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Voltage sag from FET gate driver.

Discussion in 'General Electronics Chat' started by Triode, Nov 27, 2017.

  1. Triode

    Triode Member

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    I actually changed the resistor going into the diodes that charge the capacitors to 5.1Ω as well, I wonder if that's part of the problem. Was 100Ω a good value for that?

    The flyback diodes seem like a good thing to add. I've seen it on several diagrams for similar drivers, I'll give it a try. Would that be parallel with the gate resistor? It sounds like you mean it's in parallel with the body diode, from source to drain?

    I also reduced the gate resistors to 5.1Ω, maybe that's too small?

    Thanks again for all the help! I'll post traces later so there is more to go on, I know without diagrams or traces it's kinda vague.
     
  2. dknguyen

    dknguyen Well-Known Member

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    No, 5 ohm should be good for operation for both bootstrap and gate. That's what I would have used if I bothered to use a resistor at all. OTOH, it does mean if something fails short there's not enough current limiting along those paths to keep traces from burning.

    If you have a scope, scope the gate voltage and see if there's crazy significant ringing happening whenever you switch.

    You said you can't reproduce it right? If you can reproduce it, maybe double and quadruple the gate resistor see if the failure stops at some point. If it does while the resistor is still too small to limit shorting currents from burning something out, then that means that the rise times really are fast enough to excite ringing that is popping stuff. 22AWG wire can handle 8A during normal operation so to burn one out there must have been serious current going through it.

    The flyback diode would be anti-parallel with the drain-source of each MOSFET (the direction that doesn't make a dead short across the motor supply).
     
    Last edited: Dec 29, 2017
  3. Triode

    Triode Member

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    To scope the gate do I connect the ground on the probe to source? Or do I need to do something more complicated? I had been connecting it to Vss on the chip but I just realized this isn't measuring Vgs.
     
  4. dave

    Dave New Member

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  5. dknguyen

    dknguyen Well-Known Member

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    Look at it in both. Source is better but some scopes or probes don't deal so well with the high speed common mode junk going on.
     
  6. Triode

    Triode Member

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    So maybe put a probe to gate and one to source, ground both to Vss and display the difference?
     
  7. dknguyen

    dknguyen Well-Known Member

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    I'd personally only connect one probe at a time due to that common-mode issue that some scopes have a hard time dealing with. But if your scope can handle both, go go for it.
     
  8. Triode

    Triode Member

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    EDIT: before you put too much time into this, I just found that the supply capacitor (1000uF) was failed open. This means that there was not much to smooth out the demand on the power supply. Will update once I've fixed that.

    Well, I traced it. And it looks pretty nasty. This is with two FETs installed in series.

    C1 through C3 on top frame are the high side FET gates, C4 through C6 are the low side gates. The bottom also displays F1 which is not relevant here (it was diffing the G and S, but C1-C2 are now on two different high side legs)

    I can see there is a lot of waving going on during the off period of the waveform that should not be occurring. This could be how shoot through would occur. I could be wrong but it looks like that low side "buzzing" is enough to trip the low side fets to turn on unintentionally. I'm thinking the first thing I'll try is to add more resistance to the gates.

    Switching.jpg

    Heres another if it adds any clues. In this one I put a probe on the gates of each of the two parallel high side FETs. You can see that during the high period they track eachother almost perfectly. But at the end of the fall they both do their own noise. Is that ringing?

    Here since C1 C2 are the two gates F1 now shows their difference. And its pretty noisy.

    Looks like I have a lot of work to do.

    Thanks for all the help!

    upload_2017-12-29_16-51-3.png
     
    Last edited: Dec 29, 2017
  9. Triode

    Triode Member

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    EDIT (same as edit above): before you put too much time into this, I just found that the supply capacitor (1000uF) was failed open. This means that there was not much to smooth out the demand on the power supply. Will update once I've fixed that.

    -The board I was showing below did not have a failed supply capacitor.

    By contrast, here is what the version with just one FET looks like. A lot cleaner and less noisy. Though I am still concerned with that dip at the end of switching. Note that this board does not have the flyback diodes I added to the other one.
    1 fet board.jpg
     
    Last edited: Dec 29, 2017
  10. dknguyen

    dknguyen Well-Known Member

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    The second graph in post #27 where the edge undershoots on the falling edge and oscillates (or conversely, overshoots on the rising edge and oscillates) is what ringing looks like. There is ringing in the graph in #28 as well, with that huge dip followed by a bunch of smaller oscillations.

    That huge initial dip doesn't look like it's being caused by ringing per se, since it's totally out of proportion with the much smaller and much faster oscillation that follow afterwards. It almost looks like something is receiving insufficient power. But notice that it only happens on the switch off, so that just might be remedied by adding in the flyback diodes. You should take a look at the voltage across the source-drain too and see if it looks clean.

    Ideally, you want the waveform to be critically damped for a balance between minimizing ringing while maximizing transition speed (which minimizes switching losses).

    Ringing can cause damage and other unpredictable behaviour like latching the transistors and stuff like that.

    Failing of the main bypass capacitor will cause an increase in the inductive spikes experienced by the system, especially if the wires between power supply and switches are longer. This can damage the transistors.
     
    Last edited: Dec 30, 2017

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