Switching power supply ground plane cuts (again)

You just brought it up: The advocates of surgical cuts often haven't heard the hollering in a board room after the team blew the EMC test and now the ship schedule ist toastissimo.

I belong to the faculty of solid ground plane folks :-)

For EMI, a solid plane is probably the best. Probably.

But if circulating currents in a switcher could induce sheet potentials into a ground plane, and sensitive stuff is nearby, a couple of strategic slits (either near the supply or near the victim) can fence off the currents and improve things.

Something like a thermocouple front end can be located over a ground peninsula, inside a C-shaped cut, to keep various circulating currents out.

John

Totally agree. A few mV of noise at the control chip can drive you nuts. Cheers, Harry

Cutting the ground plane is a very reasonable method to control where your high-frequency currents are flowing. For high-current switching converters it is absolutely necessary. I design this into all of my circuits. Last one I did, it met DO-160E cat L for conducted emissions the first time tested; no white wires, no re-layout. The best thing for control circuitry is to isolate the plane under this circuit and make sure all signals in and out of this area only pass over the place where the ground planes connect. I'm talking about high power switching here, like 50A at 200kHz. For small currents you might not have to worry. Basically, you do not want to create a path for high-frequency, high current through the plane which is under sensitive control circuitry.

(snip)

I also use other layer ground paths, either fills or traces, to provide specific places for large and/or fast, local ground currents to go. But the first goal is always to minimize areas surrounded by such currents.

Especially around switching power supplies, I make strategic voids in the planes to prevent large circulating currents from wreaking havoc with sensitive signals, such as output sense. I go as far as to void all layers beneath non-toroid inductors in some of my power supplies. I've seen the most bizarre behaviour from plane/signal interactions which is exacerbated at higher switching speeds, of course.

I would rather deal with increased EMI (and manage it) rather than the supply not working because transient induced problems from the local plane prevents the power supply from properly operating (I've seen it).

Solid planes have advantages, but so does shaping the plane a little for specific circumstances. As always - 'It depends' :)

Cheers

PeteS

That's exactly the point, to pay attention to where stuff is returned to ground instead of chopping up a ground plane. Most of this is related to clever component placement. Luckily I've got a layouter whom I don't have to prime on the subject. His first placement plot is usually almost right on the money, same for the traces he puts down.

The other thing a ground cut can do is stop thermal gradients.

Sometimes I do a rectangular slit that is bridged by small traces in several places. It adds useful electrical resistance (to reduce sheet currents) and thermal resistance, but is still a pretty good RF short.

John

Well, those are the higher levels of sophistication and sometimes obfuscation. Just imagine a competitor wrestling with drift and their CEO hollering "So why can Highland Technology pull that off and you #%^*!! are to freaking &^^@#!! for that?"

Careful Harry - this is a subject of Holy Wars ;)

I do appreciate the positive feedback, however

Cheers

PeteS

No, I once have been better: jump the slit with ceramics :-)

I fully agree. There are times when clever placement can completely eliminate the need to play with plane cuts because the circulating currents are neatly localised.

Some very high density designs are more difficult to accomodate, though. It can be a little tricky, if not impossible, to run a few hundred really high speed differential pairs (on a small board) and prevent the return currents from appearing in other circuitry without the judicious use of some shaping of the planes.

That's not advocating separate analog and digital ground planes, though. I've seen (in the last month alone) 5 designs done that way that have horrible problems because of plane differentials, that I am now being asked to fix without re-laying the boards out (I'm doing the consulting after the fact). Ugh.

Cheers

PeteS

And we regularly made slits of less than 10um width _into_ ceramic ;-)

So I suppose the trick is to put the ceramics where they are not needed ;-)

No, this had a technical reason. But I am not at liberty to say ;-)

High voltage PC boards sometimes have fully-routed-through slits.

Then there's Agoston Agoston's sampling head patent, where a trench is milled into a solid metal block, and the trench itself is a 3D slotline connecting a SRD impulse generator to two channels of traveling-wave diode samplers.

Got it around here somewhere. Sure looks like a dead short to me. Slotlines just defy my intuition somehow.

John

Yet the cult lives on.

John

And it bringeth us bread and butter :-D

Hmmm... Anti wires.