# Need info

1. Jan 30, 2004

I need help find a good page which talks about the physics of a tranformer. I know all about the b fields and the flux and the ratio of the windings but what I really need is the practical formulea and methods for example calculating the voltage drop on the primary if the secondary has aresistance of so much and there is a resistor in series with the primary. I could experiment with different variations and figure it out but it would be alot safer and quicker to find a reference for this material. I really would appreaciate help in this matter.

2. Feb 13, 2004

### mmwave

I've never seen a specific reference on transformers. I learned the stuff you're asking about in RF design classes. Maybe this will help:

V2 = N2/N1 * V1
I2 = N1/N2 * I1
P1 = P2

Use an ideal transformer as a circuit element & put the resistances in as circuit elements. For example if the secondary has a resistor of R2 ohms & you look into the primary the above relations will show you see a resistance of

Rin = (N1/N2)^2 * R2

This Rin would be added to any resistance in the primary winding to make a practical dc cicruit model. For ac, you apply the same idea to the capacitance of the windings using Zc = 1/(2pifc). It works well for frequencies below the self-resonant freq. of the transformer.

I think this is covered in a wonderful book by Tom Lee called
"CMOS RFIC Design".

mmwave ( writing without his EE211 notes)