Data Sheet

© 2006 Fairchild Semiconductor Corporation www.fairchildsemi.com
FSQ-Series • Rev. 1.1.3 13
FSCQ-Series — Green Mode Fairchild Power Switch (FPS™)
Functional Description
1. Startup: Figure 25 shows the typical startup circuit
and the transformer auxiliary winding for the FSCQ
series. Before the FSCQ series begins switching, it
consumes only startup current (typically 25 μA). The
current supplied from the AC line charges the external
capacitor (C
a1
) that is connected to the V
CC
pin. When
V
CC
reaches the start voltage of 15 V (V
START
), the
FSCQ series begins switching and its current
consumption increases to I
OP
. Then, the FSCQ series
continues normal switching operation and the power
required is supplied from the transformer auxiliary
winding, unless V
CC
drops below the stop voltage of 9 V
(V
STOP
). To guarantee stable operation of the control IC,
V
CC
has under-voltage lockout (UVLO) with 6 V
hysteresis. Figure 26 shows the relationship between
the operating supply current of the FSCQ series and the
supply voltage (V
CC
).
FSCQ-Series
1N4007
Rstr
V
CC
C
a1
Da
I
sup
AC line
(V
ac
min
- V
ac
max
)
C
DC
C
a2
Figure 25. Startup Circuit
I
CC
V
CC
V
STOP
=9V
I
START
I
OP
V
START
=15V V
Z
Power Up
Power Down
IOP Value
FSCQ0565RT: 4mA (Typ.)
FSCQ0765RT: 4mA (Typ.)
FSCQ0965RT: 6mA (Typ.)
FSCQ1265RT: 6mA (Typ.)
FSCQ1565RT: 7mA (Typ.)
Figure 26. Relationship between Operating Supply
Current and V
CC
Voltage
The minimum average of the current supplied from the
AC is given by:
STR
START
MIN
AC
AVG
SUP
R
V
V
I
1
2
2















(1)
where V
ac
min
is the minimum input voltage, V
START
is
the FSCQ series’ start voltage (15 V), and R
str
is the
startup resistor. The startup resistor should be chosen
so that I
sup
avg
is larger than the maximum startup
current (50 μA).
Once the resistor value is determined, the maximum
loss in the startup resistor is obtained as:
 













MAX
ACSTARTSTART
MAX
AC
STR
VVVV
R
Loss
22
2
1
2
2
(2)
where V
ac
max
is the maximum input voltage.
The startup resistor should have properly rated
dissipation wattage.
2. Synchronization: The FSCQ series employs a
quasi-resonant switching technique to minimize the
switching noise and loss. In this technique, a capacitor
(C
r
) is added between the MOSFET drain and the
source, as shown in Figure 27. The basic waveforms of
the quasi-resonant converter are shown in Figure 28.
The external capacitor lowers the rising slope of the
drain voltage to reduce the EMI caused when the
MOSFET turns off. To minimize the MOSFET’s
switching loss, the MOSFET should be turned on when
the drain voltage reaches its minimum value, as shown
in Figure 28.
V
cc
C
a1
D
a
C
DC
C
a2
GND
Cr
Drain
Ids
R
cc
R
SY1
R
SY2
Sync
+
V
DC
-
Lm Vo
C
SY
+
V
ds
-
V
co
D
SY
Np
Ns
Na
Figure 27. Synchronization Circuit