Which pcb layout is better for capacitor?

[xmen_xwk]

Which pcb layout is better for capacitor ?

which layout is better for capacitor ? do ground need to go through cap ? or vcc ? or both ?

I have seen in some smps that they are usually going through ground pin of cap while vcc is directly connected.

 

[KlausST]

Re: Which pcb layout is better for capacitor ?

Hi,

It depends.
If you want to filter a signal, then I'd prefer 1

But these are very thin wires...and with an SMPS you may expect high current peaks. Then every piece of (thin) wire adds ESR and L.
In picture 3 you show a second layer. If you have a second layer (and SMPS) ... I'd place the capacitor as close as possibke to the switching device and make both layers as "+" and "GND" planes.

Any electrical signal follows a path...a closed loop.
The DC path usually is less critical and needs to be cared about only because of trace power dissipation. But the AC (HF) path is more critical. If you don't care, then
* ringing may occur because of additional trace inductance
* EMI may be bad because the loop may act as transmitting antenna
* in filters the attenuation may be bad because of voltage drop across the traces.

Therefore it's a good idea to review the PCB layout by considering the HF loop(s).
Check
* loop trace length
* enclosed area
* trace width

Klaus

 

[xmen_xwk]

Re: Which pcb layout is better for capacitor ?

Thank you. I used default trace width, just to make an example for trace path. The usage is for DC filtering and supply enough voltage to the target component while switching. I usually use first option but sometimes I use second, because ground is easily available in PCB. Is second option bad ? I never seen second option in any PCB(yes I haven't seen all but many), but third option is commonly used in the PCBs I saw.

I wonder what are disadvantage of second or third if any ?

 

[HasHx]

Re: Which pcb layout is better for capacitor ?

Hi there! you priorities should be as follows:

1. if you have two layers, make one for GND and one for PWR which makes the third circuit a bad option.

2. If you only have one layer you should take into consideration the closed loop matter as you have to align the GND line to be as close as possible to the PWR line along their path, to ensure coupling and protect the signal itself and other signals on your board. one more thing is that you put the cap as close as possible to the current consuming terminal to get the best functionality out of it. Now the first layout and the second one don't have big differences the first is just slightly better coupled.

Ciao!