August 2012

Doc ID 12864 Rev 2

1/7

TN0023

Technical note

Discontinuous flyback transformer

description and design parameters

Introduction

The following is a general description and basic design procedure for a discontinuous

flyback transformer. It can be useful for further development of an existing design, where

some parameters are known, or for communicating changes with a transformer design

house.

The flyback transformer can be used in isolated or non-isolated power supplies for almost

any output voltage. The discontinuous flyback transformer has a triangular current waveform

and commonly uses a 50% duty cycle at full load and minimum input line. A lower duty cycle

can be used but the peak and RMS current is higher. A higher duty cycle involves more

complicated means for stability and a lower bandwidth. The frequency is selected based on

many considerations - for example, a smaller transformer can be used with higher frequency

but the switching losses are higher.

The application requirements dictate the design and include minimum AC input voltage,

design, include switching device current rating, peak current (for peak current limit),

switching device drain to source voltage and output rectifier reverse voltage (from maximum

input voltage), wire size, and auxiliary voltage if required.

This technical note presents one of the many procedures used to design a discontinuous

flyback transformer for a low power application, including application requirements.

www.st.com