Author Topic: Good PCB Design  (Read 2023 times)

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Offline MuTeD

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Good PCB Design
« on: December 31, 2014, 02:02:11 am »
I've recently watched Dave's PSU PCB Design (#244). He only briefly speaks about good design principles like good grounding and bypassing, splitting ground planes, trace width, good EMC design, ... I looked if he had a video like this but didn't really find anything and think this could be very useful. I've searched for hours about how to do good grounding, bypassing etc without any guarantee that what I am reading is indeed correct. For instance I read that you should route the grounds and then have 1 via to the main ground plane per local net but then I notice that he just made used vias for most grounds. So it would be great to get some tips from a pro! Thanks!

Offline poorchava

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Re: Good PCB Design
« Reply #1 on: December 31, 2014, 06:00:23 am »
I guess one universal tip is: every current has to come from somewhere, go through somewhere to somewhere and return all the way back from there. You need to identify this path for every single current there is on your pcb and then design it in such a way that this path is the shortest, has lowest impedance possible and does not contain high frequency components unless needed or absolutely necessary. That's all. Usually you'll do that only for signals of interest and in particularily notorious parts of a circuit (like switchmode supplies, gates and such).
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Offline Rerouter

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Re: Good PCB Design
« Reply #2 on: December 31, 2014, 09:53:30 am »
Like most things in electronics, fields overlap so learning one thing may end up with learning 5 things, take this as a warning that this is the tip of the iceberg.

After you learn ohms law most people then forget his friend power (Wattage):
- try and size your trace widths to suit the power across them (Every Trace is a resistor)
- try and size components to keep them reasonably cool where possible, or if not possible try and keep things that are effected by heat away from that area, (E.g. electrolytic capacitors)
- If you have high impedance signals, a thermal gradient can offset your voltages (Think thermocouples)

Try and visualize where each current loop is, if your device outputs 1 amp, 1 amp has to return to it to complete the loop, be it through a ground trace, or an unintended route.

Even if you have a slow signal, e.g. a user pushing a button, you can still be dealing with high MHz edge rates, as such, proper decoupling, input filtering, and such should be looked at.

With I/O where possible try and protect it, for analog/digital inputs this generally just involves a current limiting resistor, for outputs, say a transistor switch, a base resistor can act as a crude current limiter, for some other outputs, there is not much that can be done to help,

I have typed up much on this on this forum, here is one such instance just about ground planes...

Or if you want to be able to understand what is happening in a high speed circuit and traces, this is about the best thread on such a beast

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