NCP1381 ONSEMI [ON Semiconductor], NCP1381 Datasheet - Page 14

• Manufacturer Part Number: NCP1381
• Description: Low−Standby High Performance PWM Controller
• Manufacturer: ONSEMI [ON Semiconductor]

to let the converter bring the peak current up to 3.15 A at low
mains (200 VDC in follower−boost configuration).
Unfortunately, in high mains conditions, where the PFC
delivers up to 400 VDC, the controller will also allow the
same 3.15 A maximum peak current (even a little more with
the propagation delay) and the power will dramatically
increase. In these conditions, the maximum power shall
absolutely be clamped in order to avoid lethal runaways in
presence of a fault. If overpower compensation via a resistor
to the bulk capacitor offers a possible way, it suffers from the
lack of precision and good repeatability in production. It also
degrades the standby consumption.
protection that permanently senses the bulk capacitor, we
naturally have a voltage image of the bulk voltage. By
converting the BO level into a current, then routing this
current in the current sense (CS) pin, we can easily create a
As a result, we will probably calculate our sense resistor
Since our controller integrates a brown−out (BO)
130
125
120
105
100
Figure 18. Output Power Evolution with the Input
115
110
3.2
3.1
3.0
2.9
2.8
2.7
2.6
2.5
200
200
Voltage in a QR Converter at Constant Output
Figure 16. Peak Current Evolution with Input
Voltage (No Compensation)
250
250
Power (100 W)
V
V
in
in
, VOLTAGE (V)
, VOLTAGE (V)
Required I
300
300
Pmax
350
350
http://onsemi.com
400
400
NCP1381
14
variable offset that will compensate the maximum output
power. This would result in a variable I
by the dashed line on Figure 16.
power variation, with a fixed peak current (the maximum
peak the controller will authorize is 0.8 / R
quantify the difference between low and high line:
where
t
If
noncompensated output power definition and plot the result
versus the input voltage, then we obtain the following graph,
Figure 18:
P
From the peak current definition, we can extract the output
is the propagation delay (100 ns typically).
P nc (V in ) :+
I
P
we
Figure 19. A Possible Way to Compensate the
4
3
2
1
0
0
Current Excursion Lies in Offsetting the
enter
Figure 17. I
(h@(V in @(V out )V F )))
20
our
Current Floor
2
P
previous
Evolution with Output Power
40
0.8
R S
)
@ V out ) V F )
P
V in
L P
O
@ t P
parameters
60
Pmax
as exemplified
sense
LL
V in
80
N
) and thus
into
0.8 V
0.64 V
(eq. 4)
100
the
t