flyback converter help 2nd try on this forum

Flyback Converter-Need Help

hi all,
we are designing flyback converter with following parameter:
input voltage:230V
output voltage:5v
frequancy:70Khz
Diode: Fast Recovery Diode B860
transformer: primary turns:88
secondary turns:2

problem is that switch or transformer get heated when we apply voltage above 90(at that time current is 0.95A). we intechange secondary terminal also(strange thing is that we got output voltage for both the case when applied voltage is low). we Increase Frequancy up to 120Khz & duty ratio 30 to 70% to avoid core saturation. but nothing improvement.
I have use so many MOSFET 220V to 1000V(IRFP250N IRF 450 then 2SK1120) but all fail.
I have add 1k resistor between gate & Source.
also add RCD snubber to clamp but nothing could be help.
every time either MOSFET or transformer get heated when apply voltage above 80to 100V.
I checked Vdss voltage it was 40 V when my o/p is around 0.001V.
we have made hand made transformer with air-gap 2-3mm
plz help to solve the problem

Some statements are simply mysterious, e.g.

I checked Vdss voltage it was 40 V when my i/p is around 0.001V.

Click to expand...

Where do you see 0.001V?

Some information has never been mentioned in your posts, like how are you driving the switch transistor? Some points are confusing, e.g. interchanging windings terminals as if you won't know the actual winding orientation.

Observing transformer heating suggests either core saturation or a windings short. But whatever the problem is, it should be able to figure it out by looking at the voltage and current waveforms.

FvM said:

Some statements are simply mysterious, e.g.

Where do you see 0.001V?

Some information has never been mentioned in your posts, like how are you driving the switch transistor? Some points are confusing, e.g. interchanging windings terminals as if you won't know the actual winding orientation.

Observing transformer heating suggests either core saturation or a windings short. But whatever the problem is, it should be able to figure it out by looking at the voltage and current waveforms.

Click to expand...

i see 0.001V at output side across resistor. it was increase & reach to 5V when voltage at input side increase.
I have drive mosfet using ir2110 driver. generate frequency using p89v51rd2.
i have 4 to 5 transformer some i dissemble in smps of computer. so i don't know the polarity so i used trial & error method in secondary side(for some transformer). but i get the o/p voltage in both the case.

I have just sent you a link to a book which might help.

suhelshaikh21 said:

i see 0.001V at output side across resistor. it was increase & reach to 5V when voltage at input side increase.
I have drive mosfet using ir2110 driver. generate frequency using p89v51rd2.
i have 4 to 5 transformer some i dissemble in smps of computer. so i don't know the polarity so i used trial & error method in secondary side(for some transformer). but i get the o/p voltage in both the case.

Click to expand...