DC/DC converter giving acoustical noise 3

[SolarTrap]

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

 

[IRstuff]

Why did you change the inductor? Isn't it part of resonant circuit with the oscillator? Does it still make noise with the recommended value of inductance?

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[itsmoked]

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

 

[SolarTrap]

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.

 

[VEBill]

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.

 

[GregLocock]

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


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[itsmoked]

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

 

[benta]

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.

 

[SolarTrap]

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?

 

[VEBill]

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

Maybe.

 

[itsmoked]

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

 

[VEBill]

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.

 

[ScottyUK]

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.

 

[VEBill]

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.

 

[SolarTrap]

Below is the schematics. In need the buzzer so removing it is not an option. If it is high frequency EMI can I put a capacitor in parallel?

 

[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.

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

Keith Cress
kcress -

http://www.flaminsystems.com

 

[VEBill]

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.

 

[VEBill]

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

 

[itsmoked]

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

 

[SolarTrap]

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.