AN-6921 데이터 시트보기 (PDF) - Fairchild Semiconductor

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AN-6921

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

Fairchild Semiconductor 

AN-6921

APPLICATION NOTE

(Design Example) Since the output voltage is 400V for

high line and 260V for low line, the minimum

frequency occurs at high-line (264VAC) and full-load

condition. Assuming the overall efficiency is 90% and

selecting the minimum frequency as 58kHz, the

inductor value is obtained as:

L = η ⋅VLINE,MAX 2 ⋅ VO.PFC.H − 2VLINE,MAX

2 ⋅ POUT ⋅ fSW ,MIN

VO . PFC . H

= 0.9 ⋅ 2642 ⋅ 400 − 2 ⋅ 264 = 400μ H

2 ⋅ 90 ⋅ 58×103

400

The maximum peak inductor current at nominal

output power is calculated as:

I L,PK

= 2 2 ⋅ POUT

η ⋅VLINE,MIN

= 2 2 ⋅90 = 3.14A

0.9 ⋅ 90

t MAX

ON

= 2 ⋅ POUT ⋅ L

η ⋅VLINE.MIN 2

=

2 ⋅ 90 ⋅ 400 ×10−6

0.9 ⋅ 902

= 9.87μs < 20μs

Figure 8. Internal Block for ZCD

Assuming RM10 core (PC40, Ae=98mm2) is used and

setting ΔB as 0.23T, the primary winding should be:

N BOOST

≥

IL,PK ⋅ L

Ae ⋅ ΔB

=

3.14 ⋅ 400 ×10−6

98×10−6 ⋅ 0.23

= 55.7 turns

Thus, the number of turns (NBOOST) of boost inductor is

determined as 60.

[STEP-A2] Auxiliary Winding Design

Figure 9 shows the internal block for zero-current detection

(ZCD) for the PFC. FAN6921 indirectly detects the inductor

zero current instant using an auxiliary winding of the boost

inductor.

The auxiliary winding should be designed such that the

voltage of the ZCD pin rises above 2.1V when the boost

switch is turned off to trigger internal comparator as:

N ZCD

N BOOST

(VO.PFC.H

−

2VLINE.MAX ) > 2.1V

(10)

where VO.PFC.H is the PFC output voltage for high line

condition.

The ZCD pin has upper voltage clamping and lower voltage

clamping at 10V and 0.65V, respectively. When the ZCD

pin voltage is clamped at 0.65V, the maximum sourcing

current is 1.5mA and, therefore, the resistor RZCD should be

properly designed to limit the current of the ZCD pin below

1.5mA in the worst case as:

RZCD

> VIN ⋅ N AUX

1.5mA NBOOST

=

2VLINE.MAX ⋅ N AUX

1.5mA NBOOST

(11)

N ZCD

N BOOST

VIN

NZCD

N BOOST

(VO.PFC

−VIN )

Figure 9. ZCD Waveforms

(Design Example) The number of turns for the auxiliary

ZCD winding is obtained as:

N ZCD

>

2.1N BOOST

(VO.PFC.H − 2VLINE.MAX

)

=

4.7turns

With a margin, NAUX is determined as 8 turns.

Then RZCD is selected from:

RZCD >

2VLINE.MAX

1.5mA

⋅ NZCD

N BOOST

=

2 ⋅ 265

1.5 ×10−3

⋅

8

60

=

33kΩ

as 68kΩ.

© 2010 Fairchild Semiconductor Corporation

Rev. 1.0.1 • 8/24/10

5

www.fairchildsemi.com

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