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LM25149: MOS switching loss at high load

Fred Cheng
Expert 4620 points
Part Number: LM25149
Other Parts Discussed in Thread: LM25117

Hi team,

below is my customer's spec, I am surprised that the "high side" MOS power loss is way lower than what I expected and what I calculated in other tool, 

the calculator shows high side is about 1.6W at 20A,

but I use other tool found out that with similar MOS spec, the high side MOS power loss should be around 3.014W which is way higher than calculator result.

although MOS is not the exact same since this tool can only pick TI mos, but they're very similar whether Qgd or Rson .

anyway, I wonder did I type anything wrong? or is there a special mechanism for LM25149 that this tool doesn't know ?

  • 0
    TI__Mastermind 22299 points

    Hi Fred,

    The Power mosfet calculator and Quickstart should use the same equations. The Quickstart calculator has more accurate numbers for driver resistances and deadtimes.

    The FETs arent really the same. The RDSON and QGs are different.

    Also it looks like the power mosfet calculator is using worst case values for RDSON.

    Also be sure the Low-side FET QOSS is the same. The QOSS losses are actually seen on the High-side FET.

    Hope this helps,

    -Orlando

  • 0 in reply to Orlando Murray
    TI__Expert 4620 points

    Hi  Murray,

    I should stick to the  calculator .

    so one question LM25117 and LM25149 which one will have lower high-side MOS switching loss?

    although LM25149's Rdrive is lower, LM25117 VCC is higher.

  • +1 in reply to Fred Cheng
    TI__Mastermind 22299 points

    Fred,

    The high-side switching losses are related to gate drive currents.

    These devices have similar currents so the switching losses should be the same.

    The higher VCC will allow lower RDSON and lower conduction losses, however higher VCC will result in higher QG total and higher gate driver losses.

    If you have a logic level FET with Vth < 3V then I recommend LM25149. The miller plateau voltage (Qgd voltage) should be less than 4V.

    -Orlando