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TPS55340 Inductor Question

Kevin Stevens
Intellectual 350 points
Other Parts Discussed in Thread: TPS55340EVM-147, TPS55340

I have the TPS55340EVM-147 board that I am testing with. The only thing I am changing now is the inductor. Originally I did not have an inductor that met the calculated criteria for Isat. I used a Coilcraft MSD7342-472 which has an Isat of 4.1A for 10% drop. My calculation showed that it should be 8A.

I went ahead and got the same type of inductor as was originally installed on the board, just in a different inductance value. I got a Coilcraft MSD1206-472.

Originally using the 7342 I was able to get over 670mA of output (3.3V input and 12V output). However, when I replaced it with the 1206 I wasn't even able to get 470mA. I fully expected the new inductor to make the part work better, but it in fact made it worse. I looked at the specifications for both parts and I do not understand why this would happen. I can go ahead and use the 7342 since it seems to work, but I need to understand why this occurred so I can create a reliable design.

  • 0
    TI__Mastermind 35165 points

    Hi Kevin,

    First to make sure I'm looking at the right inductors, do you mean MSD1260-472 for the part number? I agree, the higher saturation current part should work better.

    Since this results is not expected the first thing I recommend trying is to make sure the coupled inductor was soldered down very well and also make sure there wasn't any issue with the setup.

    After double checking this could you capture some waveforms with each inductor to show how the part is operating? Key waveforms would be the SW pin and VOUT with a time scale that is small enough to see individual switching pulses. This may provide a hint as to what is happening.

    Thanks,
    Anthony

  • 0 in reply to Anthony F
    Intellectual 350 points

    Yes sorry about that. It is the 1260. I will check the soldering when I am in again on Thursday, but it seemed pretty solid.

  • 0 in reply to Kevin Stevens
    Intellectual 350 points

    I soldered the MSD1260-472 inductor in again and this time it works as well as the MSD7342-472.

    I guess a poor soldering job must have been the issue.

    However, I am curious as to the benefits of the larger inductor (MSD1260-472). Based on my testing I am not seeing any difference between the two in performance and physically the MSD1260-472 is much larger.

    Is there a reason I should stick with the MSD1260-472 over the MSD7342-472?

  • 0 in reply to Kevin Stevens
    TI__Mastermind 35165 points

    I'm glad to see re soldering the inductor solved this issue.

    The larger package advantage is it will be lower resistance so should give better efficiency and slightly higher output current. Looking at the specs, the MSD7342-472 is 51mΩ and the MSD1260-472 is 36mΩ. The difference in efficiency may not be too much but it should be there. If the MSD7342-472 performance is sufficient it's ok.

    One thing to be careful of with the MSD7342-472 is the saturation rating is below the switch current limit of the TPS55340. This may only impact the design during a short circuit or extreme transient conditions. For example, if the coupled inductor becomes highly saturated during short circuit conditions, with the reduced inductance the minimum on time of the TPS55340 may allow the peak current to become very high. In this case the current limit will not be able to protect the coupled inductor or the TPS55340. There is then some risk of damaging the components.

    If this is a concern, it would be good to check the current stress in both windings of the coupled inductor during a short circuit.

    Best Regards,
    Anthony