NCP1207A, NCP1207B

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10

An optocoupler is generally used to transfer the feedback

information to the FB pin while providing the necessary

isolation. It introduces a limitation in how low the skip level

can be adjusted since an optocoupler cannot pull the FB

voltage below its Vce(sat), which is usually around 150 mV.

Therefore, in order to take into account temperature and

process variations, it is not recommended to set up the skip

level below 250 mV, which corresponds to a minimum

resistor Rskip of 420

W. The 150 mV is a much lower level

than what will usually be used (it sets the peak current when

entering skip mode at 5% of the maximum peak current).

If anyway a lower skip threshold is needed, care must be

taken to select an optocoupler with a Vce(sat) guaranteed to

be below the chosen skip level with enough margin.

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

50mV 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

(NCP1207A) or 4.5

ms (NCP1207B) further to the driver

going-low transition. This prevents the switching frequency

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 NCP1207A/B 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 (10pF

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),

/ 2mA = 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 (NCP1207A) or 1.5 ms (NCP1207B)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.