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FET Killer

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

  1. Sceadwian

    Sceadwian Banned

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    mneary i and j are identical mathematically It's simply a notational oddity between mathematicians and electrical engineers.
     
  2. kinarfi

    kinarfi Well-Known Member

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    Thanks Mneary for the i j explanation, Hayato, thanks for sticking with me while I figured out what I was doing. From one of the Indiana Jones movies, where Sean Connery says Jehovah is spelled with an i in Latin and being in Brazil, I assumed j being imaginary was Spanish for i and we know ASSuME means.
    Unclejed613, does this sound about right, R = 0.0791, L = 0.000136 h, 0.1 * L/R = 0.000172, invert for a frequency of 5816 Hz
    Thanks again to all.
    Kinarfi
     
    Last edited: Nov 27, 2009
  3. Hayato

    Hayato Member

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    I know it has nothing to do with the post. But, in Brazil people speak Portuguese.

    And, indeed, Jeovah in Latin is spelled with I, like Jesus -> IESVS. But in Portuguese, j is j and i is i.
    Jeovah -> Jeová
     
    Last edited: Nov 28, 2009
  4. dave

    Dave New Member

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  5. kinarfi

    kinarfi Well-Known Member

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    I have been redoing my design and incorporating some of the stuff from this forum, I have several screen shots from my scope and found it educational, for me at least. Some of the shots have text inserted. My attempt to use a totem pole didn't seem to work at all, I used a 2n4401 & 2n4403 and it didn't work, maybe I messed up the hook up because I think at least one of them was in a TO-92-18R package, same as TO92 but EBC instead of CBE, I think. Try again tomorrow.
    Any way, It was interesting to see how the capacitance of the FET effected the oscillator output, and you could see the turn on time where the FET was linear.
    One thought about using a higher frequency for PWM versus low frequency PWM is that the FET goes linear 6000 times a second at 6Khz and only 100 time at 100 Hz which is 1/60 the heat from being linear and if you use ultrasonic frequencies, it would be even worse. When you think about it, in the USA, we have 60 Hz electrical power, 120 pulses / sec, so 100 Hz PWM seems like it may be OK.
    on the last two, the yellow trace is the FET, high is off
    2 cents
    Kinarfi
     

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    Last edited: Dec 4, 2009
  6. unclejed613

    unclejed613 Well-Known Member

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    getting back to what was happening to the FET with the switch for a moment, i think the parameter you exceeded was the max dv/dt figure for the gate. in the data sheet it's most likely listed in volts/microsecond. exceeding the max dv/dt for the gate will cause the gate insulation to break down, even if you don't exceed the Vgs of the FET.

    if you want an accurate measurement of the resistance of the wire in the motor, use another resistor and the motor winding as a DC voltage divider with a known source voltage. the resistance can be calculated from the voltage drop across the winding, the current can be calculated from the voltage drop across the resistor divided by the resistance. now that the current is known and the voltage drop across the motor, the resistance of the motor winding can be calculated
     
  7. unclejed613

    unclejed613 Well-Known Member

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    the ramping of the signal going to the gate is because of a high impedance output from whatever you're using to drive the FET. with a low output impedance (such as using a buffer amplifier), the ramping effect will be minimized, and the FET can be driven with higher frequencies (remember to stay below the dv/dt of the gate)
     
  8. Hayato

    Hayato Member

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    And don't forget to use snubbers. Sometimes a MOV does the job.
     
  9. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Are you still using an LM339? It is low power so its outputs cannot go high with enough current to quickly charge the capacitance of the gates. Its outputs go low fairly quickly as shown in your ramping.

    All the 2N4401 and 2N4403 transistors that I have seen use the American pins of EBC. Yours didn't work maybe because the pullup resistor for the outputs of the LM339 were missing.

    The 10 ohms resistors in series with the gates prevent the Mosfets from oscillating . It should be mounted very close to the gate pin to minimize inductance in series with the gate.
     
  10. Roff

    Roff Well-Known Member

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    Uncle Jed, can you point to a document that mentions MOSFET gate dv/dt sensitivity? I have never heard of it. Vds dv/dt can be a problem.
     
    Last edited: Dec 5, 2009
  11. kinarfi

    kinarfi Well-Known Member

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    Replies,
    And Thanks.
    UncleJed;"I think the parameter you exceeded was the max dv/dt figure for the gate." I agree! I'm very confident that the motor winding is .0791 ohms, post #76, thumbnail 2.
    Audioguru; "Are you still using an LM339?" Yes, the pull up resistors are 4.99K 1% and I did NOT use the 10 ohm series resistor, I will next time!
    Hayato; I haven't used the motor as a load yet, when I do, I will have a 100 ohm, 10nf snubber across the leads.

