Advanced Flyback Converter Calculator

Calculator Inputs

Minimum Input Voltage (V)

Maximum Input Voltage (V)

Output Voltage (V)

Output Current (A)

Output Diode Drop (V)

Estimated Efficiency (%)

Switching Frequency (kHz)

Reflected Voltage on Primary (V)

Allowed Output Ripple (% of Vout)

Maximum Flux Swing ΔB (T)

Core Effective Area Ae (cm²)

MOSFET Spike Margin (%)

Reset

Example Data Table

This sample row uses the default values already loaded into the form.

Vin Min	Vin Max	Vout	Iout	Frequency	Efficiency	Vref	Turns Ratio	Duty @ Vin Min	Primary L	MOSFET Stress
85.00 V	375.00 V	12.00 V	5.00 A	65.00 kHz	85.00 %	100.00 V	8.00 :1	54.05 %	230.05 µH	570.00 V

Formula Used

This page uses a practical first-pass flyback model based on reflected voltage, boundary-conduction behavior, and transformer volt-second balance.

Core equations

Turns ratio (Np/Ns) = Reflected Voltage / (Vout + Diode Drop) Duty cycle at a given input = Reflected Voltage / (Vin + Reflected Voltage) Output Power = Vout × Iout Estimated Input Power = Output Power / Efficiency Primary Peak Current ≈ max[ 2 × Pin / (Vin_min × Dmax), (2 × Iout / (1 − Dmax)) / (Np/Ns) ] Primary Inductance = Vin_min × Dmax / (fs × Ipk_primary) Energy per cycle = Pin / fs Minimum Output Capacitance ≈ Iout × Dmax / (fs × Allowed Ripple Voltage) Primary Turns ≈ Vin_min × Dmax / (ΔB × Ae × fs) Estimated MOSFET Stress ≈ (Vin_max + Reflected Voltage) × (1 + Spike Margin) Output Diode Reverse Voltage ≈ Vout + Vin_max / (Np/Ns)

These equations are suitable for preliminary sizing. Final designs still need clamp tuning, transformer optimization, thermal checks, and laboratory validation.

How to Use This Calculator

Enter the lowest and highest input voltage your supply will see.
Set the required output voltage and output current.
Add the diode forward drop and an honest efficiency estimate.
Choose switching frequency and reflected voltage to shape duty cycle and switch stress.
Enter ripple limit, flux swing target, and effective core area for transformer sizing.
Press the calculate button to show results above the form.
Review turns ratio, inductance, peak currents, capacitor need, and device stress.
Export the finished result set as CSV or PDF for documentation.

Frequently Asked Questions

1. What does this calculator estimate?

It estimates key flyback design values such as turns ratio, duty cycle, peak and RMS currents, primary inductance, output capacitance, switch stress, and rough transformer turns.

2. Is this enough to build a production supply?

No. It is a strong starting point, but final hardware still needs transformer refinement, leakage control, snubber design, thermal analysis, EMI work, and safety review.

3. Why is reflected voltage important?

Reflected voltage strongly affects duty cycle, transformer turns ratio, and MOSFET drain stress. Raising it usually lowers duty cycle, but it also increases switch voltage stress.

4. Why are there two input voltages?

Flyback stress changes across the input range. Minimum input usually drives higher duty cycle and current, while maximum input often drives the highest switch voltage stress.

5. Does the capacitor result include ESR effects?

No. The capacitor figure mainly covers charge-storage ripple. Real capacitor selection must also check ESR ripple, ripple current rating, life, temperature, and margin.

6. Can I use this for multiple outputs?

Not directly. Multiple outputs need cross-regulation analysis, winding arrangement choices, load priority rules, and separate diode and capacitor checks for each rail.

7. Why does the page estimate turns and air gap?

Primary turns come from volt-second balance and flux limit. Air gap is then estimated from inductance, turns, and core area as an early transformer planning value.

8. What should I verify after using this tool?

Verify controller limits, startup behavior, snubber stress, clamp loss, transformer temperature, MOSFET avalanche margin, diode recovery, output ripple, and oscilloscope waveforms on hardware.