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flyback transformer working

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Arpit Gupta

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i designed a flyback transformer which is working in discontinous mode...
why does oscillations happens across drain to source of mosfet on the primary when secondary current goes zero?

and the output voltage drops drastically as load increases , its output is like 8v at load current 0.9A which was designed for 25 v at nominal load current of 500mA.

what could be the reasons?
 

When secondary current falls to zero, there may still be some residual magnetic energy stored in the core which leads to a ringing waveform in the primary.

This is a result of having less than 100% perfect coupling between primary and secondary. The looser the coupling, the more likely the voltage waveforms in primary and secondary will look different.

Discontinuous energy mode pumps fixed packets of energy into the secondary.
Under no load, the output voltage will be quite high.
As you load it up, the voltage falls, because the total available flyback energy each cycle is fixed.

The way to fix this is to use voltage feedback from the secondary to control the "on" time of the primary.
 
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