Flyback SMPS Design Calculator

Optimize isolated flyback converters: compute primary inductance, peak currents, duty cycle, turns ratio, RMS values and voltage stresses. Corrected DCM current waveforms – primary current resets to zero instantly at turn-off.

Universal input: Vin_min = 100V (85VAC), Vin_max = 375V (265VAC). Vor = 100–150V for optimal duty cycle. Efficiency limited 1–99%.
? Universal 85-265VAC → 12V/2A
? Low power 5V/1A (adapter)
? PoE 48V/0.5A flyback
⚡ 230VAC only → 24V/1.5A
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Flyback Converter & Transformer Design Methodology

The flyback converter is the most popular isolated topology for low-to-medium power supplies (up to ~150W). This calculator uses Discontinuous Conduction Mode (DCM) / Boundary mode equations, ensuring complete energy transfer per cycle and simplified control loop. The waveforms reflect accurate DCM behavior: primary current rises linearly during on-time, falls to zero instantly at turn-off, and secondary current decays linearly to zero before the next cycle.

Core Design Equations (DCM/BCM)

Dmax = Vor / (Vor + Vin(min))    (Duty cycle at minimum input)

Lp = (Vin(min)² × Dmax²) / (2 × Pin × fsw) [H]   (BCM inductance)

Ipk = (2 × Pin) / (Vin(min) × Dmax)

Np/Ns = Vor / (Vout + Vf)

VDS(max) = Vin(max) + Vor + leakage spike (design margin ~20-30%)

Vdiode = Vout + Vin(max) / (Np/Ns)

Why Reflected Voltage (Vor) Matters

Vor directly influences MOSFET voltage stress and maximum duty cycle. Typical values range from 80V to 150V. Higher Vor reduces Dmax but increases VDS. The calculator now shows estimated VDS (Vin_max + Vor) and recommends a 20% margin for safe MOSFET selection.

Design verification: Ensure peak current does not saturate the core. Use RMS currents to size wire. The secondary peak current defines output diode stress. For frequencies >150 kHz, consider core losses and switching losses – a warning will appear.

Frequency & Efficiency Guidelines

Switching frequency between 50 kHz and 150 kHz is typical for ferrite cores. Above 200 kHz, the calculator will display a warning due to increased switching and core losses. Efficiency η (1–99%) strongly affects input power and peak currents. The efficiency input is now limited to reasonable values (1–99%) with an automatic warning for unrealistic extremes.

Application Case: 12V/2A Universal Input Adapter

Vin_min=100V, Vin_max=375V, Vor=120V, fsw=65kHz, η=85%. Results: Dmax≈0.545, Lp≈811µH, Ippk≈1.04A, Ispk≈9.9A, Np/Ns≈9.5, Vds(est)=495V → choose 600V FET. Diode Vr≈51V → use 100V Schottky. This BCM/DCM design is robust.

RMS Current & Thermal Significance

RMS current determines copper losses (I²R). Primary RMS: Irms_pri = Ipk × √(Dmax/3). Secondary RMS: Irms_sec = Ispk × √((1-Dmax)/3). Use these values to estimate winding loss and MOSFET conduction loss. Our tool outputs both values instantly.

Parameter Symbol Typical Range Impact
Reflected Voltage Vor 80V – 150V Duty cycle & MOSFET VDS
Max Duty Cycle Dmax 0.35 – 0.65 Energy transfer, slope compensation
Primary Inductance Lp 200µH – 2mH Peak current, energy storage (BCM value)
Switching Frequency fsw 50kHz – 150kHz Transformer size vs switching losses

Frequently Asked Questions

In DCM, the secondary current falls to zero before next switching cycle, and primary current resets completely (as shown in our corrected waveform). CCM has residual energy, requiring larger inductance. Our calculator adopts DCM/BCM equations.

Vor = (Vout+Vf)×Np/Ns. Select between 100 and 150V for 600V MOSFETs. Higher Vor reduces duty cycle but increases leakage spike. Our default 120V is balanced and shows estimated Vds.

That's the correct DCM behavior. When the MOSFET turns off, the primary current commutes to the secondary side instantly. The magnetizing current cannot change instantaneously, but the primary side current becomes zero because the secondary diode conducts. This updated plot reflects actual DCM operation.

Yes, quasi-resonant works with DCM valley switching; primary inductance and peak current calculations still apply. Tuning may be required for valley detection.
References: TI SLUA143 – Flyback Transformer Design, "Power Supply Design" by Pressman/Billings, Infineon QR Flyback guide. All calculations follow industry-proven DCM principles with corrected visual waveforms.