I have a board that uses several different switching converters with 33uH ferrite beads that take up a fair amount of space. I'd originally intended to keep them all at the top of the board (not top layer, top from a bird's eye view) and keep the 12V main power supply rail isolated near the edge of the board. However, I can cut down on about 25% of the board size if I move one of switching regulators to the bottom of the board where there's more space. In order to do this I have to route the 12V rail to another layer of the board. I'm using a 4 layer board where the top layer is a signal plane, the 2nd layer is a ground plane, the 3rd is a power plane and the 4th is mixed signal/ground plane (about 3/4 ground). I was going to route the 12V trace on the power plane. That trace would have unbroken ground planes above and below it. Additionally, the 12V rail is filtered by a ferrite bead pi filter the moment it enters the board. However, it would go beneath a noise-sensitive ADC on the top layer. Is there any danger to this?

The other part of my question relates to via selection. I'm intending to use one larger-sized via (calculated using the Saturn PCB toolkit) with an identically-sized adjacent via for the return path. I could use several smaller vias instead, which, for the same current carrying capacity, should have lower inductance and capacitance, but I don't see why this should matter for a DC supply rail.

Am I missing anything big here? I don't see why this should cause any problems, but I'd like to avoid any surprises...

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    \$\begingroup\$ Remember that even if voltage is constant, the current usually is not. It's the time-varying current, combined with inductance, that could cause problems so you might still benefit from lower inductance. How about assembly? How are you planning to put these boards together? \$\endgroup\$ – Elliot Alderson Jan 16 '19 at 23:59
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    \$\begingroup\$ What is L dI/dt on your loads? using 1nH/mm for traces or 1nH per via. What direction is max direction of B field of inductors? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 17 '19 at 0:08
  • \$\begingroup\$ 1812LS parts??.. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 17 '19 at 0:18
  • \$\begingroup\$ Bourns SRR1210A. \$\endgroup\$ – MattHusz Jan 17 '19 at 0:19
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    \$\begingroup\$ ok then current rise time is 0.35/fmax It is often best to decouple low impedance at 100kHz with ceramic caps. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 17 '19 at 0:44

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