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FET Killer

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

  1. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Your latest schematic has a 10k resistor for the load, not a light bulb.
    Where did you connect the light bulb?
     
  2. kinarfi

    kinarfi Well-Known Member

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    Sorry, I got distracted, grand kids spent the night, I meant to change it 100 ohm and I did on an edit.
    Hayato, I think you're referring the (PULL UP RESISTORS pictures from news photos on webshots), only a part of a schematic and had an explanation with it when I first used it, it was the lower left part that I was trying to use.
    I have put a 2K resistor in line with the gate of an N channel FET and use that FET to turn off and on the rest of the circuit. I know that switching the negative is considered a no no by some, but N FETs can handle a lot more current than P FETs, plus the polarity of the on/off signal, the key on a vehicle, but that is what I was investigating originally.
    Kinarfi
     
  3. kinarfi

    kinarfi Well-Known Member

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    Anyone want to look at another set of scope pictures,(13)?
    I CAN NOT get it stop oscillating!!! I have the totem with 10 ohm resistor (and I tried others) working good except every time I add a load to the FET, I get what's in the pictures. I think I read somewhere about optical isolators and something like a 9 nanosecond delay. May have to try it. FYI, as the on % out put of this circuit increases, it drives a motor which decreases the on % back to 0, any delays would be made up for by the nature of the beast.
    The green trace is the output to the gate to ground, yellow is source(ground) to drain.
    Some of the shots with letters are just pictures, shots with number are current flow, the following 2 are with the FET unloaded and loaded.
    Any ideas of how I can get this smoothed out. I'm getting frustrated.
    Thanks
    Kinarfi
     
    Last edited: Jun 25, 2014
  4. dave

    Dave New Member

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  5. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The schematic in post #28 is missing very important supply bypass capacitors. Without them the loaded Mosfets can oscillate.
    If you built the circuit on a breadboard then it will certainly oscillate.
     
    Last edited: Dec 8, 2009
  6. kinarfi

    kinarfi Well-Known Member

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    I did build it on a push the lead in the hole type bread board with full length lead on everything and the FET was about 10" from the board and another 10 or so inches from the the load,( sometime the head lamp out of a car, some time a 5, 2, or 1 ohm 50watt resistor or a combination of the 3, I even tried the motor with a 100 ohm 1/4 watt .68uf snubber and even let some smoke out of the resistor, because of the high freq, I assume.)
    Found a few more ideas to try, if all else fails, I modify a few key elements of the entire circuit and post it, see you guy can find my errors.
    Thanks for you help, thoughts and ideas,
    Kinarfi
     
  7. Hayato

    Hayato Member

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    Try to remove the capacitor snubber circuit.

    Use another kind of snubber.
     
  8. Sceadwian

    Sceadwian Banned

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    10 inches from the supply and 10 from the load without bypass caps and you're wondering why it's oscillating?
     
  9. audioguru

    audioguru Well-Known Member Most Helpful Member

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    There is your oscillator. The stray capacitance and inductance are far too high. Supply bypass capacitors might help.
     
  10. mneary

    mneary New Member

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    ROFF asked the question, but I am also eager to see a published example of this mysterious phenomenon.
     
  11. Roff

    Roff Well-Known Member

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    Thanks for bumping this question. I hate it when you challenge someone and they just totally ignore it.
     
  12. unclejed613

    unclejed613 Well-Known Member

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    i think you're right, i misread the data sheet. i saw the dv/dt listed on the line just above the gate charge spec.
    mental note to self: a new pair of reading glasses is in order ;)
     
  13. Roff

    Roff Well-Known Member

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    Thanks for coming back on this issue, unclejed613.:)
     
