Point-to-point building of medium- frequency circuits, ground plane implementation

Question

I build audio projects and small logic control circuits using point-to-point on perfboard (see pic of a recent build).

As you can see this allows me to select wire gauge in accordance to the currents at various parts. When the board is finished, tested and washed I spray paint a clear enamel on the underside. I have built circuits running up to 450V like this without problems, but these are all low frequency projects.

Now I want to build a switcher-style power supply running at 50kHz. For the moment I am only in the exploratory stages as this is terra incognita for me. Is this building method any good for medium frequency work? I seem to recall having seen HF projects built in dead-bug fashion without issues, that can't be any worse no?

The only potential issue I see could be parasitic capacitance between the copper pads. If it is a problem, I could dissolve the unneeded pads using etchant - I have done this for boards that are connected to mains in order to reduce the potential for arcing. This is done before soldering parts - I design my boards with autoCAD so I know in advance which pads need to be kept.

The second point is about the use of a ground plane; this seems mandatory for switching circuits. I can implement it with a piece of plain PCB stock under the perfboard with drilled pass-through holes for the few pins that need to connect to it but there would be a space of almost 1/8" between the two boards, can this be a problem? If so I can place the plain PCB stock on a spot that isn't under parts so it is in direct contact but it will be of smaller area of course.

Alternatively, could a large-gauge piece of wire be used instead of a ground plane? If a low-impedance path is all that is needed that could fit the bill. A 18AWG wire has as much copper as a sizeable PCB plane no? There might be problems due to skin effect in the wire though...

I could etch a bona fide PCB but I'd rather not - I hate drilling dozens of holes using those crazy-thin drill bits, and I'm not convinced at all about consistancy of home-etched PCBs tracks physical/electrical properties. This is the reason I build point-to-point on perfboard.

Sorry for the long post and thanks in advance for any help! -Joe

Answer 1

For switcher frequencies, you can forget about problems caused by excess pad capacitance, it's just not the right order of magnitude to cause trouble.

However, grounding is a more serious issue. It's not actually necessary to use a ground plane. It is necessary to have a ground return running close to all signal traces, to minimise the area of the current-switched loops that you will be creating in a switcher design, and to ensure that ground currents in one part of the design don't create stray voltages in other parts.

A gridded ground can work quite well. Something like a 10mm or 1/2" spaced grid of wires on both x and y axes, soldered together at every intersection, would be more than enough like a ground plane for switcher frequencies.

If you do use an additional board for a plane, the 1/8" gap should be fairly negligible.

What most RF engineers would do is start from a ground plane, and then mount components above it. You can get sets of pads, IC footprints, and transmission lines, on adhesive-backed copper clad. Another way to do this for cheap is to cut up some copper clad into tiny squares and strips, and epoxy or super-glue these down to the ground plane. Hint, 15 minute epoxy goes off in a few seconds if you hold a soldering iron to the pad.