Sinrace Power Supply Technology (Shenzhen) Co., Ltd.

6. Control circuits.

It’s now economical to combine through-hole and surface mount techniques in the design of AC/DC switchers. Control functions can be implemented on the underside of the printed circuit board utilizing surface mounted devices.

7. Cooling.

Where possible affix power semiconductors directly to the power supply case or U-channel. You no longer need thermal pastes. Thermal transfer components from companies such as Bergquist allow the tabs of power devices to be soldered directly to a copper pad that is already bonded to an electrically insulating but thermally conductive substrate. This approach is inherently more consistent and reliable than using thermal pastes. Where forced-air cooling is needed, you might consider the use of 3-wire intelligent fans. The fan speed varies with temperature to ensure that the fan only runs as fast as it needs to for given operating conditions. This reduces both noise and power consumption. However, the relatively high prices of these fans, and the ready availability of low cost fan control chips, makes implementing your own control circuit more cost-effective. Fan noise is another consideration. Experiment with the format and spacing of finger guards. We find that placing finger guards a few millimetres away from the panel on which the fan is mounted, rather than flush with it, can reduce fan noise by some 5dB to 6dB. In a system that uses a number of power supplies, this is a very audible reduction.

8. Digital control.

Don’t get carried away by all the hype about digital control for AC/DC power supplies, unless your application absolutely needs it. Digital control has been widely implemented in point-of-load converters but it adds considerable cost to AC/DC power supplies. The chips may now be available at 50 cents or so, but when you consider the additional components needed, manufacturing costs and the cost of connectors, implementing digital control will add $10 to $15 to the cost of an AC/DC power supply. This may be an acceptable figure for a 1kW power supply, where it is a small proportion of the total cost but for low to medium power units, the additional cost can rarely be justified.

9. Mechanical design.

There is always room for creativity in mechanical design. Consider the range of applications for which the power supply may be used at the outset. For example, it may be possible to design the overall physical dimensions so that the power supply will fit horizontally or vertically into an industry-standard enclosure format. The printed circuit board can be designed to accept plug-in connectors or screw terminals; this adds no cost but greatly improves the application flexibility. And remember the small things, like accessibility of fuses and making cable retention devices reversible, so that the walls of the system enclosure or other protuberances don’t obstruct them. Stacking components to save PCB space is another technique that is often overlooked. For example, it may be possible to stack the inductors in a filter network on top of capacitors, simply fixing them with adhesive. This can also help EMC performance by keeping filter component interconnects very short.

10. Component layout.

To look at some power supplies, you might think that the components were dropped onto the board from a great height and then connected together! Careful component layout, that follows a logical flow from input to output, is good engineering practice. The product looks better, is easier to test and service and it performs better. Remember, every bend in a PCB track adds a little inductance that can create EMI.

XP Power uses all of these techniques in the design of AC/DC power supplies. The company’s most recent product, the MFA350 350W AC/DC power supply, shown in Figure 3, uses most of them. The result in this case is an exceptionally compact 3.2” x 6.8” x 1.5”  (81mm x 173mm x 38mm) unit, with full control and monitoring features, 89% efficiency and a requirement for only 13CFM of cooling to get full output. No single design innovation has delivered this performance, it the combination of small improvements, such as those outlined above, and that makes a big difference to the end product.