POWER ELECTRONICS | CUSTOMIZABLE COUPLED INDUCTORS

Designing a Flyback Transformer: Core Principles & Practical Steps

Introduction

A flyback transformer is a specialized gapped-core coupled inductor that stores energy in its magnetic field during the MOSFET’s ON-time and releases it to the secondary during OFF-time. Unlike conventional transformers, flybacks operate in energy-storage mode, making them ideal for isolated power supplies (e.g., 5V–48V outputs).

Design parameters of Flyback transformer

Designing a flyback transformer involves calculating key parameters like primary/secondary turns, voltages, currents, and inductance based on your converter's specifications.

First calculate the following key parameters:

• Input voltage Range.

• Output voltage(Vo).

• Output Current(Io).

• Switching frequency(fSW).

• Estimated efficiency(η).

• Primary inductance(Lp).

• Turns ratio(Nps).

• Auxiliary turns(Naux).

• Core material and GAP.

Input Voltage Range

The input voltage range of a flyback transformer depends on the input AC voltage range of the power supply. The input voltage can be calculated by:

Vac = Vin_min  ~ Vin_max   (85-265V)

Vin_min_tr  = Vin_min  x √2

Vin_max_tr =  Vin_max x √2

Output Voltage

Choose an output voltage value according to the power supply demand you are designing eg., 5V, 12V, 24V.

Output Current

Choose an output current value according to the power supply demand you are designing eg., 1A, 2A, 2.5A, 3A, etc.

Switching Frequency

Choose a switching frequency eg., 110kHz.

Estimated Efficiency

Set a target efficiency you want to achieve with your power supply, eg., 80%, 85%.

Primary Inductance

The primary inductance indicates the amount of energy stored in the primary coil per cycle.

                  Vin_min x Dmax 

  Lp =       ______________

           Iinpk x fsw 

Here we choose DCM mode for calculations because of the simplicity of the equations.

Primary Current (Iin_pk)

The maximum current at minimum input voltage the input should supply to meet the required power demands:

Output Power (Po)

Output power can be calculated by multiplying output voltage and current:

Verify Energy Stored

Ensure that primary coil can store enough energy per cycle:

Transformer Turns Ratio(Nps)

This is the ratio of primary turns(Np) to secondary turns in a transformer.

                                

Auxiliary Winding (Na)

In flyback transformer the auxiliary winding serves some of the following purposes:

• Power supply for control IC.

• Feedback for voltage regulation(indirect sensing).

• Overvoltage and undervoltage protection.

• Synchronization and soft start.

---