DC fans creating electronic noise issue

[nickagian]

Hi!

In order to cool a telcom device I am using 4x 12Vdc fans from Delta Electronics (the exact type is FFB0412VHN-F00). The fans have a 3-wire interface and I operate them by sending PWM pulses at their power supply wire. The pulses are generated from a PIC-microcontroller and are "converted" to 12V pulses through two FETs (see attached schematic).

The issue is that the fans seem to disturbe the 12V power supply and as a result they generate several different problems, like bit errors at the transmissions etc, which seem to disappear when I remove the fans. A logical thought was that the pulse current drawn periodically from the fans is causing small dips in the power supply and I was able indeed to verify that with an oscilloscope. Therefore I built an LC low-pass filter in order to make the 12V quieter. And it seems to work when the duty cycle of the PWM pulses is low.

What however I still not understand is that when I give full power to the fans (the duty cycle is 100%) then I still have the same problems as before. And that actually still puzzles me because at full power there are no pulse currents drawn from the 12V... I have a feeling that the filter has worked at low duty cycle values because it creates an additional voltage drop and thus the fans receive a lower rms power supply.

Does anyone have any idea on what to do?

 

[alec_t]

You could try diode-isolated supplies, like this


Have you got a star ground system, so that fan current doesn't flow through data ground lines?

 

[nickagian]

You could try diode-isolated supplies, like this
View attachment 97534

Have you got a star ground system, so that fan current doesn't flow through data ground lines?

Well, unfortunately no we don't have a star ground system, which would of course help a lot. Actually we use the same ground surface for everything. We usually try to avoid interruptions on the ground so that our fast signals have proper return paths.

That with the diode is actually not a bad idea at all but would it really help? After all the fans will still need the current they needed. So the 12V voltage converter will need to supply this current.

 

[alec_t]

would it really help?

Don't know. Try it and see . The idea is that the fans draw peak current from their cap (the bigger the better) without pulling down the supply to the other stuff.