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The design example is based on the typical application circuit in Figure 1, where individual resonant
choke is implemented. The target specifications are summarized in Table 1.
Input voltage
V
in
Output voltage and current
Output power
P
in
η
Efficiency
Resonant frequency
Hold up time
T
h
Bulk capacitor
C
Table 1
Target application specifications
2.2
Design of Power Stage
2.2.1
System specifications
The maximum input power can be calculated as:
*
12
*
25
V
I
=
=
O
O
P
η
in
. 0
96
Based on the required 20ms hold-up time, the minimum input voltage can be given as:
2
=
V
V
_
min
_
in
in
nom
2.2.2
Selection of resonant factor m
In order to achieve the highest efficiency possible, the value of resonant factor
be set as big as possible, so that the magnetizing inductance
is small, which results in low core loss and conduction loss. On the other hand, the magnetizing current
should be big enough to discharge the
ZVS to ensure safe switching and save switching loss. In this design example,
start. The ZVS of primary side MOSFET will be confirmed later with the determination of the deadtime of
switching.
Application Note
V ,
I
o
o
f
r
out
=
312
5 .
W
2
2
P
T
−
=
−
2
400
in
h
C
out
C
ds
400VDC
12VDC, 25A
~ 300W
>96% at 100% load
>97% at 50% load
>96% at 20% load
85kHz
20ms
270uF
−
3
*
312
5 .
*
20
*
10
=
−
6
270
*
10
is big and therefore magnetizing current
L
m
of primary side MOSFET during the transitions, to realize
6
337
2 .
V
L
L
=
=
p
m
L
r
=
13
is selected as a
m
[1]
[2]
+
L
m
r
is to
L
r
2011-07-06
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