SM transformer audible noise and structure natural frequency

[Jakobster]

Hello,
it's my first posting on the site. My problem is not wholly just electronics, but hopefully someone can provide good advice.

I need to examine audible noise coming from a switch mode transformer component. In theory, the noise is created mainly because of the flux excitation affecting the ferrite core, making it vibrate.

I've been given to understand that a high-freq. transformer, such as this one, normally only creates ultrasonic freq. sound, which naturally doesn't cause noise disturbance. However, there are something called 'transient' and 'burst operation', which cause the noise freq. to come down to human hearing range. These two concepts I don't really understand.

I was first thinking of determining the natural frequency of the transformer structure and measuring the sound amplifying effect of resonance. I am wondering if it is specifically the resonance amplifying the sound that leads to disturbing levels of noise.

Any suggestions how to determine the natural frequency of the transformer's structure? Any ideas how to make the resonance less disturbing? Are there feasible ways to raise the natural frequency higher, close to or over 20kHz? Or how to keep the frequency in the ultrasonic range in this 'burst operation'?

Regards

 

[MacGyverS2000]

Of course it's the resonance that leads to annoying levels of sound, though you may get plenty of annoying sound levels before you hit resonance. Run a signal gen (with appropriate power) through the transformer and see where it sings... if you're lucky, the datasheet will have it (but don't count on it). Short of redesigning the transformer itself, you can using rubber mounting plates to reduce transfer/resonance with the mounting surface, but the best way is to ensure you're not running the transformer at audible frequencies.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[dpc]

Google up "magnetostriction" for more information on the underlying cause of the vibration.




"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[Comcokid]

I would suggest examining the control loop for your switch mode power supply. There might be a lower frequency component in the control loop that may need to be moved outside of the audible range by moving poles/zeros in your feedback or filtering circuits. Even though your device may be regulating fine, the loop may be oscillating at a low frequency. Your largest component - the transformer - is only the 'sound box' for an oscillation elsewhere.

 

[CharlieGill]

The suggestions above are all valid for natural resonance and the normal operation of a switch mode power supply. I noticed that you mentioned "burst operation". This is often associated with a lightly loaded SMPS. The control circuit will switch from continuous (at an ultrasonic or higher frequency) to non-continuous (burst) mode to maintain the output voltage under sub-optimal conditions (light load). Under these conditions, your 1 MHz SMPS may be "bursting" at 20KHz to maintain regulation and singing in the audio range.

I've had power supplies that were quiet as a mouse under full load but would squeal at light loads.

Charlie

Charlie Gill
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http://www.calibrator.com--