I was reading an announcement about a new POE Hat for the Raspberry Pi computer board. What caught my eye was mention of it using a "planar transformer" in the circuitry instead of a wire wrapped ferrite core. The planar transformer has the windings etched onto a small PC board which is then sandwiched between two pieces of ferrite to form the transformer. I have not seen this technique before.

Here is a link to the article. A picture of the transformer appears at the end.

Announcing the Raspberry Pi PoE+ HAT - Raspberry Pi

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Tim N9PUZ