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TNY274 design, occasional audible noise

Posted by: HenkC on

Hi,

I have a mains powered standby power supply built around the TNY274, see attached schematic. When switching on the equipment in which it is used, everything always starts up normally, including the main power supply. However, sometimes an annoying audible noise can be heard from this supply around the TNY274. In most cases it is quiet, but occasionally it seems to work in another mode. Operation in itself is ok. I have a few questions.

1. What could cause this?

2. Can this behavior cause damage?

3. How can this be avoided?

Thanks and regards,

Henk

Files

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Schematic with TNY274 61.34 KB

评论

Submitted by PI-Jedidiah on 01/04/2022

Hi HenkC,

The possible cause of this is your power supply design is operating in the range of audible noise frequencies. This won't cause any severe damage. Varnishing transformer will help reduce the audible noise or increasing the switching frequency range of the design.

Regards,

Jedidiah

Submitted by HenkC on 01/05/2022

Hello Jedidiah,

Thanks for your answer. According to the datasheet, the oscillator frequency is internally set to 132 kHz. Furthermore, I had seen below section in the datasheet:

"The current limit state machine reduces the current limit by

discrete amounts at light loads when TinySwitch-III is likely to

switch in the audible frequency range. The lower current limit

raises the effective switching frequency above the audio range

and reduces the transformer fl ux density, including the

associated audible noise. The state machine monitors the

sequence of enable events to determine the load condition and

adjusts the current limit level accordingly in discrete amounts."

What made me wonder is why the supply most of the time is quiet, but sometimes makes noise, even though the load is the same in both situations.

Regards,

Henk

Submitted by PI-Jedidiah on 01/05/2022

Hi HenkC,

You may try to further decrease the flux density of the transformer by increasing transformer number of turns. We're you able to capture the switching frequency waveforms? Do you have transformer design or PIXLs/PIexpert design your are using for your power supply?

Below is the link that you can use for sample computations:

https://piexpertonline.power.com/

Regards,

Jedidiah

Submitted by HenkC on 01/06/2022

Thanks Jedidiah. I will have a look at that.