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LM7480-Q1: MILSTD-1275E SOA calculator

pari ibrahim
Prodigy 10 points
Part Number: LM7480-Q1
Other Parts Discussed in Thread: TIDA-010242

Hi,

in TIDA-010242:

"To determine the MOSFET, the safe operating stress of the MOSFET must be calculated. There is a MILSTD-1275E SOA calculator that can be downloaded to help determine if a MOSFET is capable of handling the surge event."

could you please send me a link to download the SOA calculator? I can not find it anywhere.

I even would like some help with the way calculation have been performed at page 4. why multiplying 15.7 with 28V an not with 50V to calculate the power.

Best regards

  • 0
    TI__Guru 57737 points

    Hi Pari,

    Thanks for reaching out to us. Let me check and get back to you within couple of days. 

  • 0 in reply to Praveen GD
    TI__Guru 57737 points

    Hi Pari,

    Unfortunately there is no MILSTD-1275E SOA calculator available. You will have to calculate the FET SOA margin manually using the following steps. 

    FET SOA stress calculation

     

    A more detailed explanation of SOA calculation is specified in section 2.3 Understanding MOSFET’s Stress Limitations of Robust Hot Swap Design App note.

    For your understanding below is a step by step procedure (extracted from the app note),

    1. Calculate the power stress on the FET during startup. Refer to section ‘2.3.3 Checking SOA for Non-Square Power Pulses’ of the app note. 

    Derive t2. Approximation here is  that the FET is stressed for a power loss = P2 = PMAX for t2 duration.

    2. Now, estimate SOA the FET can handle for t2 duration. This value will be at room temperature. Refer to section ‘2.3.2 Checking SOA for Intermediate Time Intervals’ of the app note.

    Remember that the SOA value in the table below needs to be taken at VDS = VIN(max) in the SOA curve of the FET.

    3. Now, De-rate the SOA calculated in the above step for the Max operating junction/ case temperature of the FET. Refer to section ‘2.3.1 MOSFET SOA Curve and Thermal Model’ of the app note.

    This value can be taken as the temp. of the FET at max load condition,

       

    4. You can consider that the FET has strong SOA if

    SOA(TC) > 1.2 x (P2 / VDS)

    Where,

    SOA(Tc) is from step 3

    P2 is from step 1

    VDS = VIN(max)

    The factor 1.2 is taken to include a 20% margin.