Introducing Cutting Edge Technology into Low Power DC/DC Converters

The technology used in the manufacture of low-power board-mounted DC/DC converters has developed more slowly and separately when compared to other electronics. A typical converter has remained as a through-hole encapsulated module or an open-frame surface-mount ‘daughterboard’ for decades, and new products are still being launched with pin-outs and form factors that have been standards since the 1980s. Elsewhere though, other functional blocks such as interfaces, A/D and D/A converters, and more have continuously evolved from discrete solutions into ever-smaller ‘chips’—surface-mount ICs that are a fraction of a millimeter high and in a footprint little bigger than the size of the internal die, which itself can now have nanometer-scale trace geometries.

Why have DC/DCs not followed the trend of size reduction?

DC/DCs are just a collection of active and passive components, so why have they not become just ‘another’ integrated circuit and shrunk in the same way? One reason is that they often run with significant power dissipation, needing some surface area to dissipate from, but this is becoming less of an issue as efficiencies rise with new conversion techniques. The main reason is the magnetic components required in most converters, which have stubbornly stayed with the same fabrication technology and around the same size over decades. As a comparison, in 1988, Taiwan Semiconductor Manufacturing Company (TSMC) was providing ICs with a 3µm geometry, and today, the figure is a thousand times smaller at 3nm [1].

In the same period, typical discrete, surface-mount passive components, that are practical to machine-place, have also reduced from 1206 to as low as 01005 sizes, which is a reduction in footprint by more than 50 times. In contrast, the sizes of magnetic cores in DC/DC converter transformers and chokes have barely changed since the 80s, set by the inherent maximum flux density of the material and operating frequency, which then set the minimum number of winding turns. To be fair to the generations of power engineers, power density has improved, with lower losses from new conversion topologies, better components, and advanced thermal design. This has allowed more output power from a given DC/DC module size, by perhaps a factor of just 3x in the case of the SIP7 format, for an unregulated type (Figure 1).

The options to optimize magnetics

There have always been options to reduce the size of power conversion magnetics by increasing switching frequency, which generally reduces core size, winding turns, or both, in some combination. However, with higher switching rates, semiconductor efficiency drops and core losses increase, so overall case size does not necessarily reduce without higher internal temperatures. The solution is a more complex converter designed for high efficiency, but this has been seen as prohibitively expensive.

Magnetic parts are also relatively expensive to make and fit in a typical converter; there has been little change to assembly techniques that Faraday would have found familiar—winding insulated wire around a core and then soldering ‘flying’ wires to a substrate (Figure 2). Bobbins generally take up too much space and techniques, using printed windings, have not been practical due to the number of turns and windings needed and the unacceptable cost of multi-layer substrates, at least for low-power products.

Manufacturers have chosen simplicity for low parts cost

The route taken by most low-power DC/DC converter manufacturers has been to make the circuit as simple and low-cost as possible, for example, by using the traditional ‘Royer’ circuit (Figure 3). The savings achieved then offset the high labor cost of winding simple toroids and hand-soldering wires to a double-sided PCB, with encapsulation or over-molding, to protect the fragile terminations. The circuits and assembly techniques have been refined, over the years, so that a simple unregulated converter might only employ around ten discrete components, and a regulated version uses fifteen.

With transformer manufacture and module assembly in a low-cost location, the product is reasonably efficient, provides isolation, a wide operating temperature range, and quite accurate voltage conversion between fixed levels. An upside of the manual assembly method is that variants of the products, for different input/output voltages and power ratings, are relatively easy to achieve in the manufacturing process with a simple operator instruction to wind more or fewer turns.

There are inevitable downsides to this approach. However, manual assembly produces variation between samples, and it is difficult to provide comprehensive fault protection in simple circuits, and isolation to a safety-certified level is not practical without more complexity, cost, and larger case sizes. A basic Royer converter has no line or load regulation, and the output voltage can significantly rise at very light or no load. Additionally, labor costs only increase over time, while end-customers expect price reductions, and the labor element does not even decrease with production volume. At the same time, there is market pressure to increase functionality and efficiency and reduce the size of power converters to suit modern space-constrained applications.

Striving for the ideal

To break out of this mold, power designers have dreamt of the ideal—the incorporation of control ICs with a wide array of features including high frequency/high-efficiency operation with built-in optional active regulation and comprehensive protection. The transformer should use techniques, such as machine-placed planar cores and printed windings, in a multilayer substrate. With the necessary associated support components, the IC solution and embedded transformer have a significantly higher parts cost than the simple Royer circuit, but with flexible assembly automation and economy of scale, it is a way forward to meet the market demands of better performance and consistency, with higher power density, all at no cost penalty.

This is the approach taken by Austria-based manufacturer RECOM, who have started to incorporate such ‘cutting-edge’ technology into low-power DC/DC converters, which are designated as the ‘K’ series. The company invests heavily in automation to significantly reduce labor costs while specifying high volumes of substrates and components to keep parts costs minimal. At the same time, innovative design techniques allow for easy configuration of products, in the flexible manufacturing process, for the wide range of variants usually demanded.

Example new products on the market

An example of a product with the new technology is the RECOM RKK series in which an integrated controller and planar transformer have been implemented to improve performance and make assembly fully automated. The company has chosen to maintain the SIP7 format for compatibility and to rate the part at 1W, using the enhanced efficiency obtained to extend the operating temperature range, which is now up to 105°C without derating. 1W rating is popular for many applications, which typically power isolated communications interfaces or high-side gate drivers.

The extended temperature range opens up wider markets such as high-specification industrial and automotive. A summary of the differences and performance enhancements for the RKK series, compared with an early product, is shown in Table 1. The gains are remarkably achieved with a reduced selling price in volume. While the new product is nominally unregulated, there is a degree of compensation for input voltage changes, for example, less than +/-5% output change for +/-10% input voltage variation. An additional feature is that the parts do not need to be encapsulated, which saves weight and cost. A version of the series includes a post-regulated output when high accuracy is needed.

Other new developments

The principles of the new RECOM technology have been incorporated into their non-isolated DC/DCs as well, with an upgrade to their popular R-78 series and a drop-in replacement for linear regulators. In the new products, designated as R-78K, efficiency has been improved up to 96%, and the input range has been extended to 36V. The operating temperature is now 90°C without derating.

Further ‘K’ upgrades to existing RECOM products are in the pipeline, following the trend to replace simple traditional designs with advanced circuit techniques and manufacturing technology, without increasing costs. Look out for the new ‘K-series’ products as they are released and take advantage of the ‘cutting edge’.

References

[1] https://www.tsmc.com/english/dedicatedFoundry/technology/logic/l_3nm