Flyback Transformer Calculator (Turns Ratio & Inductance)
By Santaji GadepatilinUpdated November 9, 2026
A flyback converter stores energy in a coupled inductor while the switch is on and releases it to the output when the switch turns off, so the transformer turns ratio sets both the duty cycle and the voltage the switch must block. This flyback transformer calculator gives the turns ratio, the reflected output voltage, the drain–source voltage stress, the peak primary current and the primary inductance for operation at the edge of discontinuous mode, as described in how a flyback converter works.
How to use the flyback transformer calculator
Enter the minimum and maximum DC input voltage, the output voltage, the rectifier forward voltage, the maximum duty cycle you want at minimum input, the output power, an expected efficiency and the switching frequency. The calculator returns the turns ratio, the reflected voltage and the switch voltage, the peak primary and secondary currents, and the largest primary inductance that keeps the converter in discontinuous mode at full load (boundary mode at minimum input).
Flyback transformer formulas
n = Np ÷ Ns = Vin,min Dmax ÷ [(1 − Dmax)(Vout + Vf)] · VOR = n (Vout + Vf)
VDS = Vin,max + VOR · Ipk = 2 Pin ÷ (Vin,min Dmax) · Lp = Vin,min Dmax ÷ (Ipk fs)
The inductance and peak current are for the boundary between continuous and discontinuous conduction at minimum input and full load; any lower inductance keeps the converter in DCM.
Worked example
A 10 W, 5 V flyback from an 18–32 V DC bus, Schottky diode (Vf = 0.5 V), Dmax = 45%, efficiency 80%, 100 kHz:
- Turns ratio n = 18 × 0.45 ÷ (0.55 × 5.5) = 2.678
- Reflected voltage VOR = 2.678 × 5.5 = 14.73 V; switch voltage at 32 V input = 46.73 V (choose a ≥ 80 V MOSFET to allow for the leakage spike)
- Input power 10 ÷ 0.8 = 12.5 W, peak primary current = 2 × 12.5 ÷ (18 × 0.45) = 3.086 A
- Primary inductance Lp = 8.1 ÷ (3.086 × 100,000) = 26.24 µH; peak secondary current = 8.26 A
- At 32 V input the duty cycle drops to about 31.5%
Universal-input (85–265 V AC) designs, 20 W
Bulk-capacitor voltage 100–375 V DC, efficiency 85%:
| Output | Dmax | Turns ratio | VOR | VDS before spike |
|---|---|---|---|---|
| 5 V | 40% | 12.12 | 67 V | 442 V |
| 5 V | 45% | 14.88 | 82 V | 457 V |
| 5 V | 50% | 18.18 | 100 V | 475 V |
| 12 V | 40% | 5.25 | 67 V | 442 V |
| 12 V | 45% | 6.44 | 82 V | 457 V |
| 12 V | 50% | 7.87 | 100 V | 475 V |
| 24 V | 40% | 2.70 | 67 V | 442 V |
| 24 V | 45% | 3.31 | 82 V | 457 V |
| 24 V | 50% | 4.05 | 100 V | 475 V |
The leakage-inductance spike typically adds 30–50% of VOR, which is why 650 V or 800 V MOSFETs are used on universal-input flybacks.
Practical notes
- Clamp the leakage spike with an RCD clamp or TVS; set the clamp voltage about 1.5 × VOR and keep it below the MOSFET rating with 10–20% margin.
- The core must store ½ Lp Ipk² without saturating — flyback cores need an air gap, and the gap stores most of the energy.
- Round the turns to whole numbers and recalculate the duty cycle; an auxiliary winding for the controller supply uses the same volts-per-turn.
- Secondary RMS current is much higher than the DC output current at low duty cycle, so size the output capacitor for ripple current, not just capacitance.
Frequently asked questions
How do you calculate the turns ratio of a flyback transformer?
From volt-second balance at the minimum input voltage and maximum duty cycle, Np ÷ Ns = V_in,min × D_max ÷ [(1 − D_max) × (V_out + V_f)].
What is the reflected voltage in a flyback converter?
While the secondary conducts, the output voltage plus the diode drop appears on the primary multiplied by the turns ratio, V_OR = n × (V_out + V_f). It adds to the input voltage across the switch, so the switch sees V_in,max + V_OR plus a leakage-inductance spike.
How is the primary inductance chosen?
For discontinuous (DCM) operation all the energy ½ L I_pk² must be delivered every cycle, so L_p ≤ (V_in,min × D_max)² ÷ (2 × P_in × f_s). A larger inductance pushes the converter into continuous mode (CCM) with lower peak current but a right-half-plane zero in the control loop.
What duty cycle should a flyback use?
Typically 40–50% at minimum input. A higher duty cycle lowers the primary peak current but raises the reflected voltage and the switch voltage stress; with universal AC input this usually means a 600–800 V MOSFET and an RCD clamp.