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Flyback Converter Transformer Magnetizing Inductance Calculator

Flyback converters are the dominant isolated power supply topology for USB-C chargers, LED drivers, and auxiliary supplies up to 100 Watts.

Minimum rectified bulk capacitor DC voltage.

Secondary isolated DC output voltage.

Maximum continuous output power delivered to load.

PWM operating frequency (typically 65 to 130 kHz).

Maximum allowed primary duty cycle (typically 0.40 to 0.50).

Estimated conversion efficiency (typically 80% to 90%).

Calculated Result
265.7 µH

Primary Magnetizing Inductance

Primary Magnetizing Inductance (L_m)

265.7 µH

Peak Primary Current (I_pk)

2.21 A

RMS Primary Winding Current (I_rms)

0.86 A

Transformer Turns Ratio (N_p / N_s)

5.48:1

Energy Stored per Cycle

651.6 µJ

Calculation Breakdown

  1. Input Power & Peak Current FormulationP_in = 36 W / 0.85 = 42.4 W, I_pk = 2 × P_in / (V_in × D_max) = 2.21 A
  2. Primary Core Inductance SizingL_m = (V_in × D_max) / (I_pk × f_sw) = (85 × 0.45) / (2.21 × 65000) = 265.7 µH
  3. Turns Ratio SynthesisN_p / N_s = [ V_in / (V_out + V_d) ] × [ D_max / (1 - D_max) ] = 5.48:1

Flyback Transformer Design Parameters

Interactive visualization based on your current inputs

Value
0.016334965L_m (µH / 10)I_pk (A)I_rms (A)Turns RatioEnergy (µJ / 10)ParameterValue

What Is the Flyback Converter Transformer Magnetizing Inductance Calculator?

The Flyback Converter Transformer Magnetizing Inductance Calculator sizes the energy storage core inductance, turns ratios, and primary currents for isolated flyback power supplies.

How Does the Flyback Converter Transformer Magnetizing Inductance Calculator Work?

It computes input power based on efficiency, derives peak primary currents for DCM operation, and solves for gapped core inductance and turns ratio.

Flyback Converter Transformer Magnetizing Inductance Calculator Formula & Variables

The core mathematical equation utilized by this calculator is expressed as:

I_{pk} = \frac{2 P_{in}}{V_{in} D_{max}}, \quad L_m = \frac{V_{in} D_{max}}{I_{pk} f_{sw}}, \quad \frac{N_p}{N_s} = \frac{V_{in}}{V_{out} + V_d} \left(\frac{D_{max}}{1 - D_{max}}\right)

Discontinuous Conduction Mode (DCM) magnetic energy storage where all magnetizing energy transfers to secondary each clock cycle.

How to Use the Flyback Converter Transformer Magnetizing Inductance Calculator

  1. Enter minimum rectified input DC voltage and output voltage.
  2. Specify continuous output power in Watts.
  3. Input switching frequency and maximum allowable duty cycle.

Step-by-Step Example Calculation

36W 12V Universal AC-DC Flyback Adapter

Input Values:

inputVoltageMinVin:85
outputVoltageVout:12
outputPowerWatts:36
switchingFrequencyKhz:65
maximumDutyCycleDmax:0.45
targetEfficiencyPercent:85
Worked Steps: Requires 265.5 µH primary magnetizing inductance, handling 2.21 A peak primary current with a 5.48:1 turns ratio.

Understanding Your Result

Magnetizing Inductance (L_m): Primary winding inductance to specify.

Peak Primary Current (I_pk): Maximum current the primary MOSFET must conduct without core saturation.

Turns Ratio (Np/Ns): Primary-to-secondary turns ratio.

Factors That Affect the Result

  • Higher switching frequencies proportionally shrink required core inductance and physical transformer size.
  • Wider duty cycles lower peak currents but raise reflected voltage stress on the primary switch.

When Should You Use This Calculator?

  • Isolated offline switch mode power supplies (SMPS), auxiliary power rails, LED driver ballasts, and telecom PoE power converters.

Assumptions & Limitations

  • Assumes boundary/discontinuous conduction mode (DCM) and 0.7V secondary diode forward drop.

Frequently Asked Questions

Calculation Accuracy & Reference Note

Adheres to Texas Instruments SLVA463 and Power Integrations design application notes.

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