Data Sheet

© 2010 Fairchild Semiconductor Corporation www.fairchildsemi.com
FSFR2100 • Rev.1.1.0 11
FSFR2100 — Fairchild Power Switch (FPS™) for Half-Bridge Resonant Converters
5. Current Sensing
Current Sensing Using Resistor: FSFR2100 senses
drain current as a negative voltage, as shown in Figure
24 and Figure 25. Half-wave sensing allows low power
dissipation in the sensing resistor, while full-wave
sensing has less switching noise in the sensing signal.
Figure 24. Half-Wave Sensing
Figure 25. Full-Wave Sensing
Current Sensing Using Resonant Capacitor Voltage:
For high-power applications, current sensing using a
resistor may not be available due to the severe power
dissipation in the resistor. In that case, indirect current
sensing using the resonant capacitor voltage can be a
good alternative because the amplitude of the resonant
capacitor voltage (V
cr
p-p
) is proportional to the resonant
current in the primary side (I
p
p-p
) as:
2
p
p
p
pp
Cr
s
r
I
V
f
C 



(6)
To minimize power dissipation, a capacitive voltage
divider is generally used for capacitor voltage sensing, as
shown in Figure 26.
delay d d
TRC
pk
sense B
pp
Cr sense B
VC
VCC



2
pk
sense
CON
V
V
Figure 26. Current Sensing Using Resonant
Capacitor Voltage
6. Protection Circuits
The FSFR2100 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 27.
6.1 Auto-restart Mode Protection: 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 11.3 V, the protection is reset. The FPS
resumes normal operation when LV
CC
reaches the start
voltage of 14.5 V.
Control
IC
CS
SG PG
Ns
Np
Ns
R
sense
Ids
Cr
I
ds
V
CS
V
CS
Control
IC
CS
SG
PG
R
sense
Ids
V
CS
I
ds
V
CS
Ns
Np
Ns
Cr