The transformer is a passive device that changes voltage, current or impedance to the required parameters. As demonstrated by a basic physic's experiment - iron filings sprinkled on paper by a conductor carrying current will form a circular field or flux with no definitive end. Changing the magnetic field by changing the current will alter this flow or "flow". This change is the basis for transformer operation. The intensity of this flux is determined by the number of turns in the coil multiplied by the current. It is also affected by the magnetic conductivity of the area around the coil. This magnetic conductivity is permeability. The force needed to create the flow is called magnetomotive force. To get the current to flow from one winding to another, the magnetic force must be changed.
A transformer is ideal for many applications although should not be used in circuits requiring high-fidelity reproduction of audio or video signals. It may also pose a problem if size and weight are at a premium.
A transformer is composed of a core, windings, and insulation, which are determined by the power requirements and the frequency of operation.

Cores

Laminations
Supplied in stamped letter shapes such as: "EI", "EL", "EE", "F", or "UI". They are composed of silicon iron and nickel alloys. Audio and telecommunication transformers use nickel alloys. Silicon iron is generally used for line frequency power magnetics.
Ferrite
Available in a variety of shapes and sizes, these ceramic magnetic cores are composed of ferric oxide and a combination of maganese, zinc, or nickel. The shapes "EE", "PQ", "UU", "ETD" and dual-slab are used for high frequency power applications. Telecommunications and low power applications use pot cores, touch tone cores, "EP" and "RM". Slugs, rods, and beads are used for radio frequency applications.
Toroids
These circular, nonradiating, magnetic cores are available in a large selection of materials such as nickel and cobalt alloys, metallic glass, and, ferrite and powdered iron. Popular for its low cost and small size, these are ideal for nonradiating power transformers and current-sensing transformers.
Windings
Copper wire, based on the American Wire Gauge (AWG) Standard, copper foil or litz wire (made by twisting together multiple strands of wire) are the most common conductors used. The current load determines the wire gauge. Voltage, type of core material, and wave form will determine the number of wire turns.
Insulation
Properly insulated transformers can withstand severe environmental conditions and remain in service for many years. The temperature of operation and the dielectric withstanding voltage (hipot) with determine the type and amount of insulation needed. Bobbins that hold the wire are manufactured with either thermoset or thermoplastic. To insulate the windings from each other, Mylar &spr, Kapton &spr, or Nomex &spr are used.

Linear Power Transformers

These transformers operate between 47Hz to 400Hz. Called isolation, step-up, step-down, or rectifier, they alter the output voltage or current needed for the equipment load.
Power losses resulting in heat generated by transformers are a major design consideration. Efficiency, defined as the ratio of Power Out vs. Power In, is influenced by frequency of operation, choice of core material, wire gauge, and current. Resistive or reactive loads (inductive or capacitive) influence power losses and efficiency. With resistive loads, voltage and current are out of phase resulting in higher losses, since the current in the transformer is in phase with only the voltage across the resistive portion of the transformer windings.
Voltage and current out-of-phase conditions are caused by reactive loads resulting in "apparent" power as opposed to true power. Apparent power is referred to as Volt/Amperes and can be misleading when determining power consumption unless the Power Factor (phase angle) is considered. These phase angle power losses limit transformer ratings.
Linear power supplies using capacitor input filters increase the transformer secondary current up to two times the DC current. An inductive input filter results in a unity ratio between DC and transformer current. Choke input filters are normally not cost effective below 1,000 Volt/Amps.

Switching Power Transformers

Switching power transformers are called buck, boost, converters, and inverters. These are specified for high efficiency, small size, and low weight applications. Operated from DC power that is typically switched at a rate of 50 to 100 KHz. Switching speeds of almost 1 MHz can be achieved with the new magnetic materials. Input power is supplied by batteries, system DC power, or, a rectified AC line. The switched DC power is seen as a square wave AC at the transformer. Switching power transformers are designed for a specific application such as flyback, forward, push-pull, or bridge.
Flyback
Actually an isolated storage inductor, a flyback transformer is a combination of an isolating transformer, output inductor, and a flywheel diode. These use a gapped core and have a power capability of 100 VA. Storing energy in the gap when the switch is on and delivering energy to the load when off, they do not perform like standard transformers.
Forward
The forward transformer operates by transferring the power to the load during the on-time and resetting in the off-time. A clamp diode and clamp winding are used in the off-time to reset the core. This transformer has a limitation of 500 VA.
The disadvantages of using the flyback or forward transformer are that a larger transformer is required since the power is only transferred during half of the input cycle.
Push-Pull
Push-pull delivers power to the load during the whole input cycle. This transformer has a center tapped primary and secondary used alternatively with each input cycle. These can achieve power levels in excess of 1 KVA. Push-pull transformers are practical at low input voltages and higher output power. They are not advisable for off-line converters because the power switches operate at collector stress voltages of twice the supply voltage.
Half-Bridge
A dual, forward-converter, using two power switches can also be called a half-bridge. Power, which does not exceed the supply voltage, is delivered to the load only during half the input cycle. This design permits the use of a smaller transformer.
Full-Bridge
Four power switches are used in a full-bridge and usually utilizes a single primary winding. Full supply voltage is obtained in both directions and utilizes the core and windings more effectively. Voltage on the switches does not exceed the supply voltage.
Current Transformers
Usually used in a sensing device, current transformers customarily have a one turn primary. The number of secondary turns is determined by the sensitivity required and is terminated with a resistor. Toroidal in shape, cores of silicon steel, nickel alloy, or ferrite are used. Choice of core material influences cost and accuracy.
Telecommunications Transformers
For connecting a piece of equipment to the phone line, a transformer is used. Its function is to isolate the equipment from the phone line, improve common-mode noise rejection, and match impedance.

System Requirements

System requirements should be reviewed to determine transformer parameters including: total power, temperature, humidity, voltage, current, and insulation. Domestic or off-shore applications will determine safety requirements for different line voltage and line frequency.
Open-Circuit Inductance
Specifying only the minimum inductance required will result in an economical transformer. Tightly specified inductance value, i.e., (+-)5%, the more costly the transformer will be.
Leakage Inductance
The size and number of windings in a transformer have an effect on leakage inductance. The physical parameters of a coil cannot be precise, therefore, a tolerance of typically 30% can be maintained and accommodate circuit requirements without resulting in drastic cost increases.
Capacitance
A coil's size, number of coil-layers/windings, and dielectric, will determine leakage capacitance. As with leakage inductance, a maximum value should be specified to keep costs within bounds.
Resistance
Wire gauge and length, determined by the number of turns required in an inductor, will determine resistance. A tolerance of (+-)15% for resistance values of 10 Ohms or more can be used to maintain cost. Tighter tolerances will drive costs up substantially.
Open-Circuit Voltage or Turns Ratio
A ±3% tolerance is standard for this parameter and can be accomplished with current coil-winding machines having a resolution of ±1 turn.
Full Load Voltage
Variations in winding resistance, turns ratio, and leakage can cause minor discrepancies in output voltage. A difference of ±5% is acceptable. Tighter tolerances will increase costs.
Mechanical Specifications
Physical specifications ensure proper fit. Fewer critical parameters require less fixturization, resulting in lower costs.

Magnetics