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FET Killer

Discussion in 'General Electronics Chat' started by kinarfi, Nov 20, 2009.

  1. kinarfi

    kinarfi Well-Known Member

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    Why is this killing my FETS and then shorting out the power supply?

    EDIT
    First drawing was bad, second is correct, supplied by Sceadwian.

    What happens is I close the switch and I end up with a short across the power supply, I open the switch and the circuit is on and the FET is dead, Shorted.

    Thanks,
    Kinarfi
     

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    Last edited: Nov 22, 2009
  2. Sceadwian

    Sceadwian Banned

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    Because the FET is in the wrong place, you're running it as a high side fet, which means you need a higher gate drive than your VCC can provide to keep it on when the load is using current. You need to connect the source directly to ground, with the load on the drain side, then it will work properly.

    The gate on voltage requirement of a FET is ALWAYS related to it's source voltage, in your case you have a load on the source side which means it forms a voltage divider network, so the source voltage is elevated above ground requiring a higher gate voltage that otherwise needed.
     
    Last edited: Nov 20, 2009
  3. Sceadwian

    Sceadwian Banned

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    Your schematic is also missing +/- terminals on the power source so you could have it hooked up backwards for all we know.
     
  4. dave

    Dave New Member

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  5. Sceadwian

    Sceadwian Banned

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    Wait a second.. I just looked at that again. You DO have it hooked up backwards AND the fet is in the wrong place. Where did you get that circuit from? I'll post a corrected on shortly.
     
  6. Sceadwian

    Sceadwian Banned

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    Something like this.

    That's the basics of it at least.
     

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  7. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A Mosfet works best when it has something to turn it off.
    The original schematic is a hopeless mess.
     
  8. Sceadwian

    Sceadwian Banned

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    It's not hopeless, it's just completely wrong. The basic ideas are there, the voltage source connected to the fet, the load on one side of it, and even a gate resistor, just hooked up completely wrong, there's even a switch there attached to the gate to turn it on/off (his intention not the practical circuit)

    The poster obviously has no idea what they're doing so lets explain it and move on without the wry commentary.
     
    Last edited: Nov 20, 2009
  9. Sceadwian

    Sceadwian Banned

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    We are a school for cooks. The poster had all the correct ingredients, just baked em wrong. My post more than well sufficed to inform the poster of the original errors and correct them so we don't need to be arguing in this thread or even saying anything else until the original poster comments on the original post and the responses. If they still have no clue what's going on or express a desired to learn then abandon the post, otherwise all we're gonna do is get wry comments and useless messages.
     
  10. Roff

    Roff Well-Known Member

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    Sceadwian, your circuit is missing a resistor from gate to ground. The FET may never turn off when the switch is opened.
     
  11. Sceadwian

    Sceadwian Banned

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    I knew I should have put that in =P This is what I get for quick and dirty =) Didnt realize I forgot it till I'd posted it, and the OP already had a gate resistor, was hoping they'd figure it out.
     
    Last edited: Nov 20, 2009
  12. kinarfi

    kinarfi Well-Known Member

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    Be nice you guys, I was in the middle of my reply and had to look at something and lost it all. Maul these over while I retype every thing, the FET was in correctly the drawing was incorrect. My next post will explain.
     

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    Last edited: Nov 21, 2009
  13. kinarfi

    kinarfi Well-Known Member

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    The black - and red + are the power supply output and it's turned down as far as it can be and I still toasted/ cooked:) to 100 amp FETs.

    Now explain this if you can, you may even want to verify it. Using a Fluke 87, set for diode check, black lead on source, red lead on drain. reading OL ---red lead to gate, reading OL ---- red lead back to drain, reading .042 v reverse red and black leads, reading is.021v ---- the gate cap got charged, easy to understand. Now some weirdness
    Black lead to drain, red to source, reading .510 -- the diode is forward biased, ----red lead to gate, read OL ---- red lead back to source, reading .022v ---- reverse leads, reading OL
    -- doesn,t conduct in both directions

    Explain that to me FET is IRF 2805
     
    Last edited: Nov 21, 2009
  14. Sceadwian

    Sceadwian Banned

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    Void please.
     
    Last edited: Nov 20, 2009
  15. kinarfi

    kinarfi Well-Known Member

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    In the photo, the blue wire is negative, the white wires go to a 10Kresistor, the red is to the on switch and tab is heat sinked, or sunk, with a terminal on it to the rest of the circuit.

    It is my guess that the gate gets reversed charge through the forward biased zener diode and when you turn the switch on, it kills the FET.

    thanks
    kinarfi

    It is my guess that the gate gets reversed charge through the forward biased zener diode and when you turn the switch on, it kills the FET.-----That don't make sense, does it!!!
     
    Last edited: Nov 22, 2009
  16. audioguru

    audioguru Well-Known Member Most Helpful Member

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    If you do not do it correctly then you might as well don't do it.
     

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  17. Sceadwian

    Sceadwian Banned

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    If your load is inductive you need to deal with the flyback voltage. Relying on the body diode will... well kill the FET.
     
    Last edited: Nov 20, 2009
  18. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You drew the Mosfet wrong because you had almost everything upside-down.
    The zener diode in your Mosfet was upside-down.

    You did not say what is your load. If it is inductive then the voltage spike it makes when it turns off should be surpressed with a diode.
     
  19. kinarfi

    kinarfi Well-Known Member

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    I was only driving and H bridge with nothing hooked to it, just checking the out put of some op-amps, and it has killed 2 IRF 3205 FETs so far. That's why I'm here, to see if I can get some ideas,
    Nice drawing Audioguru, I'll try to do better in the future.
     
  20. ke5frf

    ke5frf New Member

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    There is nothing wrong with being a newbie, everyone starts somewhere.

    Consider this though, an important philosophy in electronics is that knowledge is secondary to knowing what you DON'T know. When in doubt, seek the answers.

    That appears to be what the OP is doing, seeking answers. And with each answer, his knowledge will only grow.
     
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  21. unclejed613

    unclejed613 Well-Known Member

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    try limiting Vgs to 10V, using a voltage divider. with the 10k resistor you already have, a 4.7 k resistor going from the gate to the switch should suffice. the 4.7k resistor and the gate capacitance will also serve to debounce the switch (to debounce it better add a 1 nF or 10nF cap across the 10k resistor). there are noise spikes every time you operate the switch, which the FET amplifies, and the burst of noise could be reflected by the load back into the FET. what is your load? is it a motor or other highly inductive device? if so, you might get an inductive spike that could kill the FET. yes there is a body diode in the FET, but that doesn't automatically protect the gate from being sent outside the +/-20Volt Vgs limit. when you check the D-S "junction" of a MOSFET, the gate should always be connected to the source during the test. also you said the device was heatsinked, which in the photo, it's not...
     
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