Power transformers

WHAT????

• Passive device that transfers alternating current (AC) from one circuit through electromagnetic induction
• Normally consist of a ferromagnetic core and two or more coils ( windings)

HOW???

• Changing current in the primary winding creates an alternating magnetic field in the core. The core multiplies this field and couples most of the flux through the secondary windings. This in turn induces an alternating voltage (emf) in each of the secondary coil in accordance to Faraday’s law

TYPES

• Power transformers


•  Laminated core
• Toroidal
• Autotransformer
• Variac
• Induction regulator
• Stray field transformer
• Polyphase transformer
• Resonant transformer
• Constant voltage transformer
• Ferrite core
• Planar transformer
• Oil cooled transformer
• Cast resin transformer
• Isolating transformer

• Instrument transformers
• Current transformer     
• Voltage transformer

• Pulse transformers
• RF transformers
• Air core transformers
• Ferrite-core transformers
• Transmission-line transformer
• Baluns (specifically to connect balanced and unbalanced circuits)
• Audio transformer

etc.

This article’s main concern is power transformers. What is a power transformer? Power transformers are made from high-grade materials, a very special type of electrical steel for the core, high-grade copper for the windings, and cellulose-based paper for insulation.

A power transformer in switch mode power supply (SMPS) is designed to change amplitude of high-frequency pulses by the turns ratio and to provide isolation between circuits. Note that it can't transfer a DC component of the pulse voltage: in a steady state mode net volt-seconds across each winding should be zero, otherwise the core will saturate. DC output voltage is obtained only by using rectifiers. Nevertheless, an average voltage across a real coil's terminals can be non-zero due to non-zero coil's resistance. This DC offset can be used for lossless sensing of an average current across an inductor or a transformer winding. In general, ideal SMPS transformers need to transfer all energy instantaneously from one winding to another while storing no or little energy in the process.

Conversely, a power inductor is used in SMPS as an energy storage device. It accumulates energy in the magnetic field as current flows through it, and then transfers all or a portion of this energy into another circuit during the alternate part of the switching cycle. In power supplies the inductors are also used for filtering out high frequency currents (often called chokes). That is all the information I have for now. Will post more when I get more information.