FSCQ FAIRCHILD [Fairchild Semiconductor], FSCQ Datasheet - Page 14

• Manufacturer Part Number: FSCQ
• Description: Green Mode Fairchild Power Switch (FPS)
• Manufacturer: FAIRCHILD [Fairchild Semiconductor]

FSCQ-SERIES
The minimum drain voltage is indirectly detected by
monitoring the Vcc winding voltage as shown in Figure 6
and 8. Choose voltage dividers, R
peak voltage of the sync signal (V
OVP voltage (12V) to avoid triggering OVP in normal
operation. It is typical to set V
voltage by 3-4 V. To detect the optimum time to turn on
MOSFET, the sync capacitor (C
that T
T
where L
and N
winding and Vcc winding, respectively, V
diode forward voltage drops of the output winding and Vcc
winding, respectively, and C
capacitance of the MOSFET and the external capacitor, Cr.
14
Q
V
I
V
ds
are given as, respectively
s
gs
d
Figure 7. Quasi-resonant Operation Waveforms
R
s
is the same with T
V
m
T
T
and N
co
Q
R
is the primary side inductance of the transformer,
=
=
=
π
R
N
---------------------------------------- - V
a
SY2
⋅
a
are the number of turns for the output
⋅
MOSFET
(
L
V
I
m
⋅
Off
pk
N
o
C
⋅
s
+
SY
C
V
eo
Q as shown in Figure 8
⋅
FO
ln
)
eo
⎛
⎝
V
V
V
-------- -
–
2.6
SY
sypk
RO
RO
co
MOSFET
is the sum of the output
) should be determined so
Fa
On
⋅
SY1
sypk
---------------------------------- -
R
to be lower than OVP
SY1
V
and R
) is lower than the
R
DC
Fo
SY2
+
R
and V
SY2,
SY2
. The T
so that the
⎞
⎠
Fa
are the
R
and
In general, the QRC has a limitation in a wide load range
application, since the switching frequency increases as the
output load decreases, resulting in a severe switching loss in
the light load condition. To overcome this limitation, the
FSCQ-Series employs an extended quasi-resonant switching
operation. Figure 9 shows the mode change between normal
and extended quasi-resonant operations. In the normal quasi-
resonant operation, the FSCQ-Series enters into the extended
quasi-resonant operation when the switching frequency
exceeds 90kHz as the load reduces. To reduce the switching
frequency, the MOSFET is turned on when the drain voltage
reaches the second minimum level, as shown in Figure 10.
Figure 8. Normal Quasi-Resonant Operation Waveforms
90kHz
45kHz
Vds
Vs ync
MOS FET Gate
2V
Figure 9. Extended Quasi-Resonant Operation
frequency
ON
Switching
R O
T
T
Extended QR operation
Q
R
Vrh (4.6V)
Vrf (2.6V)
V
ON
sypk
Normal QR operation
Output power