• State Resolved
  • Locked Locked
  • Replies 9 replies
  • Subscribers 9 subscribers
  • Views 207 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM34936: heat generated by the external FET

Yuta Nishie
Genius 3655 points
Part Number: LM34936
Other Parts Discussed in Thread: LM51772, LM5175

Tool/software:

Hi,

I designed the board using LM34936. However, the amount of heat generated by the external FET is a big problem.

I would like you to check the circuit diagram and measurement results, so I would like to send it by private message. Is it possible for you to handle it?

Best Regards,

Nishie

  • 0
    Genius 3655 points

    Hi,

    I would appreciate it if someone could help me.

    Best Regards,

    Nishie

  • 0 in reply to Yuta Nishie
    TI__Mastermind 48711 points

    Hi Yuta,

    sorry, It looks like the answer did not get publish.

    You can write a private message if you right click on my name and select private message.

    Please always drop a note here that you have sent a private message so that i can check this private message box or if I am not available someone else from the team can get in contact with you.

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Genius 3655 points

    Hi Stefan-san,

    Thank you for your support.

    I sent you a private message. I would appreciate it if you could check it.

    Best Regards,

    Nishie

  • +1 in reply to Yuta Nishie
    TI__Mastermind 48711 points

    Hi Nishie,

    please check the undershoot on the SWx node - this seems to have undershoots of > 10V while per datasheet the max allowed value is -5V with transients less the 20ns

    This can be improved by adding Gate resistors and/or Snubber on the SWx nodes.

    It looks like the had been some trial - incl. adding gate resistors. How did this impact the thermal issue?

    Note: adding 10Ohm gate resistor would be to large as this interacts with the dead time sensing - use less the 5 Ohms 

    Also the layout will have an big impact on undershoots as this are mainly due to parasitic on the PCB.

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Genius 3655 points

    Hi Stefan-san,

    Thank you for your support.

    The customer added a 10 ohm gate resistor, but it did not solve the thermal problem.

    I am asking the customer to confirm the problem with a 5 ohm gate resistor. I am also asking the customer to confirm the layout. When I know the result, please let me talk to you again.

    Best Regards,

    Nishie

  • 0 in reply to Yuta Nishie
    Genius 3655 points

    Hi Stefan-san,

    I am sorry for my late reply.

    The customer changed the gate resistance value to 5Ω, and was able to reduce the heat generation a little, but the heat generation is still large. So, could you tell me about the following questions?

    1. Do you have data on the minimum load current value that can reduce the heat generation, including the power supply IC and FET? As the heat generation increases as the load becomes lighter, I would like to know the relationship between the heat generation and the load current.

    2. With the input/output conditions below, could you tell me about other ICs that generate less heat under light load? I would appreciate it if you could tell me which products are more efficient because they are battery driven.

    Input voltage: 18 V~28 V (battery), Output voltage: 24 V, Maximum output current: 10 A

    3. Could you tell me which FET can reduce heat under light load (almost no load)? Should I make a thread on FET for this question?

    Best Regards,

    Nishie

  • +1 in reply to Yuta Nishie
    TI__Guru 61275 points

    Hello Yuta,

    Let me try to answer your questions

    1. It is not normal that the MOSFETs get hot with a reduced load. It should be the other way around. More power, more heat because of higher losses. Are all MOSFETs getting hot at the same time? If this is the case, please measure the gate voltages of all MOSFETs (if possible at the same time). Be aware that the gate signals of the top MOSFETs need to be measured relative to the switch nodes.

    2. We have newer devices that could be used, e.g. LM51772. On the other hand, you might use LM5175 as it has a power save mode for low output power. LM34936 dos not have a power save mode. The heat at light load is not anything that is caused by the device. The converter design has to be changed to overcome this issue.

    3. If you check the quickstart calculator tool LM34936-BUCK-BOOST-QUICKSTART-TOOL Calculation tool | TI.com In the lowest part you find an estimation for the power losses in the FETs. You could check out different FETs there.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Genius 3655 points

    Hi Brigitte-san,

    Thank you for your support.

    I would like to proceed with the board revision based on the information you provided. If there are any unclear points during the revision, please let me consult with you again.

    Best Regards,

    Nishie

  • 0 in reply to Yuta Nishie
    TI__Prodigy 540 points

    Hi Nishie-san,

    Please feel free to reach out via e2e when you have follow up questions.

    Best regards

    Yezi