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LM5002: Flyback topology SW Pin pulse waveform abnormal

Part Number: LM5002

Hi,

I designed a flyback power by LM5002. 

Vin: 24V

Vout: 200V

The transformer turns ratio is 1:5 and the primary inductor is 80uH

Switching frequency is 250KHz

Load is 100K ohm (0.4W)

C123 is ceramic capacitance

The schematic is as below(Picture 1). When i test Pin3 (SW Pin), the pulse waveform is abnormal(Picture 2).

But when i change input voltage from 24V to 20V, the pulse waveform is fine(Picture 3).(Could you help to check whether the waveform is normal?)

and when i change the switching frequency to 130KHz, the the pulse waveform is fine too(Picture 4).(Could you help to check whether the waveform is normal?)

So, could you help to explain the phenomenon and principle? why the two ways could solve the issue.

If other better solutions, please help to advise.

Picture 1. Schematic

Picture 2. Abnormal pulse waveform

Picture 3. Input Voltage 20V

Picture 4. Switching Frequency 130KHz

Thanks

Walsh