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Functional Description
1. Basic Operation
Gain
FSFR2100 is designed to drive high-side and low-side
MOSFETs complementarily with 50% duty cycle. A fixed
1.8
f min
f normal
f max
f ISS
dead time of 350 ns is introduced between consecutive
1.6
transitions, as shown in Figure 16.
1.4
1.2
1.0
Soft-start
0.8
0.6
60
70
80
90
100
110
120
130
140
150
Figure 16. MOSFETs Gate Drive Signal
freq (kHz)
Figure 18. Resonant Converter Typical Gain Curve
2. Internal Oscillator
FSFR2100 employs a current-controlled oscillator, as
shown in Figure 17. Internally, the voltage of R T pin is
regulated at 2 V and the charging/discharging current for
RT
LV CC
VDL
the oscillator capacitor, C T , is obtained by copying the
current flowing out of R T pin (I CTC ) using a current mirror.
Therefore, the switching frequency increases as I CTC
increases.
R max
R mi R ss
C ss CON
Control
IC
SG
PG
Figure 19. Frequency Control Circuit
The minimum switching frequency is determined as:
Figure 17. Current Controlled Oscillator
f min ?
5.2 k ?
R min
? 100( kHz )
(1)
f max ? (
?
) ? 100( kHz )
3. Frequency Setting
Figure 18 shows a 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 19 shows the typical circuit
configuration for R T pin, where the opto-coupler transistor
is connected to the R T pin to modulate the switching
frequency.
? 2010 Fairchild Semiconductor Corporation
FSFR2100 ? Rev.1.1.0
9
Assuming the saturation voltage of opto-coupler
transistor is 0.2 V, the maximum switching frequency is
determined as:
5.2 k ? 4.68 k ?
(2)
R min R max
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 I S S ) 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 19. FSFR2100 also has an internal soft-start for
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