Planar transformers facilitate DC/DC converter design

Transformers have long been a pain point and difficulty for DC/DC converter designers. Because they are large, expensive and difficult to make. By reading this article, you can learn how flat transformers can improve quality and reduce low power DC/DC converter design costs.

What is the difference between a planar transformer and a toroidal transformer?

Transformers are the primary core of any DC/DC converter design and are a key component that determines overall performance.

Traditional transformers and inductors are wound components. Most production low-power DC/DC converters use hand-wound toroidal transformers; It is extremely difficult to develop an automated manufacturing process for transformers that require up to six individual windings and have core diameters as small as 6 mm and apertures as small as 3 mm. Manufacturers such as RECOM have developed automatic winding machines to make such miniature toroidal transformers, but this solution is very difficult.

Flat magnets have been around since the 1980s, but the early manufacturing process was expensive, so flat transformers had limited market penetration outside of professional applications. However, with the advancement of manufacturing technology, the cost of planar transformers has been reduced, so new application markets can be developed.

Design and structure of planar transformer

Planar transformers or inductors use copper layers of multilayer printed circuit boards (PCBS) to form windings; The ferrite core surrounds the winding, as shown in Figure 1. In order to form the required number of turns, the PCB layers are connected using hidden through-holes. Connecting multiple holes in parallel can carry enough current to make a miniature power transformer.

How to form 6 turns of winding by stacking layers connecting hidden through holes. Observe how this arrangement places the ends of the winding on the outside of the ferrite core. The primary and secondary windings are usually interleaved to reduce leakage. Printed circuit boards provide a mechanically stable and consistent winding structure suitable for automated production. In addition, the evolution of multilayer PCB structures has allowed the epoxy insulation between the windings to withstand high isolation voltages between the primary and secondary circuits.

Graphic design does present some challenges for design and manufacturing teams. Because it is difficult to lay out multiple lines on a multi-layer PCB, planar transformers are best suited for DC/DC converter topologies that do not require the use of complex transformers, while half-bridge topologies are a common choice.

Even so, there are practical and cost limitations to the number of layers, so planar magnets use high-frequency PWM modulators and drive circuits to reduce the number of turns and layers required. The skin effect caused by these higher frequencies is the main problem they cause. As the frequency increases, charge carriers move more and more to the conductor surface, reducing the effective cross section and increasing the I2R loss. Designing the conductors to be flat (rectangular) prevents this skin effect.

The multi-layer structure has high coupling capacitance, which affects the high frequency PWM control. The coupling between the planar transformer and the conventional circuit must be carefully managed to avoid termination losses. The proximity of the core air gap and the layered winding results in significant eddy current losses, and because of the different turn ratios, separate PCBS must be designed and tested for each input/output combination.

Once these issues are resolved, planar technology allows for the construction of extremely flat transformers to transmit high power, which in turn allows DC/DC converters to become thinner: the 15W converter in Figure 1 has a package height of only 9.9 mm (0.4 in)!

Other advantages of flat design are better winding heat dissipation, high density design, low leakage induction and high power density.