Data Sheet

© 2010 Fairchild Semiconductor Corporation www.fairchildsemi.com
FSFR-XS Series • Rev.1.0.2 10
FSFR-XS Series — Fairchild Power Switch (FPS™) for Half-Bridge Resonant Converter
It is typical to set the initial frequency of soft-start two to
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:
SSSS
CR 
(4)
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. C
SS
starts to discharge until V
Css
across C
SS
drops to V
CssL
. 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 V
Css
is under V
CssL
and V
CssH
level, the switching is terminated immediately, V
Css
continues to increase until reaching V
CssH
, then C
SS
is
discharged by M1.
Figure 20. Internal Block of AR Pin
After protections trigger, FSFR-XS is disabled during the
stop-time, t
stop
, where V
Css
decreases and reaches to
V
CssL
. The stop-time of FSFR-XS can be estimated as:

 kRRCt
MINSSSSSTOP
5||
(5)
The soft-start time, t
s/s
can be set as Equation (4).
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.
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 10 V or AR signal is HIGH, the protection is
reset. The FSFR-XS resumes normal operation when
LV
CC
reaches the start voltage of 12.5 V.
Figure 22. Protection Blocks
5.1 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.
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 -0.9 V.
LV
CC
I
Cr
V
AR
t
stop
t
S/S
V
CssH
(a) (a)(a)(b) (b)
(a) Protections are triggered, (b) FSFR-US restarts
V
CssL
(b)