# 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)

Schematic of the circuit

2. ### Papabravo Expert

Feb 24, 2006
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Last edited: Jan 14, 2017
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3. ### ronv AAC Fanatic!

Nov 12, 2008
3,692
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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
3,692
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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

On Spike

Off Spike

P-P led ripple is 26.7mA

P-P inductor current ripple 112.85mA

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:

Current waveform inductor:

Current ripple diode:

Current ripple inductor:

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
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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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