flyback

Flyback refers to a principle and its implementation in electrical power conversion, most commonly embodied by the flyback transformer used in switch-mode power supplies and CRT displays. The core idea is to store energy in the transformer's magnetic field while current flows in the primary winding, and then release that energy to the secondary when the primary current is interrupted. This can produce high-voltage pulses or regulated outputs, depending on the design.

The name derives from the way energy stored in the core “flies back” to the secondary during turn-off, creating voltage transients that can be stepped to higher levels. The core is typically ferrite, and a deliberate air gap is included to allow energy storage and to limit magnetizing current, enabling the transformer to function as an energy-storage device rather than a simple mutual inductor.

Operation in a flyback converter involves a switching device, such as a transistor, that connects the primary to the input source. During the on-time, energy is stored in the core. When the switch turns off, the stored energy is transferred to the secondary windings through magnetic coupling, and the output is rectified and filtered. The design provides galvanic isolation between input and output and can supply multiple outputs from different secondary windings, though regulation is achieved by controlling the duty cycle of the switching device.

Applications and characteristics: Flyback transformers are common in low- to medium-power supplies, CRT television and monitor circuits, CCFL backlights, and battery chargers. They offer simplicity, isolation, and multi-output capability, and can accommodate wide input ranges, but may exhibit higher peak currents and electromagnetic interference.

Design considerations and safety: Key concerns include leakage inductance, winding insulation, core saturation, and thermal management. High-voltage secondaries require careful insulation and creepage distances. Modern flyback converters use dedicated controllers to regulate output via feedback and duty-cycle control.