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FAN6921 Datenblatt(PDF) 9 Page - Fairchild Semiconductor

Teilenummer FAN6921
Bauteilbeschribung  Integrated Critical Mode PFC / Quasi-Resonant Current Mode PWM Controller
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Hersteller  FAIRCHILD [Fairchild Semiconductor]
Direct Link  http://www.fairchildsemi.com
Logo FAIRCHILD - Fairchild Semiconductor

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AN-6921
APPLICATION NOTE
© 2010 Fairchild Semiconductor Corporation
www.fairchildsemi.com
Rev. 1.0.1 • 8/24/10
9
Once Lm is determined, the maximum peak current and
RMS current of the MOSFET in normal operation are
obtained as:
..
max
min
.
PK
OPFC L
DS
mS QR
VD
I
Lf
=
(24)
max
3
RMS
PK
DS
DS
D
II
=
(25)
The MOSFET non-conduction time at heavy load and low
line is obtained as:
max
.
.
(1
)
OFF L
min
SQR
D
t
f
=
(26)
The MOSFET non-conduction time at heavy load and high
line is obtained as:
..
..
..
..
.
.
O PFC L
O PFC H
RO
OFF H
OFF L
O PFC H
O PFC L
RO
VV
V
tt
VV
V
+
=⋅
+
(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:
min
PK
mDS
P
e
LI
N
AB
=
Δ
(28)
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 N
S so that the
resulting N
P is larger than Np
min as:
min
PS
P
Nn NN
=⋅
>
(29)
The number of turns of the auxiliary winding for VDD is
given as:
()
nom
DD
FA
AUX
S
OF
VV
NN
VV
+
=
+
(30)
where VDD
nom 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:
max
mLIM
sat
eP
LI
B
B
AN
=<
(31)
Bmax should be smaller than the saturation flux density. If
there is no reference data, use B
sat =0.35~0.40T.
(Design Example)
Setting the minimum frequency
is 52kHz and the falling time is 0.8µs:
min
max
.
..
36
(1
)
130
(1 52 10 0.8 10 ) 0.319
130 260
RO
SQR
F
RO
O PFC L
V
Df
t
VV
=⋅ −
+
=⋅ − ×
×
=
+
2
..
max
min
.
2
3
()
2
0.95 (260 0.319)
700
252 10 90
QR
O PFC L
m
SQR
O
VD
L
fP
H
η
μ
⋅⋅
=
⋅⋅
==
⋅ ×⋅
63
260 0.319
2.28
70010
5210
PK
DS
I
A
==
×⋅ ×
max
.
3
.
(1
) 10.319
13
52 10
OFF L
min
SDD
D
ts
f
μ
==
=
×
..
..
..
..
.
.
260 400 130
13
11.48
8
400 260 130
O PFC L
O PFC H
RO
OFFH
OFFL
O PFC H
O PFC L
RO
VV
V
tt
VVV
s
ss
μ
μμ
+
=⋅
+
+
=⋅
=
>
+
Assuming POT3319 (Ae=159mm2) core is used and
the flux swing is 0.26T


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