Terminology clarification for transformer with a third primary tap

Question

I am looking over the reference design from Microchip app note 46102, page 4, that shows a reference schematic for the M90E26 energy meter. They are using a transformer that has a series tap along the primary winding to pull 35Vrms for rectification before passing through a 3.3V linear voltage regulator. What do you call such a transformer? A search on digikey has yielded nothing, and the part number seems to be long out of production. Does anyone know which search parameter you might use to find such a device?

Answer 1

It's likely a custom transformer for this application. There is no problem getting a transformer made with the windings and materials you request, provided you buy some thousands and/or pay a premium. In this case there is a tap brought out that has just a few turns (probably fewer than 10) between pins 1 and 3.

I would wager most transformers sold are custom. Ones you can buy from distributors tend to be very expensive, and switching supply transformers are rarely available at all.

If you wanted to make just one or two units you could substitute two transformers, one for the supply for the meter chip and and one dual winding one for the two isolated supplies.

In any case, the transformer is a mains power transformer with dual secondaries and a tapped primary. A transformer made for dual voltage US/Japan would be close but the tap would be more like 20VAC, which is too high for the 3.3V supply.

The actual transformer they used (from an older version of the app note) appears to have Chinese hanzi on it:

This (from maker 裕正 Electric in Hangzhou) may be the actual manufacturer, or a competitor: