Current Spike In Buck Converter

Discussion in 'Power Electronics' started by Gamer91, Jan 14, 2017.

  1. Gamer91

    Thread Starter New Member

    Sep 23, 2016
    24
    1
    What's causing this current ripple when the PWM is turned low?

    I've calculated everything up and came up with the following.

    Vin – 12vdc
    Vout – 3.1vdc
    Iout– 350mA
    Led driver – LT3517
    Led – Cree Xlamp XM-L Color (White Die)
    IC frequency – 500kHz
    Pwm frequecy - 500kHz

    Inductor selection:

    Dbuck = Vled / Vin

    3.1/ 12 = .2583

    Tsw = 1/Frequency

    1/500,000 = .000002

    ΔI = Percentage of current ripple x Iout

    .30 x .350 = .105

    L(µH) = Dbuck • Tsw(µs) • (Vin – Vled) / ΔI

    .2583 x .000002 x 8.9 / .105 = L(µH) = 0.000043788

    Inductor = 43.7uH

    There isn't a calculation for the output capacitor in the document for the IC, so I sourced a document from a google search and found a Texas Instrument document on buck converter design and it says to use the following to calculate the output capacitor. Also read in that same document that 25mV is a good set point for output voltage ripple as the following equation require you to put in a value for the
    desired output voltage ripple.

    Capacitor selection:

    Cout(min) = ΔIL / 8 x Fsw x Δvout

    .105 / 8 x 500,000 x .025 = .00000105

    Capacitor Out = 1.05uF

    Voltage probed after mosfet and before LED = Green(n003)
    Voltage probed after led before inductor = Teal(n004)
    PWM voltage = Red(n012)
    Current through LED = Blue(D2)
    Current through inductor = Purple(Inductor)
    [​IMG]
    Schematic of the circuit
    [​IMG]
     
  2. Papabravo

    Expert

    Feb 24, 2006
    12,040
    2,590
    Last edited: Jan 14, 2017
    Gamer91 likes this.
  3. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Can you post your .asc file?
     
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  4. Gamer91

    Thread Starter New Member

    Sep 23, 2016
    24
    1
    Thanks for sourcing the documents, I'll start digesting those, ha ha.

    When I click upload a file nothing is showing up but folders.
     
  5. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Okay, I put it in Spice. One thing I think is you PWM frequency is to high. Try like a 10MS period.
    Lots of little switching spikes. Which one is bothering you?
     
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  6. Gamer91

    Thread Starter New Member

    Sep 23, 2016
    24
    1
    Hey man, thanks for the response, and taking the time to punch it all into LTspice.

    I sent an email the moderator that sent me the welcome message to try and figure out why I can't upload any attachments.

    After you letting me know that I read in the IC document that min on/off time is 4 x Tsw of the micro controller. At 500kHz that would be 8uS, I had it set to 2uS.

    Thanks man! Made the square wave square!

    I am still having current spikes, but now at the beginning and end of the pwm switching.

    Led current spike on/1.4a off/.788a
    [​IMG]

    On Spike
    [​IMG]

    Off Spike
    [​IMG]

    P-P led ripple is 26.7mA
    [​IMG]

    Max current inductor about 408mA
    [​IMG]

    P-P inductor current ripple 112.85mA
    [​IMG]

    Thanks again man, much appreciated!

    Any idea what's causing the spikes?
     
    Last edited: Jan 16, 2017
  7. ronv

    AAC Fanatic!

    Nov 12, 2008
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    The big one is from charging the cap when the PWM turns back on. I'm not clear yet on the little one.
     
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  8. eetech00

    Senior Member

    Jun 8, 2013
    1,470
    289
    Hi

    Just a couple of questions...
    Why reduce the switching frequency? You can use smaller value components if you keep the 1 Mhz switching frequency.
    Why are using a 500 Khz PWM frequency? Why not use a much lower frequency like 500 Hz?

    I think the spike is caused by the model. If you insert a 1k resistor in series with the gate, it greatly reduces the spike.
     
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  9. Gamer91

    Thread Starter New Member

    Sep 23, 2016
    24
    1
    If I move Cfilter to the left of the mosfet or take it completely out of the circuit, the current waveform and ripple is the exact same on the LED and inductor @112mA p-p ripple. But does do away with the current ripples at turn on and off.

    Am I messing up on capacitor calculations?

    Current waveform diode:
    [​IMG]

    Current waveform inductor:
    [​IMG]

    Current ripple diode:
    [​IMG]

    Current ripple inductor:
    [​IMG]

    I can increase the frequency, there's really no reason other than EMI, for me keeping the frequency low. If you guys have any suggestions like that
    let me know. I appreciate the help and knowledge I'm gaining from you guys.

    I slowed the frequency to 500hz, plan to drive this from Arduino that I will integrate into the PCB.
     
    Last edited: Jan 17, 2017
  10. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,692
    2,837
    The cap gets rid of the high frequency ripple current thru the diode. I'm not sure how important that is since you can't see it anyway. Try a 4.7Ufd. ceramic.
    If your worried about the spike, I don't think I would. It's only a few nanoseconds in length. Not enough to do any damage.
     
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