What Is a Flyback Transformer?
A flyback transformer is the core magnetic component of the flyback converter — one of the most widely used topologies in low-to-medium power switch mode power supplies (SMPS). Unlike a conventional transformer, it operates more like a coupled inductor: energy is stored in the air gap during the switch-on period and transferred to the output during the switch-off period. Flyback transformers are found in phone chargers, LED drivers, laptop adapters, auxiliary power supplies and countless industrial control boards.
How Does a Flyback Transformer Work?
During the switch on-time, current ramps through the primary winding and energy is stored in the magnetic field, with the secondary diode reverse-biased so no energy is transferred. When the switch turns off, the primary current path opens, the magnetic field collapses, the secondary diode conducts, and the stored energy is delivered to the output capacitor and load. The transformer therefore provides isolation, energy storage and voltage conversion in a single component.
Flyback converters can operate in discontinuous conduction mode (DCM), where the secondary current falls to zero before the next switch cycle, or continuous conduction mode (CCM), where energy remains in the core between cycles. DCM is simpler and common at low power; CCM reduces peak currents and is preferred for higher power. The transformer design — especially its primary inductance and air gap — determines which mode the converter runs in.
Key Parameters of a Flyback Transformer
- Turns ratio (Np:Ns): sets the output voltage together with the duty cycle and reflected voltage.
- Primary inductance (Lp): controls the stored energy per cycle and the operating mode (DCM or CCM).
- Air gap: stores energy and prevents core saturation; gap length is a core design decision, not an afterthought.
- Leakage inductance: causes voltage spikes on the switch; low leakage improves efficiency and EMI and reduces snubber losses.
- Saturation current: the peak primary current the core can carry before inductance collapses — critical for CCM operation.
- Isolation and safety ratings: reinforced insulation, creepage and clearance for mains isolation (e.g. 3 kV to 4 kV AC withstand).
- Operating frequency: typically 65 kHz to 150 kHz for modern flyback designs; core material must suit the frequency.
Flyback vs. Forward Transformer
A forward transformer transfers energy directly during the switch on-time and needs a reset winding or reset mechanism; a flyback transformer stores energy and transfers it during the off-time. Forward converters suit higher power with better transformer utilization, but add a reset winding and output choke. Flyback converters are simpler — no output choke, lower parts count — which is why they dominate below roughly 100 W. For higher power, a forward or LLC design usually becomes more economical.
How to Choose the Right Flyback Transformer
- Define input and output requirements — input voltage range, output voltage and current, and isolation requirements.
- Select the operating frequency — higher frequency shrinks the core and winding, but increases switching loss and EMI.
- Choose the core material and shape — ferrite (e.g. PC40/PC47 or equivalent) with an appropriate gap is standard for 65–150 kHz; EE, EI, RM, PQ and EP shapes suit different power levels and PCB footprints.
- Calculate turns and inductance — determine Np:Ns from the reflected voltage and output, and Lp from the energy and mode (DCM or CCM).
- Size the wire for current density and skin effect — use litz or multiple strands at high frequency to control AC resistance.
- Verify insulation and EMI margins — triple-insulated wire for secondary on mains-isolated designs, proper creepage, and a low-leakage winding structure (sandwich winding) to reduce spikes.
Applications of Flyback Transformers
- Phone and tablet chargers, USB-C adapters
- LED drivers and lighting power supplies
- Laptop and notebook adapters
- Auxiliary and standby power supplies for appliances and industrial controls
- Electric meter and IoT device power modules
- Isolated gate driver and communication power supplies
Shaanxi SiRui Technology Co., Ltd Flyback Transformer Range
Shaanxi SiRui Technology Co., Ltd designs and manufactures flyback transformers on ferrite cores for switching power supplies, covering EE, EI, RM, PQ and EP packages with custom turns ratios, inductance, air gaps and reinforced insulation, plus full OEM/ODM support for your converter design. Products are manufactured under ISO 9001:2015 with RoHS and REACH compliance.
FAQ
Why does a flyback transformer need an air gap?
The air gap stores energy and controls the BH curve so the core does not saturate at the required peak current. Without a sufficient gap, a flyback transformer would saturate quickly and the converter would lose regulation or fail.
What is the difference between DCM and CCM flyback?
In DCM the secondary current reaches zero before the next cycle, giving lower stored energy, simpler control and smaller inductance; in CCM energy remains in the core, giving lower peak currents and better utilization at higher power, but requiring a larger inductance and careful saturation design.
Why is leakage inductance a problem in flyback converters?
Leakage inductance stores energy that cannot be transferred to the output, so it produces a voltage spike on the switch when it turns off. Low-leakage winding construction (for example sandwiching the secondary between primary halves) and a proper RCD snubber keep the spike within the switch rating.
Related Articles
- High-frequency transformer design: key considerations for switching power supplies
- Transformer core materials: ferrite, iron powder, sendust, MPP & nanocrystalline
Talk to Our Engineers
Need a flyback transformer designed or customized for your power supply? Contact Shaanxi SiRui Technology Co., Ltd — our engineers will help with turns, inductance, core and winding selection, provide samples and share detailed datasheets.