Problem in Flyback Transformer

Hi friends,

I am trying to design a Flyback converter(Vin=15 volts, Vout=15 volts, fsw=140kHz) in general purpose PCB. The Mosfet(IRF510) is being driven from TL494. I have checked that the mosfet is being successfully driven by TL494. But after connecting the transformer(E 16/7/6) in sec side, I observed some notches for usec duration instead of a square wave voltage. I have checked the xformer separately after winding it by giving signal from a SIGNAL GENERATOR, where the output is coming correctly.
Can anyone suggest me any tests that can be done to sort out the cause of the xformer malfunctioning, which will be of great help to me.
Thanks in advance.

Sumanta

Start by posting your schematic and scrrenshot of the waveform adn where and how you measured it.

Thanks for the information. In the next post I will attach the image of the hardware ckt.