Data Sheet

© 2011 Fairchild Semiconductor Corporation www.fairchildsemi.com
FSFR1800 / FSFR1700-HS • Rev.1.0.1 12
FSFR-HS Series — Advanced Fairchild Power Switch (FPS™) for Half-Bridge Resonant Converter
C
Filter
can be used to reduce some noise induced from
transformer or switching transition. Generally, hundreds
of pico-farad to tens of nano-farad is adequate,
depending on the quantity of noise.
The start and stop input-voltage can be calculated as:
][
2
21
,
V
R
RR
VV
LINESTOPlinkdc
+
×=
−
(6)
][1
,,
VRIVV
LINESTOPlinkdcSTARTlinkdc
×+=
−−
(7)
Figure 25. Half-Wave Sensing
7. Simple Remote-On/Off: The power stage can be
shutdown with optional Auto-Restart Mode, as shown in
Figure 26.
To configure an external protection with Auto-Restart
Mode, an opto-coupler and the LS pin are used. When
the voltage on the LS pin is pulled below V
LINE
(2.5 V),
the IC stops during the status holds. However, the opto-
coupler stops pulling down and the IC can perform the
auto-restart operation itself.
Figure 26. External Protection Circuits
8. Current-Sensing Methods: FSFR-HS series employs
negative voltage sensing to detect the drain current of
MOSFET, which allows a low-noise resistive sensing
using a filter with low time-constant and capacitive
sensing method.
8.1 Resistive Sensing Method: The IC can sense
drain current as a negative voltage, as shown in Figure
27 and Figure 28. Half-wave sensing allows low power
dissipation in the sensing resistor; while full-wave
sensing has less switching noise in the sensing signal.
For a time constant range for the filter, 3/100~1/10 of
the operating frequency is reasonable.
Figure 27. Half-Wave Sensing
Figure 28. Full-Wave Sensing
8.2 Capacitive Sensing Method: The drain current
can be sensed using an additional capacitor parallel
with the resonant capacitor, as shown in Figure 29.
During the low-side switch turn on, the current, i
CB
through C
B
, makes V
SENSE
across R
SENSE
. The i
CB
is
scale-down of i
p
by the impedance ratio of C
r
and C
B
.
Generally, 1/100~1/1000 is adequate for the ratio of C
B
against C
r
. R
D
is used as a damper for reducing noise
generated by switching transition. Several hundreds of
ohm to a few of kilo-ohms can be normally used.
V
SENSE
can be estimated as;
][VR
Cr
C
IV
sense
B
pk
Crsense
⋅=
(8)
Control
IC
CS
SG
PG
R
sense
I
DS
V
CS
I
DS
V
CS
Ns
Np
Ns
C
r
Control
IC
CS
SG PG
Ns
Np
Ns
R
sense
I
DS
C
r
I
DS
V
CS
V
CS