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6.2 Latch-Mode Protection: Once this protection is
triggered, switching is terminated and the MOSFETs
remain off. The latch is reset only when LV CC is
discharged below 5 V.
6.6 Over-Voltage Protection (OVP) : When the
LV CC reaches 23 V, OVP is triggered. This protection is
used when auxiliary winding of the transformer to supply
V CC to FPS? is utilized.
6.7 Thermal Shutdown (TSD) : The MOSFETs and
the control IC in one package makes it easy for the
control IC to detect the abnormal over-temperature of the
MOSFETs. If the temperature exceeds approximately
130 ? C, the thermal shutdown triggers.
7.
PCB Layout Guidelines
Duty unbalance problems may occur due to the radiated
noise from main transformer, the inequality of the
Figure 27. Protection Blocks
6.3 Over-Current Protection (OCP) : When the
sensing pin voltage drops below -0.58 V, OCP is
triggered and the MOSFETs remain off. This protection
has a shutdown time delay of 1.5 μs to prevent
premature shutdown during startup.
6.4 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 5 V.
6.5 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 26 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:
secondary side leakage inductances of main transformer,
and so on. Among them, it is one of the dominant
reasons that the control components in the vicinity of R T
pin are enclosed by the primary current flow 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 T
pin, which makes the turn-on duration of each MOSFET
different. It is highly recommended to separate the
control components in the vicinity of R T pin from the
primary current flow pattern on PCB layout. Figure 28
shows an example for the duty balanced case.
Figure 28. Example for Duty Balancing
V CON ?
C B
2( C B ? C sense )
V Cr p ? p
(7)
where V Cr
voltage.
p-p
is the amplitude of the resonant capacitor
? 2010 Fairchild Semiconductor Corporation
FSFR2100 ? Rev.1.1.0
12
www.fairchildsemi.com
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