Introducing Cutting Edge Technology into Low-Power DC/DC Converters

DC/DC converters have not followed the same size-reduction trend because their magnetic components cannot be miniaturized at the same rate as semiconductor devices. Unlike ICs, converters rely on transformers and chokes that require a minimum physical size to handle flux density, switching frequency, and heat dissipation. Although higher efficiency reduces some thermal constraints, magnetics remain the main bottleneck. For comparison, Taiwan Semiconductor Manufacturing Company (TSMC) scaled IC geometries from 3µm in 1988 to 3nm today [1], while magnetic components have changed little in fabrication method or size over the same period.

In contrast, discrete surface-mount passive components suitable for machine placement have shrunk from 1206 to as small as 01005 sizes, reducing footprint by over 50 times. In contrast, the sizes of magnetic cores in DC/DC converter transformers and chokes have changed little since the 1980s, constrained by the maximum flux density of the material and operating frequency, which determine the minimum winding turns.

To be fair to power engineers, power density has improved through lower losses from new conversion topologies, better components, and advanced thermal design. This has allowed higher output power from a given DC/DC module size, by roughly a factor of three in the case of the SIP7 format for an unregulated type (Figure 1).

One of the most effective ways to optimize magnetic components is to increase the switching frequency, which allows the transformer core and the number of winding turns to be reduced. However, higher switching rates reduce semiconductor efficiency and increase core losses, so overall case size does not necessarily decrease without higher internal temperatures. The solution is a more complex, high-efficiency converter, but this has often been considered prohibitively expensive.

Magnetic components are also relatively costly to manufacture and assemble in typical converters. Assembly techniques have changed little from those Faraday would recognize—winding insulated wire around a core and soldering flying leads to a substrate (Figure 2). Typical wire size is 0.18mm, with a core diameter of 6mm outside and 3mm inside. Bobbins occupy too much space, and printed winding techniques are impractical due to the number of turns and windings required, as well as the high cost of multi-layer substrates for low-power products.

Most low-power DC/DC converter manufacturers have focused on keeping circuits simple and low-cost, for example, using the traditional ‘Royer’ circuit (Figure 3). The savings achieved offset the high labor costs of winding simple toroids and hand-soldering wires to a double-sided PCB, with encapsulation or over-molding to protect fragile terminations. Over the years, assembly techniques have been refined so that a simple unregulated converter may require only around ten discrete components, while a regulated version uses fifteen.

With transformer manufacturing and module assembly in low-cost locations, products remain reasonably efficient, provide isolation, a wide operating temperature range, and accurate voltage conversion between fixed levels. Manual assembly allows easy creation of variants for different input/output voltages and power ratings by simply adjusting the number of winding turns.

There are inevitable downsides. Manual assembly produces sample variation, makes comprehensive fault protection difficult, and achieving safety-certified isolation requires additional complexity, cost, and larger case sizes. A basic Royer converter lacks line or load regulation, and output voltage can rise significantly under very light or no load. Labor costs continue to increase while customers expect lower prices, and labor does not decrease with production volume. Simultaneously, market pressure demands higher functionality and efficiency and smaller power converters for space-constrained applications.