AN-8027 — FAN480X PFC+PWM Combination Controller Application

AN-8027
[STEP-11] Coupled Inductor Design for the
PWM Stage
When the forward converter has more than one output, as
shown in Figure 20, coupled inductors are usually employed
to improve the cross regulation and to reduce the ripple.
They are implemented by winding their separate coils on a
single, common core. The turns ratio should be the same as
the transformer turns ratio of the two outputs as:
NS 2 NL
2
(47)
N N
S1 L1
N
p
N S2
N S1
L 2
N L2
N L1
L 1
Figure 20. Coupled Inductor
V N
N
POUT S1
0
0
P
V N
N
POUT S 2
0
P
V N
N
D1
D2
N
V V V
POUT S1
P
L M
L1
L2
N S1
O2 O2 O1
NS
2
N
I I
N S 2
O2 O2
NS1
I SUM
L LK
L LK
D1
D2 N
Normalized
V O1
I O1
V O2
I O2
V O1
I O2
V N
O2
N IO2 Figure 21. Normalized Coupled Inductor Circuit
VO2
V O1
One way to understand the operation of coupled inductor is
to normalize the outputs to one output. Figure 21 shows
how to normalize the second output (VO2) to the first
output (VO1). The transformer and inductor turns are
divided by NS2/NS1, the voltage and current are adjusted by
NS2/NS1. It is assumed that the leakage inductances of the
coupled inductor are much smaller than the magnetizing
inductance and evenly distributed for each winding.
The inductor value of the first output can be obtained by:
L1 VO1( VO1 VF
1)
ISUM
fSW ( PO 1PO2) ISUM
(1 DMIN)
(48)
where:
MIN
VBOUT
DMINDMAX VBOUT
PO1PO2 ISUM
V
(49)
© 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com
Rev. 1.0.2 • 6/21/13 12
O1
Then, the ripple current for each output is given as:
IO1ISUM1
I 2 I
(50)
O1 O1
IO2 ISUMNS1
1
(51)
I 2 N I
O2 S 2 O2
(Design Example) The minimum duty cycle of
PWM stage at nominal input (PFC output) voltage is:
MIN
VBOUT
310
DMINDMAX 0.45 0.36
V
389
BOUT
The sum of two normalize output current is:
P P
243
O1 O2
ISUM 48.6A
VO1
5
Assuming 16% p-p ripple current in LSUM, the inductor
for the first output is obtained as:
V ( V V
)
O1 O1 F1
L1DMIN ISUM
fSW ( PO 1PO2) ISUM
3
6510 (5 9 12 16.5) 0.16
(1 )
5(5 0.45)
(1 0.36) 6.9uH
Then, the ripple current for each output is given as:
IO1ISUM148.60.161 43%
I 2 I 2 9
O1 O1
IO2 ISUMNS1 1 48.6 0.16 3 1
10%
I 2 N I 2 7 16.5
O2 S 2 O2