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LM25149: Design Spreadsheet FET loss calculation

Part Number: LM25149
Other Parts Discussed in Thread: CSD18540Q5B, LM5149, , LM5149-Q1EVM-400, LM5148

I'm working a design with the LM25149 with Vin = 36V (nom) Vout = 12V @ 15A max load.

Webench generates a design at Fsw = 250 kHz using CSD18540Q5B FETs. Power loss of control FET is almost 2.2W and estimated Tj of 159 degC. (Ta = 50 degC).

If I use the design spreadsheet with the same component values, but use max values for the FET parameters (where known), the Control FET power loss is closer to 3.3W which leads to excessive junction temp (>200 degC) with theta jA of 50 degC/W.

It seems Webench probably uses nominal FET parameters as I can get close to its results when using nominal parameters in the spreadsheet.   

Is it wrong or overly conservative to use max values for all of the FET parameters when using the design spreadsheet? I'm trying to avoid recommending going to parallel FETs or a dual -phase design, but worried that using nominal results may not be realistic.    

  • Mark, 

    It's fairly reasonable, even 2.2W could be a bit much, when comparing to the results from the default quickstart calculator.

    By the way, the LM5149 quickstart calculator is meant to model the LM25149-Q1EVM-2100, and that EVM user guide has a thermal image of both FETs and inductor.

    Same for LM5148 quickstart calculator, modeled after LM5149-Q1EVM-400.

    Also inductor losses, the inductor can be the dominant loss at lower frequency (higher inductance = higher DCR) and higher current.

    I'd recommend trying out your design on LM5149-Q1EVM-400 EVM, and seeing preliminary temperatures at ~10A.

    Hope this helps,