Flyback transformers

What Are Flyback Transformers?

Flyback transformers are magnetically coupled energy-storage inductors used in flyback power converter circuits to transfer energy from the primary circuit to a galvanically isolated secondary output. Unlike conventional power transformers, which transfer energy while both the primary and secondary windings carry current simultaneously, a flyback transformer stores energy in its magnetic core during the switch-on interval and releases that energy to the secondary circuit only after the primary switch opens. The term "flyback" refers to the voltage reversal on the secondary winding that occurs at switch turn-off, which forward-biases the output diode and allows current to flow. This operating principle enables isolated, multi-output power supplies with a minimal component count.

Flyback transformers are core components of switched-mode power supplies (SMPS) in the 5 to 150 W power range and were historically the defining element of high-voltage power stages in cathode ray tube (CRT) televisions and monitors, where they generated the kilovolt-level deflection and anode voltages required.

Magnetics Design and Ferrite Cores

The energy storage function of a flyback transformer requires a core with a controlled air gap, because ungapped ferrite would saturate rapidly with the unidirectional magnetizing current that builds during the primary conduction interval. Ferrite cores made from manganese-zinc or nickel-zinc spinel compositions are the standard choice for switching frequencies from approximately 20 kHz to several hundred kilohertz. The gap, introduced into the center leg of an E or EE core assembly, stores the majority of the magnetic energy and sets the effective inductance. The primary inductance Lp, switching frequency fs, and duty cycle D together determine the peak current and the energy per cycle delivered to the output.

Coupled winding turns ratios set the output voltage for each secondary: a turns ratio n = Np/Ns, combined with the input bus voltage and duty cycle, determines the regulated output voltage. High-voltage secondary windings wound with appropriate creepage and clearance between layers generate the several kilovolts needed for CRT anode supplies without breakdown.

Cathode Ray Tube Applications

In CRT-based display systems, the flyback transformer served a dual role: it provided the regulated high-voltage (15 to 30 kV) supply for the electron beam accelerating anode, and its secondary winding coupled to the horizontal deflection yoke to sweep the electron beam across the screen at the line rate (15.625 kHz in PAL, 15.734 kHz in NTSC). The transformer's leakage inductance resonated with the deflection yoke capacitance, shaping the sawtooth-like current waveform needed for linear horizontal deflection. Additional secondary windings on the same core provided lower voltages for focus electrodes and screen grids. The electron beam current drawn by the tube created a load-regulation challenge because the high-voltage output was sensitive to picture brightness, requiring additional regulation circuits for broadcast-quality displays.

Switched-Mode Power Supply Operation

In modern isolated SMPS, flyback converters are favored for their simplicity: a single switching transistor, the transformer, a diode, and an output capacitor constitute the basic topology. The converter operates in either discontinuous conduction mode (DCM), where the core fully demagnetizes before the next switching cycle begins, or continuous conduction mode (CCM). DCM simplifies control loop design and is typical at lower power levels; CCM offers lower peak currents and reduced output ripple at higher power levels. Feedback is taken from the secondary output, either directly via an optocoupler or indirectly via an auxiliary winding, and returned to the primary-side pulse-width modulation (PWM) controller. Commercial PWM controller ICs from suppliers including Texas Instruments, Infineon, and ON Semiconductor integrate the switching transistor, protection circuits, and startup functions into single packages.

Applications

Flyback transformers have applications in a range of power electronics and display systems, including: