A new Ceramic Capacitor Piezoelectric Effect.

[montcler]

Hello all,

A few days ago I was given a circuit board that I developed to control a 900W motor PWM, this circuit board works with the A3941 Allegro driver. My surprise was when I started the engine, everything worked but there was a background sound.
Checking the whole circuit board until finding the sound as it spread all over the circuit board, at the end were CBoot capacitors, these capacitors are 1.5uF 50V (Ref digikey: 445-12701-1-ND) I realized that they are Of the CGA series, it may be that this influences.

The capacitors are near the driver as indicated by the datasheet.

I could do it so they would not make so much noise.

Here are some pictures attached.

THX.
Here are some pictures attached.

[w2aew]

It's possible that your PWM frequency happens to hit a mechanical/acoustic resonance of the caps and board.  Are you able to experiment with changing the PWM frequency to see if the volume of the sound changes (moves away from a resonant point)? 

[tautech]

Watch this vid for some addition understanding of cap behaviour:
https://www.eevblog.com/forum/chat/capacitors-explained-by-james-lewis-of-kemet/

[montcler]

Hi,

I have been changing the frequency of PWM (50hz, 100hz) and seems to put less attenuates enough but still heard whisper.
The PWM frequency of the motor is between 15Khz to 20khZ.

I have seen in digikey different series of capacitors ... C, CC, CL, GRM, CGA etc ...
It is not possible that switching to another series does not make noise ..

THX.

[Audioguru]

Film capacitors are used in audio circuits because they are not microphonic like ceramic capacitors.

[T3sl4co1l]

Bootstrap capacitors are unlikely to produce much sound, because the ripple voltage should be small.  This may give you some clue what to do, if they aren't.

It is true, the Piezoelectric effect is symmetrical, so that mechanical strain induces voltage, and vice versa.

Inductors often produce sound as well, for analogous reasons, though this time the effect is magnetostriction, which doesn't lead to as much microphonic behavior because it is a square-law effect.

Tim

[montcler]

Hi,

I'll do more tests and see what happens. Any recommendation...?

THX.