Application Note
9
2011-07-06
[11]
The peak current is
A
I
I
rms
r
pk
r
91
.
2
*
2
_
_
=
=
. The OCP level is set with about 20% margin:
A
I
I
pk
r
pk
ocp
49
.
3
*
2
.
1
_
_
=
=
The actual leakage inductance (
leak
L
) measured at primary side with one of the secondary side winding
shorted is around 13uH. Therefore, the inductance for the independent resonant choke is:
If a magnetic core with specs of RM10/PC95 is selected, where
2
min
_
90
mm
A
e
=
, and
max
B
is selected
to be 0.08T to reduce core loss, the minimum turns can be given as:
[12]
2.2.7 Transformer
design
From Figure 2, the normalized frequency
35
.
0
min
=
F
has been located to achieve maximum gain
28
.
1
=
pk
G
. Accordingly the actual minimum frequency
min
f
is:
kHz
f
F
f
r
30
10
*
85
*
35
.
0
*
3
min
=
=
=
The voltage across the primary winding can be calculated as
)
(
f
o
p
V
V
n
V
+
=
. The half switching cycle
period is around:
min
2
1
f
t
=
. According to Faraday’s law:
B
A
N
f
V
V
n
e
p
f
o
Δ
=
+
min
2
)
(
The minimum number of turns at primary side can be found:
B
A
f
V
V
n
N
e
f
o
p
Δ
+
=
*
2
)
(
min
min
[13]
Where
2
161
mm
A
e
=
with PQ3230 core.
T
B
62
.
0
=
Δ
is selected to avoid magnetic saturation.
Then
min
p
N
can be calculated as:
33
62
.
0
*
10
*
161
*
10
*
30
*
2
)
1
.
0
12
(
*
5
.
16
6
3
min
=
+
=
−
p
N
The number of turns at primary side is selected as
33
min
=
p
N
. The secondary side turns can be
calculated accordingly:
A
V
P
I
rms
in
in
rms
in
06
.
2
79
.
151
*
96
.
0
300
*
min
_
_
max
_
_
=
=
=
η
4
.
19
10
*
90
*
08
.
0
49
.
3
*
10
*
40
6
6
min
max
_
_
min
=
=
⋅
⋅
=
−
−
L
L
pk
r
choke
r
L
A
B
I
L
N
uH
L
L
L
leak
r
choke
r
40
_
=
−
=