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TPS2493: MOSFET breakage during turnoff

Andrea Picillo
Prodigy 40 points
Part Number: TPS2493
Other Parts Discussed in Thread: LM5069, CSD19536KTT

Hi,

I would like to receive support regarding TPS2493. We are using the schematic that follows:

We are using the "Blade1_MOS_OFF" signal to force a shutdown of the TPS2493.

I'm here to ask you how to improve the current design, if it's possible.

Currently the problem is that when we force low the UVEN pin through the Q34 MOS, the Q32 and/or Q33 breaks down. This happens when an electronic load is used and a current of 40A is set.

It's not clear to me why this happens.

Furthermore we are worried about the ability of the TPS2493 to protect the power MOSFETs in case of short circuit. Since Soft-start is used, the turn-off time is quite huge to support a short-circuit for such a long time. How can we handle that?

Thanks for your support.

Greetings,

A.Picillo

  • 0
    TI__Guru** 105361 points

    Hi Picillo,

    Welcome to E2E!

    Can you please help with filled up design calculator to review the design. It is available under TPS2493 product folder at ti.com.

    Please share test waveforms of Vin, GATE, input current and Vout during turn-off event. I suggest to use dvdt softstart circuit shown in front page of LM5069 datasheet to discharge 220nF locally to facilitate fast turn-off.

    Regards, Rakesh

  • 0 in reply to Rakesh
    Prodigy 40 points

    Dear Rakesh,

    thank you very much for you suggestions.

    At the moment I'm no able to give you some waveforms.

    Here is my Design calculator sheet 

    https://we.tl/t-SnCB59Vxjf

    Please, could you provide me some hints in short-circuit handling? Is it enough comparing the SOA graph with my system current and voltages? It's quite hard to predict the actual current during the shor-circuit event.

    Thank you again.

    Greetings.

    Andrea

  • 0 in reply to Andrea Picillo
    TI__Guru** 105361 points

    Dear Andrea,

    Can you please attach the design calculator. I am not able to access the link due to security reasons. 

    Regards, Rakesh

  • 0 in reply to Rakesh
    Prodigy 40 points

    Dear Rakesh,

    here is the file.

    Is there some solution to turn off the MOSFET in a faster way?

    I'm worried about the situation in which the mosfet is turned off through 2.3mA sink current, in opposition to 125mA current.

    Thank you very much.

    TPS249x_Calc_Prj1 - FDB86363-F085.xlsx

  • 0 in reply to Andrea Picillo
    TI__Guru** 105361 points

    Hi Picillo,

    Here are my comments on the design. 

    1. Set current limit just above your max load current. so, 1 mOhm sets min current limit of 45A

    2. F53 should be the Rds_on value at Tj

    3. During turn-off the dvdt cap 220nF should also be discharged. 1k series resistor limits the rate of discharge. so, I suggest to reduce 1k to 220 Ohm and then test again. If you are going to redesign the board, I suggest to change the RC circuit to PNP type as shown in front page of LM5069 datasheet https://www.ti.com/lit/ds/symlink/lm5069.pdf

    4. The FET still looks weak from SOA point of view. I suggest to use stronger FET similar to CSD19536KTT.

    8284.TPS249x_Calc_Prj1 - FDB86363-F085.xlsx

    Best Regards, Rakesh

  • 0 in reply to Rakesh
    Prodigy 40 points

    Dear Rakesh,

    thank you very much for your support.

    1. We have a nominal current of about 40A, with some surge up to 60A. We should use 1mohn as you suggested but the problem is that the system could have a fault.

    The solution is to increase the timer capacitor. Increasing that capacitor means having more trouble in case of short circuit. Is this means tp have a more rugged mosfet during short-circuit? In that case what do I have to look for? SOA parameter is enough?

    2. Which value of Tj? The one you would reach at the rated current?

    3. I will absolutely use your suggested circuit. Is there a way to make the discharge even faster, in order to further decrease the turn-off time?

    Do you think it could be a nice idea to substitute the TPS2493 with a different IC, maybe with a higher gate sink current?

    4. The MOSFET you suggested is very interesting. Thank you very much, I will take it into account

    We are considering this MOSFET at the moment. What do you think about? Thanks

    TPS249x_Calc_Prj1 - IPB020N10N5LF.xlsx

    Thank you again

    Best regards,

    Andrea

  • 0 in reply to Andrea Picillo
    TI__Guru** 105361 points

    Hi Andrea,

    I would recommend LM5069 over TPS2493 due to the reasons mentioned in https://e2e.ti.com/support/power-management/f/196/t/837745?TPS2493-TPS2493-recommendation-to-use-LM5069-instead

    1. If you need to support 60A, set F30 = 59A because the device won't trigger fault till 60A current and SOA margin should be check at current close to current limit value

    2. Tj is Maximum steady state FET Junction Temperature 

    3. the soft-start ciruit with PNP transistor would be good enough.

    4. IPB020N10N5LF does not look strong. considering power rating of your system, I suggest CSD19536KTT or PSMN4R8-100BSE https://e2e.ti.com/blogs_/b/powerhouse/archive/2018/04/06/how-to-select-a-mosfet-hot-swap

    Best Regards, Rakesh