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LM5176: LM5176 efficiency improvement

Jian Wei Lai
Prodigy 130 points
Part Number: LM5176


Hi TI team,

We are preparing use LM5176 in our new project. The operating condition is VIN=9-48V / VOUT=20V/IOUT=12A (max).

And we got a design file to simulation LM5176 system, we found that when VIN=9V and IOUT around 11A the efficiency is drop to under 75%.

We tried to modify the boost mosfet parameter, but don’t have any improvement.

Please help to review attached file and give some advice. 

Let me know if you need any further clarification.

LM5176 Buck-Boost Quickstart Tool_JAO_20240109.xlsm

  • 0
    TI__Genius 10385 points

    Hi Jian,

    I will get back to you as soon as possible. Currently my bandwidth is quite short, so please allow me some time to get back to you. I will have a response for you earlier next week.

    BR,

    Haroon

  • 0 in reply to Haroon Wardak
    TI__Guru 61011 points

    Hi Jian,

    the problem is the RDSon of the high side Buck MOSFET. Due to the high losses there 

    Input current * input Current * RDSon = 29A * 29A * 5.4mOhm = 4.54 W

    With a Theta JA of 56 C/W this will result in  250 Degree which again impact the RDSon ....

    So first step would be to optimize the RDSon of this MOSFET.

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 130 points

    Hi Stefan,

    Thanks for your feedback. We wonder to know parallel a same N-MOSFET on high side BUCK MOSFET to halve the RDSon is a suitable solution?

    Cause parallel a N-MOSFET will double Qg,Qgs and QGD. 

    Best regards,

    Jian

  • 0 in reply to Jian Wei Lai
    TI__Guru 61011 points

    Hi Jian,

    this can be done (see also the LM5176EVM-HP where the footprint for a 2nd MOSFET is already there) but as you already mentioned you need to check the losses again for that solution. 

    -> You get less losses in conduction but more switching losses.

    In the Excel calculator you can have the RDS,on and double Qg, Qgs and Qgd to get the for the loss estimation.

    But I also see quite high loss on the Boost Side. The transition losses are very high and handling a loss of 14W will not work with passive cooling. => 14W * Thata JA  = 784 Degree !!!

    So selecting a another MOSFET might be required here as well.

    Best regards,

     Stefan