FAN7621B Fairchild Semiconductor, FAN7621B Datasheet - Page 11

• 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
Auto-Restart Mode Protection: Once a fault condition
is detected, switching is terminated and the MOSFETs
remain off. When LV
10.0V, the protection is reset. FAN7621B resumes
normal operation when LV
12.5V.
Latch-Mode
triggered, switching is terminated and the gate output
signals remain off. The latch is reset only when LV
discharged below 5V.
Current Sensing Using Resistor: FAN7621B senses
drain current as a negative voltage, as shown in Figure
25 and Figure 26. Half-wave sensing allows low power
dissipation in the sensing resistor, while full-wave
sensing has less switching noise in the sensing signal.
O CP
OV P
OLP
CON
1 0 / 12.5V
C
L V
DL
CC
R
V
sense
20k
good
CS
LV
12
CON
CC
CS
RT
Figure 25. Half-Wave Sensing
Figure 24. Protection Blocks
+
-
Protection:
LV
SG
CC
HV
CT R
LV
HO
LO
CC
PG
CC
Aut o-restart
CC
protec tion
I
ds
good
R
S
F/ F
falls to the LV
-Q
Q
CC
V
Once
reaches the start voltage of
R E F
V
I
ds
CS
protec tion
Latch
Q
-Q
this
F/F
Internal
Bias
CC
S
R
stop voltage of
protection
Shutdo wn
LV
CC
AOCP
TSD
< 5V
CC
is
is
11
Current Sensing Using Resonant Capacitor Voltage:
For high-power applications, current sensing using a
resistor may not be available due to the severe power
dissipation in the resistor. In that case, indirect current
sensing using the resonant capacitor voltage can be a
good alternative because the amplitude of the resonant
capacitor voltage (V
current in the primary side (I
V
Cr
p p
−
C
DL
=
V
2
CS
I
π
p
CON
RT
CS
Figure 26. Full-Wave Sensing
p p
f C
−
s
LV
SG
r
CC
HV
CTR
HO
LO
CC
PG
cr
p-p
) is proportional to the resonant
I
ds
p
p-p
R
V
I
sense
) as:
ds
CS
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(6)