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NCP1251

Current-Mode PWM Controller for Off-line Power Supplies

ON Semiconductor 

NCP1251

enough current (30 mA) at low line will turn a converter in

fault into an auto-recovery mode since the SCR won’t

remain latched. To build a sufficient design margin, we

recommend to keep at least 60 mA flowing at the lowest input

line (80 V rms for 85 V minimum for instance). An excellent

solution is to actually combine X2 discharge and start-up

networks as proposed in Figure 13 of application note

AND8488/D.

Internal Over Power Protection

There are several known ways to implement Over Power

Protection (OPP), all suffering from particular problems.

These problems range from the added consumption burden

on the converter or the skip−cycle disturbance brought by

1 v(24)

the current−sense offset. A way to reduce the power

capability at high line is to capitalize on the negative voltage

swing present on the auxiliary diode anode. During the

power switch on−time, this point dips to −NVin, N being the

turns ratio between the primary winding and the auxiliary

winding. The negative plateau observed on Figure 42 will

have an amplitude dependant on the input voltage. The idea

implemented in this chip is to sum a portion of this negative

swing with the 0.8 V internal reference level. For instance,

if the voltage swings down to −150 mV during the on time,

then the internal peak current set point will be fixed to 0.8 −

0.150 = 650 mV. The adopted principle appears in Figure 42

and shows how the final peak current set point is

constructed.

40.0

off−time

20.0

N1(Vout +Vf)

1

0

−20.0

−N2Vbulk

−40.0

on−time

464u

472u

480u

488u

496u

time (s)

Figure 41. The Signal Obtained on the Auxiliary Winding Swings Negative During the On−time

Let’s assume we need to reduce the peak current from

2.5 A at low line, to 2 A at high line. This corresponds to a

20% reduction or a set point voltage of 640 mV. To reach this

level, then the negative voltage developed on the OPP pin

must reach:

VOPP + 640m * 800m + −160 mV (eq. 6)

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