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One of my MOSFET source-drain shorted after I connect them in parallel?

So I was making an ARC speaker yesterday, and I used one MOSFET ( irf540), and it got really hot, but it did not blow up, so I decided to put another MOSFET connected in parallel, and before I connected them I have tested both MOSFET and both of them worked normally, but when I connected them in parallel, the whole setup shutdown immediately due to overloading, and I found that one of my MOSFET was source-drain shorted that's why the power supply shut down to protect itself.

How do you connect the MOSFETs? Just normal parallel circuit gate to gate, drain to drain, source to source, .connected with a piece of wire

PWM controller? TL494CN, powered by a battery, completely isolated from the MOSFET side

MOSFETs? IRF540 from IR

As you can see in this video there are two MOSFETs on the heat sink but the lead was disconnected because it was shorted. By the way this video is not specially made to show the setup so it would be more focused on the fly-back.

Here is the full schematic and the PWM board.

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    \$\begingroup\$ You MUST provide a circuit diagram showing your exact circuit and identify all components. A description of the transformer and the load also needed. You also need to explain "ARC speaker" as most people will not know what that is. | The transformer stores LARGE amounts of inductive energy. When it is turned off something else will "store" this energy. If you do not provide some means of dealing with it it may get "stored" [tm] in your MOSFET(s) - which is probably what happened. \$\endgroup\$ – Russell McMahon Oct 28 '15 at 4:39
  • \$\begingroup\$ Connecting MOSFETs in parallel will not damage them (though it's not advisable). The fact that one of the MOSFETs was shorted after you connected them in parallel was probably a coincidence, the root cause would be heat from previous overloads and / or soldering. \$\endgroup\$ – Dmitry Grigoryev Oct 28 '15 at 7:55
  • \$\begingroup\$ the circuit is right there now \$\endgroup\$ – Tonny Cassidy Oct 28 '15 at 9:34
  • \$\begingroup\$ The TL494 isn't capable of driving a MOSFET at a fast speed - what operating frequency are you running at? \$\endgroup\$ – Andy aka Oct 28 '15 at 10:06
  • \$\begingroup\$ not quite sure,around 20Khz \$\endgroup\$ – Tonny Cassidy Oct 28 '15 at 10:15
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A pretty normal explanation for this is as follows: -

You said the first MOSFET (on its own) got hot but continued working and my 1st observation is that you may not have been driving the gate "hard" enough. This would have caused some switching losses and possibly some conduction losses causing the device to get hot but it still worked.

So then you put another device in parallel and in doing so you have "shared" the gate drive current - the gate capacitance has effectively doubled and the rise time and fall time of the gate signal has doubled increasing switching losses for the device that begins to turn on first (not all FETs are identical). It rapidly overheats, melts and short circuits.

If you want this confirming, produce a circuit diagram of what you actually did.

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