Flyback transformer design looks simple on a schematic, yet it is the part of a small power supply that most often decides efficiency, EMI behaviour and reliability. The flyback is the usual topology for isolated supplies up to roughly one or two hundred watts, the range handled by a typical SMPS transformer manufacturer, because it needs only one switch, one magnetic component and no output inductor. The price of that simplicity is that the transformer must do several jobs at once.

This article sets out the basic design sequence in plain terms, using only standard relations, so that engineers and buyers can judge a design or prepare a clear specification. It is written by NG Electronics, an ISO 9001:2015 certified manufacturer in Pune, India, which builds custom ferrite core transformers for flyback, forward and push-pull converters and tests every unit electrically before dispatch.

How a flyback transformer stores and delivers energy

Unlike a forward or bridge transformer, a flyback transformer is really a coupled inductor. While the switch is on, input voltage is applied across the primary and current ramps up according to V = L di/dt; the secondary diode is reverse biased, so no power reaches the output. Energy accumulates in the magnetic field, mostly in the air gap, following E = 1/2 L I^2. When the switch turns off, the stored energy transfers to the secondary and the output.

Two operating modes exist. In discontinuous conduction mode the core is fully demagnetised every cycle, which gives a simple control response and a smaller inductance but higher peak and RMS currents. In continuous conduction mode some energy remains in the core, lowering peak current at the cost of a larger inductance and a more demanding control loop. Our comparison of flyback and forward transformers covers the alternatives.

Flyback transformer design inputs to fix first

A sound design starts with a complete requirement. Before any core is chosen, the items below should be written down, because each one changes the turns, the gap or the insulation system. Gaps in this list are the most common reason for a second prototype round, and they also make quotations from different suppliers difficult to compare on equal terms, since each supplier fills the blanks with different assumptions.

With these fixed, the electrical targets for the transformer follow directly: primary inductance with tolerance, turns ratios, peak and RMS currents in each winding, and the maximum leakage inductance that the snubber can tolerate. A specialist flyback transformer manufacturer can then propose a core, bobbin and winding build, or review a design that already exists and check that it can be produced consistently in volume.

• Input voltage range, including the minimum DC bus voltage at full load.
• Each output voltage and current, and which output is regulated.
• Switching frequency and operating mode at minimum input.
• Maximum duty cycle and the voltage rating of the switch.
• Peak primary current limit set by the controller.
• Isolation voltage, insulation class and the safety standard that applies.
• Ambient temperature, cooling and maximum dimensions.

Turns ratio, primary inductance and peak current

The turns ratio sets the reflected voltage, which is the output voltage plus the diode drop multiplied by the primary-to-secondary ratio. During the off-time the switch sees the input voltage plus this reflected voltage plus a leakage spike, so the ratio is chosen to keep the total within the switch rating at maximum input, typically with a clear margin. A higher reflected voltage allows a larger duty cycle and lower primary peak current, but raises switch stress.

Primary inductance is then set from the current ramp. During the on-time the current rises by the input voltage multiplied by the on-time and divided by the inductance, so a larger inductance gives a smaller ripple and pushes the design towards continuous conduction. The peak current that results, at minimum input and maximum load, is the figure that the core must carry without saturating, and the current limit of the controller must sit above it with some allowance for tolerance.

Core size, air gap and saturation margin

Peak flux density in a flyback core equals the primary inductance multiplied by the peak current, divided by the primary turns and the effective core area. The designer selects turns so that this value stays below the saturation limit of the ferrite at its hottest operating temperature, where saturation flux density is lower than at room temperature. The consequences of getting this wrong are described in our guide to transformer core saturation.

Once the turns are fixed, the air gap is adjusted to give the required inductance. The gap, usually ground into the centre leg, stores most of the energy and makes inductance far less dependent on ferrite permeability, as explained in the article on the air gap in inductors. A larger gap raises the saturation current but increases fringing flux, which heats nearby copper, so turns are typically kept slightly away from the gap or the winding is spaced with tape.

Winding structure, leakage and insulation

Leakage inductance does not transfer its energy to the output; it is dissipated in the snubber or clamp every cycle and produces the voltage spike at turn-off. It is reduced by winding the primary and secondary across the full bobbin width, in few layers, and close together, often by splitting the primary so that it sandwiches the secondary. Our article on transformer leakage inductance explains the trade-off against interwinding capacitance and common-mode noise.

Insulation between primary and secondary must satisfy the applicable safety standard, using margin tape with layered insulation or triple-insulated wire, and the build must be repeatable. NG Electronics winds, tapes, varnishes and tests flyback transformers in-house, with 100% of units electrically tested as described on the quality assurance page. A free sample for lab approval is part of the normal process, and the prototype manufacturing service typically delivers custom samples within 7 to 10 working days.

Frequently asked questions