PCB design guidance. Is AC in a power plane a bad idea?

Question

I have a 4 layer PCB board that I am working on a new revision for with the following general layer setup:

• Top=signal = signal layer
• middle1 = 5V power plane (with 24VAC subdivisions)
• middle2 = gnd plane (and a subdivision for a external gnd)
• bottom = signal layer

For most of the non-5V voltages, we get them to their destinations with top/bottom traces. There were just a handful of relays that needed 24VAC. The previous engineer added two power plane subdivisions to the 5VDC power plane (one on the outside for the return, and one that essentially bisected the board for the power.)

My question is: Is this type of configuration a bad idea? The boards function, but there has always been some amount of noise. We are constantly finding ways to improve it. This is the latest to get our attention. Would it have been strictly better to just have these as appropriately sized traces? Is an inner AC plane just a noise generating antenna that would compromise signal integrity more than if place as a signal trace?

I am primarily concerned with noise issues and not current, as we only run around a few amperes maximum and we have no concerns here at the moment.

Answer 1

24Vac 50Hz AC is not hard to prevent crosstalk unless any part of a high current loop is near a very high impedance voltage loop.

Consider 10MHz with 250MHz harmonics on a square wave, a greater AC concern using guard ground tracks.

The Relay flyback current may be a bigger concern with high dV/dt until the diodes conduct. So these tracks must not share pwr,gnd current paths with other analog circuits and are decoupled and guarded properly.