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LM5145: High current converter with LM5145

dimitar partenov
Prodigy 10 points
Part Number: LM5145

Tool/software:

Hello, I made a converter with LM5145 24V, based on the recommendations from the datasheet and the  quickstart calculator. The circuit is the same as shown in the datasheet, except for the transistors. I used as low and high side transistors FDBL86063-F085. The circuit works at 150 kHz, and the inductor is 12 μH, with a saturated current of 50A. Input power supply is 1000 W at 60 V. The circuit works well up to 15A. But my goal was to reach 25, 30A. When I load the circuit with a load of 25A, the output voltage drops to 8 V, and with a current of 8A. Any idea or guess as to the reasons?

  • 0
    TI__Genius 11425 points

    Hi Dimitar,

    please fill in the quick start calculator for this device and share with me www.ti.com/.../LM5146DESIGN-CALC. Please also share the schematic. 

    As a rule of thumb - the power levels that you are targeting here ~600W/720W - a simple single phase buck topology using LM5145 will be thermally limiting, I would recommend you look at more advanced topologies like: https://www.ti.com/tool/PMP23591; https://www.ti.com/tool/PMP23380

    Thanks,

    Best Regards,

    Taruy

  • 0 in reply to Taru Kapoor
    Prodigy 10 points

    Thank you for the quick response. I am attaching the schematic file and the quick calculator..board_24V.pdfquick_start_board_24_V.pdf

  • 0 in reply to dimitar partenov
    TI__Genius 11425 points

    Hi Dimitar,

    Most likely the device is hitting thermal limits here. The efficiency curve in the quick start shows a lot of power loss in the HS FET. At 25A - there's ~4.5W of power dissipation which is very high for a single FET to handle. Furthermore, the FET Rdson will increase as the temperature increases - this will make the FET losses even larger. For your application - please refer to the recommended reference designs shared in my previous reply. 

    I also have few other comments on the design:

    1. The output caps are rated for 25V. For a 24V Vout - there will be significant DC Bias Derating in the Caps which will significantly reduce the net output capacitance - this will change the loop response of your circuit. I would recommend you use 50V rated output caps. 

    2. The design is also marginally stable (45 degrees) - with derated Cout - the design may be getting into instability. 

    Thanks,

    Best Regards,

    Taru

  • 0 in reply to Taru Kapoor
    Prodigy 10 points

    Hello Tru,

    Thanks for the answer. I will migrate to another topology. If I put two transistors in parallel, will this bypass the thermal mode, or will LM5145 not have power to drive the transistors?