AN-9741
APPLICATION NOTE
DCM operation at minimum output voltage should be also
checked. The MOSFET conduction time at minimum output
voltage is given as:
[STEP-6] Output Voltage and Current Setting
The nominal output current is determined by the sensing
resistor value and transformer turns ratio as:
t
ON
@
C
=
1
V
DL
@
C
min
×
2
×
P
IN
_
T
@
C
×
L
m
f
SR
(30)
R
Sense
=
N
P
N
N
S
×
I
O
×
8.5
(36)
where f
SR
is the reduced switching frequency to prevent
CCM operation.
Then, the non-conduction time at minimum output voltage
is given as:
The voltage divider R
1
and R
2
should be determined such
that VS is 2.5V at the end of diode current conduction time,
as shown in Figure 8.
t
OFF
@
C
V
DL
@
C
N
1
=
−
t
ON
@
C
(1
+
P
×
min
)
f
SR
N
S
V
O
+
V
F
min
(31)
R
1
V
O
N
=
×
A
−
1
R
2
V
ref
N
S
N
(37)
Select 1% tolerance resistor for better output regulation.
It is recommended to place a bypass capacitor of 22~68pF
closely between the VS pin and the GND pin to bypass the
switching noise and keep the accuracy of the sampled
voltage for CV regulation. The value of the capacitor affects
the load regulation and constant-current regulation. Figure
10 illustrates the measured waveform on the VS pin with a
different VS capacitor. If a higher-value VS capacitor is
used, the charging time becomes longer and the sampled
voltage is higher than the actual value.
The non-conduction time should be larger than 3 s (10% of
the switching period), considering the tolerance of the
switching frequency.
[STEP-5] Calculate the Voltage and Current of
the Switching Devices
Primary-Side MOSFET
The voltage stress of the MOSFET was discussed when
determining the turns ratio in STEP-3. Assuming that drain-
voltage overshoot is the same as the reflected output
voltage, maximum drain voltage is given as:
V
DS
max
=
V
DL
max
+
V
RO
+
V
OS
(32)
The RMS current though the MOSFET is given as:
I
DS
rms
=
I
DS
PK
×
t
ON
×
f
S
3
(33)
Secondary-Side diode
The maximum reverse voltage and the RMS current of the
rectifier diode are obtained, respectively, as:
V
F
=
V
O
+
N
N
S
max
×
V
DL
N
P
(34)
Figure 10. Sampling Voltage with Different VS Capacitors
(35)
I
F
rms
=
I
DS
rms
V
N
×
DL
×
P
V
RO
N
S
min
FL103 is able to control brownout voltage by VS resistors.
When the current through VS (I
VS
) is typically 175 A, the
FL103 triggers brownout protection. At that time, V
S
is
1.13V. The brownout voltage is obtained, respectively, as:
N
V
A
=
V
DL
× −
A
N
P
(38)
I
VS
=
VS VS
−
V
A
+
R
2
R
1
(39)
When input voltage is low line & output load is heavy, I
VS
should be larger than 227 A.
© 2011 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 6/27/11
www.fairchildsemi.com
6
FAIRCHILD [ FAIRCHILD SEMICONDUCTOR ]
