Commercial ac-dc supplies from a reputable manufacturer often are taken for granted when used as basic drop-in, no-headache components. Making some basic mistakes, however, can inadvertently transform the power supply into a source of problems, both at the prototype stage and in the field. Here are the five most common issues, which can be circumvented rather easily.
Inadequate Air Cooling
Inadequate cooling becomes a major problem-maker when integrating a power supply into a design. Power supplies generate heat, and how that’s handled affects performance and reliability.
First, consider the cooling air passed over the supply, whether forced air (fans) or by convection airflow. Where is it coming from? Is it initially pre-heated by passing over hot ICs before being used to cool the supply? If so, its effectiveness drops off significantly whether or not the power-supply manufacturer’s airflow requirements are met.
Second, obstructions in the cooling path greatly impede the airflow. When the airflow stalls, the fans actually speed up, but they push virtually no air. Ensuring maximum flow requires careful planning of component placement, air-path layout, and inlet and exhaust routing and sizing.
Incorrect Supply Sizing
An undersized supply’s output may become erratic or struggle to provide more current than its rated value. Some power supplies have a safety feature that will cause the power supply to restart when encountering an overload condition. This will result in unexpected power cycling of the system driven by the power supply. Even worse, power supplies that lack an over-power safety feature may become damaged under an overload condition.
Implementing an oversized supply won’t solve the problem, though. Aside from unnecessary material cost, it will result in inefficient operation, generating extra heat to dissipate and increasing operating costs over the product’s lifetime. Most power supplies operate at their peak efficiency when supplying 80% to 95% of their rated output, so the supply size should be selected accordingly.
Voltage Drop From Cables
Significant losses can result from resistance in conducting lines or circuitry between the power supply and the load. One foot of #10 AWG has resistance of about 1 mΩ. While this may not seem like much, it’s important to remember Ohm’s Law (V = I × R).
Calculating for Ohm’s Law reveals a drop that can put the delivered voltage at the load outside of the rail tolerance (both the high side and return wires must be included). Several possible solutions include altering the nominal output voltage to be a little higher at the supply, using remote sensing, or reducing the wire length. It’s always important to choose a proper gauge wire for the design.
Cabling-related problems are easily avoided with good design practices: twist supply and return lines to minimize EMI, and dress and strain-relieve them properly. Cables that move may eventually come in contact with other installed components, and flexing due to ordinary vibration can lead to tiny cracks in the copper itself, potentially developing into intermittent or open circuits.
Open-Frame Supply Issues
Open-frame supplies have exposed components on their underside. The key is to ensure that they don’t touch the enclosure or chassis—standoffs will provide adequate clearance in this case.
Also, there should be enough space to avoid possible interference caused by any motion and minute flexing of the system during normal use. A basic understanding of agency requirements when integrating a power supply doesn’t hurt either.
Using Multiple Supplies
When supplies are connected in parallel, recognizing the difference between current sharing and redundancy becomes essential. In current sharing, failure of a single supply means there may not be enough capacity. In redundant and N+1 designs, a single-supply failure is invisible to the load because there’s enough excess capacity to carry the entire load.
Current sharing refers to two or more supplies used in parallel to deliver more current than a single supply’s rated output. The total output of the shared supplies is necessary to meet the load requirement. Some supplies are inherently designed to support this configuration, while others need a small “sharing” interface to equalize per-supply loading. Multiple small supplies are often chosen when the physical system layout lacks room for a single large supply, when heat sources must be dispersed, or if the initial supply turned out to be undersized for the final design.
In contrast, redundancy or N+1 occurs when multiple supplies are wired so that they share the load. If one supply is lost, the remaining supplies have enough aggregate capacity to support the entire load without any “switchover” lag when the operating supplies pick up the total load. Again, some supplies are designed for this capability, while others need additional circuitry.
Bill Lurie, vice president of engineering, holds a bachelor of science in mechanical engineering from the University of California at Santa Barbara (UCSB), and completed Technical and Program Management studies at University of California at Los Angeles and UCSB.