Data Sheet
© 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com
FAN7621 • Rev. 1.0.3 11
FAN7621 — PFM Controller for Half-Bridge Resonant Converters
resumes normal operation when LV
CC
reaches the start
voltage of 14.5V.
5.2 Latch-Mode Protection: Once this protection is
triggered, switching is terminated and the gate output
signals remain off. The latch is reset only when LV
CC
is
discharged below 5V.
Figure 24. Protection Blocks
5.3 Current Sensing Using Resistor
: FAN7621
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.
R
sense
FAN7621
LV
CC
RT
CON
CS
SG PG
CTR
HV
CC
HO
LO
C
DL
V
CS
I
ds
I
ds
V
CS
Figure 25. Half-Wave Sensing
R
sense
FA N7621
LV
CC
RT
CON
CS
SG PG
CTR
HV
CC
HO
LO
C
DL
V
CS
I
ds
I
ds
V
CS
Figure 26. Full-Wave Sensing
5.4 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
cr
p-p
) is proportional to the
resonant current in the primary side (I
p
p-p
) as:
2
pp
p
pp
Cr
s
r
I
V
f
C
π
−
−
=
(6)
To minimize power dissipation, a capacitive voltage
divider is generally used for capacitor voltage sensing,
as shown in Figure 27.
I
p
V
Cr
V
sense
V
Cr
p-p
V
sense
pk
t
Delay
=R
d
C
d
pk
sense B
pp
Cr sense B
VC
VCC
−
=
+
2
pk
sense
CON
V
V=
V
CON
V
sense
pk
FAN7621
LV
CC
RT
CON
CS
SG PG
CTR
HV
CC
HO
LO
C
DL
I
p
C
sense
V
sense
C
B
C
r
100
Figure 27. Current Sensing Using Resonant
Capacitor Voltage
5.5 Over-Current Protection (OCP)
: When the
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 Abnormal Over-Current Protection (AOCP): If
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
CC
is pulled down below 5V.
LV
CC
good
12
11/ 14V
V
RE F
In
t
ernal
Bias
LV
CC
good
LV
CC
OLP
OVP
+
-
-Q
Q
R
S
F/F
LV
CC
<5V
La tch
prote
c
tion
-Q
Q
R
S
F/F
A
uto-
r
es
t
art
protection
Shutdo
w
n
CON
20k
OCP
A
OCP
TSD