flyback mosfet problem

[electros6]

I tried to design a 48 v to 12v fly back converter using PWM controller. When I connect the transformer and power up the circuit even when the gate to the mosfet is removed the mosfet conduct and short the source.I used 34v dc from a transformer  and rectifier. I tried different voltage mosfet eg IRF840 , IRF540 , IRF44n ctc also ex-bit the same characteristics. can somebody spot out

[Marco]

"even when the gate to the mosfet is removed the mosfet conduct"

You need to short the gate to source to be sure it is off and stays off.

[Seekonk]

That is called phantom switching.  Even if you short the gate, a FET will draw power when it is first applied.  This is due to the Miller Effect.  Generally it is not a problem.  In really high power switchers it can be overcome by using a negative bias in the gate.  The "phantom gate voltage" pulse still  happens but it remains too low to turn on the FET.

[planet12]

It's very hard to know what's going on without seeing a schematic - can you provide one?

Along with the miller capacitance pulling the gate up, as discussed above, the other option is possible reversal of the drain-source pins, meaning the body diode will conduct - this would also cause the symptoms you're seeing.