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LM5175: Mosfet Selection Questions

Ryan McDaniel
Prodigy 55 points
Part Number: LM5175

I'm struggling a bit with MOSFET selection for a buck-boost converter built with the LM5175.   (16-36V VIN, 24V VOUT at 10A)  Web bench only seems to allow selection of particular sets of M1/M2  or M3/M4.  When I choose the M1 I want, the M2 FET that comes with it is not adequate for the design (conduction losses are too high in buck mode at max Vin.  I would like to choose a different FET there, but that forces a change to M1 as well, which I don't want.

Further complicating things, the datasheet doesn't mention how to calculate the switching losses for the M2 (QL1) FET, just the conduction losses: 

 

Since WebBench seems to force me to pick a FET with a relatively high RDSon but lower gate charge for M2 given my M1, I assume the switching losses are significant for this FET in at least one mode of operation.   Is there a way to get the FET combinations I want in WebBench or an equation to manually calculate my losses for M2?  (Note: I haven't started looking at M3/M4 yet, but they are also linked and the options available to choose from are limited depending on what I choose for M1/M2) 

  • +1
    TI__Guru 50431 points

    Hi Ryan,

    the losses of the Buck low side MOSFET can be calculated with:

    P_switch.loss =  2 * Deadtime * Fsw * Vsd * I_Out

    Deadtime can be set to 50ns

    Vsd = Forward Voltage of MOSFET Body diode

    I may also use the Quickstart calculator which lets you set the MOSFET individually:

    https://www.ti.com/tool/download/LM5175QUICKSTART-CALC

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 55 points

    Thanks for the response Stefan!  I'm trying to understand this circuit a little better, so if you don't mind, I have some follow up questions.  So that equation covers the body diode losses during the deadtime, but isn't there some switching losses associated with the turn-on/off of that FET as well?  Or is that so small as to be irrelevant because the current is flowing backwards through the body diode keeping the Vds small?  I don't understand why WebBench picks a relatively high Rdson FET that seems inappropriate for that location.  Or is that just a bug in the program and I should ignore it?

    I played with the quickstart calc a little more and figured out how to use it to check the different operating conditions for the FETs and can see the FET loss chart to see how changing a particular parameter affects it.  That should help me work out a better FET selection.  Thanks for the suggestion!  I see it also has the ability to add external Schottky diodes to reduce the deadtime losses in Q2/Q3 as well if needed.

  • 0 in reply to Ryan McDaniel
    TI__Guru 50431 points

    HI Ryan,

    right, there are also losses with controlling the Gate signal for the MOSFET but physically this are associated to the Gate driver in the LM5175 as the losses appear here and the power dissipation for that needs to be considered for the LM5175. 

    Nevertheless this are to an certain degree defined by the MOSFET and the selection of it and finally also impacts the overall efficiency.

    Best regards,

     Stefan