NCP1237 datasheet(18/34 Pages) ONSEMI

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NCP1237

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Compensation for Overpower Detection

The power delivered by a flyback power supply is

proportional to the square of the peak current:

POUT +

@ h @ LP @ FSW @ IP 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 13.

This leads to a significant difference in the maximum output

power delivered by the power supply.

Figure 13. Line Compensation for True Overpower Protection

time

High

Line

Low

Line

ILIMIT

tdelay

IP 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 (IOPC): by adding an external resistor (ROPC) 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 ROPC 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 VFB(OPC), as shown in

Figure 15.

Figure 14. Schematic Overpower Compensation Circuit

V to I

t LEB

blanking

To CS

block

VDD

IOPC = 0.5m x

(VHV − 125)

−

VFB(OPC)

HV Sensing

ROPC

Rsense

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