Design reliable flyback windings for wide input ranges. Include drops, duty cycle, and core area. Get clean results, export files, and prototype confidently now.
| Vin(min) (V) | Vout (V) | Dmax (%) | fs (kHz) | Ae (mm²) | ΔB (T) | Np (rounded) | Ns (rounded) |
|---|---|---|---|---|---|---|---|
| 12 | 24 | 45 | 100 | 80 | 0.20 | 34 | 98 |
| 18 | 12 | 42 | 65 | 125 | 0.18 | 52 | 39 |
| 90 | 15 | 40 | 130 | 95 | 0.22 | 126 | 29 |
Example rows are illustrative for learning and layout. Your core, losses, and temperature limits may require different ΔB and duty targets.
Primary turns from flux swing
Np = Vin(min) · Dmax / (ΔB · Ae · fs)
This limits the peak-to-peak flux change in the core by controlling volts-seconds per turn.
Reflected voltage during OFF-time
Vref = Vin(min) · Dmax / (1 − Dmax)
This comes from volt-second balance across the magnetizing inductance.
Secondary turns from reflected voltage
Ns = Np · (Vout + Vd) / Vref
The diode drop is included so the winding supports the required load voltage.
A flyback stores energy while the switch is ON, then delivers it when OFF. Primary turns set volts-per-turn, core flux swing, and saturation margin. Secondary turns set reflected voltage and output headroom. Small changes can shift duty cycle and losses.
Turns are usually sized at the lowest input voltage because it demands the highest duty cycle. Many designs cap duty near 40–50% to preserve control margin and reduce peak currents. Very high duty tends to push flux swing upward.
Allowed flux swing ΔB depends on core material, temperature rise, and loss limits. A common starting range is 0.15–0.25 T for ferrite, then refined with loss curves. Lower ΔB increases primary turns and reduces core stress, but may increase copper loss and winding window usage.
Core effective area Ae comes from the datasheet and is typically given in mm². Switching frequency is often 50–200 kHz, depending on controller and EMI limits. Higher frequency reduces turns for the same ΔB, but increases switching loss and may demand better layout and snubbing.
The secondary equation uses (Vout + Vd) so the winding supports the load voltage after rectification. For Schottky diodes, a typical drop may be 0.3–0.6 V, while fast recovery diodes can be higher at current and temperature. If you use synchronous rectification, model the effective drop as the conduction loss equivalent.
Reflected voltage Vref sets a major part of switch drain stress during OFF-time. A first-pass estimate is roughly Vin(max) + Vref, then add leakage spikes and ringing. Many engineers keep 20–30% headroom below the MOSFET rating using turns ratio, clamps, and snubbers.
An auxiliary winding often powers the controller after startup, reducing standby loss. This calculator estimates auxiliary turns by scaling from the secondary using (Vaux + Vd_aux). In practice, regulation depends on coupling and load, so validate the aux rail under extremes.
Real transformers use integer turns, so rounding is unavoidable. After rounding, verify ΔB actual and implied duty in the results. If implied duty exceeds your limit, increase Np or adjust Ns. If ΔB rises above target, add primary turns.
Finally, treat turns as one checkpoint in a full design flow. Confirm insulation and creepage, winding window fill, wire gauge for RMS and peak current, and expected leakage inductance. Leakage drives clamp and snubber requirements, which in turn can influence your preferred Vref and turns ratio. Validate with measurements on a first prototype.
Use Vin(min) for primary turns because it produces the highest duty cycle and the greatest volts-seconds demand. Then verify stress at Vin(max) by checking reflected voltage and MOSFET headroom.
Many designs begin around 0.15–0.25 T, then refine using the core loss curves and temperature rise limits. Lower ΔB improves saturation margin but may increase copper loss and winding size.
The transformer must produce enough secondary voltage so the rectified output reaches the regulated value under load. Adding Vd accounts for rectifier conduction losses and helps avoid under-voltage at rated current.
Reflected voltage is the secondary voltage reflected onto the primary during OFF-time. It largely sets MOSFET drain stress and is controlled by the turns ratio and output voltage requirements.
Yes. Rounding turns changes the effective turns ratio, which changes Vref and therefore the duty needed for volt-second balance. Adjust Np and Ns so the implied duty stays within your chosen limit.
You can calculate the main secondary turns first, then scale additional outputs from that secondary using their target voltage and diode drop. Final regulation depends on cross-regulation and coupling, so bench verification is required.
No. It focuses on turns from volts-seconds and turns ratio. Magnetizing inductance, peak current, air gap, and copper fill are separate design steps that should be validated with your power level and controller limits.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.