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DC/DC converter giving acoustical noise

DC/DC converter giving acoustical noise

(OP)
Hello All,

I am using a DC/DC converter from 24V to 5V for my Arduino project. The converter is producing a high pitch sound that I would like to silence. I am using a TI MC34063AD in the Step-Down configuration on page 8 but coil 330mH instead of 220mH.

Any Suggestions?

Thanks
Markus

RE: DC/DC converter giving acoustical noise

Got a picture of your board?

Is this an FRP board?

Are the components exactly what's on page 8?

Is everything surface mount?

Keith Cress
kcress - http://www.flaminsystems.com

RE: DC/DC converter giving acoustical noise

(OP)
I have attached a pic: The Dc/DC is in the lower right corner of the red box - you can see the coil.
I am not exactly sure what the material of the PCB is. The components are as on page 8 but I can also supply the circuit drawings if that helps.

RE: DC/DC converter giving acoustical noise

Under the 'Remove Seal After Washing' sticker is probably a beeper. Make sure that's not been accidentally programmed on. Unlikely, but mentioned for completeness.

Next is the coil. If the windings are vibrating, then maybe a bit of glue would hold them fast, and thus stop the noise.




RE: DC/DC converter giving acoustical noise

I've got a buck boost converter that runs at 44 kHz. Either I've turned into a bat or it creates subharmonics, I can definitely hear a high pitched (>5 kHz) tone. Anyawy, getting rid of these high frequencies is easy. Mass. Put it in a heavy box with no holes in it.

Cheers

Greg Locock


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RE: DC/DC converter giving acoustical noise

SolarTrap: Comments. First you have very well thought out board there. Nicely done. Kudos.

That is an FRP board. Fiber Reinforced Plastic The standard green PCB

There are three possibilities for the noise.
1) The inductor coils as VE1BLL suggests. It would likely be the wires moving and glue would help.

2) Capacitors with high freq currents can also produce noise. However it's more usually ceramic caps that are surface mounted to the board and not aluminum caps like what I'm seeing.

3) Your piezo noise maker is REALLY close to the high current high frequency traces. You could have some of that current exciting it or its drive circuit.

In all cases you should be able to hunt down your culprit using simple mechanical means. Use a pencil eraser to apply some pressure to all the various suspects. Listen for any obvious audio responses to your probings.

If it is the inductor there are a lot of different style inductors that will be similar in size but radically different in construction. I'd consider trying a few to find a quieter one. Look to Coilcraft.

Keith Cress
kcress - http://www.flaminsystems.com

RE: DC/DC converter giving acoustical noise

Could be that you are drawing too little current from the converter. This makes it operate in discontinuous mode, resulting in audible magnetic contractions in the inductor.

Benta.

RE: DC/DC converter giving acoustical noise

(OP)
Keith had the right idea: the piezo buzzer is making the noise.
Any suggestions how to fix that - except putting the entire PCB into a sealed box?

RE: DC/DC converter giving acoustical noise

Add a diode in series, and adjacent, to 'block' low voltage noise.

Maybe.

RE: DC/DC converter giving acoustical noise

Thanks Solar for coming back with results. It's very informative for us to get confirmations.

I'm not sure how you're driving the piezo.. Hard to offer suggestions. Can you put up at least that part of your schematic?

VE1BLL; Not sure what you're describing. I'm lousy at brief text schematics. LOL

Keith Cress
kcress - http://www.flaminsystems.com

RE: DC/DC converter giving acoustical noise

If the high current traces are coupling onto the traces feeding the piezo, then the induced voltage at the piezo might actually be quite low. Maybe a series diode, installed near the piezo, might be enough to stop the induced current. The old Vf trick.

RE: DC/DC converter giving acoustical noise

A blob of RTV in the piezo will stop its noise permanently. Just be sure you don't want to use it later. Or if it's a through-hole device simply remove it.

RE: DC/DC converter giving acoustical noise

If you haven't already, pull off the 'Remove Seal After Washing' board wash seal from the piezo beeper. This seal is just to keep water out when washing the board after soldering. If it's stil in place, it may be blocking the full volume of the sound, thus causing confusion.

Once peeled, if the beeping is revealed as being full volume, then perhaps it's trying to tell you something.

It'd be surprising if induced EMI would achieve full volume. EMI would normally be a relatively small signal, of course YMMV.

You could measure the voltage / waveform on the piezo to make sure.


RE: DC/DC converter giving acoustical noise

Ah.. Well for starter's, not having a resistor from the FET's gate to ground is a rookie mistake. There is absolutely no guarantee that the output pin of the processor won't leak some small current even when 'turned off'. You're always supposed to include a resistor to ground to 'bleed' this leakage current away.

