DPAK and D2PAK power dissipation

I assume you mean solder mask. Thermally, it doesn't matter.

Lots of vias will conduct heat from the topside pour (around the dpak) to the bottom ground plane. Six or eight 30+ mil vias, close around the dpak, should work... something like that.

One big plane!

John

But watch out for relay coil noise on the digital ground!

Cheers! Rich

Digital ground? Ground is ground. And there can't be noise on ground; you just define it to be zero volts!

John

I predict that you are going to find that life is full of surprises. ;-)

Yes. in theory you can define a 'ground' point as being at zero.

However any real real circuit that has currents flowing through wires will have non zero voltages due to the resistance of the wires, inductive effects when the currents change (i.e. relay noise or other switching noise effects) and cross talk due to magnetic field coupling between signals, etc.

At high frequencies there are other problems that have to be considered related to signals bouncing off impedance mismatches, etc. Ask anyone that works with RF, microwave, or modern high speed computer circuits. (FM radio is at 100 Mhz, this is a low frequency by modern computer standards.)

Yes. in theory you can define a 'ground' point as being at zero however if you look at any other point in your circuit, the voltages will not be zero even on wires connected to your 'ground' point.

close around -- or right beneath it. Anything wrong with that?

robert

Does anybody sell that ground? I've been looking for it on some of my recent projects.

Either is OK. For production, people mostly prefer to not have vias under the part, because they slurp solder paste away from the pad. If you hand solder, they're OK... just use lots of solder, and filling the vias will hugely reduce their thermal resistance too.

But for best thermal performance, the vias should be spread out some. Dumping all the heat into a small patch of the backside ground plane suffers from the thermal spreading resistance of the groundplane copper... the foil is thin and isn't a perfect heat conductor, 70K/W per square for 1 oz copper. If it's a small board and the chip dissipation is low, it's no big deal.

How much power will the dpak dissipate? and how much groundplane area is available?

John

Oh, life certainly is. And splitting ground planes multiplies the surprises nicely.

Exactly!

Sure. Just deal with it.

John

That's kinda what I was getting at - watch that the supply return to the sensitive digital stuff doesn't cross/share paths with the 24V relay supply return. and that neither crosses/shares with the sensitive analog stuff. I guess this can be done by visualizing current paths - it seems a split plane is kinda the lazy person's way out. ;-)

Chers! Rich

Go to an aviation boneyard, and pick up one of those boxes of dirt that they put in the tails of airplanes to ground their avionics to. ;-)

Cheers! Rich

Send me a purchase order, and I'll make you a deal on 1000.

John

I read in an app note somewhere that suggested instead of using large holes, use the smallest hole size that would at least 85% fill with solder to make a better thermal connection. You should be able to ask your board house what this size is.

Marc

Well, if you have a noisy digital ground from a 2Ghz running processor with an 133Mhz memory bus, some fast 200Mhz buses tied to a DSP, a switching power supply at 2Mhz, some analogic transcievers running in

greetings, Vasile