FSCQ-SERIES
Vds
MOSFET
On
MOSFET
Off
2VRO
Vgs
TQ
VRO
VRO
Vsync
Vsypk
Vd
s
VDC
Vrh (4.6V)
Vrf (2.6V)
TR
Ids
Ipk
MOSFET Gate
ON
ON
Figure 7. Quasi-resonant Operation Waveforms
Figure 8. Normal Quasi-Resonant Operation Waveforms
The minimum drain voltage is indirectly detected by
monitoring the Vcc winding voltage as shown in Figure 6
and 8. Choose voltage dividers, RSY1 and RSY2, so that the
Switching
frequency
peak voltage of the sync signal (V ) is lower than the
sypk
OVP voltage (12V) to avoid triggering OVP in normal
operation. It is typical to set V to be lower than OVP
Extended QR operation
sypk
voltage by 3-4 V. To detect the optimum time to turn on
MOSFET, the sync capacitor (C ) should be determined so
SY
Q as shown in Figure 8
90kHz
Normal QR operation
that T is the same with T
. The T and
R
R
T
are given as, respectively
Q
45kHz
Vco
RSY2
⎛
⎝
⎞
⎠
-------- ----------------------------------
TR = RSY2 ⋅ CSY ⋅ ln
⋅
2.6
RSY1 + RSY2
TQ = π ⋅ Lm ⋅ Ceo
Na ⋅ (Vo + VFO
Output power
Figure 9. Extended Quasi-Resonant Operation
)
----------------------------------------
– VFa
Vco
=
Ns
where L is the primary side inductance of the transformer,
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.
m
and N and N are the number of turns for the output
s
a
winding and Vcc winding, respectively, V and V are the
Fo Fa
diode forward voltage drops of the output winding and Vcc
winding, respectively, and C is the sum of the output
eo
capacitance of the MOSFET and the external capacitor, Cr.
14
FAIRCHILD [ FAIRCHILD SEMICONDUCTOR ]
