regulated PS grounding scheme questions

[CharlieLaub]

I'm designing a regulated (LM317/LM337) power supply for some active circuits that I want to use. I have everything worked out and laid out, but the grounding scheme is more like a ground plane than single traces and star ground. I am wondering how much this matters for this type of circuit. The terminals for the ground and +/- rails are located near the final caps that are "after" the linear regulator ICs. Is this sufficient? Is this not much of a worry, since the ground plane will give plenty of low-resistance pathways and the return currents are not very high (e.g. not like a power amp PS)?

The ground plane is connected to earth at one point, at the opposite end of the board from where the rails and ground connect.

This is for +/- 15VDC rails, at 500mA or less current.

Charlie

[hesener]

Hi, can you post a picture? From your description it sounds like you have a good, low-impedance ground plane so the PS itself should be fine.
You may want to use star grounding at the input of the device you are building, and lead the ground connection from the output connector of your board as well as your earth connection there.
just my two cents...

[Conrad Hoffman]

The key thing, assuming it's powered from AC of some sort, is to look at the current loop through the transformer windings, diodes and filter caps. Look at the ground point. You have to arrange things such that zero AC current passes through the DC circuit ground and takeoff points. Or as close to zero as is possible. Ground planes are usually a disaster there, even on low power circuits, because the cap charging currents are still significant. You should lay it out or wire it exactly the way power amps are done; stop trying to justify/convince yourself it will work OK. It may work, but it won't be nearly as good as it could be.

[CharlieLaub]

I've attached the board layout as a TIF image. Red is bottom copper layer = ground plane. AC-GND-AC enters at top via terminal block, regulated rails and grounds connect at bottom.

-Charlie

[DF96]

Ground planes are lovely for RF, but not so good for audio PSU because you can't force the charging pulse return current to go exactly where you want it to.

A quick way to improve your PCB is to almost put a cut in the ground plane just below C1/C2, but with a connection in the centre to the rest of the board. This keeps charging currents in the top half. Alternatively, get rid of the ground plane at the top and have proper traces for C1/C2 ground return/trafo CT, with a single connection from there to the ground plane in the lower part.

[CharlieLaub]

Originally Posted by DF96

Ground planes are lovely for RF, but not so good for audio PSU because you can't force the charging pulse return current to go exactly where you want it to.

A quick way to improve your PCB is to almost put a cut in the ground plane just below C1/C2, but with a connection in the center to the rest of the board. This keeps charging currents in the top half. Alternatively, get rid of the ground plane at the top and have proper traces for C1/C2 ground return/trafo CT, with a single connection from there to the ground plane in the lower part.

Thanks for your post. This is the kind of info I was hoping to get!

I'd like to understand a little bit better about what you are saying, and the function of the "ground" connection(s) in various parts of the circuit. There is the transformer center tap (which is not strictly a ground) that I assume must be connected to the rectifier and caps C1/C2 to return the currents back through the transformer windings. So, as you say, the top part of the ground plane as shown is handling currents and pulses from the rectification and smoothing going on there, and that this should be kept separate from the grounds in the lower part of the circuit. Is that correct?

Also, when you say:

Originally Posted by DF96

...get rid of the ground plane at the top and have proper traces for C1/C2 ground return/trafo CT, with a single connection from there to the ground plane in the lower part.

Is this because a "star" type ground should always return to the main smoothing caps that follow the rectifier? I seem to recall reading that somewhere...

What about earth ground? I have always been a little unsure of exactly how/where that should be connected to line level circuit power supplies.

Should earth ground be connected to:

The transformer center tap?
The ground points on the lower part of the PCB (e.g at caps C8/C9), and not the transformer center tap?
At caps C1 and C2 to make something like a star connection there?

Any insight on what should be connected where and why is very much appreciated. Thanks!

-Charlie

[discrete]

This is a very good read and answers many of your questions:


Audio Component Grounding and Interconnection

[CharlieLaub]

Thanks, that is a good article!

I also found this thread on the forum, regarding "ground" and "earth/safety ground" connection that cleared up a few things for me:
best Place for Star Ground?

-Charlie