Help with PSU Power Traces

[Caterpiggle]

Hi guys,

I've been working on a very simple power supply that provides 3 voltage rails. The +/-12V rails can pull 625mA each, and the +5V rail can pull 1.5A (2.75A at some points on the board). I've designed a PCB to allow for these currents, but I'm a bit worried since I don't know the standards for this sort of thing.

The large square on the bottom is a thermal pad for the large DC-DC converter that provides the +/-12V rails. The thermal pad is connected to the ground pour on the bottom, with thermal vias to pull heat towards that side.

The large traces are 2mm wide, and the smaller are 0.508mm. Some of the power traces I did by hand to provide more current (while still looking clean). Is this necessary, or possibly bad practice? Is there a better way to do this?

I'm also concerned about the ground paths for some of the connections on the bottom. They have to run across the whole board and take some long paths. I made sure to make the thermals thicker for higher current.

I appreciate any input or help you guys can provide.

Thanks!

[viperidae]

With a 7805, isn't your 5v rail going to be limited to 1A?

[Caterpiggle]

The datasheets of both the LM7805 and the uA7805 say it can handle up to 1.5A. The LM7805 even mentions it can handle more than 1.5A with adequate heatsink, but that seems like a stretch. I don't expect to pull more than 500mA regardless, but it's best to design for the limits I suppose.

[T3sl4co1l]

You don't need to expose copper to get a heat-sinking pour, indeed it's rather undesirable as the uneven soldered surface will not be easy to heatsink (if it needs one applied).  Soldermask is utterly negligible insulation under convection cooling.

A couple amps is easily handled by 20 mil traces at 1oz thickness and 20C temp rise.  Not a problem.

Tim

[Caterpiggle]

Thanks for the tips!

So what I've done with the traces is essentially overkill? I suppose the bigger traces won't hurt anything.

Is it bad practice to heat sink into a ground pour like I've done? Should I be creating a separate isolated 'thermal' pour? I'm guessing the pcb hardly insulates the copper pours in the first place, so this probably wouldn't work anyway.

[T3sl4co1l]

The PCB is actually a quite good insulator thickness-wise.  It's... made of layers of plastic.

It's an okay conductor sideways (layers of glass fabric -- mineral, not plastic), which helps keep thinner traces cool.  The sideways conductivity doesn't matter much for wide traces and pours, though.

Copper is a few hundred times more conductive, so despite it being a fraction of the thickness of the board, it dominates thermal conductivity.  Ground fill is a good idea for this reason, even if you don't need it for EMI purposes!

Even more so, having copper pour on inner layers.  If the inner layer is 5 or 10 mils under the surface, that's a lot less insulation, and it can effectively heatsink traces on the surface.  This can about double the current capacity of single surface traces.  A 20 mil trace might fuse at, say, 10A (I don't know what one typically fuses at), but 15 or 20A with an inner copper plane.

Obviously, this only affects getting heat away from a trace, and doesn't reduce the heat it's actually dissipating.  The board overall gets warmer.  If you have a lot of traces (or really wide traces) carrying a lot of current in total, all that extra heat makes everything that much hotter, and your total current capacity goes back down.

Don't worry about thermals.  They make soldering considerably easier, and have little effect, thermally and electrically.  (Calculate it!)

Tim

[Caterpiggle]

Thanks so much for this! This clears all my questions up.

I should've realized that fiberglass is a pretty good insulator!