FAN7621B Datasheet, PDF (10/17 Pages) Fairchild Semiconductor – PFM Controller for Half-Bridge Resonant Converters

proportional to the switching frequency, the soft-start is
implemented by sweeping down the switching frequency
from an initial high frequency (f ^{I S S} ) until the output
voltage is established. The soft-start circuit is made by
connecting R-C series network on the RT pin, as shown
in Figure 19. FAN7621B 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:
f ^ISS = (5.2^kΩ + ^5.2kΩ ) ×100 + 40 (kHz)
(3)
R_min
R_SS
It is typical to set the initial (soft-start) frequency of two ~
three times the resonant frequency (fO) of the resonant
network.
The soft-start time is three to four times the RC time
constant. The RC time constant is as follows:
T_SS = R_SS ⋅ C_SS
(4)
f_s
f ^ISS
40kHz
Control loop
take over
time
Figure 20. Frequency Sweeping of Soft-Start
4. Control Pin: The FAN7621B has a control pin for
protection, cycle skipping, and remote on/off. Figure 21
shows the internal block diagram for control pin.
CON ⁶
LV_CC
I_OLP
-
0.4 / 0.6V
+
+
OLP
5V
-
LV_CC
+
23V
-
OVP
LV_CC good
Stop Switching
SQ
R -Q
Auto-restart
protection
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: FAN7621B 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:
SKIP ₌ ^5.2k ₊ ^4.16k _x100(kHz)
(5)
R _min R _max
V_CC
R_{m ax}
R_{m in}
R_SS
C_SS
LV_CC
HV_CC
RT
HO
CON
CTR
LO
CS
SG PG
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
FAN7621B 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.
Figure 23. Remote On / Off Circuit
5. Protection Circuits: The FAN7621B 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.
© 2009 Fairchild Semiconductor Corporation
FAN7621B • Rev. 1.0.1
10
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