Functional Description
1. Basic Operation. FSFR-XS series is designed to
drive high-side and low-side MOSFETs complementarily
with 50% duty cycle. A fixed dead time of 350ns is
introduced between consecutive transitions, as shown in
Figure 15.
Figure 15. MOSFETs Gate Drive Signal
2. Internal Oscillator: FSFR-XS series employs a
current-controlled oscillator, as shown in Figure 16.
Internally, the voltage of RT pin is regulated at 2V and the
charging / discharging current for the oscillator capacitor,
CT, is obtained by copying the current flowing out of the
RT pin (ICTC) using a current mirror. Therefore, the
switching frequency increases as ICTC increases.
Figure 17. Resonant Converter Typical Gain Curve
LVCC
VDL
RT
Rmax
Rmin
Rss
Css
AR
CS
PG
SG
Figure 18. Frequency Control Circuit
Figure 16. Current-Controlled Oscillator
To prevent excessive inrush current and overshoot of
output voltage during startup, increase the voltage gain
of the resonant converter progressively. Since the
voltage gain of the resonant converter is inversely
proportional to the switching frequency, the soft-start is
implemented by sweeping down the switching frequency
from an initial high frequency (fISS) until the output
voltage is established. The soft-start circuit is made by
connecting R-C series network on the RT pin, as shown
in Figure 18. FSFR-XS series also has a 3ms internal
soft-start to reduce the current overshoot during the initial
cycles, which adds 40kHz to the initial frequency of the
external soft-start circuit, as shown in Figure 19. The
initial frequency of the soft-start is given as:
3. Frequency Setting: Figure 17 shows the typical
voltage gain curve of a resonant converter, where the
gain is inversely proportional to the switching frequency
in the ZVS region. The output voltage can be regulated
by modulating the switching frequency. Figure 18 shows
the typical circuit configuration for the RT pin, where the
opto-coupler transistor is connected to the RT pin to
modulate the switching frequency.
The minimum switching frequency is determined as:
5.2kΩ
Rmin
f min
=
×100(kHz)
(1)
Assuming the saturation voltage of opto-coupler
transistor is 0.2V, the maximum switching frequency is
determined as:
5.2kΩ 5.2kΩ
f ISS = (
+
)×100+ 40 (kHz)
(3)
Rmin
RSS
5.2kΩ 4.68kΩ
f max = (
+
)×100(kHz)
(2)
Rmin
Rmax
© 2010 Fairchild Semiconductor Corporation
FSFR-XS Series • Rev.1.0.1
www.fairchildsemi.com
9
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