NCP1237 ONSEMI [ON Semiconductor], NCP1237 Datasheet - Page 18

• Manufacturer Part Number: NCP1237
• Description: Fixed Frequency Current Mode Controller for Flyback Converters
• Manufacturer: ONSEMI [ON Semiconductor]

Compensation for Overpower Detection
proportional to the square of the peak current:
(in discontinuous conduction mode).
protection, an offset proportional to the input voltage is
added to the CS signal by turning on an internal current
source (I
series between the sense resistor and the CS pin, a voltage
offset is created across it by the current. The compensation
can be adjusted by changing the value of the R
The power delivered by a flyback power supply is
To compensate this and have an accurate overpower
OPC
): by adding an external resistor (R
P
OUT
+
I
1
2
HV
LIMIT
FB
@ h @ L
I
P
Figure 13. Line Compensation for True Overpower Protection
P
Figure 14. Schematic Overpower Compensation Circuit
@ F
+
V
High
Line
FB(OPC)
SW
+
−
@ I
P
2
HV Sensing
OPC
t
delay
OPC
resistor.
http://onsemi.com
(eq. 1)
) in
To CS
block
18
blanking
the logic, the actual peak current is higher at high input
voltage than at low input voltage, as shown in Figure 13.
This leads to a significant difference in the maximum output
power delivered by the power supply.
order of magnitude as the current sense signal, it must be
removed. Therefore the compensation current is only added
when the FB voltage is higher than V
Figure 15.
t
t LEB
delay
Unfortunately, due to the inherent propagation delay of
Since in light load conditions this offset is in the same
I
OPC
(VHV − 125)
V to I
= 0.5m x
Low
Line
VDD
time
CS
compensated
I
P
R OPC
to be
R
sense
FB(OPC)
, as shown in