  14. kinarfi

    kinarfi Well-Known Member

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    I can't explain the "dv/dt" question, but it still sounds right and maybe I'm up in the night, but has any one built a replica circuit and tried it to see if it kills a Power FET on not, I do believe that it got two of mine and "dv/dt" just sound logical, Maybe we found something we should tell the industry about, maybe I've gone nut, yup, yup, yup ,yup!!!!!
    Got some more scope picts. that look absolutly great, same circuit, same scattered set up, no totem, different FET, IRF4905 P channel, Why does it work and the IRF2805 is giving me fits? I put my fluke on the gate and source and measured .0077uf, I thought that was interesting. Here's the 2nd of this last series, the name implies 2v across the motor and 1500 ma and it is turning over very slowly, most of the names refer to current through the motor, again, why is the P FET working so well and the N FET oscillating so much, may have to PWM the P side of my H bridge instead of the N side, Even if the N can handle more current, My next test may be to try a lower amperage FET. oh yea schottky diode and snubber protected, LM339 driven.
    Someone build that replica circuit and test it and let us know, Please, I wonder too, but won't risk another FET.
    Kinarfi
    [​IMG]
     
    Last edited: Dec 9, 2009
  15. kinarfi

    kinarfi Well-Known Member

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    Sorry Roff, I didn't think I had ignored your questione, I very strongly doubt static electricity got my FETs and as I said before the load was a test light, no induction and I,m sure it was connected correctly. I wanted to be able to turn my design on and off with the ignition key and feeding that voltage to the gate of a powerful N FET is the ticket for what I want, I just hadn't put a series resistor between the gate and key, just one from gate to source. Added a cap for switch bounce too.
    I didn't want to make a challenge, but I guess I did, for someone else to make the replica circuit and see if they kill a Power FET like I did, Twice.
    And I also said This is my guess as to what happens, and I DON'T KNOW
    kinarfi
     
    Last edited: Dec 9, 2009
  16. kinarfi

    kinarfi Well-Known Member

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    Roff, mneary, please reread my post, especially the last part. Please build circuit and test and report back.
    Thanks
    kinarfi
     
  17. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Your problem is that the circuit is built on a breadboard and the supply bypass capacitors are missing.
     
  18. kinarfi

    kinarfi Well-Known Member

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    What is it that I need to bypass? Because my power supply is limited to 5.7 amps and I'm working toward driving a motor that can pull upwards of 50 amps and more if I could stall it, I have a really big gel cel sitting next to my breadboard with #16 wire from battery to the board and the power supply keeps the battery at 14 as if in the vehicle with the engine running. The circuit works great with a p FET, and I plan to play with it today. Expect more scope picts:D If I wasn't ashamed of my messy bench, I'd even throw in a photo of the bread boarded circuit:eek:
    Thanks
    Kinarfi
     
    Last edited: Dec 9, 2009
  19. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The circuit must be built on a compact pcb with very low stray capacitance and low inductance in the short pcb copper traces. The supply bypass capacitors on the pcb stop the effect of inductance in the wires from the power supply.
     
  20. kinarfi

    kinarfi Well-Known Member

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    Well I'll be damned, Put 2 8000uf 15vdc and 2 12000uf 15vdc and 1 50,000uf 50vdc caps on the rails of my bread board and all my problems ppisadeared except for why my FETs died, here's just 2 of the many scope shot I took, but since they all looked the same except of on and off time, you only get to see 2 of them, They are the output from from my LM339 with a 1K pull up resistor and the out from the drain of an IRF2805 and IRF3205 with a schottky diode drain to negative and 120 ohm, .1uf snubber across the motor leads.
    Audioguru, I figured that since my power was coming from a 27AH battery with only 18 to 24" of #16 wire, the battery would be filter enough, guess not, when in service there will be 2 #10 wires about 6 feet each straight from the battery, do you think I should design a filter cap for that also? If so, how big?
    Thanks All,
    Kinarfi
    did anyone try to see if dv/dt was valid or not?
     

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  21. unclejed613

    unclejed613 Well-Known Member

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    have you put your scope probe on the power supply rail when the circuit is oscillating? a lot of people ignore the power rails as a feedback path when trying to track down oscillation, but power supply rails can often be a major contributor to oscillation. your power supply, if it's a regulated supply can often become part of the oscillator if it has foldback current limiting and isn't properly bypassed at the load and is being pushed beyond it's limiting current.
     

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