400EAFBA0 datasheet(36/47 Pages) ONSEMI

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NCP12400

www.onsemi.com

Internal Overpower Protection

The power delivered by a flyback power supply is

proportional to the square of the peak current in

discontinuous conduction mode:

POUT +

@ h @ LP @ FSW @ IP 2

(eq. 3)

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, leading to a significant

difference in the maximum output power delivered by the

power supply.

Figure 67. Needs for Line Compensation For True Overpower Protection

time

Ipeak

High

Line

Low

Line

ILIMIT

tdelay

DIpeak to be

compensated

To compensate this and have an accurate overpower

protection, an offset proportional to the input voltage is

added on the CS signal by turning on an internal current

source: by adding an external resistor 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 resistor.

But this offset is unwanted to appear when the current

sense signal is small, i.e. in light load conditions, where it

would be in the same order of magnitude. Therefore the

compensation current is only added when the FB voltage is

higher than VFB(OPCE). However, because the HV pin is

being connected to ac voltage, there is needed an additional

circuitry to read or at least closely estimate the actual voltage

on the bulk capacitor.

Figure 68. Overpower Protection Current Relation to Feedback Voltage

VFB

IOPC

VFB(OPCE)

VFB(OPCF)

VHV

Figure 69. Overpower Protection Current Relation to Peak of Rectified Input Line AC voltage

VHV

IOPC

365 V

125 V

IOPC(125)

IOPC(365)

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