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