FSCQ0565RT Fairchild Semiconductor, FSCQ0565RT Datasheet - Page 15

• Manufacturer Part Number: FSCQ0565RT
• Description: A Quasi-Resonant Converter (QRC) typically shows lower EMI and higher power conversion efficiency compared to conventional hard-switched converter with a fixed switching frequency
• Manufacturer: Fairchild Semiconductor

FSCQ-Series Rev. 1.1.2
The minimum drain voltage is indirectly detected by mon-
itoring 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 3–4V. To detect the optimum time to turn on
MOSFET, the sync capacitor (C
mined so that T
8. The T
where L
former, and N
output winding and V
V
winding and Vcc winding, respectively, and C
sum of the output capacitance of the MOSFET and the
external capacitor, Cr.
V
T
Fa
V
I
T
V
ds
gs
R
ds
Q
co
are the diode forward voltage drops of the output
=
=
Figure 7. Quasi-Resonant Operation
=
R
R
m
N
----------------------------------------- - V
SY2
and T
a
is the primary side inductance of the trans-
L
m
s
V
C
R
Q
N
and N
o
SY
MOSFET
is the same with T
s
C
are given as, respectively:
+
I
eo
pk
V
Off
In
FO
Waveforms
CC
a
are the number of turns for the
V
-------- -
2.6
–
winding, respectively, V
co
V
V
Fa
RO
RO
MOSFET
sypk
SY1
--------------------------------- -
R
SY1
On
R
SY
and R
sypk
to be lower than OVP
Q
SY2
+
V
) should be deter-
as shown in Figure
DC
R
) is lower than the
SY2
SY2,
so that the
eo
Fo
is the
and
15
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 switch-
ing loss in the light load condition. To overcome this limi-
tation, 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
Figure 9. Extended Quasi-Resonant Operation
90kHz
45kHz
MOSFET Gate
V
V
ds
2V
sync
Switching
frequency
RO
ON
Figure 8. Normal Quasi-Resonant
Operation Waveforms
Extended QR operation
T
T
Q
R
Vrh (4.6V)
Vrf (2.6V)
V
Normal QR operation
sypk
ON
Output power
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