For the past few years as a design and field applications engineer at Wurth Electronics Midcom, I have been working with engineers to develop custom transformers. In the world of custom magnetics, it is sometimes nick-named ‘black magic’, thanks to its complexity and interdependence of multiple electrical and mechanical parameters. Transformers are mainly used for power transformation and isolation. In more than 90% of the cases I have seen, the most common SMPS topology across the electronics industry for isolated converters with power levels less than 100W is the flyback. This is a popular topology in today’s world and is used in different conduction modes like continuous, discontinuous and boundary. Flyback converters are known for their simple design, small size, stability and high efficiency.
When it comes to low power isolated products like bias power supplies for applications in areas like telecom, industrial controls, e-meters, programmable logic circuits (PLC) and so on, I typically recommend the flyback topology. If the low power levels of the 3.3V and 5V isolated supplies will work, then I recommend TI's small SN6501 push-pull solution. The most popular non-isolated topology used in the industry is the buck which has advantages in size, cost and ease of regulation when compared to the flyback topology. But alas, it is not an isolated topology. So why not combine the best of both worlds, isolated flyback with small size and ease of regulation of the buck, and create a Fly-Buck™! This topology is derived from synchronous buck topology by adding coupled isolated windings to the primary inductor. The Fly-Buck converter is much easier to design, smaller in size and more efficient when it comes to multi-output isolated power supplies.
The Fly-Buck transformer design is not very different from the flyback transformer. The primary winding is the regulated output winding. The design of the primary winding is as simple as designing the inductor used in the buck topology and is analogous to the regulated secondary winding in a flyback transformer. The calculation of the inductance value is identical to the calculations used for the buck topology, including ripple current, input voltage and switching frequency. The standard transformer design theory of turns ratio is used to calculate the secondary turns on the unregulated secondary windings. Coupling among different windings is critical to permit a smooth cross-regulation throughout all layers, minimizing leakage inductance and hence, improving transformer efficiency.
The challenging part of the design is the extra attention needed to account for the peak current. The peak current affects the core saturation and rms current which can cause heating effects. The peak inductor current does not only include the primary winding’s current, but also the reflected currents from all the outputs, and the maximum peak-to-peak ripple current.
Wurth Electronics Midcom in collaboration with TI developed a series of three transformers for TI's LM5017 multiple output Fly-Buck solution. This reference design caters to a wide range of applications because of its wide input voltage range from just a few volts up to 100V. The design includes two dual output transformers and one quad output transformer for typical output voltage requirements of +/-5V and +/-15V. The most attractive part of this design is the versatility offered to designers. It allows them to swap out a dual output transformer with another dual output or quad output transformer based on their requirements, power levels and application. Read an article in Planet Analog, "Product How-to: Fly-Buck adds well-regulated isolated outputs to a buck without optocouplers".
As of now, there are no design calculators available to design a Fly-Buck transformer but needless to say, for low power isolated DC/DC applications, with the emphasis always being on small form factor, high power density, versatility in terms of multiple outputs, the Fly-Buck converter is a solution that offers many advantages over the popular flyback topology. Watch a video about the technology here.