+ Post New Thread
Results 1 to 5 of 5
  1. #1
    Full Member level 3
    Points: 2,319, Level: 11
    Achievements:
    7 years registered

    Join Date
    Jan 2011
    Posts
    156
    Helped
    5 / 5
    Points
    2,319
    Level
    11

    Spikes in output voltage in Buck Converter

    Hi All,

    I am testing a buck converter rated at 120V input and 48V output with 750W output power. The switching frequency is 100kHz. I am observing that the output voltage is having spikes during the switching . The LC filter of buck converter is 100uH and 100uF. The waveform is yellow one shown in figure . Its captured in AC Coupling.



    How can I eliminate spikes. Do I need to add another LC filter apart from the existing one. Thanks.

    •   AltAdvertisement

        
       

  2. #2
    Super Moderator
    Points: 80,248, Level: 69
    Achievements:
    7 years registered
    Awards:
    Most Frequent Poster 3rd Helpful Member

    Join Date
    Apr 2014
    Posts
    16,283
    Helped
    3692 / 3692
    Points
    80,248
    Level
    69

    Re: Spikes in output voltage in Buck Converter

    Hi,

    The LC filter of buck converter is 100uH and 100uF.
    "100uF" does sound like an electrolytic capacitor. These are not suitable for fast spikes. They are for low frequencies only.

    --> use a ceramics capacitor or a foil capacitor (value 100nF ... 1uF) in parallel to the electrolytics capacitor. Connect the scope directly at the ceramics/foil capacitor.


    Klaus
    Please donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



    •   AltAdvertisement

        
       

  3. #3
    Advanced Member level 5
    Points: 12,438, Level: 26

    Join Date
    Aug 2015
    Location
    Melbourne
    Posts
    2,031
    Helped
    747 / 747
    Points
    12,438
    Level
    26

    Re: Spikes in output voltage in Buck Converter

    your 100uF should be very low ESR/ESL - but, putting that to one side - it is just as likely your scope is picking up common mode impulses - try shorting the probe and connecting to pos out - if the spikes are still there - you have CM pickup ...!



    •   AltAdvertisement

        
       

  4. #4
    Advanced Member level 3
    Points: 6,500, Level: 19

    Join Date
    Feb 2014
    Posts
    934
    Helped
    312 / 312
    Points
    6,500
    Level
    19

    Re: Spikes in output voltage in Buck Converter

    "it is just as likely your scope is picking up common mode impulses"

    Yes what Easy said. I just started typing exactly these words.



  5. #5
    Advanced Member level 5
    Points: 40,508, Level: 49

    Join Date
    Mar 2008
    Location
    USA
    Posts
    6,531
    Helped
    1919 / 1919
    Points
    40,508
    Level
    49

    Re: Spikes in output voltage in Buck Converter

    Additionally the probe "loop area" (between tip+shaft,
    and ground clip) is an inductive pickup for all the
    inductor leakage field, and your choice of ground-
    point embeds more or less I*R, L*dI/dt voltage
    noise from circulating ground "plane" currents to what
    the 'scope channel sees.

    Output filter wants low ESL/ESR, but the inductor also
    contributes spiking through its interwinding capacitance.
    If you think the switching noise is unexpected, try adding
    a shunt C to your inductor model that corresponds to the
    advertised SRF. This is what your output filter must knock
    down. If you have 100V input and you want 100mV spike
    amplitude, that's 1000:1 attenuation and if the winding
    has 10pF shunt C, you'd need >= 10nF of really good HF cap
    returned as tightly as possible to prime ground (along with
    the fat baseband cap to soak up the 100kHz designed
    ripple current).


    2 members found this post helpful.

--[[ ]]--