NCP1238
http://onsemi.com
25
Compensation for Overpower Detection
The power delivered by a flyback power supply is
proportional to the square of the peak current:
P
OUT
+
1
2
@ h @ L
P
@ F
SW
@ I
P
2
(eq. 1)
(in discontinuous conduction mode).
Unfortunately, due to the inherent propagation delay of
the logic, the actual peak current is higher at high input
voltage than at low input voltage, as shown in Figure 50.
This leads to a significant difference in the maximum output
power delivered by the power supply.
Figure 50. Line Compensation for True Overpower Protection
time
I
P
High
Line
Low
Line
I
LIMIT
t
delay
t
delay
I
P
to be
compensated
To compensate this and have an accurate overpower
protection, an offset proportional to the input voltage is
added to the CS signal by turning on an internal current
source (I
OPC
): by adding an external resistor (R
OPC
) in
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
OPC
resistor.
Since in light load conditions this offset is in the same
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
FB(OPC)
, as shown in
Figure 52.
Figure 51. Schematic Overpower Compensation Circuit
HV
V to I
CS
t LEB
blanking
To CS
block
VDD
I
OPC
= 0.5m x
(VHV − 125)
FB
−
+
+
V
FB(OPC)
HV Sensing
R
OPC
R
sense
