FAN7631 Fairchild Semiconductor, FAN7631 Datasheet - Page 12

• Manufacturer Part Number: FAN7631
• Description: The FAN7631 is a pulse-frequency modulation controller for high-efficiency half-bridge resonant converters that includes a high-side gate drive circuit, an accurate current-controlled oscillator, and various protection functions
• Manufacturer: Fairchild Semiconductor

© 2011 Fairchild Semiconductor Corporation
FAN7631 • 1.0.2
initial switching frequency is not affected by R
solely determined as six times the minimum switching
frequency set by R
switching frequency is also internally limited at 600kHz.
When V
the high threshold of V
V
(5V). The soft-start time is given as:
4. Current Sensing
FAN7631 employs a negative voltage sensing method to
sense the drain current of the MOSFET. This allows
sensing the current without a leading edge spike caused
by the low-side MOSFET’s driving current. Therefore, the
resistive-sensing method requires only a small RC filter.
The capacitive-sensing method is also available.
4.1. Resistive Sensing Method
The FAN7631 can sense the drain current as a negative
voltage, as shown in Figure 33. An RC filter with a time
constant of 1/30~1/10 of the operating period is typical.
SS_END
t
SS
SS
while V

Figure 32. Soft-Start Waveforms
reaches V
C
Figure 33. Resistive Sensing
SS
3 10
SS

2.6
keeps increasing until it reaches V
min

SS_END
5
as in Equation (1). The maximum
CT
comparator, V
(4.2V), soft-start ends. Then,
TH
, is clamped at
max
and is
(2)
SSC
12
4.2. Capacitive Sensing Method
The MOSFET drain current can be sensed using an
additional capacitor in parallel with the resonant
capacitor, as shown in Figure 34. While the low-side
switch is turned on, the current, I
introduces V
the transformer primary-side current, I
the current divider with capacitors C
Generally, 1/100~1/1000 is adequate for the ratio of
C
generated by the switching transition. To prevent the
damping resistor from affecting the current divider ratio,
the resistor should be much smaller than the impedance
of C
B
Then, V
/C
B
V
i
r
R
. R
CB
at the switching frequency, calculated as:
Sense
D


D
SENSE
C

is used as a damper for reducing noise
2
Figure 34. Capacitive Sensing
r
SENSE
C
C
C


B
r
B
can be obtained as:
C
1
f C
S
R
B
sense p
across R
i
B
p
i

C
C
B
r
i
SENSE
p
. The I
r
and C
CB
p
CB
, determined by
, through C
is a fraction of
B
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as:
(3)
(4)
(5)
B