(a)
(b)
( a) (b) (a)
(b)
It is typical to set the initial frequency of soft-start two to
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:
LVCC
VAR
VCssH
VCssL
τ = RSS •CSS
(4)
ICr
tstop
tS/S
(a) Protection s are trigge red, (b) F SF R-U S restarts
Figure 21. Self Auto-Restart Operation
5. Protection Circuits: The FSFR-XS series has several
self-protective functions, such as Over-Current Protection
(OCP), Abnormal Over-Current Protection (AOCP), Over-
Voltage Protection (OVP), and Thermal Shutdown (TSD).
These protections are auto-restart mode protections, as
shown in Figure 22.
Figure 19. Frequency Sweeping of Soft-Start
4. Self Auto-Restart: The FSFR-XS series can restart
automatically even though any built-in protections are
triggered with external supply voltage. As can be seen in
Figure 20 and Figure 21, once any protections are
triggered, the M1 switch turns on and the V-I converter is
disabled. CSS starts to discharge until VCss across CSS
drops to VCssL. Then, all protections are reset, M1 turns
off, and the V-I converter resumes at the same time. The
FSFR-XS starts switching again with soft-start. If the
protections occur while VCss is under VCssL and VCssH
level, the switching is terminated immediately, VCss
continues to increase until reaching VCssH, then CSS is
discharged by M1.
Once a fault condition is detected, switching is terminated
and the MOSFETs remain off. When LVCC falls to the LVCC
stop voltage of 10V or AR signal is HIGH, the protection is
reset. The FSFR-XS resumes normal operation when
LVCC reaches the start voltage of 12.5V.
Figure 22. Protection Blocks
5.1 Over-Current Protection (OCP): When the
sensing pin voltage drops below -0.58V, OCP is
triggered and the MOSFETs remain off. This protection
has a shutdown time delay of 1.5µs to prevent
premature shutdown during startup.
Figure 20. Internal Block of AR Pin
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 is triggered. AOCP is triggered
without shutdown delay if the sensing pin voltage
drops below -0.9V.
After protections trigger, FSFR-XS is disabled during the
stop-time, tstop, where VCss decreases and reaches to
VCssL. The stop-time of FSFR-XS can be estimated as:
tSTOP = CSS •{(RSS + RMIN
)
|| 5kΩ
}
(5)
The soft-start time, ts/s can be set as Equation (4).
© 2010 Fairchild Semiconductor Corporation
FSFR-XS Series • Rev.1.0.1
www.fairchildsemi.com
10
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