A flyback transformer (FBT), also called a line output transformer (LOPT) is a special type of step-up transformer that is used to convert electrical energy efficiently from one part of the circuit to the other part at constant power. It generates high-voltage, high-frequency electric signals, referred to as sawtooth signals. Unlike conventional transformers, which require an AC input, flyback transformers can be powered by DC pulses, making them suitable for a broader range of applications. They are commonly used in various electronic applications, including CRT displays, LED fixtures, solar micro-inverters, telecom systems, and AC-DC power supplies.

The term "flyback" originates from the use of flyback transformers in cathode-ray tube (CRT) displays. A flyback transformer can be energized with a very low voltage. When the primary winding is excited with a low-value sawtooth voltage, it rapidly energizes and de-energizes due to the sawtooth waveform's nature. This rapid energization causes the electron beam in the CRT to "fly back" from the right side of the screen to the left. This distinctive characteristic led to the transformer being named a flyback transformer.

Design and Construction of Flyback Transformer

A flyback transformer consists of primary and secondary windings with an inductive gap in between. The primary winding is driven by a transistor switch from a DC supply. The transformer has a ferrite rod or core that minimizes leakage inductance. The secondary winding is wound layer by layer with enameled wire and Mylar film between the layers. An air gap is present between the rod and the ferrite frame, which increases the reluctance and helps in energy storage.

Working Principle of Flyback Transformer

Flyback transformers work by storing energy in the magnetic field during the first half of the switching cycle and then releasing it to the secondary winding connected to the load.

                         Schematic of Flyback Transformer  

The primary winding is driven by a transistor switch from a DC supply. When the switch is turned on, the primary inductance causes the current to build up in a ramp. The current flow creates a magnetic field in the core, which increases over time. This magnetic field stores energy in the core. The core has an air gap, which increases the reluctance and helps in energy storage. The magnetic field flux density varies up and down in value but keeps the same direction.

When the switch is turned off, the current in the primary falls to zero. The energy stored in the magnetic field collapses, causing the magnetic field in the core to collapse. The energy released from the magnetic field is transferred to the secondary winding, inducing a voltage in the secondary. A diode connected in series with the secondary winding prevents the formation of a secondary current that would oppose the primary current ramp. The secondary current flows through the load, which can be a resistor, a capacitor, or an inductor.

Modes of Operation of Flyback Transformer

• Continuous Conduction Mode (CCM): In this mode, the secondary current never reaches zero. The switch is turned back on before all of the flyback energy is transferred to the secondary.  

• Discontinuous Conduction Mode (DCM): In this mode, the secondary current reaches zero before the end of the period. The switch is turned on after the stored energy is fully depleted into the secondary.  

Applications

Flyback transformers are used to control the horizontal movement of the electron beam in CRT displays, such as television sets and computer monitors. They are commonly used in switch-mode power supplies (SMPS), which are prevalent in consumer electronics and in various signaling applications in telecommunications. Flyback transformers also find application in aeronautics for high-voltage electric supply and are also used in industrial motors, generators, and pumps.

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