EMC Question of the Week: March 27, 2023

A poor buck converter layout

The 1-MHz buck converter layout in this figure is poorly done. Layer 2 is a solid GND plane. Cin connects to GND. Cout connects to PGND. Every ground pin of the converter IC connects to both PGND and GND. Which component is grounded incorrectly?

  1. CIN
  2. COUT
  3. the converter IC
  4. the inductor


The best answer is “b.” COUT should never share an isolated ground with CIN or any other component. In fact, this layout shouldn't even have a PGND. Currents at MHz frequencies and higher take the lowest inductance path and don't need isolated returns to prevent them from spreading uncontrollably. In a buck converter layout, CIN and COUT should both connect directly to the GND plane on Layer 2.

Another problem with this layout is that the switch-node (SW) is larger than it needs to be. The surface area of the switch-node should always be minimized and kept away from other traces or components that could carry the switching noise away from that part of the board. A better converter layout is shown below.
A better buck converter layout

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