**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.