Data Sheet

© 2010 Fairchild Semiconductor Corporation www.fairchildsemi.com
FSFR-XS Series • Rev.1.0.2 9
FSFR-XS Series — Fairchild Power Switch (FPS™) for Half-Bridge Resonant Converter
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 350 ns 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 R
T
pin is regulated at 2 V and
the charging / discharging current for the oscillator
capacitor, C
T
, is obtained by copying the current flowing
out of the R
T
pin (I
CTC
) using a current mirror. Therefore,
the switching frequency increases as I
CTC
increases.
Figure 16. Current-Controlled Oscillator
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 R
T
pin, where the
opto-coupler transistor is connected to the R
T
pin to
modulate the switching frequency.
The minimum switching frequency is determined as:
min
min
5.2
100( )
k
f
kHz
R


(1)
Assuming the saturation voltage of opto-coupler
transistor is 0.2 V, the maximum switching frequency is
determined as:
max
min max
5.2 4.68
()100()
kk
f
kHz
RR

 
(2)
Figure 17. Resonant Converter Typical Gain Curve
Figure 18. Frequency Control Circuit
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 (f
ISS
) until the output
voltage is established. The soft-start circuit is made by
connecting R-C series network on the R
T
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 40 kHz 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:
min
5.2 5.2
( ) 100 40 ( )
ISS
SS
kk
f
kHz
RR



(3)