FAN7621 Fairchild Semiconductor, FAN7621 Datasheet - Page 12

• 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
5.7
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.
5.8
LV
used when auxiliary winding of the transformer to supply
V
5.9
of the junction exceeds approximately 130 ° C, the
thermal shutdown triggers.
V
CON
CC
CC
to the controller is utilized.
=
reaches 23V, OVP is triggered. This protection is
2(
Cr
Overload
Over-Voltage Protection (OVP) : When the
Thermal Shutdown (TSD) : If the temperature
C
p-p
B
C
+
is the amplitude of the resonant capacitor
B
C
sense
)
V
Protection
Cr
p p
−
(OLP) :
Overload
(7)
is
12
6.
Duty imbalance problems may occur due to the radiated
noise from main transformer, the inequality of the
secondary-side
transformer, and so on. Among them, it is one of the
dominant reasons that the control components in the
vicinity of R
pattern on PCB layout. The direction of the magnetic
field on the components caused by the primary current
flow is changed when the high-and-low side MOSFET
turns on by turns. The magnetic fields with opposite
direction from each other induce a current through, into,
or out of the R
each MOSFET different. It is strongly recommended to
separate the control components in the vicinity of R
from the primary current flow pattern on PCB layout.
Figure 28 shows an example for the duty-balanced case.
The yellow and blue lines show the primary current flows
when the lower-side and higher-side MOSFETs turns on,
respectively. The primary current does not enclose any
component of controller.
It is helpful to reduce the duty imbalance to make the
loop configured between CON pin and opto-coupler as
small as possible, as shown in the red line in Figure 28.
Figure 28. Example for Duty Balancing
PCB Layout Guideline
T
pin are enclosed by the primary current flow
T
pin, which makes the turn-on duration of
leakage
inductances
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of
main
T
pin