how to remove noise generated in the inductor of a buck regulator

Good day to all,
Please I need your help on how to remove the noise generated in the inductor of a buck Regulator.I used the buck Regulator (A6213 from Allegro microsystem)to drive string of Leds.Everything is working except the noise that is coming from the inductor.Whenever i increase the brightness of the Leds,the noise increases while a decrease in brightness decreases the noise.I have used the recomended board layout in the datasheet to avoid this Problem but unfortunately seems it does not solve the problem.
I will be glad if somebody could put me through on how to solve this Problem.Thank you for the usual Support.
Best regards.

John,

I assume that the noise you are referring to is flickering of the LEDs or excessive current ripple. If this is the case, Figure 13 of the data sheet should tell you where to set your frequency operating point or inductance value based on the current you are trying to supply. The data sheet has all the design information you should need.

The data sheet can be found here: "A6213()":http://www.allegromicro.com/en/Products/Regulators-And-Lighting/LED-Drivers-For-Lighting/A6213.aspx

The circuit should be in a metal screened enclosure, such as a tin can. If it is not the inductor coil will be susceptible to ambient radiation. Inspect the place where you are operating the circuit. Is it near an AC-DC power supply, a PC computer or screen, a mobile phone, a radio, an MP3 player, etc? The normal 40KHz frequency of a switching PSU (your circuit) is similar to digital audio stereo at 44KHz. It is also similar to other switching PSU's.

If EM radiation is the issue you are having as Nick suggests you should also use a toroid for your inductor, that will keep most of the magnetic flux contained.

thank you for the Response.The Leds do not flicker.Everything is stable except the fact the inductor keeps making a Kind of noise/Vibration(hmmmmmmmm).

Since you indicated in one response that it is ACOUSTIC noise you're talking about, the solution would seem to be to replace the inductor. Load current pulses certainly can have magnetic effects in an inductor which might cause it to 'sing,' by mechanically vibrating some part of the inductor itself or its relation to its neighbors. Changing the operating frequency of the circuit would probably confirm this. But it might be easier to change to a different inductor -- a different brand, a different configuration, a different orientation on the circuit board. Maybe just put a glob of RTV on it to dampen the vibration, if nothing else works.

In the field of electronic engineering, the term "noise" usually refers to unwanted fluctuations in an electrical signal, rather than sound, so we were rather caught out at first !

So the problem is audio frequency physical vibrations in your inductor.

1) Are you able to determine the frequency of the buck regulator circuit (using an oscilloscope or DMM with frequency range), which (as others have indicated) is usually well above an audible range. An oscilloscope would show if there is an additional frequency component caused by some fault in the circuit.

2) Are you sure the supply is not overloaded by the number of LEDs ?

3) Have you tried adding extra capacitance across the output terminals ?

4) As Mark indicates, loose parts in the components can be a problem. If this is so, a possible solution may be a drop of glue to solidify the relative positions of the coil and core. (Epoxy resin would be ideal.)

(Old transformers often quieten down a lot if loose core laminations are glued together.)

5) It is unlikely to be your problem, but for interest: see Core losses - magnetostriction on: http://en.wikipedia.org/wiki/Transformer#Energy_losses

You certainly threw us all, electronics engineers assume the word 'noise' is electronic / radiation related. You now say 'hmmmm' implying audible hum, likely to be 50Hz or 100 Hz (double for rectified but not yet smoothed). In US use 60 and 120.
If an inductor is giving off audible noise there is something mechanically loose. Exactly what will depend on your inductor type, its physical construction design. Someone mentioned laminations which is the most common cause of hum. These rectangular style inductors are not normally used in switching PSU's.
Do you have the correct component? Your device sounds old and tired. Someone else mentioned using a toroid, a good suggestion for many reasons.

I took "noise" to mean ringing, since it was referenced to a "inductor". If so, then what you probably need is a snubber circuit. But, it would help if we could see a schematic of what you have, and more info on this "noise", i.e. is it an audible noise, or is it something that you see on an oscilloscope, or ??? And things like, how you know it's coming from the inductor. If you see this on an oscilloscope, may we see a screenshot (or similar) and an indication on a schematic of where you are probing when you see this.

