Trade-Offs in Flyback Transformers

How can you balance the trade-offs of size, efficiency and power loss when choosing a transformer in designs that demand the best of each? Flyback transformers, for instance, offer multiple outputs for efficiency and flexibility, but the size and winding losses can be hard to minimize. Find out how Bourns approaches creating a win-win in our recent white paper entitled How Litz Wire Maximizes Offline Multi-Output Flyback Transformer Efficiency for Battery Charger Applications. It provides background on flyback transformers and lays out several ways to reduce temperature rise of a multi-output offline 60 W flyback transformer, including using Litz wire.

Flyback transformers are designed based on hand calculations, circuit simulators, analytical tools, and finite element analysis (FEA) software. They are optimized based on size, winding losses and core losses.  The white paper follows this method by highlighting four designs. The first design does not consider any high-frequency effects, while the other three designs presented address techniques that can reduce AC resistance and power loss.

The first design uses reinforced insulation in its secondary windings, which isolates the transformer from hazardous voltages at the expense of a 20-30 % increase in size. The second design reduces copper loss by accounting for the fringing effect, and results in a 28 % improvement in total power loss. In the third design, the proximity effect is addressed to reach a 31 % power loss improvement. Finally, the fourth design uses Litz wire, which is constructed of multiple fine wires woven together to imitate typical copper wire, to reduce the skin effect. In our tests, the Litz wire design achieves a power loss improvement of 85 %.

At Bourns, we know that each application has unique requirements, and thus there is no “one size fits all” solution. You can learn which design is likely most appropriate for your next application, and whether the use of Litz wire may be beneficial. We provide details of the physical design, modifications made for that step, and the results, including the individual shortcomings. We evaluate each method on winding, core and power loss, and AC resistance in approachable detail.

You are invited to read the full white paper to learn more about how Litz wire helps maximize offline multi-output flyback transformer efficiency. With the continuing trend of energy efficiency, we predict that Litz wire’s popularity will grow.

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