Free Flyback SMPS Transformer Calculator

Calculate primary inductance (Lp), peak switch current, primary-to-secondary turns ratio, and magnetic core air gap length for isolated power supplies.

⚡ Power Supply Input & Output Specs

Min Input V_in(min) (V) Rectified DC bus min

Max Input V_in(max) (V) Rectified universal 265VAC

Output Voltage V_out (V)

Output Current I_out (A)

Switching Freq (kHz)

Max Duty Cycle D_max Typically 0.40 - 0.48

Ferrite Core Selection (Cross Section Area Ae)

📊 Calculated Transformer Parameters

Required Primary Inductance (L_p)

560 μH

30.0W Output Power (35.3W Input @ 85% eff)

Peak Switch Current (I_pk)

1.75 A

MOSFET rating min: 3.5A

Turns Ratio (N_p / N_s)

6.4 : 1

Reflected voltage: 81.8V

Estimated Primary Turns (N_p): 58 Turns (B_max = 0.28 T)

Secondary Turns (N_s): 9 Turns (for 12.0V + 0.7V diode)

Required Center Air Gap: 0.52 mm (20.5 mils)

Drain-Source Peak Voltage: 556V (Use 650V/700V MOSFET)

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Understanding the Flyback Transformer

Strictly speaking, a flyback transformer is not a true transformer—it is a coupled inductor. Energy is stored in the magnetic core's air gap during the primary MOSFET ON-time, and then released into the secondary load circuit during the OFF-time.

The Discontinuous Mode (DCM) Equations

P_{\text{in}} = \frac{P_{\text{out}}}{\eta} \quad (\eta \approx 0.85)

L_p = \frac{V_{\text{in(min)}}^2 \cdot D_{\text{max}}^2}{2 \cdot P_{\text{in}} \cdot f_{\text{sw}}} \quad \text{(Primary Inductance in Henries)}

I_{\text{pk}} = \frac{2 \cdot P_{\text{in}}}{V_{\text{in(min)}} \cdot D_{\text{max}}} \quad \text{(Peak Primary Switch Current)}

l_g \approx \frac{\mu_0 \cdot N_p^2 \cdot A_e}{L_p} \quad \text{(Center Leg Air Gap Length)}

Why the Air Gap is Critical

High-permeability ungapped ferrite cores saturate at low energy storage ($B_{\text{sat}} \approx 0.35\text{ Tesla}$). Grinding an air gap into the center core leg reduces effective permeability, forcing almost all magnetic energy ($E = \frac{1}{2} L I^2$) to be stored in the air gap volume without driving the ferrite core into saturation.

Frequently Asked Questions

Why do flyback primary and secondary windings have opposite dot polarities?

The opposite polarity dots guarantee that when the primary switch turns ON and current charges the primary inductor, the secondary diode is reverse-biased, preventing energy transfer. When the primary switch abruptly turns OFF, the collapsing magnetic field inverts winding voltage polarities, forward-biasing the secondary diode to dump energy into the output capacitor.

What causes high voltage ringing spikes on the MOSFET drain?

Not all magnetic flux couples between primary and secondary; roughly 1% to 3% remains as leakage inductance ($L_{\text{leak}}$). Because the secondary cannot clamp leakage energy, an RCD snubber network across the primary winding is mandatory to clamp the voltage spike below the MOSFET breakdown rating ($V_{\text{DS}}$).

How does continuous conduction mode (CCM) compare to DCM?

In CCM, inductor current never falls to zero between cycles, resulting in lower peak currents and smaller filter capacitors. However, CCM introduces a right-half-plane (RHP) zero in the feedback control loop and requires ultra-fast secondary recovery diodes.