    I found the scope shots interesting, especially the circled area in this one. I notice the wave form changed very little with load, and the FET did get warm to the touch with the 2 ohm resistor, it was heat sinked at the edge of a 3/16 aluminum plate with about 1/2 the body in free air, but it did cool quickly when turned off.
    Again, Thank You,
    Kinarfi
     

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  12. Roff

    Roff Well-Known Member

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    Post deleted, see below.
     
    Last edited: Dec 5, 2009
  13. Roff

    Roff Well-Known Member

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    kinarfi wrote:
    I hate to belabor this point, but why do you agree with this? Is it in the MOSFET's datasheet?
     
  14. kinarfi

    kinarfi Well-Known Member

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    Reread post #28
    PULL UP RESISTORS pictures from news photos on webshots
    "In the bottom left corner is an N FET drawing similar to what I had when I Killed my FETs, minus the cap. I figured there was something weird going on and the dv/dt sounds correct, I was suspecting that the gate was charging backwards (blue circle) when off and when the switch was closed, it discharged and recharged 'violently' punching a hole in the gate insulator and killing the FET.
    originally attributed to noise, dv/dt makes real good sense to me. I reverse charged the gate with a diode checker and got conduction in one direction only, doing so with the power of closing a switch to, or touching a wire to, the gate would create a high dv/dt, considering dt is extremely small.
    I guess I just believe that it was dv/dt that killed a couple of high power FETs when current was limited to < 1 amp! and no, I haven't been able to find any specs about dv/dt on the data sheet.
    kinarfi
     
    Last edited: Dec 5, 2009
  15. Roff

    Roff Well-Known Member

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    As I said before, MOSFETs can be damaged by excessive dv/dt applied between drain and source.
    I'll just state unequivocally that MOSFETs cannot be damaged by excessive dv/dt applied between gate and source, and see if anyone can refute this.
     
  16. kinarfi

    kinarfi Well-Known Member

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    I ask as nicely as I can, would you please build the circuit referred to in previous post without the cap and attached and try it, see if you lose power FET also or not? and let me/us know. Remember, it's the closing of the switch after the gate has reversed charged that kills the FET. Good Luck.
    Thanks
    Kinarfi
     

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  17. Roff

    Roff Well-Known Member

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    MOSFETs are fragile if not applied correctly. When a novice kills a MOSFET, the last failure mode I would consider is one that I cannot find documented anywhere.
    Static electricity may have killed your transistor.
    If you didn't have a diode across an inductive load, flyback voltage could have killed it.
    If you had the source and drain swapped, that could have killed it.
     
  18. Hayato

    Hayato Member

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    Try to use a small resistor at the Gate. Like 10 ohms or 47 ohms.
     
  19. kinarfi

    kinarfi Well-Known Member

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    The attached drawing is exactly what I had, the switch was a mini clip and the load was a test light which draws about 150 ma. If you build the circuit using a 100 ohm resistor, and use an on/off switch or just touch wires together, you will prove a point, I don't know which point, but if the FET dies, that's one point, if the FET lives and works, that's the other point.
    This is my guess as to what happens, with the switch open, a reverse charge builds up on the gate as shown in the attached drawing, maybe there should be a bunch more + in other areas, I DON'T KNOW, then when the switch is closed and the gate charge is drained off and the polarity of the gate charge is reversed, the fragile insulation is damaged because it happened so fast and the FET dies.
    This is purely a guess, and I have only my own reasoning to pull from.
    I don't want to throw challenges out, but if you are correct. building and trying the circuit can't hurt.
    Kinarfi
     

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    Last edited: Dec 6, 2009
  20. Hayato

    Hayato Member

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    I'm looking to the last picture link you posted.

    There is a useless IRF2805, with the Drain connected to the GND and the Source connected to 0V, and the Gate connected to a ON/OFF switch. This FET is not working. It is just a high resistance piece of silicon.


    The output driver is driving a NFET and PFET at the same time with a 2.5k (2 5k in //) resistor at the OC output. I do not feel confortable with it.
    Why not use the 339 to drive a Totem Pole?
     
  21. Hayato

    Hayato Member

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    Without the Gate resistor you can have transients as high as 25V, piercing the gate.
     

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