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FSCQ1565RP Datasheet(PDF) 14 Page - Fairchild Semiconductor

Part No. FSCQ1565RP
Description  Green Mode Fairchild Power Switch (FPS) for Quasi-Resonant Switching Converter
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Maker  FAIRCHILD [Fairchild Semiconductor]
Homepage  http://www.fairchildsemi.com
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FSCQ1565RP Datasheet(HTML) 14 Page - Fairchild Semiconductor

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FSCQ1565RP
14
Figure 4. Quasi-resonant operation waveforms
The minimum drain voltage is indirectly detected by
monitoring the Vcc winding voltage as shown in Figure 6
and 8. The voltage divider RSY1 and RSY2 should be chosen so
that the peak voltage of sync signal (Vsypk) is lower than the
OVP voltage (12V) in order to avoid triggering OVP in
normal operation. It is typical to set Vsypk to be lower than
OVP voltage by 3-4 V. In order to detect the optimum time
to turn on MOSFET, the sync capacitor (CSY) should be
determined so that TR is the same with TQ as shown in Figure
8. The TR and TQ are given as, respectively
where Lm is the primary side inductance of the transformer,
Ns and Na are the number of turns for the output winding
and Vcc winding, respectively, VFo and VFa are the diode
forward voltage drops of the output winding and Vcc
winding, respectively, and Ceo is the sum of the output
capacitance of MOSFET and external capacitor Cr.
Figure 5. Normal quasi-resonant operation waveforms
Figure 6. Extended quasi-resonant operation
In general, quasi-resonant converter 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. In order to get over
this limitation, FSCQ1565RP employs extended quasi-
resonant switching operation. Figure 9 shows the mode
change between normal quasi-resonant operation and
extended quasi-resonant operation. In the normal quasi-
resonant operation, the FSCQ1565RP enters into the
extended quasi-resonant operation when the switching
frequency exceeds 90kHz as the load reduces. Then, the
MOSFET is turned on, when the drain voltage reaches the
second minimum level as shown in Figure 10, which reduces
the switching frequency.
V
DC
V
RO
V
RO
I
pk
Ids
Vds
Vgs
MOSFET
off
MOSFET
on
T
R
R
SY 2
C
SY
V
co
2.6
---------
R
SY2
R
SY1
R
SY2
+
-----------------------------------


ln
⋅⋅
=
T
Q
π
L
m
C
eo
=
V
co
N
a
V
o
V
FO
+
()
N
s
-----------------------------------------
V
Fa
=
Vs ync
Vds
MOS FET Gate
2V
RO
Vrh (4 .6 V)
Vrf (2 .6 V)
ON
T
Q
T
R
ON
V
sypk
Output power
Switching
frequency
Normal QR operation
Extended QR operation
90kHz
45kHz


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