Try a 10k or a 47k ohm resistor there.

Keith Cress
kcress - http://www.flaminsystems.com

RE: DC/DC converter giving acoustical noise

Using any old DVOM, measure the voltage at pin 2 of the beeper. If it's close to Vcc, then the Q2 FET is off. If it's close to 0v, then the FET is ON.

RE: DC/DC converter giving acoustical noise

BTW, great job with the in-line images. Fantastic. Makes these threads so easy. Just great.

RE: DC/DC converter giving acoustical noise

Hi VE1BLL. Keep in mind that if the processor heats up things cold change from room temp. e.g. the FET could THEN turn on.

Keith Cress
kcress - http://www.flaminsystems.com

RE: DC/DC converter giving acoustical noise

(OP)
I have tried you suggestions and this are the results:

- various resistors across the G-S pins of the mosfet: no effect. I even shorted the connection.

Then I took a metal plate and inserted it between the buzzer and the cap next too it (the cap between the coil and the buzzer) and that lowered the noise. It seems to me that the noise is introduced only by the EM field of the coil -> buzzer is too close and I have to re-design the board for the next revision.

RE: DC/DC converter giving acoustical noise

Quote (itsmoked)

Ah.. Well for starter's, not having a resistor from the FET's gate to ground is a rookie mistake. There is absolutely no guarantee that the output pin of the processor won't leak some small current even when 'turned off'. You're always supposed to include a resistor to ground to 'bleed' this leakage current away.

I can't tell what processor you have in there, but if it's a GPIO that FET is connected to, it's likely the GPIO line has an internal pull-up/-down that you can enable by flipping a register bit.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: DC/DC converter giving acoustical noise

(OP)
It is an ATmega2560 and is has a pull-up mode - but I dont see how this would help because it would turn on the buzzer.

BTW: I have just another problem with my PCB and I started another thread: http://www.eng-tips.com/viewthread.cfm?qid=368902

Thanks for all your help so far!

RE: DC/DC converter giving acoustical noise

In our next design, make the high keep the buzzer off... one less component to put on the board.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: DC/DC converter giving acoustical noise

I just thought of something... if you truly think radiated EMI is what is causing your problem, swap out the coil for a shielded version. A few pennies more, but it likely means not having to redesign the entire board for one simple component.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: DC/DC converter giving acoustical noise

MArkus; Even if the resistor isn't fixing the immediate problem 'good design' requires it so don't leave it out of any rev you do.

Have you tried a 200 ohm resistor across the piezo to swamp the magnetic leakage?

Keith Cress
kcress - http://www.flaminsystems.com

RE: DC/DC converter giving acoustical noise

Oh, and Dan has an excellent point about using a sheilded coil instead of that positively un-sheilded inductor you have.

Keith Cress
kcress - http://www.flaminsystems.com

RE: DC/DC converter giving acoustical noise

(OP)
I tried even to shorten the contacts of the buzzer but that has no audible effect. It seems this is entirely caused by radiated EMI.
In the next rev I will have to pick a different coil and a new location for the buzzer.

Thanks to everyone looking into this!

RE: DC/DC converter giving acoustical noise

Before you worry about the next design, you need to fix the current one. Don't assume a shielded can or a moved buzzer will fix the issue, only to find it's something else. Swapping out the inductor for a shielded version is a very easy test. Putting the buzzer and control circuitry on longer leads is also pretty easy in the grand scheme of things.

Try before you buy...

Dan - Owner
http://www.Hi-TecDesigns.com

RE: DC/DC converter giving acoustical noise

(OP)
I have a spare buzzer and just holding it next to the coil and the cap makes sound. Unfortunately the coil is SMD and I don't know how to get it off the board.

RE: DC/DC converter giving acoustical noise

Try to cover coil (or coil + capacitor even) by a piece of copper or aluminum.
Buzzer is too close to switching converter.
Maybe more easy, as sugested, is to move buzzer away by twisted wires of minim 10cm.

RE: DC/DC converter giving acoustical noise

Had a thought. Some PWM outputs on the Arduino can never be fully turned off. It is a little glitch in the timer software. I found in in some notes about using the PWM.

RE: DC/DC converter giving acoustical noise

OP says he can simply hold another beeper in his hand near the circuit, unconnected, and it will emit sound.

Perhaps use a less-amazingly-sensitive beeper.

RE: DC/DC converter giving acoustical noise

Quote (OperaHouse)

Some PWM outputs on the Arduino can never be fully turned off. It is a little glitch in the timer software. I found in in some notes about using the PWM.
Yeah, it's a PITA when you want to drive a motor but still keep absolute positioning without a feedback mechanism. But I don't think that's the issue here...

Dan - Owner
http://www.Hi-TecDesigns.com

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