Data Sheet

© 2006 Fairchild Semiconductor Corporation www.fairchildsemi.com
FSQ-Series • Rev. 1.1.3 14
FSCQ-Series — Green Mode Fairchild Power Switch (FPS™)
V
DC
V
RO
V
RO
I
pk
Ids
Vds
Vgs
MOSFET
Off
MOSFET
On
Figure 28. Quasi-Resonant Operation Waveforms
The minimum drain voltage is indirectly detected by
monitoring the V
CC
winding voltage, as shown in Figure
27 and Figure 29. Choose voltage dividers, R
SY1
and
R
SY2
, so that the peak voltage of the sync signal (V
sypk
)
is lower than the OVP voltage (12 V) to avoid triggering
OVP in normal operation. It is typical to set V
sypk
to be
lower than OVP voltage by 3–4 V. To detect the
optimum time to turn on MOSFET, the sync capacitor
(C
SY
) should be determined so that t
R
is the same with
t
Q
, as shown in Figure 29. The t
R
and t
Q
are given as:










21
2
2
6.2
SYSY
SYCO
SYSYR
RR
RV
InCRt
(3)
eomQ
CLt 
(4)
 
Fa
s
FOOa
CO
V
N
VVN
V 


(5)
where:
L
m
is the primary side inductance of the transformer;
N
s
is the number of turns for the output winding;
N
a
is the number of turns for the V
CC
winding;
V
Fo
is the diode forward-voltage drop of the output
winding;
V
Fa
is the diode forward-voltage drop of the V
CC
winding; and
C
eo
is the sum of the output capacitance of the
MOSFET and the external capacitor, C
r
.
Vsync
Vds
MOSFET Gate
2V
RO
Vrh (4.6V)
Vrf (2.6V)
ON
t
Q
t
R
ON
V
sypk
Figure 29. Normal QR Operation Waveforms
Output Power
Switching
Frequency
Normal QR
Operation
Extended QR
Operation
90kHz
45kHz
Figure 30. Extended Quasi-Resonant Operation
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 30
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 90 kHz 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 31. Once the FSCQ series enters into
the extended quasi-resonant operation, the first sync
signal is ignored. After the first sync signal is applied,
the sync threshold levels are changed from 4.6 V and
2.6 V to 3 V and 1.8 V, respectively, and the MOSFET
turn-on time is synchronized to the second sync signal.
The FSCQ series returns to its normal quasi-resonant
operation when the switching frequency reaches 45 kHz
as the load increases.