AN-6094
© 2012 Fairchild Semiconductor Corporation
www.fairchildsemi.com
Rev. 1.0.0 • 9/27/12
15
[STEP-12] Protection Setting
Output OVP:
In STEP-5, the voltage divider for VS is determined such
that VS sampling voltage is about 2.5 V in normal
operation. The voltage divider also determines the output
over-voltage protection (OVP) level. The OVP is
triggered when VS sampling voltage is above 2.8 V. The
OVP trip point is given as:
1
2
.
2
2.8
OVP
S
VS
VS
O
F SH
A
VS
N R
R
V
V
N
R
+
=
−
(54)
Pulse-by-Pulse Current Limit:
Since FAN302 employs current-mode control, the
MOSFET drain current is regulated properly by the error
amplifier output in normal operation. During the load
transient or abnormal condition such as output short, the
error amplifier can be saturated HIGH and the drain
current is regulated by the pulse-by-pulse current limit,
which forces the MOSFET gate to be turned off when the
current-sensing voltage reaches 0.7 V. The flux density of
the transformer during the pulse-by-pulse current-limit
mode should be checked to make sure that the flux density
is below 0.4~0.42T to prevent severe core saturation.
/
OCP
m
STH
CS
MAX
P
e
L V
R
B
N A
=
(55)
(Design Example)
With 91 k
Ω
and 40 k
Ω
for R
VS1
and
R
VS2
from STEP-5, the OVP trip point is obtained as:
1
2
.
2
2.8
5.63
OVP
S
VS
VS
O
F SH
A
VS
N R
R
V
V
V
N
R
+
=
−
=
The flux density during pulse-by-pulse current limit is
given as:
/
0.36
OCP
m
STH
CS
MAX
P
e
L V
R
B
T
N A
=
=
