TN0023 Datasheet, PDF (1/7 Pages) Bothhand USA, LP. – 10/100 BASE -T MAGNETICS MODULES

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,
maximum output power (P_Omax), efficiency (η), operating frequency (f), output voltage (V_o)
and diode drop (V_do). Some of the other circuit considerations, which are a result of the
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.
August 2012
Doc ID 12864 Rev 2
1/7
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