Data Sheet

© 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com
FAN7621 • Rev. 1.0.3 10
FAN7621 — PFM Controller for Half-Bridge Resonant Converters
implemented by sweeping down the switching frequency
from an initial high frequency (
f
ISS
) until the output
voltage is established. The soft-start circuit is made by
connecting R-C series network on the R
T
pin, as shown
in Figure 19. FAN7621 also has an internal soft-start for
3ms to reduce the current overshoot during the initial
cycles, which adds 40kHz to the initial frequency of the
external soft-start circuit, as shown in Figure 20. The
initial frequency of the soft-start is given as:
min
5.2 5.2
( ) 100 40 ( )
ISS
SS
kk
f
kHz
RR
ΩΩ
=+×+
(3)
It is typical to set the initial (soft-start) frequency of two ~
three times the resonant frequency (
f
O
) of the resonant
network.
The soft-start time is three to four times the RC time
constant. The RC time constant is as follows:
SS SS SS
TRC=⋅
(4)
f
s
time
Control loop
take over
40kHz
f
ISS
Figure 20. Frequency Sweeping of Soft-Start
4. Control Pin
The FAN7621 has a control pin for protection, cycle
skipping, and remote on/off. Figure 21 shows the internal
block diagram for control pin.
OLP
LV
CC
good
6
CON
LV
CC
OVP
+
-
+
-
-Q
Q
R
S
Auto-restart
protection
+
-
0.4 / 0.6V
5V
23V
LV
CC
I
OLP
Stop Switching
Figure 21. Internal Block of Control Pin
Protection
: When the control pin voltage exceeds 5V,
protection is triggered. Detailed applications are
described in the protection section.
Pulse Skipping: FAN7621 stops switching when the
control pin voltage drops below 0.4V and resumes
switching when the control pin voltage rises above 0.6V.
To use pulse-skipping, the control pin should be
connected to the opto-coupler collector pin. The
frequency that causes pulse skipping is given as:
() kHz100x
R
k16.4
R
k2.5
maxmin
SKIP
+=
(5)
FAN7621
V
CC
LV
CC
RT
CON
CS
SG PG
CTR
HV
CC
HO
LO
R
max
R
min
R
SS
C
SS
Figure 22. Control Pin Configuration for
Pulse Skipping
Remote On / Off
: When an auxiliary power supply is
used for standby, the main power stage using FAN7621
can be shut down by pulling down the control pin
voltage, as shown in Figure 23. R1 and C1 are used to
ensure soft-start when switching resumes.
OP1
R
min
FAN7621
Main
Output
Main Off
Aux
Output
R1
C1
OP1
R
T
CON
Figure 23. Remote On / Off Circuit
5. Protection Circuits
The FAN7621 has several self-protective functions, such
as Overload Protection (OLP), Over-Current Protection
(OCP), Abnormal Over-Current Protection (AOCP),
Over-Voltage Protection (OVP), and Thermal Shutdown
(TSD). OLP, OCP, and OVP are auto-restart mode
protections; while AOCP and TSD are latch-mode
protections, as shown in Figure 24.
5.1 Auto-Restart Mode Protection: Once a fault
condition is detected, switching is terminated and the
MOSFETs remain off. When LV
CC
falls to the LV
CC
stop
voltage of 11.3V, the protection is reset. FAN7621