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CSD19534Q5A: IDS measurmant method for ASO (Are of Safety Operation) confirming.

Kazuya Nakai59
Guru 20530 points
Part Number: CSD19534Q5A
Other Parts Discussed in Thread: LM5145

 Hello guys,

 One of my customers is evaluating LM5145 on their own PCB with CSD19534Q5A as external MOSFET.

 Now they are trying to check whether CSD19534 are working in area of Safety Operation(ASO) of the MOSFET. For that, they tried to measure the MOSFET drain current with a current probe of oscllioscope.

But because they added a jumper wire for the current measurement, big spike noise was observed.

 Do you have any idea to measure the drain current with smaller spike noise? 

 Or is the spike noise not impact to measure ASO? I think it is good way to use a lowpass filter. Is it not problem to use low-pass filter?

 Your reply would be much appreciated?

 Best regards,

 Kazuya Nakai.

  • 0
    TI__Mastermind 31195 points
    Hello Nakai san,
    Thank you for promoting TI FETs at your customer. Measuring MOSFET drain current is challenging. You have to break the current path and insert either a loop for a current probe or a low ohm current shunt (resistor) to make the measurement. Why is the customer concerned about safe operating area (SOA) in a switch mode power supply application? Usually, SOA is more of a concern where the FET is operated in the saturation region where there is simultaneously voltage across the drain-source and drain current thru it leading to high power dissipation. Typical applications include hot swap/efuse, load switch and linear regulator. In a buck converter, the MOSFET alternates between the cut off (OFF) and linear (ON) regions with very short periods in the saturation region during switching transitions. SOA can be a measure of MOSFET robustness but in switch mode applications, it is more important to insure that VDS and IDS do not exceed the absolute max ratings in the datasheet and that the power dissipated in the FET does not cause the junction temperature, TJ, to exceed the max rated in the datasheet. You can learn more about SOA from the following blog: e2e.ti.com/.../understanding-mosfet-data-sheets-part-2-safe-operating-area-soa-graph.

    When using a current probe to measure drain current, make sure that the loop is just large enough to accommodate the current probe. I don't know a good way to use a low pass filter other than limiting the bandwidth of the current probe and/or the scope.
  • 0 in reply to John Wallace1
    Guru 20530 points
    Hello John,

    Thank you for the prompt reply. Your reply is very helpful for the customer and me. I will ask the customer why the customer need to check SOA.

    Could I ask you a few additional questions about CSD19534Q5A parameter definition?

    Q1. Does td(on) mean that the time between Vgs=5V and Ids=5A when FET is turned on?
    Q2. Does td(off) mean that the time between Vgs=5V and Ids=5A when FET is turned off?
    Q3. Does tr mean that the time between Ids=1A and Ids=9A when FET is turned on?
    Q4. Does tf mean that the time between Ids=9A and Ids=1A when FET is turned off?

    Thank you again and best regards,
    Kazuya Nakai.
  • 0 in reply to Kazuya Nakai59
    TI__Mastermind 31195 points
    Hello Kazuya Nakai,
    Thanks for your interest. Please see this blog for more information for a detailed explanation of switching parameters: e2e.ti.com/.../understanding-mosfet-data-sheets-part-5-switching-parameters. This is a multi-part series on understanding MOSFET datasheets: e2e.ti.com/.../MOSFET+Blog+Series
  • 0 in reply to John Wallace1
    Guru 20530 points

    Hello John,

    Thank you very much for your reply and I'm sorry for my late respose.

    As I said in the first of this thread, the customer uses CSD19534Q5A for external FET of LM5145 buck controller and they are evaluating those on their own PCB. So far, they didn't observe any shoot through on the evaluation. But they calculated CSD19534Q5A on/off timing of high/low side base on the FET and LM5145 datasheets. The calculation result is the attached PDF file. According to the calculation, two shoot through timings are found.  

    LM5145 datasheet (SNVSAI4)

    http://www.ti.com/lit/ds/snvsai4/snvsai4.pdf

    CSD19534Q5A datasheet (SLPS483)

    http://www.tij.co.jp/jp/lit/ds/symlink/csd19534q5a.pdf

    They worry that the shoot trough may be happend after production start though any shoot through was not observed at current evaluation.

    So could you give me your comments or advice for the follwing questons?

    Q1. Is the customer calculation correct?

    Q2. Is there any  possibility of shoot through problem?

    Q3. Their calculation uses all typical data. How can the customer calculate shoot through timing in case that the  temperature is changed to high or low from 25C with manufacturing variation?

    Q4. If you have any reccomend FET, could you please let me know?

    Thank you very much again and best regards,

    Kazuya Nakai.    /cfs-file/__key/communityserver-discussions-components-files/196/LM5145-with-CSD19534Q5A_5F00_on_5F00_off_5F00_delay_5F00_020819.pdf

  • 0 in reply to Kazuya Nakai59
    TI__Mastermind 31195 points
    Hello Kazuya Nakai,
    I have asked the LM5145 controller team to take a look at this for you. I believe this is more of a controller question than a FET question.
  • 0 in reply to John Wallace1
    Guru 20530 points
    Hello John,

    Thank you very much for your forwarding my questions to LM5145 team.
    But I didn't have any answer from the team.

    John, could I ask you a few questions?

    Q. Only typical data are described for td(on),td(off), tr,tf of dynamic caracteristics in CSD19534Q5A datasheet. Do you have any data for the minimum and the maximum. How much are those parameters variation? Is it small? For example about +/-20%?

    Thank you and best regards,
    Kazuya Nakai.
  • 0 in reply to Kazuya Nakai59
    TI__Guru 54282 points

    Nakai-san,

    As we discussed the MOSFET deadtime with the LM5145 controller via email (with no issues identified in your circuit configuration), we can go ahead and finish this thread. Unless there's something else, please click "This resolved my issue" to close this out.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Guru 20530 points
    Tim,

    Thank you for your strong suppots. I see. I reply with cliking the box.

    Best regards,
    Kazuya Nakai.