NCP1207A

http://onsemi.com

10

Demagnetization Detection

The core reset detection is done by monitoring the voltage

activity on the auxiliary winding. This voltage features a

FLYBACK polarity. The typical detection level is fixed at

50 mV as exemplified by Figure 17.

POSSIBLE

RE−STARTS

50 mV

Figure 17. Core reset detection is done through a

dedicated auxiliary winding monitoring

7.0

5.0

3.0

1.0

−1.0

0 V

Figure 18. Internal Pad Implementation

TO INTERNAL

COMPARATOR

Aux

ESD2

ESD1

4

5

2

4

1

3

1

An internal timer prevents any re−start within 8.0

ms

further to the driver going−low transition. This prevents the

ms)) but also

avoid false leakage inductance tripping at turn−off. In some

cases, the leakage inductance kick is so energetic, that a

slight filtering is necessary.

The 1207 demagnetization detection pad features a

specific component arrangement as detailed by Figure 18. In

this picture, the zener diodes network protect the IC against

any potential ESD discharge that could appear on the pins.

The first ESD diode connected to the pad, exhibits a parasitic

capacitance. When this parasitic capacitance (10 pF

and the possibility to switch right in the drain−source wave

exists. This guarantees QR operation with all the associated

calculated to limit the maximum current flowing through

pin 1 to less than +3 mA/−2 mA. If during turn−on, the

auxiliary winding delivers 30 V (at the highest line level),

/ 2 mA = 14.6 k

W. This value will be further increased to

introduce a re−start delay and also a slight filtering in case

of high leakage energy.

power.

Figure 19. The NCP1207A Operates in

Borderline / Critical Operation

400

300

200

100

0

Overvoltage Protection

The overvoltage protection works by sampling the plateau

voltage 4.5

ms after the turn−off sequence. This delay

guarantees a clean plateau, providing that the leakage

inductance ringing has been fully damped. If this would not

be the case, the designer should install a small RC damper

across the transformer primary inductance connections.

Figure 20 shows where the sampling occurs on the auxiliary

winding.

Figure 20. A voltage sample is taken 4.5

ms after

the turn−off sequence

8.0

6.0

4.0

2.0

0

SAMPLING HERE

4.5

ms

When an OVP condition has been detected, the

NCP1207A enters a latchoff phase and stops all switching

operations. The controller stays fully latched in this position

is cycled down 4 V, e.g. when the user unplugs the power

supply from the mains outlet.

By default, the OVP comparator is biased to a 5 V

reference level and pin1 is routed via a divide by 1.44

network. As a result, when Vpin 1 reaches 7.2 V, the OVP

comparator is triggered. The threshold can thus be adjusted

by either modifying the power winding to auxiliary winding

turn ratios to match this 7.2 V level, or insert a resistor from

Pin 1 to ground to cope with your design requirement.