Electronic Design

Fresh Design Ideas Benefit AC-DC Supplies

Is it possible to say anything new about designing ac-dc supplies? Frank Rene, president of global product development at XP Power, sent along a list of five incremental enhancements that added up for his company.

First, solder power semiconductors directly to the pc board and then bond them to the chassis, rather than insulating them and bolting them to the chassis. Countering the expense of thermal bonding materials, assembly costs go down. This also reduces size of the supply and reduces device junction temperatures by about 10°C, roughly doubling expected mean-time before failure (MTBF).

Second, go for silicon-carbide (SiC) power diodes in boost converters. This eliminates six components in the snubber normally required to deal with reverse current spikes in conventional diodes. For a 1000-W supply, parts for the SiC-diode approach cost around 37% higher. But assembly costs are lower, reliability is up, and efficiency increases by about 1%. Rene also notes that prices for Infineon's and Cree's SiC diodes have been dropping lately.

Third, "float" power semiconductor heatsinks instead of connecting them to the chassis. This yields three advantages. There's reduced electromagnetic interference (EMI) because interference isn't conducted to the chassis. There's no need for the metal-oxide varistors that are usually needed to deal with surges. And, there's lower leakage current, which is particularly important in medical applications.

Fourth, look for unconventional approaches to mechanical design. For instance, don't place toroidal chokes for EMI filters alongside the filter capacitors on the board. Instead, stack them above the caps. This not only saves board space, it also results in shorter interconnects between filter components for better filter performance.

Also, a little creativity with fan guards can make fans field-replaceable, reducing servicing costs and assisting planned maintenance programs. XP found that increasing the gap between the fan and the fan guard reduced fan noise by 4 dB. Another benefit of making fans fieldreplaceable is that the fan does not have to be accounted for in MTBF calculations because it's not considered an integral part of the unit.

And fifth, use a single pc board for the main part of the power supply, EMI filtering, and control/interface circuitry. This does away with a number of failure-prone connectors and can shrink physical size.

XP used all of these approaches in its configurable fleXpower ac-dc supply series, with versions for industrial and medical applications (see the figure). Hundred-piece unit pricing runs from $314 to $1208.

XP Power

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