Transformer Cheat Sheet

· A transformer is used to transform from one voltage to another. 

· The voltage ratio is proportional to the turns ration.  Current is inversely proportional to the turns ratio.

· The power ratio of an ideal transformer is 1:1.  In other words, if the voltage goes up, the current must go down.

· Transformers only work with AC, Alternating Current.

Dots and phase

· The dots can be thought of as voltage polarity.

· When current flows into the dot on the primary side, an induced current flows out of the dot on the secondary.

· The voltage is in phase at the dots, but the current is flowing in the opposite direction.

Impedance Matching

Common Mode Choke

When configured as a common mode choke, a transformer is seen as a low impedance to differential current and high impedance to common mode current. 

· When common mode current flows through the windings, the flux adds, creating more inductance.

· When differential current flows through the windings, the flux cancels, reducing the inductance.

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