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LM5155-Q1: Quick start tool breaks

Viktorija Cecil
Mastermind 18471 points
Part Number: LM5155-Q1
Other Parts Discussed in Thread: LM5155

Team, My customer is observing that when he uses a LM5155-Q1 with a very low gfs he is breaking the Quickstart tool… Fet in question is: 

https://www.onsemi.com/pub/Collateral/NVTFS015N04C-D.PDF

Any guidance would be appreciated. 

Thanks

Viktorija

  • 0
    TI__Guru* 78070 points

    Hi Viktorija,

    Thank you for posting.  What do you mean by breakign the Quickstart tool?  The gfs is for linear mode operation but the FET is used with the LM5155 will be fully turned on, why does it matter much?   I reviewed the FET datasheet and I don't see any problem of using the part. 

    Thanks,

    Youhao Xi

  • 0 in reply to Youhao Xi
    TI__Mastermind 18471 points

    Youhao, 

    The gfs seems to have a large effect on what the estimate power loss across the device is. The fet in question had a gfs of 2 which caused the power loss calculation to go negative. I would agree that the gfs shouldn’t matter, but it is in the tool and seems to have a pretty big effect on the results.

    Can you try to duplicate it on your side?

    Let me know what additional information you need please.

    Thanks

    Viktorija

  • 0 in reply to Viktorija Cecil
    TI__Guru* 78070 points

    Hi Viktorija,

    I have plugged the referred MOSFET parameter into the calculator and I even further reduced the transconductance, and I do not get any negative power losses. You can download the calculator and have a try.

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    Prodigy 30 points

    Hello Youhao

    I have been working with Viktorija on this. Here are the values i am inputting into the calculator. From 0A up to just befroe 2A on the output side of the controller the MOSFET power loss is negative. I can send you the entire calculator with the values filled out if necessary. The issue seems to resolve around the gate threshold voltage being low. When i use the minimum gate threshold voltage from the datasheet i get these results. If i use 3.5V which is the maximum for the MOSFET in question it looks ok or at least feasible. 

    On-State Resistance, RDS(on)  14.4 mΩ
    Total Gate Charge, QG  6.3 nC
    Gate-Drain Charge, QGD  1.2 nC
    Gate-Source Charge, QGS  2 nC
    Gate Resistance, RG  2 Ω
    Transconductance, gFS  2 S
    Gate-Source Threshold Voltage, VTH  2.5 V
  • 0 in reply to Lucas Ritter
    TI__Guru* 78070 points

    Hi Lucas,

    Thank you for posting.  Negative loss is obvious wrong. 

    Anyway, just let you know that the quick start calculator is intended to offer the good start point for your design. Bench evaluation is always recommended. It is possible excel spreadsheet has bugs in it, but so far I cannot figure it out. Can you share your calculator, because I plugged your number into the calculator and I always get >0W? 

    I am passing this to the engineer who created the calculator.   However, he is currently on a business trip.  So please wait for couple of weeks for his response.

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    Prodigy 30 points

    I attached the tool in another reply and will try to get my work completed using the pspice model. Let me know if you or your team find a solution. Hopefully it is just some wierd case that can be easily resolved. 

    Thanks, 

  • 0 in reply to Lucas Ritter
    TI__Guru* 78070 points

    Hi Lucas,

    It looks like some bug in the calculator.  Our engineer will take a look into it once he comes back to office, in two weeks.  

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    TI__Guru* 78070 points

    Just let you know that our engineer should look into this issue next week. 

    Thanks,

    Youhao

  • 0 in reply to Lucas Ritter
    TI__Mastermind 21720 points

    Lucas,

    The switching loss calculation is based on adjusting the threshold voltage of the MOSFET in conjunction with the input current, gate drive voltage and the trans-conductance of the MOSFET. This is a good model when the trans-conductance of the MOSFET has higher values. Typically the trans-conductance of the power MOSFET is around 50S. Since trans-conductance of the selected MOSFET  is low the model results in a negative rise and fall time.

    I can adjust this model in the next revision of the calculator. For right now I suggest setting the trans-conductance to 50S as this should provide a fairly accurate estimation of the transition times.

    Please let me know if you have any questions.

    Thanks,

    Garrett