[sdiy] PCB Transformers

Richie Burnett rburnett at richieburnett.co.uk
Fri Jan 20 10:21:12 CET 2023

> Has anyone ever tried putting transformers in pcb layers? I guess it
> wouldn't be audio frequencies (due to size)...

Occasionally transformers are made like this in switch-mode power supplies 
but it often needs lots of PCB layers and a ferrite core looping through the 
PCB to achieve enough inductance in the hundreds of kHz.

Air-cored planar PCB transformers are common at higher frequencies though, 
such as the "balun" transformers on the input and output side of this 2500W 
VHF (FM broadcast) power amplifier:


The single turns of the thick silver and blue PCB traces provide sufficient 
inductance and couples RF to one or more turns on the other side of the 
board.  These transformers achieve impedance matching and also allow power 
to be combined from two transistors in push-pull style amplifiers.  At 
frequencies of hundreds of MHz a single turn of copper provides ample 
inductance without requiring any ferrite, and leakage inductance due to 
relatively poor coupling can easily be compensated out with a capacitor.

In a high-power amplifier they typically do a fair job of heating up the PCB 
though! :-)


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