EMC Question of the Week: December 16, 2024
An 8-layer circuit board has solid half-ounce copper planes on Layers 2, 3, 5 and 7. At 10 kHz, magnetic field lines passing through the circuit board induce eddy currents primarily in which layer(s)?
- Layer 2
- Layers 2 and 7
- Layers 2, 3, 5 and 7
- Layers 1 - 8
Answer
The best answer is “c.” The skin depth of copper at 10 kHz is approximately 0.65 mm (much more than the thickness of all 4 copper planes combined). Magnetic field lines passing through the board induce eddy currents of approximately equal magnitude in every solid copper plane.
At much higher frequencies (e.g., > 10 MHz), the outer planes, or the planes closest to the source, carry more of the eddy current. At 100 MHz and higher, the eddy currents are largely confined to the outer surface of the outer planes.
Occasionally, an application note or data sheet will recommend a separate "ground" plane under power converter components for carrying the eddy currents. The idea being that eddy currents are "noisy" and must be kept isolated from the signal-return plane. This line of reasoning is faulty on many levels. Eddy currents are place-bound and unlikely to couple noise into a circuit sharing the same solid plane. And even if the eddy currents were a problem, simply providing a separate "ground" on another layer doesn't significantly reduce these currents at power circuit frequencies.
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