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LM3150 12V PSU voltage rise

Krystof Vanek
Prodigy 30 points
Other Parts Discussed in Thread: LM3150

Hello,

We've been working on LM3150 based PSU designed by webench. We've come to last stage of testing but theres one problem which occured while testing 12V output (The board can be used for 5V/3V/12V). 
While using 10k2 / 191k resistor divider the PSU starts at iddle output voltage 12,2V. Then under load it rises up to 12,55V which is rather unexpected behavior. Its supposed to start at 12,2V but then drov to about 11,8V which is not happening.

Could you please help us find out what's wrong? Thank you very much

 

Used Layout:

  • 0
    TI__Genius 16025 points

    Hi Krystof,

    Can you supply the BOM list for your PCB?

    Thanks,

    Anston

  • 0 in reply to Anston Lobo
    Prodigy 20 points

    Hello Anston,

    BOM is attached. But there is one change: 

    We changed Cout ceramic capacitors C3225X7R1C226M (Qty.:10) for aluminium capacitor 16SVPE180M (Qty.:1).
    With ceramic capacitors we wasn't able to make it working correctly. When switched on with ceramic capacitors the output voltage get up to 16-18V and then was slowly(ca. 5s) discharging to 12V, after reaching 12V output voltage rises again to 16-18V. When load connected, no current was going through and cycle was still the same as described.

    With aluminium capacitor it works well, but there is problem, which my colleague described in first post... And this problem remains even if Cac, Cr and Rr removed and Cff 2,2nF connected. 

    Thank you for helping us with this problem
    Have a nice day,

    Zbysek

    BOM.pdf
  • 0 in reply to zbysek.vanek
    TI__Genius 16025 points

    Hi Zybeek,

    I'm sorry for the delay, as I was out on business. I'm evaluated your BOM and found the Ron to be quite high! Also since this controller uses the COT architecture with emulated ripple mode, please provide both ceramic and AlumElect caps on the output. This design needs a small amount of ESR on the output caps to work correctly since the IC uses it to sense the load using ripple.

    Please try reducing your Ron to about 100-500k and checking the stability. If it is still not stable, then try increasing the Cbst cap to 1uF. Let me know.

    Thanks,

    Anston

  • 0 in reply to Anston Lobo
    Prodigy 20 points

    Hello Anston,

    we have already tested some changes during the time from our last post and there was a noticeable change, when combined Aluminium and ceramic caps. Now its starting on Vout 12.08V with load 0.5A, going down to 12.03V - 2.5A and raising again to 12.07V with load 8A. 

    About the Ron, this one 1M15 is webench calculated. We have this setup: Vin 20-28V, Vout 12V, I max 8A, Fs 100kHz. You can check it here. Maybe a webench bug?

    My last question is, what would happen if the PSU were staying in "rising mode"(no load=lower Vout than with 8A load). Would it shorten the lifetime of the PSU or damage some parts? 

    Thank you

  • 0 in reply to zandy
    TI__Genius 16025 points

    Hi Aneta,

    From your post I gather the part seems to be operating correctly now. The Ron value should be okay. I didn't see that value in the datasheet so was curious. 

    To answer your last question, there is no damage to the PSU or parts due to rising mode. The controller on it's own is unaffected and standard reliability is to be expected. As far as lifetime of the passives and FET's, you will have to refer that to the datasheets of the respective parts.

    Thanks,

    Anston

  • 0 in reply to Anston Lobo
    Prodigy 20 points

    Thank you, 

    have a nice day