AN4070
DC-DC converter design
Doc ID 022934 Rev 1
11/53
2.
Calculation of the maximum duty cycle:
In this topology the switch duty cycle is always >0.5. Let's call D the time beyond Ts/2
for which the MOSFET is still closed:
Equation 2
3.
Calculation of maximum input power:
Equation 3
4.
Calculation of maximum average input current:
Equation 4
5.
Calculation of the maximum switch RMS current:
Equation 5
6.
Calculation of maximum average output current:
Equation 6
7.
Calculation of transformer turns ratio:
Equation 7
where Vin is chosen as the average between the maximum and minimum input voltage
value. The final transformer turns ratio has been chosen equal to 2.6.
8.
Calculation of minimum current value for CCM operation:
Equation 8
to ensure CCM for I>I
lim
.
s
on
s
on
T
t
T
t
D
⋅
=
=
−
=
7
.
0
2
.
0
5
.
0
max
max
max
1
2
0
1
2
N
N
D
V
V
in
⋅
−
⋅
=
W
P
P
out
in
7
.
257
97
.
0
250
=
=
=
η
A
V
P
I
in
in
in
8
.
12
20
7
.
257
min
max
=
=
=
A
I
D
I
in
rms
6
.
8
8
.
12
2
2
.
0
2
2
2
max
max
=
⋅
−
=
⋅
−
=
A
V
P
I
out
out
out
67
.
0
370
250
min
max
=
=
=
53
.
2
30
2
)
2
.
0
1
(
2
380
2
)
1
(
2
1
2
=
⋅
−
⋅
=
⋅
−
⋅
=
in
out
V
D
V
N
N
A
V
P
I
W
P
P
P
in
out
625
.
0
30
5
.
12
2
1
25
250
1
.
0
%
10
lim
lim
lim
lim
=
=
⋅
=
=
⋅
=
=
