AN-8027 データシートの表示（PDF） - Fairchild Semiconductor
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AN-8027 Datasheet PDF : 17 Pages
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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:
NS2 = NL2
NS1 N L1
(47)
L2
N
NL2
p
NS2
VO2
NL1
L1
NS1
VO1
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 + VF1 )
f SW
( PO1
+
PO 2
)
ΔI SUM
I SUM
⋅ (1− DMIN )
(48)
where:
DMIN
= DMAX
V MIN
BOUT
VBOUT
I SUM
=
PO1 + PO2
VO1
(49)
Then, the ripple current for each output is given as:
ΔIO1 = ΔISUM ⋅ 1
IO1
2 IO1
(50)
ΔIO2 = ΔISUM ⋅ NS1 ⋅ 1
IO2
2 NS2 IO2
(51)
Figure 20. Coupled Inductor
D1
L1
VPOUT ⋅ NS1
NP
0
D2
VPOUT ⋅ NS 2
L2
NP
0
VO1
IO1
VO2
IO2
VO 2 N
=
N S1
NS2
VO2
= VO1
IO2N
=
NS2
NS1
IO2
ISUM
LM
LLK
VPOUT ⋅ NS1
NP
0
LLK
Normalized
D1
VO1
IO2
D2N
VO2N
IO2N
(Design Example) The minimum duty cycle of
PWM stage at nominal input (PFC output) voltage is:
DMIN
= DMAX
V MIN
BOUT
VBOUT
= 0.45 310 = 0.36
389
The sum of two normalize output current is:
I SUM
=
PO1 + PO2
VO1
=
243 = 48.6A
5
Assuming 16% p-p ripple current in LSUM, the inductor
for the first output is obtained as:
L1 =
VO1 (VO1 + VF1 )
fSW
(PO1
+
PO 2
)
ΔI SUM
I SUM
⋅ (1− DMIN )
=
5(5 + 0.45)
⋅ (1− 0.36) = 6.9uH
65×103 (5× 9 +12 ×16.5) ⋅ 0.16
Then, the ripple current for each output is given as:
ΔIO1 = ΔISUM ⋅ 1 = 48.6× 0.16 ⋅ 1 = 43%
IO1
2 IO1
29
ΔIO2 = ΔISUM ⋅ NS1 ⋅ 1 = 48.6 × 0.16 ⋅ 3 ⋅ 1 = 10%
IO2
2 NS2 IO2
2 7 16.5
Figure 21. Normalized Coupled Inductor Circuit
© 2009 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 8/26/09
12
www.fairchildsemi.com
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