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Power supplies

Step by step for an optimised flyback design

October 25, 2011 | Michele Sclocchi | 222903558
Step by step for an optimised flyback design Michele Sclocchi of Texas Instruments in this first part of a two part series offers a step by step guide on how to approach optimising a flyback design.
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Flyback is the most known topology for isolated power supply since it can provide multiple isolated outputs with a single low side switching transistor and limited number of external components. However, a flyback power supply presents some peculiarities that could limit its overall performances if they are not fully understood and analyzed by the designer.

This series of articles dedicated to this topology demystifies any flyback power supply design with a simple mathematic approach, guiding the designer to a well-optimized design.

The Flyback Converter

DC-DC applications, both multiple outputs and output isolation may need to be implemented depending on the application. In addition, input to output isolation may be required to meet safety standards or provide impedance matching.

Isolated power supplies not only protect users from potentially lethal voltages and currents, but they also provide performance benefits. Isolated supplies preserve instrument accuracy by interrupting ground loops, and they easily provide positive regulated voltages from a negative power bus without compromising the benefits of that bus.

Flyback topology has traditionally been the designer's choice for power isolated converters with output power below 100 W. It requires only one magnetic component and one output rectifier, so it has the advantages of simplicity and low cost. Multiple outputs are easily implemented.

Disadvantages of the flyback topology are the need of a high-value output capacitor, the high current stress in the power switch and in the output diode, high eddy current loss in the air-gap area, a large transformer core, and potential EMI problems.

Flyback converter derives from the buck-boost topology and it shares the main disadvantages: the energy is only collected from the source during the ON time of the switching MOSFET. And then later during the off time, this energy from the primary winding is delivered from the inductor to the output. This is a unique peculiarity of the flyback and buck-boost topology. (Figure 1)

Flyback transformer does not work as a traditional transformer where primary current and secondary current flow at the same time, and only a small position of the energy is actually stored in the transformer (magnetizing energy).

A flyback-transformer is more like multiple inductors on the same core rather than a typical transformer. Ideally a transformer doesn't store energy, all the energy is transferred instantaneously from the primary to the secondary.

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