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NCP1631
depending on the presence or absence of an effective
in ? rush limiting circuitry. If the MOSFET turns on during
this severe transient, it may be over ? stressed and finally
damaged. That is why, the NCP1631 permanently monitors
the input current and delays the MOSFET turn on until the
in ? rush current has vanished. This is the function of the I CS
comparison to the I in ? rush threshold (14 m A typical). When
I CS exceeds I in ? rush , the comparator output (“In ? rush”) is
high and prevents the PWM latches from setting (see block
diagram). Hence, the two drivers cannot turn high and the
MOSFETs cannot switch on. This is to guarantee that the
MOSFETs remain off as long as if the input current exceeds
10% of its maximum value. This feature protects the
MOSFETs from the possible excessive stress it could suffer
from if it was allowed to turn on while a huge current
flowed through the coil as it can be the case at start ? up or
during an over ? load transient.
The propagation delay (I CS < I in ? rush ) to (drive outputs
high) is in the range of few m s.
However when the circuit starts to operate, the NCP1631
disables this protection to avoid that the current produced
by one phase and sensed by the circuit prevents the other
branch from operating. Practically, some logic grounds the
In ? rush protection output when it detects the presence of
current cycles with a zero current detection signal provided
by the auxiliary winding (Figure 13).
EMI
Filter
V IN
C IN
I IN
Curr e nt
Mirror
I CS
I ILIM1 = 210 m A
OC P
V aux1
L 1
V aux2
L 2
D 1
D 2
V OUT
CS
I CS
I in ? rush = 14 m A
The pin voltage
is maintained
to 0 V
9
R OCP
I CS
Negative clamp
(I CS is proportional to the coil current)
I CS (from ZCD
I CS I n ? rush block)
Q ZCD1
Q ZCD2
DRV 1
DRV 2
M 1
DRV 1
M 2
DRV 2
C BULK
R CS
I IN
The CS block performs the over ? current protection and the in ? rush current detection.
Figure 13. Current Sense Block
Over ? Voltage Protection
While PFC circuits often use one single pin for both the
Over ? Voltage Protection (OVP) and the feed ? back, the
NCP1631 dedicates one specific pin for the under ? voltage
and over ? voltage protections. The NCP1631 configuration
Vout (bulk voltage)
allows the implementation of two separate feed ? back
networks (see Figure 15):
1. One for regulation applied to pin 2.
2. Another one for the OVP function (pin 8).
Vout (bulk voltage)
Rout1
FB
1
2
16
15
Rout1
FB
1
2
16
15
Rout3
3
4
14
13
Rovp1
Rout2
3
4
14
13
5
12
5
12
Rout2
OVP
6
7
8
11
10
9
Rovp2
OVP
6
7
8
11
10
9
Figure 14. Configuration with One Feed ? back
Network for Both OVP and Regulation
http://onsemi.com
14
Figure 15. Configuration with Two
Separate Feed ? back Networks
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