PCB Help - Power & Ground Planes in 4-Layer Board

[adamey]

I'm using ExpressPCB to design a board for a project. The board will have both 12V and 5V circuits. The two 12V circuits will draw a maximum of about 6A each (PWM motor drivers) with typical draw being around 1.5-2 amps. They also have relays to turn them on/off (more of a safety thing separate from the Mosfets). The 5V circuits are a microcontroller and several sensors that also work on 5V.

If I'm making a 4 layer board with power & ground planes, which one should I put on the power plane? The 12V supply with the higher current or the 5V supply low current?

It would make things much easier to have the 12V as the power plane, but I'm wondering if the PWM and higher current will make any "noise" or cause problems with my low current signals.

 

[ronv]

You have lots of layers to work with. I think I would take care of my low level signals, but you may find out you can do both without much trouble. Just keep the gate to source loop to the fets short.

 

[crutschow]

You can also flood the other layers with power or ground planes between the traces. This helps reduce noise (and reduces the amount of copper the PCB maker has to remove).

 

[adamey]

Let's say I made my power plane 12V. I place a pad to provide power to my load (a PCB mounted relay) and I connect this pad to the 12V power plane.

How much current could I reasonably expect to pull from a single pad connected to an inner power plane? I'm sure the plane itself could provide significant current, but I'm wondering about the area where this plane actually meets the pad.

 

[adamey]

In looking over some commercial PCB's I've seen where a high current wire enters a board they often have several "vias" placed directly beside the pad with short traces to the pad.

I'm going to assume this is to allow additional paths for the current from a power plane to get to the pad, indicating to me that a single pad to a power plane cannot really pull that much current without some additional "help".

 

[crutschow]

If you Google "pcb via current capacity" you will get numerous hits about the subject.

 

[adamey]

^ Thanks, it seems a via can handle more current than I thought.

Also found out in ExpressPCB I can have more than one power plane and have them split (I thought you couldn't modify inner planes). So I can have a high-current 12V plane on one half of my board and a 5V low current plane on the other side.

Next time I'll read a little more before posting.