If it's ringing, then here are some URLs to information how to design a snubber:

* http://www.cde.com/tech/design.pdf
* http://www.ti.com/ww/en/analog/power_management/snubber_circuit_design.html
* http://www.nxp.com/documents/application_note/AN11160.pdf
* or just Google "How to design a snubber circuit"

If it's audible noise, then Colin has the right idea. Then be sure of where the noise is coming from, try using a vinyl or rubber hose to locate the source of the sound [one end of the hose in your ear and then probe the running circuit with the other end of the hose until the sound is the loudest, etc]. Then you can try replacing the component, using a different brand, etc. But, if the switching frequency is in the audible range, then you may be out of luck. But, judging by the datasheet, it looks like this thing won't run below 500 kHz [or is not intended to be run lower than that], so I have my doubts that "noise" means "audible" ;)

If it's ripple, then you may need to increase the value of the filter capacitor or use several capacitors in parallel to reduce the ESR or select a different type of capacitor. You can also increase the amount of inductance. Increasing the frequency will also reduce ripple, as long as the inductor and other electronics can function at the higher frequency. The datasheet has more information about this. [_see my addendum, below_]

Also, make sure you are using an appropriate inductor. One thing the datasheet appears to leave out is the fact that the inductor you choose must have a self resonant frequency [SRF] that is higher than the highest frequency it will operate at. How high is high enough? That's on the edge of my knowledge, so I can only suggest the higher the better, with things like physical size and cost as a limiter. You're probably not going to run this thing at more than 1 to 2 MHz, so an inductor with an SRF of 10MHz or more should be fine.

BTW, there will always be ripple. And with a switch mode LED driver [such as what you have going] some ripple is tolerable, since, at the frequency range this device is intended to be run at [500kHz - 2 MHz] there will be no visual perception of any brightness fluctuation in the LEDs caused by ripple. So, as the datasheet advises: design for a ripple of 10% to 30%.

Also, as the datasheet advises, you can throw a capacitor across the LEDs to reduce ripple, which might reduce stress on the LED[s] if you're running them at or near their specified maximum current. This would be the "filter capacitor" I referred to in my last comment. But, as I noticed after I hit "Submit" on this addendum, the data sheet advises against using an output filter capacitor for the reasons specified.

One other possibility. If this is noise seen on an oscilloscope or visual flickering in the LEDs, then it's possible that the thermal shutdown feature is kicking in. Possible reasons:

* Not enough heatsinking -- consider the thermal pad on the bottom of the chip -- might need better contact with copper, etc. See the "Thermal Dissipation" and "Optimizing Thermal Layout" in datasheet.
* Running that too high a current.
* Improper PCB layout -- see "Component Placement and PCB Layout Guidelines"

Good day to all,
Thank you all for taking the time to answer my question.I really appreciate your sense of understanding and Support.
The problem was eventually solved by adjustment of PWM SIGNAL FREQUNCY as suggested above.I went through the datasheet of the chip again and i realised that the frequncy range of pwm-signal should be between 100Hz and 2Khz.My Initial frequency was more than 2Khz so adjusting it to 100Hz removed the noise.
Once again thank you for the Support and guide.
Best regards.

Really -- 100Hz? As in 100 Cycles Per Second? May I ask what size inductor you're using? And, we're still talking about the "A6213 from Allegro", right?

Ohhh.. [slapping hand to forehead] .. the PWM _dimming_ signal -- now, that makes more sense! My bad :P

I was still hung up on your reference to the "inductor".

So, as pointed out by some of the others, if the noise is coming from the inductor, then, as they said, you might try a different one.

And just an FYI, one drawback of using such a low frequency is the increased potential for flicker. 100Hz is very close to the border line [for most people] for what is know as the "flicker fusion threshold". This is the point at which the eye[brain] no longer perceives flicker in things like TV screens, and apropos to this discussion: LED lighting. The thing is, different people have different flicker fusion thresholds, and some may experience fatigue and/or headaches under lighting at or even above 100Hz. Plus, at such a low frequency as 100Hz, there will be much more of a "strobing" effect -- especially if your light[s] will be moving. And even for a stationary light, there can be strobing - caused by the eye moving across the light. Also, peripheral vision is more sensitive to flicker and can see it at higher frequencies than center vision. Just something to consider.

Here's more on the repercussions of Flicker Frequency:

* http://www.ormenww.com/assets/flickering.pdf

Especially scroll down to the end of the document where you'll find Table 1 -- _Flicker and Biological Effects_.

This is much more of a problem with LED lights because there is no _persistence_. In other words, when an LED is pulse width modulated it is either all the way on or all the way off. LEDs switch _very_ fast. With most other forms of AC driven lighting, such as incandescent and florescent, the light producing element does not immediately go dark, like LEDs do. When the AC voltage dips to zero in an incandescent light, the filament, due to thermal inertia, continues to glow and only dims very slightly. Same with a florescent light, but for a different reason. The light in a florescent bulb comes from glowing phosphors that convert UV emitted by mercury vapor in the tube, into visible light. When the AC wave goes to 0 and the mercury is no longer radiating, the phosphors continue to glow, dimming only slightly (but more so than in incandescent lighting).

Light sources that have emitters that persist, can function at lower frequencies without causing annoying [and in some cases health compromising] effects. But, even with florescents, with a degree of persistence, at 100Hz, there are documented health effects. So, at 100Hz an LED light is going to be even more problematic when dimmed.

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