When is it considered that too much is too much when it comes to copper fills on DC-DC switchers. I am currently laying out a Buck, 3 Amp max output. Every literature that I have scanned before advises me to pour and fill solid regions on the input node, the switch node(where the maximum peak currents happen in a Buck) and the output. But when should i start backing off?
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There is no glaring downside to overfilling the input and output nodes of a SMPS (if you have room) but the switching node of the supply should not be egregiously* oversized.
This node is switching very fast (500kHz - 2MHz are common in most ICs) and usually from ground up to your input voltage so it generates a lot of high frequency noise. It is essentially a capacitor and the larger you make the plane pour, the more capacitance it has and the more likely it is to cause problems by coupling into your ground plane or high speed sensitive trace.
*On most designs I generally size it about 1.5x - 2x as big as it needs to be (and am very careful not to run any other traces under this node on any layer) for a couple of reasons listed below.
If I accidentally short the output the trace (hopefully) doesn't pop and the circuit is useable/salvageable.
This can also protect you from throwing out the PCB due to requirement creep if you need more output current than initially thought
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1\$\begingroup\$ "should not be egregiously oversized" -> Make it as small as possible \$\endgroup\$– FunkyguyCommented May 7, 2015 at 18:29
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2\$\begingroup\$ @Funkyguy I disagree, as explained in the last paragraph. Obviously it varies from design to design, but if no sensitive traces are nearby I like to make it a little bigger than needed if requirements creep (Oh I said the load was only 1A, we actually need 2A) or if I short out something when debugging I don't need to throw out the board cause a copper trace burned up. \$\endgroup\$– ACDCommented May 7, 2015 at 18:32
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\$\begingroup\$ True, it does change from design to design. I just never have a case where it needs to be bigger apart from current. Yea, if there aren't any small signals around it, make sure it is large enough for the current capacity. I've never had any issues with a popping or shorting although the techs must test it before they hand it over to the engineers \$\endgroup\$– FunkyguyCommented May 7, 2015 at 18:34
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\$\begingroup\$ @ACD the last line is a key observation. Running traces directly underneath a switching node is asking for trouble as those high frequency noise can easily couple in to sensitive low amplitude single-ended traces. Some switcher parts explicitly state that in their app notes. \$\endgroup\$ Commented May 7, 2015 at 18:46
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\$\begingroup\$ I tried to cleanup the answer a little bit addressing the concerns in everyone's comments. \$\endgroup\$– ACDCommented May 7, 2015 at 18:49