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DRV8701EVM: DRV8701 - MOSFET selection

Prodigy 20 points

Replies: 5

Views: 281

Part Number: DRV8701EVM

Hello together,

conditions:

DRV8701E + 4x CSD18532Q5B (TI) / NTMFS5C628NLT1G (ON) + 37V DC power supply + DC Motor

Discription:

As a part of our development we have to find some second source components beside the main components. Therfor we measured some different NFETS from different suppliers.

But with all mosfets we have the same problems. They are all dieing (shortcircuit vom drain to source) if we run the motor with pwm less then 100%.

We measured a lot of signals like the gatevoltage, the voltage from U_DS to GND or with an differential probe. We measured the motorvoltage, the current and the power supply. But we see no spikes or extrem voltages/currents.

We changend the IDRIVE level (normaly we use the 25mA/50mA setting) and calculated the level as discribed in the datasheet.

Then we concentrated us from the DRV timings. But all control times are looking good.

We compared the datasheets from the mosfets and we are not sure which value from the second source mosfet is responsible for our problem.

Concrete quenstions:

  • Can you analyse our attached dokument/pictures to help us?
  • is it possible that the free wheeling diode is responsible for that?
  • How important is the value "reverse recovery charge" from the integrated mosfet diode and how he has to be.
  • is it possible that the mosfets are dieing because of the "Self On MOSFET Phänomen"?

More infos you can see in the attached dokument. comparison of the mosfets.pdf

Thanks for helping

Regards Boris from Germany

  • Hi Boris,

    The FETs chosen have very low Vgs thresholds and could be turning on.

    In the first image of page 2 please note the spikes on Source High 2. Should there be spikes?

    If not, this could be an indication of coupling causing the high side FET to try to turn on. Please examine the layout with particular attention to the GH2 trace.

    Also, if possible, please lower the voltage to 12V and monitor the gates of the FETs.

    If everything looks good, slowly increase the VM voltage and watch for signs of trouble.

    These signs could be the opposite side FET trying to turn on too soon, or some type of coupling in the system.

    Best Regards,
    Rick Duncan
    Motor Applications Team

  • In reply to Rick Duncan:

    Hey Rick,

    Thanks for your answer! Youre right with the low Vgs threshold, but the CSD18540Q5B has also an very low Vgs.

    Fo checking our layout, we soldered the ON Mosfets on your demoboard and did the same measurements. The result is the same. Reducing VM has better results. With every step of increasing the motorvoltage the spikes are getting bigger.

    We observe that it depends also on the motorcurrent. So, if the VREF Level is at 4,8V (no current regulation) the fets are damaged. But if the VREF level is at about 2,5V (I_chop ~ 12A) the fets are still alive. So it seems to be that the fets are dying a thermal dead, because for a short moment (spikes on the gates) both fets are on and the motorcurrent is "very high".

    The other thing we saw, is that the spikes are at the time we turn off the fets. So has to do with the induktion-/voltage and -current when the motor works like an generator. Turning on the mosfets seems to be good.

    For testing we soldered 2,2nF capacitors at the Drain-Source at every fet. It helps a little bit to keep the spikes smaler. But it is not the solution, because the fets are still "burning" if VREF is near on or at the maximum.

    And i am honest. I do not understand why they are burning. I have not a good feeling, also not with the CSD18540Q5B. What is, when we use the fets close to the border of there limits. Therfore i have to understand what is going on. I will measure some more and will send you morde detailed results to compare and diskuss.

    Thanks

    Boris

  • In reply to Boris Buecker:

    Hi Boris,

    We will await your detailed results.

    Please provide as many scope captures as possible. If you can measure the Vgs of the FETs during this time, it will be especially helpful.

    Best Regards,
    Rick Duncan
    Motor Applications Team

  • In reply to Rick Duncan:

    Hi Boris,

    Do you have any updates? Thank you.

    Best Regards,
    Rick Duncan
    Motor Applications Team

  • In reply to Rick Duncan:

    Hi Boris,

    We have not heard from you in some time, and are assuming your issue has been solved.

    If it has not, please reply. If the thread is locked please select "+ Ask a related question".

    Best Regards,
    Rick Duncan
    Motor Applications Team

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