FAN7621 Fairchild Semiconductor, FAN7621 Datasheet - Page 11

• 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
resumes normal operation when LV
voltage of 14.5V.
5.2
triggered, switching is terminated and the gate output
signals remain off. The latch is reset only when LV
discharged below 5V.
5.3
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.
OCP
OLP
OVP
CON
11/ 14 V
LV
C
C
CC
DL
DL
20k
good
LV
Latch-Mode Protection : Once this protection is
Current Sensing Using Resistor : FAN7621
V
V
R
CS
CS
12
sense
CC
CON
Figure 25. Half-Wave Sensing
RT
CON
Figure 26. Full-Wave Sensing
CS
RT
CS
Figure 24. Protection Blocks
+
-
LV
LV
SG
SG
CC
CC
LV
HV
CTR
HV
CTR
HO
LO
HO
LO
CC
CC
PG
PG
CC
Auto-restart
protection
I
good
R
S
ds
F/F
I
ds
-Q
Q
V
RE F
R
V
V
sense
I
I
ds
ds
CS
CS
protection
La tch
Q
-Q
CC
F/F
In ternal
Bias
R
reaches the start
S
Shutdown
LV
CC
AOCP
TSD
CC
< 5V
is
11
5.4
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
resonant current in the primary side (I
To minimize power dissipation, a capacitive voltage
divider is generally used for capacitor voltage sensing,
as shown in Figure 27.
5.5
sensing pin voltage drops below -0.6V, OCP is triggered
and the MOSFETs remain off. This protection has a
shutdown time delay of 1.5µs to prevent premature
shutdown during startup.
5.6
the secondary rectifier diodes are shorted, large current
with extremely high di/dt can flow through the MOSFET
before OCP or OLP is triggered. AOCP is triggered
without shutdown delay when the sensing pin voltage
drops below -0.9V. This protection is latch mode and
reset when LV
V
Cr
V
V
CON
p p
I
−
sense
p
V
Figure 27. Current Sensing Using Resonant
Cr
=
Current Sensing Using Resonant Capacitor
Over-Current Protection (OCP) : When the
Abnormal Over-Current Protection (AOCP) : If
2
I
π
p
C
p p
f C
−
s
DL
CC
V
V
sense
Cr
r
p p
is pulled down below 5V.
−
pk
=
V
CON
RT
CS
C
sense
sense
Capacitor Voltage
SG
C
LV
t
+
B
Delay
C
CC
C
sense
B
HV
CTR
PG
HO
LO
=R
CC
cr
d
C
p-p
d
100
V
) is proportional to the
sense
2
pk
C
p
B
I
=
p-p
p
V
) as:
CON
C
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r
V
V
V
sense
sense
Cr
(6)
p-p
pk
pk