Flyback Converter Calculator
Find the turns ratio, magnetizing inductance, and peak switch currents for flyback converter design in CCM and DCM switch-mode power supplies.
🔄 What is a Flyback Converter?
A flyback converter is a type of isolated DC-DC switch-mode power supply (SMPS) that stores energy in a transformer's magnetic field during the switch on-time and transfers it to the output through the secondary winding during the switch off-time. Unlike a forward converter where energy flows to the output continuously, the flyback topology uses a single switching transistor and a coupled inductor (the flyback transformer) to provide galvanic isolation between input and output while stepping voltage up or down.
Flyback converters are the most widely used isolated converter topology for output powers from 1 W to approximately 150 W. They appear in phone chargers, laptop adapters, set-top box power supplies, LED driver circuits, industrial 24 V isolated rails, and any application requiring a low-cost isolated output. Multiple isolated secondary windings can provide several independent output voltages from a single converter, making flyback designs especially versatile for multi-rail power supplies.
A common misconception is that the flyback transformer works like a conventional mains transformer. It does not. The flyback transformer is functionally a coupled inductor, and energy stored in the core's air gap during the on-time is the key operating mechanism. The turns ratio determines voltage conversion and switch voltage stress, not just impedance transformation. The magnetizing inductance directly sets the peak current, stored energy, and whether the converter operates in Discontinuous Conduction Mode (DCM, where the core fully demagnetises each cycle) or Continuous Conduction Mode (CCM, where residual flux remains).
This calculator covers the two most critical design steps: finding the transformer turns ratio and estimating the minimum magnetizing inductance for DCM operation. The Turns Ratio mode handles converter topology analysis and MOSFET selection, while the Inductance and Current mode outputs the inductance value to specify to a transformer manufacturer and the currents needed for winding wire sizing and core selection.
📐 Formulas
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 - 24 V to 5 V USB Charger (Turns Ratio Mode)
Vin = 24 V DC, Vout = 5 V, Duty Cycle D = 0.40
Example 2 - 48 V Telecom Bus to 12 V (Turns Ratio Mode)
Vin = 48 V DC, Vout = 12 V, Duty Cycle D = 0.45
Example 3 - 10 W Charger Inductance and Current (Inductance Mode)
Vin = 24 V, Pout = 10 W, D = 0.40, Fs = 100 kHz, Efficiency = 85%
❓ Frequently Asked Questions
🔗 Related Calculators
What is the turns ratio formula for a flyback converter?
For a flyback converter in CCM, the turns ratio is N = Np divided by Ns = Vin times D divided by (Vout times (1 minus D)). For Vin = 24 V, Vout = 5 V, D = 0.4: N = 24 times 0.4 divided by (5 times 0.6) = 9.6 divided by 3 = 3.20. This means the primary has 3.20 times as many turns as the secondary.
How do I calculate the duty cycle for a flyback converter?
Rearrange the voltage conversion equation: D = N times Vout divided by (Vin plus N times Vout), where N = Np/Ns. For a 3:1 turns ratio, 24 V input, 5 V output: D = 3 times 5 divided by (24 plus 15) = 15 divided by 39 = 0.385. Always verify that D stays below 0.5 to ensure adequate demagnetisation time in the transformer.
What is magnetizing inductance in a flyback transformer?
Magnetizing inductance (Lm) is the inductance seen at the primary terminals when the secondary is open circuit. It determines how much energy is stored in the core during the switch on-time and sets the boundary between DCM and CCM operation. Smaller Lm means higher peak current and more certain DCM operation but also higher conduction losses.
What is the minimum Lm for DCM operation in a flyback converter?
Lm_min = Vin squared times D squared times eta divided by (2 times Pout times Fs). For Vin = 24 V, D = 0.4, eta = 0.85, Pout = 10 W, Fs = 100 kHz: Lm = 576 times 0.16 times 0.85 divided by 2 million = 39.17 uH. A primary magnetising inductance at or below this value ensures the core fully demagnetises before the next switch turn-on.
What is the peak primary current in a flyback converter?
Peak primary current sets the core saturation limit and the MOSFET current rating. For DCM operation: Ip_peak = 2 times Pout divided by (eta times Vin times D). For 10 W output at Vin = 24 V, D = 0.4, eta = 0.85: Ip_peak = 20 divided by 8.16 = 2.451 A. The transformer core must not saturate at this current level.
How do I calculate the peak MOSFET voltage stress in a flyback converter?
The theoretical minimum switch voltage is Vds = Vin plus Vreflected, where Vreflected = Vout times N (Np/Ns). For Vin = 24 V, Vout = 5 V, N = 3.2: Vds = 24 plus 16 = 40 V. In practice, leakage inductance causes additional voltage spikes at turn-off. Add at least 50 percent margin: select a MOSFET rated at 60 V or higher for this example.
What is the difference between DCM and CCM in a flyback converter?
In Discontinuous Conduction Mode (DCM), the transformer core fully demagnetises before the next switching cycle, so primary current starts from zero each cycle. In Continuous Conduction Mode (CCM), residual magnetising current remains at the start of each on-time. DCM is simpler to stabilise, gives lower switch voltage stress, but requires higher peak currents. CCM gives lower peak current but increases switch voltage stress and right-half-plane zero complexity.
Why is the reflected output voltage important in flyback design?
The reflected output voltage (Vreflected = Vout times Np/Ns) adds directly to the input voltage to create the peak switch voltage stress during turn-off. A high reflected voltage means a higher MOSFET Vds rating is needed, which usually means higher on-resistance and more conduction loss. Designers choose the turns ratio to balance reflected voltage against peak primary current.
What switching frequency should I use for a flyback converter?
Common ranges are 65 kHz to 500 kHz for consumer power supplies and 100 kHz to 1 MHz for compact industrial designs. Higher frequency allows a smaller transformer and output capacitor but increases switching losses in the MOSFET and diode. Most cost-optimised flyback designs targeting 5 to 65 W operate at 65 to 130 kHz, a range that balances core loss, transformer size, and EMI compliance.
How do I convert turns ratio to actual primary and secondary turns?
Choose the number of secondary turns first based on the minimum feasible winding (often 3 to 10 turns for standard output voltages). Then multiply by the calculated N: primary turns = N times secondary turns. For N = 3.2 and Ns = 5 turns: Np = 3.2 times 5 = 16 turns. Always round to integer turns; the resulting slight deviation in turns ratio adjusts the duty cycle slightly at steady state.
What core material should I use for a flyback transformer?
MnZn ferrite grades such as Ferroxcube 3C90, 3C95, or TDK PC40 are standard for flyback transformers operating at 65 to 300 kHz. NiZn ferrite is preferred above 500 kHz. Select a core size with sufficient energy storage: the core must handle Lm times Ip_peak squared divided by 2 joules without saturating. Always verify the operating flux density B against the material's saturation limit, typically 300 to 400 mT for MnZn at room temperature.
Can a flyback converter be used for multiple outputs?
Yes. Additional secondary windings can be added for extra isolated outputs. Cross-regulation between outputs is a known limitation since the feedback loop controls only one output (usually the main 5 V or 12 V rail). Auxiliary outputs track the main output but vary with load. Synchronous rectifiers and post-regulation with LDOs are common solutions for tight-regulation auxiliary outputs.