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24 Vdc @ 10A LM5118 based DC DC converter

Bruno Rodrigo Diaz Vargas
Prodigy 20 points
Other Parts Discussed in Thread: LM5118

Hello

I´m designing a buck-boost  DC DC converter with the next input and output requirements

Vin:  9-30 Volts DC

Vout: 24 Volts @ 10 A

I  want to use the LM5118 ...... Introducing these values into WEBENCH tool it says "Can´t find suitable FET for the desing" .  

Is there any way to use WEBENCH to complete the desing (obtain the  rest of components commercial values ) and extra  MOSFETS gate drivers  ICs´??

Bruno

  • 0
    TI__Intellectual 2155 points

    Hi Bruno,

    If you just want to see an example BOM, you can try lowering the current on the WEBENCH design:

    Vin: 9-30V, Vout: 24V @3A

    Regards,

    Kevin Ip

    WEBENCH Applications Engineer

    Texas Instruments

  • 0 in reply to Kevin Ip
    Prodigy 20 points

    Thank´s Kevin:

    What I would like to know is if  a  design based on  LM5118  (according to your experience ) can provide this amount of current considering the input and output  requirements or if I should migrate to another IC,   and if so, what´s the best  method to calculate the  main components  (inductor, mosfets, gate drivers  ic, etc).

    Thank´s in advance

    Bruno

  • 0 in reply to Bruno Rodrigo Diaz Vargas
    TI__Intellectual 2155 points

    Hi Bruno,

    The IC selection is important, but your design requirements present challenges in finding the right inductor, and creating a PCB layout that keeps everything cool.

    Your inductor will need to be rated for around 50A, based on the inductor peak current calculation here:

    Dmin(buck-boost) = Vout/(Vin+Vout) = 24/(9+24) = 73%

    ILavg = Iout / (1-D) = 10 / (1-0.73) = 37A

    Assume 30% ripple current, ILpp = 11A

    ILpk = ILavg + 1/2 * ILpp = 42.5A

    The mosfets and diodes will also need to be rated to handle this max current. If you end up using parallel FETs on the high side or low side because a single FET can not handle the current, then the trade-off is increasing the total gate charge of the switches. So, you'll have to pick a low enough frequency so that the driver will not overheat.

    Proper PCB layout is going to very important so that the FETs do not overheat, since you are going to be running an average current of 37A in the traces delivering the output power.

    If we can assume everything works fine on your layout at fsw=200KHz, which is optimistic, then the estimated inductance on L1 would be here:

    L1 = (Vin_min * Dmin) / (fsw * Ipp) = 9V * 0.73 / (200KHz * 11A) = 3uH

    So you will need a 3uH rated for 50A. If you cannot find one on the market you may need to wind your own.

    Regards,

    Kevin Ip

    WEBENCH Applications Engineer

    Texas Instruments