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DC/DC Converter Layout Discussion

Updated: Nov 12, 2021

This LinkedIn discussion generated a lot of interest and even the experts couldn’t agree on whether there should be a ground plane under the switch node and whether copper planes should be placed under the output filter inductor.


We designed 4 variants of a simple, open loop DC/DC converter module to compare the results of these changes. We made this an open source project, so you can make your own board from our FREE Gerber Files and BOM (see zip file below), purchase a finished board set for your own experiments or modify the design for your own needs.


Some are interested in thermal aspects, some noise, efficiency and many EMI. Whatever your interest, the data should be posted here so all that are interested can see the available results or post their own.


Click here if you'd like to purchase a finished board set.


Download the open source files here.


Visit picotest.com to view all Picotest products.



4 Comments


Comments from Scott McMorrow, Samtec:


Nice experiment and results.

I’d suggest several changes to this board.

  1. Fill unused space with ground plane and stitch with vias to contain emissions from currents that pass down to the ground plane.

  2. Additional parallel MLCC across the input of the converter will help to reduce ringing. 0201 is preferred for low inductance

  3. In actual system applications, 12V input will often come from lower planes. That high edge rate ringing off the FETs will radiate throughout the board through the power vias. It’s important to provide a ring of ground vias around 12V/24V/48V input vias to isolate them from radiating in the power/ground plate cavity. (I once diagnosed a system that was failing catastrophical…



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"DC-DC Converters - Solid Return Plane or Cutouts Under Switch Node and Inductor?" {Preliminary Report / March 10, 2021}


By Ken Wyatt, Wyatt Technical Services

With Steven Sandler, Picotest


See the LinkedIn post here.

View the pdf here.

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Hi, do you have any update on the testing of these 4 different layouts please? I am currently working on a switching regulator design and would be interested to see some results to inform whether to remove GND from under the inductor/switch node, or not! thanks very much, Nick

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steve
steve
Mar 10, 2021
Replying to

Hi Nick,


Data is just starting to trickle in, much slower than we expected, but coming. Here is my best guidance for you at this time


1) Minimize the size of the switch node. This is the #1 instigator

2) Make sure you know the SRF/capacitance of the inductor. You want it to be a few pF max. The one on our board isn't specified and measures a whopping 200pF.


Look at the different in ripple and spectrum comparing the ferrite to a toroid I custom designed:


3) Use a shielded inductor if possible


4) If you can, avoid copper on the surface plane directly below the inductor. This will reduce the induction heating and also the radiated noise


Having…


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