AN-9741
APPLICATION NOTE
Description
Maximum Rectifier Output Voltage
Output
N
A
/N
S
Ratio 1
N
A
/N
S
Ratio 2
N
A
/N
S
Ratio 3
Transformer Design
Non-Conduction Time at Point B
Input
Transformer Core Cross-Sectional Area
Maximum Flux Density
Determine Secondary Side Turns
MOSFET Conduction Time at Point A
Inductor Discharge Time at Point A
Non-Conduction Time at Point A
MOSFET Conduction Time at Point B
Inductor Discharge Time at Point B
MOSFET Conduction Time at Point C
Inductor Discharge Time at Point C
Output
Non-Conduction Time at Point C
Transformer Primary-Side Inductance
Peak Drain Current
Minimum Primary-Side Turns
Primary-Side Turns
Auxiliary Winding Turns
Final N
P
/N
S
Ratio
Final N
A
/N
S
Ratio
Selection Switching Device
Input
MOSFET Overshoot Voltage
MOSFET Maximum Drain-Source Voltage
Output
MOSFET RMS Current
Maximum Second Diode Voltage
Second Diode RMS Current
Setting Output Voltage and Current
Determine VS High-Side Resistor
Input
Determine VS Low-Side Resistor
Determine Current-Sensing Resistor 1
Determine Current-Sensing Resistor 2
Calculate VS High-Side Resistor
Auxiliary Voltage at Low Line
Output
Auxiliary Current at 90V
AC
DC Link Voltage at Brownout
Calculate Current-Sensing Resistor
Symbol
V
RO
N
A
/N
S
N
A
/N
S
1
2
Value
80
0.50
0.24
0.49
4.00
31.0
0.30
23
7.66
8.24
4.10
4.60
11.40
5.08
15.25
9.98
1.21
0.55
71.13
74
16
3.22
0.70
40
495
0.20
140
0.65
91
16
2.4
2.2
90.85
-27.52
379.59
38.83
1.08
Unit
V
3
N
A
/N
S
t
OFF@B
A
e
B
sat
N
S
t
ON
t
DIS
t
OFF
t
ON@B
t
DIS@B
t
ON@C
t
DIS@C
t
OFF@C
L
m
I
DS
N
p
PK
min
us
mm
T
Turns
us
us
us
us
us
us
us
us
mH
A
Turns
Turns
Turns
2
N
p
N
A
N
P
/N
S
N
A
/N
s
V
OS
V
DS
I
DS
I
F
max
rms
V
V
A
V
A
K
K
V
F
rms
R1
R2
R
sense1
R
sense2
R1
cal
1
V
A
low
I
VS
K
V
uA
V
V
DL
BO
R
sense
© 2011 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 6/27/11
www.fairchildsemi.com
9
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