FAN7621 Fairchild Semiconductor, FAN7621 Datasheet - Page 9

• Manufacturer Part Number: FAN7621
• Description: The FAN7621 is a pulse frequency modulation controller for high-efficiency half-bridge resonant converters
• Manufacturer: Fairchild Semiconductor

© 2009 Fairchild Semiconductor Corporation
FAN7621 • Rev. 1.0.3
Functional Description
1. Basic Operation
FAN7621 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
FAN7621 employs a current-controlled oscillator, as
shown in Figure 17. Internally, the voltage of R
regulated at 2V and the charging / discharging current
for the oscillator capacitor, C
current flowing out of R
Therefore, the switching frequency increases as I
increases.
3. Frequency Setting
Figure 18 shows the typical voltage gain curve of a
resonant
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
coupler transistor is connected to the R
the switching frequency.
8
gate drive
MOSFET
High-side
MOSFET
gate drve
Low-side
R
V
T
REF
Figure 17. Current Controlled Oscillator
I
Figure 16. MOSFETs Gate Drive Signal
CTC
converter,
I
2I
-
+
CTC
CTC
2V
T
where
pin (I
C
T
T
, is obtained by copying the
CTC
3V
1V
the
Dead t ime
) using a current mirror.
T
pin, where the opto-
gain
+
-
+
-
Counter
(1/4)
T
pin to modulate
is
R
S
Gate drive
F/F
inversely
T
time
-Q
Q
pin is
CTC
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
proportional to the switching frequency, the soft-start is
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
70
R
R
min
k
min
min
k
Ω
Ω
the
×
+
80
100(
f
4.68
min
saturation
R
C
max
R
kHz
SS
90
SS
k
R
Ω
sense
)
) 100(
f
×
100
normal
CON
RT
CS
Frequency (kHz)
voltage
LV
kHz
110
SG
CC
)
f
HV
CTR
HO
LO
120
max
Soft-sta rt
CC
PG
of
130
www.fairchildsemi.com
opto-coupler
f
140
ISS
150
(1)
(2)