Free Flyback Transformer Inductance & Turns Calculator
Calculate primary magnetizing inductance (Lp), primary and secondary turns (Np/Ns), peak switch current, and core air gap for flyback SMPS converters.
⚡ Converter Electrical Specs
📊 Magnetics & Winding Specs
Recommended Tools & Equipment
Tested hardware and components for high reliability
How to Design Flyback Transformers
Unlike forward, push-pull, or bridge converters where a true transformer transfers energy instantaneously from primary to secondary, a flyback "transformer" is actually a coupled inductor. Current never flows through both windings at the same instant.
1. Primary Inductance Equation (DCM Mode)
In Discontinuous Conduction Mode (DCM), all stored magnetic energy is completely transferred to the output before the next clock cycle begins:
L_p = [V_in,min² × D_max²] / [2 × P_in × f_sw]
2. Turns Ratio & Peak Current
The peak primary current is:
To prevent magnetic core saturation, the primary turns ($N_p$) must satisfy Faraday's Law using the core's effective cross-sectional area ($A_e$):
Frequently Asked Questions
Why does a flyback core require an air gap?
Ferrite material has extremely high permeability and saturates at very low currents. An air gap has high magnetic reluctance, allowing the core to store substantial energy (Energy = 1/2 * Lp * Ipk^2) in the air gap volume without driving the ferrite past its saturation flux density (0.25-0.35 Tesla).
What is the reflected voltage (VOR) in a flyback converter?
When the primary MOSFET turns off, the secondary output voltage plus diode drop (Vout + Vd) is transformed back across the primary windings by the turns ratio: VOR = (Vout + Vd) * (Np / Ns). The MOSFET sees a total drain-source voltage equal to Vin_max + VOR + leakage inductance spike.
How do I minimize primary leakage inductance in a flyback?
Use sandwich winding construction: wind half of the primary winding first, then the complete secondary winding, and finish with the remaining half of the primary on top. This interleaving cuts leakage inductance by 50% to 75%, greatly reducing RCD snubber dissipation.
What is the difference between DCM and CCM flyback operation?
In DCM (Discontinuous Conduction Mode), inductor current returns to zero every cycle, eliminating diode reverse recovery losses and simplifying control loop stability. In CCM (Continuous Conduction Mode), current never drops to zero, allowing smaller peak currents and lower ripple, but introducing a Right-Half-Plane (RHP) zero that restricts loop bandwidth.