FAN7621B Fairchild Semiconductor, FAN7621B Datasheet - Page 12

• 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
To minimize power dissipation, a capacitive voltage
divider is generally used for capacitor voltage sensing,
as shown in Figure 27.
5.1 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
V
CON
V
sense
V
I
p
Figure 27. Current Sensing Using Resonant
Cr
C
DL
V
V
V
sense
Cr
p p
sense
CON
CS
−
RT
pk
=
SG
C
LV
sense
C
C
Capacitor Voltage
T
CC
sense
B
delay
+
HV
C
CTR
PG
HO
B
LO
=
CC
R C
d
d
100
V
sense
2
pk
C
=
V
B
I
p
CON
startup.
V
V
V
sense
sense
Cr
p-p
pk
pk
12
5.2 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
5.3 Overload Protection (OLP): Overload is defined as
the load current exceeding its normal level due to an
unexpected abnormal event. In this situation, the
protection circuit should trigger to protect the power
supply. However, even when the power supply is in the
normal condition, the overload situation can occur during
the load transition. To avoid premature triggering of
protection, the overload protection circuit should be
designed to trigger only after a specified time to
determine whether it is a transient situation or a true
overload situation. Figure 27 shows a typical overload
protection circuit. By sensing the resonant capacitor
voltage on the control pin, the overload protection can be
implemented. Using RC time constant, shutdown delay
can be also introduced. The voltage obtained on the
control pin is given as:
where V
voltage.
5.4 Over-Voltage Protection: (OVP): When the LV
reaches 23V, OVP is triggered. This protection is used
when auxiliary winding of the transformer to supply V
to the controller is utilized.
5.5 Thermal Shutdown (TSD): If the temperature of the
junction exceeds approximately 130°C, the thermal
shutdown triggers.
V
CON
=
2(
Cr
C
p-p
B
C
+
is the amplitude of the resonant capacitor
B
C
CC
sense
is pulled down below 5V.
)
V
Cr
p p
−
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CC
CC