MOSFET oscillation - audible turn on/off click?

[ACharnley]

Hi All,

Here's one I haven't come across before. I have a voltage protection circuit using a FET which disconnects the load from the input at about 17v, waits for the output to discharge to a low level (about 5v) and then reconnects. This induces an oscillation of about 2-3Hz.

I've noticed there's a noticeable clicking noise when the SOT-23 FET connects and I'm wondering if this can occur normally or if I should be worried. The load is of very low ESR (lots of ceramics) and I'm pondering if that click is an electrical spark inside the FET, in which case could that cause the material inside it to wear and break down?

 

[ACharnley]

Sorry I should mention, there's no concernable power draw (my power supply isn't even registering 2mA) and the FET temperature isn't rising at all (ambient).

 

[ACharnley]

Ah now I have it, connected an additional bank of mlcc's externally and it's the source. Not had this noise from ceramics before but after some research it's normal.

 

[rjenkinsgb]

It's probably coming from the ceramic caps - the dielectric tends to have piezoelectric properties, so a rapid change of voltage could possibly cause them to make some sound?

I know for a fact the reverse is true - some cheap & nasty microphones have multilayer ceramics in the audio path and they can still pick up sound to some extent (eg. tapping the mic body) with the actual mic capsule disconnected...

Another possibility is the influence of magnetic fields on conductors during a short high current pulse. That could be causing slight movement of any loose wiring?

 

[ACharnley]

You're right it's the ceramics, something new every day!