AN-6094
© 2012 Fairchild Semiconductor Corporation
www.fairchildsemi.com
Rev. 1.0.0 • 9/27/12
9
(Design Example)
By setting the non conduction time at
operating point B as 1.6 µs, the MOSFET conduction
time is obtained as:
@
@
min
@
@
1/
2.15
(1
)
S
OFF
B
ON
B
DL
B
S
P
O
B
F
f
t
t
s
V
N
N
V
V
μ
−
=
=
+
⋅
+
The transformer primary-side inductance is calculated
as:
min
2
@
@
. @
(
)
527
2
DL
B
ON
B
m
S
IN T
B
V
t
L
f
H
P
μ
⋅
=
⋅
=
Assuming V
SH@A
is 2.5 V, the switching frequency at the
minimum output voltage is obtained as:
@
.
@
.
(2.15 2.5
)
45
O
C
F SH
S
S
C
S
N
SH
O
F SH
V
V
f
f
f
V
V
V
kHz
+
Δ
= −
−
⋅
Δ
+
=
The MOSFET conduction time at minimum output
voltage is obtained as:
. @
@
min
@
@
2
1
1.84
IN T
C
m
ON
C
DL
C
S
C
P
L
t
s
V
f
μ
=
=
The non-conduction time at minimum output voltage:
min
@
@
@
@
@
1
(1
)
10.33
DL
C
S
OFF
C
ON
C
S
C
P
O
C
F
V
N
t
t
f
N
V
V
s
μ
=
−
+
⋅
+
=
The peak drain current at maximum load condition is
given as:
. @
2
423
IN T
A
PK
DS
m
S
P
I
mA
L
f
=
=
⋅
EE12.5 core is selected for the transformer. The
minimum number of turns for the transformer primary
side to avoid the core saturation is given by:
min
6
6
527 10
0.423
63.5
0.3 12.88 10
PK
m DS
P
sat
e
L I
N
B A
−
−
=
×
⋅
=
=
⋅
×
Determine the proper integer for
N
s
such that the
resulting
N
p
is larger than
N
p
min
; given as:
min
13.27
13.27 5 66
P
S
P
N
N
N
=
×
=
× =
>
The auxiliary winding turns, N
A
, is obtained as:
1.6 5 8
A
A
S
S
N
N
N
N
=
×
=
× =
[STEP-5] Set the Output Current and V
S
Sensing Resistor
The nominal output current is determined by the sensing
resistor value and transformer turns ratio as:
2
P
CCR
CS
N
S O
N
V
R
N I
K
×
=
×
(31)
where V
CCR
is 2.43 V and K=12 and 10.5 V for UL and
HL, respectively.
The voltage divider R
VS1
and R
VS2
should be determined
so that V
S
is about 2.5 V at 85% of diode current
conduction time as:
1
.
2
@
(
)
1
N
VS
O
F SH
A
VS
S
SH
A
R
V
V
N
R
N
V
+
=
−
(32)
The FAN302 indirectly senses input voltage using the VS
pin current while the MOSFET is turned on, as illustrated
in Figure 15. Since the VS pin voltage is clamped at 0.7 V
when the MOSFET is turned on, the current flowing out
of the VS pin is approximately proportional to the input
voltage, calculated as:
.
1
2
1
1
0.7
(
0.7)
A
A
DL
VS ON
DL
P
VS
VS
P
VS
N
N V
I
V
N
R
R
N R
=
+
+
≅
(33)
Figure 15. VS Pin Current Sensing
FAN302 modulates the minimum on-time of the
MOSFET such that it reduces as input voltage increases,
as shown Figure 16. This allows smaller minimum on
time for high-line condition, ensuring Burst Mode
operation occurs at almost the same power level
regardless of line voltage variation. The V
S
current needs
to be higher than 150 µA.
Increasing the minimum on-time by increasing R
VS1
and
R
VS2
allows FAN302 to enter Burst Mode at a higher
power level. This reduces the standby power consumption
by increasing power delivered to the output per switching.
However, this also increases the output voltage ripple by
increasing the time interval between switching bundles in
Burst Mode. Thus, the minimum on-time should be
determined by a trade-off between standby power
consumption and output voltage ripple. When selecting
R
VS1
and R
VS2
, 150 µA is the VS current level to consider
seriously. If the VS current is lower than 150 µA, t
on_min
won’t be larger.
