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LM5146-Q1: Buck-Converter for Automotive EV application

Rajesh RAJESH
Prodigy 40 points
Part Number: LM5146-Q1

Tool/software:

We are currently using the LM5146-Q1 for our 240W converter setup (VIN 35-60V, OUT 14V, 20A), and we are experiencing heat issues with both the MOSFET and inductor, where temperatures exceed 130°C.

We are using the following components:
- MOSFET: IAUC90N10S5N062ATMA1 (configured in parallel for high-side and low-side, two units)
- Inductor: 4.7µH (also in parallel, two units)

Our current switching frequency is set at 169kHz, but have tried increasing it, resulting in higher temperatures in the MOSFET. Also, we are currently utilizing a two-layer PCB.

We have been following the values from your Excel design file (Quick Calculator) and would appreciate some clarifications to help us optimize our design and address the heating issues we are encountering.

Could you please provide guidance or insights regarding the following points?
1. Are there any recommended adjustments in the design variables that could help reduce heat in the selected components?
2. Would increasing / decreasing the switching frequency yield better thermal performance?
3. Are there any additional thermal management practices you’d suggest given our current setup?

Thank you in advance for your assistance. I look forward to your prompt response.

For better understanding please arrange one technical meeting with your team 

  • 0
    TI__Expert 6600 points

    Hi Rajesh,

    Please use the quick start calculator here to get an understanding of the power loss that you will expect in the design: https://www.ti.com/lit/zip/snvc220. 

    Some thumb rules and comments:

    1. Higher the switching frequency - worse is the efficiency. You can bring down the switching frequency further - this should marginally help with the overall efficiency.

    2. While the IC doesnt place a limitation on the current that needs to be driven - the max power of the design is being limited due to the DC resistance of the Inductor (which is leading to core losses and hence heating). Similarly, the conduction losses in the MOSFET will play a major role for the MOSFET being heated up. You can use the quick start calculator to get an understanding on what MOSFET to use. Also ensure you do not use FETs with very big packages since they come with high parasitic inductance, high Rdson, and high Qg and Qrr which make the efficiency worse. 

    3. Considering ~94% efficiency at Full Load (Output Power - 280W - Power loss - that is around 18W of power dissipation - the bulk of which will happen in the MOSFETs - hence choosing a MOSFET with a low enough Rdson is very important - especially on the low side since your application has a high step down ratio). 

    4. It is important to keep the layout optimized as well where the switching loop is as small as possible. Look at:  http://www.ti.com/lit/snva803

    Thanks,

    Best Regards,

    Taru

  • 0 in reply to Taru Kapoor
    Prodigy 40 points

    This IC capable for 14v and 20A ? 

    (LM5146QRGYRQ1 )

  • 0 in reply to Rajesh RAJESH
    TI__Expert 6600 points

    Hi Rajesh,

    The IC has no restriction on the output current since that is determined by the passives being used. As long as the LM5146-Q1 can drive the FETs (considering their gate capacitance, Vth, etc.). However, it may be difficult to run 280W (considering the availability of the right passives) and you may want to consider an interleaved/dual output approach for better thermals. 

    Please see the following reference designs:

    1. https://www.ti.com/tool/PMP23262
    2. https://www.ti.com/tool/PMP23420

    Thanks,

    Best Regards,

    Taru

  • 0 in reply to Taru Kapoor
    Prodigy 40 points

    PCB should be four or two layer do have any guidelines ?and also please let me know which one better thermal heating transparent 

  • 0 in reply to Rajesh RAJESH
    TI__Expert 6600 points

    Hi Rajesh,

    It is recommended to use 4-layer PCB. Please refer to section 11.1.4 Thermal Design and Layout in the datasheet for more information. 

    A four-layer PCB generally offers better heat dissipation due to the additional copper layer dedicated to ground planes, allowing for more efficient heat distribution and improved thermal management compared to a two-layer board with limited heat pathways

    I would also recommend you to look at the EVM layout and the layouts of the reference designs I have shared. 

    Thanks,

    Best Regards,

    Taru