rev. 1.0.0 • March 10, 2011

9

www.fairchildsemi.com

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