FAN7621B Fairchild Semiconductor, FAN7621B Datasheet - Page 9

• Manufacturer Part Number: FAN7621B
• Description: The FAN7621B is a pulse frequency modulation controller for high-efficiency half-bridge resonant converters
• Manufacturer: Fairchild Semiconductor

© 2009 Fairchild Semiconductor Corporation
FAN7621B • Rev. 1.0.1
Functional Description
1. Basic Operation: FAN7621B 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 16.
2. Internal Oscillator: FAN7621B employs a current-
controlled oscillator, as shown in Figure 17. Internally,
the voltage of R
/ discharging current for the oscillator capacitor, C
obtained by copying the current flowing out of R
(I
frequency increases as I
3. Frequency Setting: Figure 18 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 19 shows
the typical circuit configuration for R
opto-coupler transistor is connected to the R
modulate the switching frequency.
8
gate drive
MOSFET
CTC
High-side
MOSFET
gate drve
Low-side
R
V
T
) using a current mirror. Therefore, the switching
REF
Figure 17. Current Controlled Oscillator
I
Figure 16. MOSFETs Gate Drive Signal
CTC
I
2I
+
-
CTC
CTC
2V
T
pin is regulated at 2V and the charging
CTC
C
increases.
T
3V
1V
Dead t ime
+
-
+
-
Counter
T
(1 /4)
pin, where the
R
S
Gate drive
F/F
T
time
-Q
pin to
Q
T
T
, is
pin
9
The minimum switching frequency is determined as:
Assuming
transistor is 0.2V, the maximum switching frequency is
determined as:
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
Gain
f
f
1.8
1.6
1.4
1.2
1.0
0.8
0.6
Figure 18. Resonant Converter Typical Gain Curve
min
max
R
60
max
=
=
V
5.2
(
CC
5.2
R
Figure 19. Frequency Control Circuit
R
70
min
R
k
min
k
min
Ω
Ω
the
×
+
80
100(
f
4.68
min
saturation
R
C
max
kHz
R
90
SS
R
k
SS
Ω
sense
)
) 100(
f
×
100
normal
CON
RT
CS
Frequency (kHz)
voltage
kHz
110
LV
SG
CC
)
f
HV
CTR
120
max
Soft-sta rt
HO
LO
CC
PG
of
130
www.fairchildsemi.com
opto-coupler
f
140
ISS
150
(1)
(2)