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NCP1251

Current-Mode PWM Controller for Off-line Power Supplies

ON Semiconductor 

NCP1251

15 V. Under 3 mA and neglecting the series diode forward

drop, it requires a series resistor of:

ROVP

+ Vlatch * VVOP + 18 * 3 + 15 + 5 kW

VOVPńROPPL

3ń1k 3m

(eq. 16)

In nominal conditions, the plateau establishes to around

14 V. Given the divide−by−6 ratio, the OPP pin will swing

to 14/6 = 2.3 V during normal conditions, leaving 700 mV

margin. A 100 pF capacitor can be added between the OPP

pin and GND to improve noise immunity and avoid erratic

trips in presence of external surges. Do not increase this

capacitor too much otherwise the OPP signal will be affected

by the integrating time constant.

A second solution for the OVP detection alone, is to use

a Zener diode wired as recommended by.

D3

15V

11

ROPPU

421k

D2

1N4148

OPP

10

4

ROPPL

C1

1k

5

22pF

Vlatch

1

OVP

OPP

VCC

9

8

aux.

winding

Figure 48. A Zener Diode in Series with a Diode Helps to Improve the Noise Immunity of the System

For this configuration to maintain an 18 V level, we have

selected a 15 V Zener diode. In nominal conditions, the

voltage on the OPP pin is almost 0 V during the off time as

the Zener is fully blocked. This technique clearly improves

the noise immunity of the system compared to that obtained

from a resistive string as in Figure 47. Please note the

reduction of the capacitor on the OPP pin to 10 pF − 22 pF.

This capacitor is necessary because of the potential spike

coupling through the Zener parasitic capacitance from the

bias winding due to the leakage inductance. Despite the 1 ms

blanking delay at turn off. This spike is energetic enough to

charge the added capacitor C1 and given the time constant,

could make it discharge slower, potentially disturbing the

blanking circuit. When implementing the Zener option, it is

important to carefully observe the OPP pin voltage (short

probe connections!) and check that enough margin exists to

that respect.

Over Temperature Protection

In a lot of designs, the adapter must be protected against

thermal runaways, e.g. when the temperature inside the

adapter box increases above a certain value. Figure 49

shows how to implement a simple OTP using an external

NTC and a series diode. The principle remains the same:

make sure the OPP network is not affected by the additional

NTC hence the presence of this isolation diode. When the

NTC resistance decreases as the temperature increases, the

voltage on the OPP pin during the off time will slowly

increase and, once it passes 3 V for 4 consecutive clock

cycles, the controller will permanently latch off.

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