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I was learning about power mosfet and i decided to try some switching circuits in multisim, so i chose a random n-channel Power mosfet and set up this circuit enter image description here

when 10V is connected to the gate 15 A flows and there is no problem, but when ground is connected, 2 A flows, i didn't know what's wrong so i tried again using IRFP351 ( and decreased 350 to 340 V) , which is ( according to the datasheet) the same but lower voltage rating ( 350 instead of 400) and it worked fine. So i have no idea what's the problem???, i am pretty sure it's not the setup because i used the same with IRFP351. Is there something about ratings i dont understand?

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  • \$\begingroup\$ uWhat defines the timing for SW1,SW2 to prevent shorting out the supply and prevent measurements during a floating gate. Look at a few good schematics to see how FET drivers are drawn. Other than this , no explanation possible for error in result. \$\endgroup\$ – Sunnyskyguy EE75 Sep 4 '17 at 16:17
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You have some sort of MultiSim error. Note that the RMS current is zero, and the pk-pk current is zero. Since pk-pk is zero, the current has no AC components, and the RMS current must be equal to the DC current. I have no idea why your simulator has barfed like this, but you should disregard this particular example.

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If current is flowing, then either you wired the MOSFET in the wrong direction (and the diode is forward conducting,) or your MOSFET is in "avalance breakdown" mode which means you over-volted it and it's eroding itself.

Assuming the wiring is correct, and assuming your 350V is DC (not AC!) then it's likely that your first device has been damaged.

Also, you're not specifying anything about your heat sinking solution, which is a critical part of this design. Look at the "Fig 8. - Maximum Safe Operating Area" diagram in the datasheet: http://www.vishay.com/docs/91225/91225.pdf It's on page four, in the lower-right corner. You do not get to push 16A through it when the voltage is at the maximum rated. This is because of internal heating that the internal construction of the device can't move away fast enough, even if you had an infinite heatsink (which you don't.)

Some other things to note:

The voltage rating you're looking at is "absolute maximum." Even a single volt over that rating MAY permanently damange the device, and even staying at maximum for any longer time may harm the device. It is always a good idea to specify with a fair bit of margin if you want robust circuits.

The resistance of the device with 10V gs is 300 mOhms. At 16 A, with 300 mOhms load, the device will need to dissipate 77 Watts. The 23 Ohm load will additionally need to dissipate about 6 kW. I'm assuming the resistive load is a heater of some sort, but unless you have a cooling solution for the MOSFET with a better (lower) rating than 1.5 C/W, you are likely to overheat the MOSFET (which, again, will damage it.)

You should always have a "strapping" resistor that sets the gate to a known value if none of the input switches are closed. A 47 kOhm resistor from gate to ground will make sure the device defaults off, safely, without dissipating much current while on. This also protects against accidental turn-on because of static build-up.

Additionally, it's usually a good idea to put a 12-15V Zener across the gate/source of a MOSFET, to protect the gate against any accidental voltage transients. (If robustness matters. Which it should!)

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