The flyback transformer is the heart of every flyback converter — one of the most popular topologies for offline power supplies from 5 W to 150 W. Unlike a normal transformer, a flyback transformer stores energy in the core during the on-time and releases it during the off-time, so its design balances turns ratio, primary inductance, core size, and air gap. This guide walks through the key formulas and core selection criteria for a reliable flyback transformer design.
How a Flyback Transformer Works
During the switch on-time, the primary winding is connected across the input and the core stores magnetic energy. The output diode is reverse-biased, so no energy transfers to the load. During the off-time, the switch opens, the primary current collapses, and the stored energy is released through the secondary winding to the output. The transformer therefore behaves like a coupled inductor: the air gap dominates the energy storage, and the turns ratio determines the reflected output voltage on the primary.
This operating principle means a flyback transformer must have a controlled air gap (in the ferrite core) or a distributed gap (powder core) so that the required energy can be stored without saturating the core at the maximum peak current.
Key Design Parameters
1. Turns ratio (Np:Ns). The turns ratio sets the reflected output voltage and the primary/secondary voltage stress. For a given input range and output voltage, the turns ratio must keep the MOSFET drain voltage within its rating while maximising efficiency. A common starting point is to choose the turns ratio so that the reflected voltage is roughly 60-80% of the input voltage at the low line.
2. Primary inductance (Lp). Lp determines the peak current and the converter’s operating mode (CCM or DCM). Lower Lp gives higher peak current and more energy per cycle; higher Lp pushes the converter toward CCM and reduces peak current but requires more turns.
3. Peak current and saturation. The core must not saturate at the maximum peak current plus margin. The saturation flux density of ferrite is about 0.3-0.45 T at 100 °C, so the air gap must be sized to keep the operating flux below this limit over the full temperature range.
4. Leakage inductance. Leakage energy causes voltage spikes on the switch and increases losses. Good winding techniques (interleaving, proper layer order, tight coupling) keep leakage below about 1-3% of primary inductance.
Turns Ratio Calculation
For a flyback converter with input voltage Vin, reflected voltage VR = N·(Vout + VD), the turns ratio is:
N = Np / Ns = VR / (Vout + VD)
where VD is the output diode forward drop. The maximum drain voltage is then roughly Vin,max + VR + leakage spike. For example, a 24 V output with a 120 V reflected voltage and VD = 1 V gives N ≈ 120/25 = 4.8.
The primary inductance is selected from the required power and switching frequency:
P = ½ · Lp · Ipk² · fsw (DCM), or via the ripple ratio k in CCM design.
Core Selection for Ferrite Core Flyback Transformers
1. Core material. Mn-Zn power ferrites (e.g., PC40/PC44 class, with permeability around 2000-2500) are the standard choice for flyback transformers from 50 kHz to 200 kHz. At higher frequencies, lower-loss grades (PC95/3C95) reduce heating. The material must maintain high saturation flux density at the operating temperature (typically 100 °C).
2. Core size and area product. Size the core so that the area product (Ae × Aw) satisfies the power and current density requirements, and so that the core loss stays within the thermal budget. For a 60 W flyback at 65 kHz, a core around EE25-EE28 or PQ2620 is typical.
3. Air gap. Grind the centre leg to achieve the target Lp with the chosen turns. The gap also linearises the inductance, which stabilises the converter over load and temperature.
Practical Design Steps
Step 1. Define input range, output voltage/current, and switching frequency. Step 2. Select the operating mode (DCM or CCM). Step 3. Calculate the turns ratio from voltage stress limits. Step 4. Calculate Lp from the power equation and ripple specification. Step 5. Choose the core material and size, then calculate the minimum turns from saturation. Step 6. Determine the air gap for the target Lp. Step 7. Design the windings for low leakage and good insulation (safety isolation for offline applications). Step 8. Build, test, and verify at minimum and maximum input voltage.
Sirui Flyback Transformer Manufacturing
Shaanxi SiRui Technology Co., Ltd manufactures custom flyback transformers and ferrite core transformers for offline power supplies, LED drivers, chargers, and industrial control. We support the full range of ferrite materials, custom turns ratios and air gaps, and provide free design review with winding and insulation recommendations. Our factory is ISO 9001:2015 certified and all products are RoHS / REACH compliant.
For related reading, see our Flyback Transformer: Working Principle, Design Considerations & Selection Guide, Transformer Core Materials Selection Guide, and High-Frequency Transformer Design Guide. To request a quotation or a design review, contact sales@sirui-tech.com.
FAQ
Why does a flyback transformer need an air gap?
The air gap stores magnetic energy and prevents core saturation at the peak current. Without a gap, the effective permeability is too high, the inductance per turn is large, and the core saturates well below the required peak current.
What is the difference between a flyback transformer and a normal transformer?
A normal transformer transfers energy continuously (simultaneously on primary and secondary). A flyback transformer stores energy during the on-time and releases it during the off-time, so it behaves as a coupled inductor and requires a gapped core.
How do I reduce flyback transformer leakage inductance?
Use interleaved windings, keep the primary and secondary close (bobbin design), minimise the number of layers, and keep the winding area full. A well-designed flyback transformer keeps leakage at 1-3% of Lp.