Why my IRF is overheating ?

[ydanzig]

i am trying to understand why my IRF is getting very hot..
as you can see.. Vd is getting very high.
and when it's off.. it's not fully off.
also if the freq is lower you can see some signal swinging.
why is that? why the high peak? why the swinging?
somthing with the inside diode?
hope you can help me to understand this and mayb to solve the problem.
thank you,
yoni

 

[Zekeymonkey]

IRF GETTING HOT.

Yoni,

Let me see if I can help you. First, let me ask you a few question. Is there any form of snubber or voltage clamp in parallel with the flyblack transformer? Also, what is the turns ratio of the flyback? Finally, do you have a more complete schematic?

 

[xaccto]

Re: IRF GETTING HOT.

Vd high peaks is as expected when no snubber in the primary circuit.

when Vg switches on, notice the Vd dropping towards 0V, then ramps up as current flows
through the inductor.
When Vg switches off, the circuit current must continue in the opposite direction, so it
rises until the Max Vds of the mosfet is reached, then it conducts across the mosfet. It continues to ring between that and the body diode allowing conduction also.

So, put a snubber circuit in.

You should also investigate Vgs not going fully to 0V, it needs to if you want any
decent efficiency.
search for "totem pole driver"