Free Flyback Transformer Inductance Calculator
Size flyback transformer primary inductance (Lpri), calculate DCM/CCM peak current, turns ratio (Np/Ns), and ferrite core air gap length.
⚡ Converter Specifications & Voltage Rails
Turns Ratio (Np / Ns): 6.41 : 1
• Energy Stored per Cycle: 0.54 mJ (E = ½ L I_pk²).
• Recommended MOSFET Breakdown: ≥ 650V or 800V (Requires RCD snubber clamp).
A Gapped Inductor, Not a Transformer: Flyback transformers do not transfer power simultaneously like forward transformers. They store magnetic energy in an air gap during $t_{on}$, then release it to the secondary during $t_{off}$.
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Flyback Converter Design Principles
The flyback converter is the dominant isolated SMPS topology for consumer power supplies under 100 Watts (wall adapters, appliances, auxiliary supplies). Sizing calculations depend on key governing relations:
Governing Design Equations
Turns Ratio: n = N_p / N_s = V_RO / (V_out + V_F)
Input Power: P_in = (V_out × I_out) / η
DCM Primary L: L_pri = (V_in,min × D_max)² / (2 × P_in × f_sw)
Peak Primary Current: I_pk = (V_in,min × D_max) / (L_pri × f_sw)
MOSFET Voltage Stress: V_ds = V_in,max + V_RO + V_leakage_spike (typically +100V spike)
DCM vs. CCM Trade-offs
- Discontinuous Mode (DCM): All magnetic energy empties before the next switching cycle begins. Eliminates secondary diode reverse recovery losses and removes the Right-Half-Plane Zero (RHPZ), allowing fast loop compensation. Requires a smaller inductance but higher peak switch current.
- Continuous Mode (CCM): Current remains in the core at all times. Reduces RMS and peak currents, lowering conduction heating. However, it introduces an RHPZ and severe diode reverse-recovery spikes.
Frequently Asked Questions
Why does a flyback transformer require a physical air gap?
Ferrite core material has high magnetic permeability and saturates quickly under unipolar DC bias. An air gap reduces effective permeability and stores >95% of the magnetic energy (W = B^2 / (2 * mu0) * Volume), allowing high DC energy storage without saturating the magnetic core.
What is Reflected Output Voltage (V_RO)?
When the primary MOSFET turns off, the secondary diode conducts, clamping the secondary winding at Vout + Vf. This voltage transforms back across the primary winding as V_RO = n * (Vout + Vf). V_RO sits directly on top of the DC input voltage, adding to the MOSFET drain voltage stress.
Why is an RCD snubber required across the primary winding?
A real flyback transformer has 1% to 3% leakage inductance that does not couple to the secondary. When the switch opens, this trapped leakage energy cannot discharge into the load; it creates an inductive voltage spike (V = L_leak * di/dt) that will destroy the switch without an RCD clamp.