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LM74930-Q1: Regarding the calculation formulas in “INRUSH_FET_SOA Margin Calculator.xlsx.”

Masamu Ogawa
Prodigy 60 points

Part Number: LM74930-Q1

Hi,

Please tell me the calculations (formulas) used internally in “INRUSH_FET_SOA Margin Calculator.xlsx.”
The reason is that the tool does not support calculations for MOSFETs connected in parallel.

In the middle block, there is an item called “Number of MOSFETs in parallel driven by HGATE,” but its result is reflected only in “FET Power dissipation at full load (per FET)” and “Maximum steady-state FET Junction Temperature.”

I would like to perform calculations such as “SOA margin during start-up” when using multiple MOSFETs in parallel, so please tell me the internal calculations performed in “INRUSH_FET_SOA Margin Calculator.xlsx.”

Thank you. 

  • 0
    TI__Guru 58405 points

    Hi Masamu,

    The tool doesnt consider MOSFET in parallel because in general, even if multiple FETs are used, it is considered that all the stress happens at 1 single FET during startup.

    Multiple FETs help only in steady state.

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Prodigy 60 points

    Hi Shiven Dhir,

    I understand. 
    In other words, the inrush behavior of a single MOSFET determines the upper limit of the load this system can handle (particularly for capacitive loads), correct?

    After surveying many commercially available MOSFETs and performing the calculations, I found that there is no MOSFET suitable for an environment of 60 V output, Cout = 60 mF, and a resistive load of 1.2 Ω (50 A).
    (With the settings “Maximum Input DC Operating Voltage: VIN(MAX) = 60 V,” “Maximum Output Load Capacitance: COUT = 60,000 µF,” “Startup Load Value = 1.2 Ω (60 V / 1.2 Ω = 50 A),” and “Load Turn-On Threshold = 2 V,” I could not find any MOSFET whose SOA margin during start-up exceeds 1, regardless of the Cdv/dt value chosen.)

    Do you have any modules in your product lineup that can support a configuration operating under the conditions of 60 V output, Cout = 60 mF, and a 1.2 Ω (50 A) load, or any modules that allow MOSFETs to be used in parallel to increase the inrush-current capability?

    Thank you.

  • 0 in reply to Masamu Ogawa
    TI__Guru 58405 points

    Hi Masamu,

    Driving 50A load with 60mF capacitor is ridiculously huge power dissipation. I would agree that no FET can support it.

    Hence, it has to be designed such that first we charge the entire capacitors and then turn-on the loads. We can't do it together.

    Regards,

    Shiven Dhir