How this calculator works
In a flyback the coupled winding stores energy during switch-on, then supplies the secondary during switch-off. Volt-second balance gives the voltage ratio in continuous conduction and at its boundary. The sizing modes specifically assume triangular currents starting at zero and ending at zero with no idle interval. They do not calculate an arbitrary continuous-conduction peak current. Leakage inductance, clamp losses and winding resistance are excluded.
Formula
n = Ns/Np; Vo + Vd = n Vi D/(1-D). At boundary: Is,pk = 2Io/(1-D); Ip,pk = n Is,pk; Lp = Vi D/(fs Ip,pk); Ls = n² Lp.
Worked example
For 12 V in and out, equal turns and no diode drop, D = 0.5. At 1 A output and 100 kHz, both peak currents are 4 A and the primary inductance is 0.000015 H.
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Frequently asked questions
Is the winding ratio Ns/Np or its inverse?
This page uses secondary turns divided by primary turns. Both turns fields remain visible wherever they are known.
Why specify boundary conduction?
Without a magnetizing-current valley or a conduction-mode assumption, output current alone cannot determine peak currents and inductance.
Does this check transformer insulation and core saturation?
No. Core area, allowable flux swing, gaps, creepage, leakage and thermal limits need separate transformer design checks.