Gary Bocock of XP Power explains how to reduce size and increase performance in AC-DC power supply design 

Improvements in AC-DC power supply design are evolutionary, rather than revolutionary. It is easy to settle for tried and tested approaches because no one new design technique is likely to yield great benefits.

The design challenge is to establish what small enhancements can be made in various parts of the design to achieve worthwhile improvements in power density, efficiency and EMC performance.

Solder power semiconductors directly to the printed circuit board then bond them to the chassis, rather than insulate them and clamp them to the chassis with a conventional nut-and-bolt fixing. Good thermal bonding materials are relatively expensive but the technique reduces assembly costs, reduces size and will typically result in 10°C cooler junctions.

Furthermore, thermal performance is more predictable and consistent. With less heat to worry about, designers can decide to take the increased MTBF advantage – a rule-of-thumb says that MTBF doubles with every 10°C reduction in temperature – or push the power supply to higher power levels without reducing the original calculated MTBF.

In boost converter designs, replace power diodes with silicon carbide (SiC) types. A major disadvantage of conventional diodes is reverse current spikes. High reverse current produces wasted power in the diode and switching transistor that needs to be dissipated using a snubber circuit (figure 1, left).

Six additional components are used to dissipate the power generated by the unwanted reverse current. The resulting PCB area is shown in figure 2 with the relevant components highlighted with blue dots.

Figure two: Blue dots show the power dissipation circuit

The SiC diode has very low reverse current so can be used without additional components, resulting in space saving on the board lower assembly costs and greater reliability. In addition, because almost no power is lost due to reverse current, the efficiency of the power supply is increased by around one per cent - a significant improvement.

With respect to cost, the SiC version is now comparable to using a conventional diode and snubber circuit (see table below).

Do not connect power semiconductor heatsinks to the chassis – let them ‘float’ electrically. This has three major advantages.

Firstly, it reduces EMI because interference is not conducted to the chassis. Secondly, it removes the need for metal oxide varistors (MOVs) that are usually needed to deal with surges because when floating the heatsink surges are not transferred to the power supply.

Finally, it reduces leakage current – something that is particularly important for medical applications.

Do not settle for conventional approaches to mechanical design. Here are some examples where creative thinking has yielded benefits:

Toroidal chokes for EMI filters can be stacked above filter capacitors, rather than placed alongside them on the PCB. This not only saves board space but results in shorter interconnects between filter components and better filter performance.

Conventional fan guards, fitted flush to the chassis, can create considerable air turbulence and noise. The custom-designed fan guard pictured is punched out of sheet metal and raised up to create a 4mm gap between the fan and the fan guard. This reduces fan noise by 4dB - a welcome improvement in many operating environments.

Make fans field replaceable. This has two advantages. Fans are potentially the most unreliable part of any power supply, so making them field replaceable reduces servicing costs and assists planned maintenance programs. Furthermore, if the power supply is for sale commercially, it means that the fan does not have to be taken into account in any MTBF calculations because it is not considered an integral part of the unit. This boosts the calculated MTBF figure.

Gary Bocock

Minimise the number of PCBs in the design. Many designs use separate boards for the main part of the power supply, EMI filtering and control/interface circuitry.

With a little care, all of this can be done put onto one PCB, greatly improving reliability through minimising the number of interconnect parts and reducing overall power supply size.

None of the techniques described above is in itself unique, but together they were recently applied to the design of XP Power’s ‘fleXPower’ configurable power supply and produced a product that is ten per cent smaller and one per cent more efficient, with better EMI and leakage current performance, lower component count, reduced manufacturing costs, easier maintenance, and lower acoustic noise than its predecessor.

Nothing revolutionary perhaps, but a very much better power supply nevertheless.

Gary Bocock is technical director at XP Power

www.xppower.com