NCP1308

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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.

Figure 17. Core Reset Detection is Done through

a Dedicated Auxiliary Winding Monitoring

POSSIBLE

RESTARTS

50 mV

7.0

5.0

3.0

1.0

−1.0

0 V

An internal timer prevents any restart within 10

μs

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 NCP1308 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

created and the possibility to switch right in the

drain−source wave exists. This guarantees QR operation

with all the associated benefits (low EMI, no turn−on losses

current flowing through Pin 1 to less than +3 mA / −2 mA:

if during turn−on, the auxiliary winding delivers −30 V (at

defined by: (−30 + 0.7. This value will be further increased

to introduce a restart delay and also a slight filtering in case

of high leakage energy.

Figure 18. Internal Pad Implementation

TO INTERNAL

COMPARATOR

Aux

ESD

ESD

4

1

Figure 19 portrays a typical Vds shot at nominal output

power.

Figure 19. The NCP1308 Operates in

Borderline/Critical Operation

400

300

200

100

0

Overvoltage Protection

comparator and a reference voltage. Figure 20 portrays the

internal arrangement:

Figure 20. OVP Section Circuitry

+

−

50 us

FILTER

+

16 V

OVP

COMPARATOR

TO LATCH

A 50

ms time−constant filter prevents any parasitic spikes

comparator. When the OVP comparator output goes high,

the NCP1308 fully latches off and stays latched, being

self−supplied by the DSS. The user must unplug the power

voltage of typically 4 V.