PFC SPM
®
Design Guide
© 2005
Rev. 1.0.1-2012
Fairchild Semiconductor – System Engineering Group
3
1. System Configurations
Control IC
V
ac
N /F
R elay
LVIC
V
TH
R
TH
S
R
P
R
N
N
SENSE
V
AC-
Shunt
Resistor
NTC
Thermistor
SPM
IN
(S)
IN
(R)
Fig.1 Typical block diagram of PFC SPM® system
An inrush-current prevention circuit is required due to the large DC link capacitance as shown in Fig. 1.
The relay of the circuit should be closed after DC link capacitor is charged far enough. PFC SPM®, mini-
SPM and control IC can share single GND stage. Usually, this GND and the N
SENSE
terminal of PFC SPM
should have the same potential. Large surge voltage is easily produced between P and N terminals by large
current switching. To reduce surge voltage it is important to shorten the DC link bus wiring between PFC
SPM and DC link capacitor. In addition, good high frequency characteristic capacitor, such as polypropylene
film capacitor should be mounted near to P and N terminals as a snubber.
2. Protection Circuits
Following Fig. 2 shows the timing chart of protection function. There are two kind of protection level
for both OCP and OVP. Generally, PFC control ICs have its own OCP and OVP function. Also, user can
make the PFC SPM® stop and output the FO signal under preset OC, OV condition using its Csc input.
Over Current Protection (OCP)
[OCP Level1 –PFC SPM®]
PFC SPM can protect from over current situation. When OC(over current)
situation happens, the PFC SPM stops operating and generates fault out signal during fault-out duration
time(set by C
FOD
). And then after the duration, it works again according to the input command. Its total
propagation delay time may depend on outer op-amp speed. We recommend using a low cost slow op-amp
solution with fast protection. It is the OCP level2 protection described in next paragraph.
[OCP Level2 (SCP) –PFC control IC]
By the peak current limit function of PFC control IC, the system
is protected from SC(Short Circuit) situation. The recommended current limit of OCP level 2 is higher than
that of OCP level 1. It doesn’t generate the fault out signal but its response is very fast. It will protect the
system from short circuit situation during the propagation delay time of OCP level1.
