There’s nothing quite like a power supply datasheet for providing vague or misleading specifications: two apparently similar units can deliver widely differing performance and reliability. With cheaper power supplies usually cheaper for a reason, what questions need to be asked to make datasheets unambiguous?
Most ac/dc supplies are designed for a ‘universal’ input of 90 to 264V ac. If the supply is to be used near its full rating, check that full power is available at the lower end of this range. Here, input current rises, switching current in the boost converter becomes relatively high and overall efficiency falls. Available output power may be reduced by up to 30% at 90V ac, compared with the quoted nominal power output. If the datasheet doesn’t state something like ‘full power from 90 to 264V ac’, look for a derating specification or graph.
Inrush current figures – often specified as ‘cold start’ inrush current – are more open to ‘specmanship’. Here, a thermistor controls the inrush current – its resistance falls as it warms. Switching the supply off and on after it has been running for a while can produce an inrush current many times the cold start figure. This higher current needs to be taken into account when selecting fuses and switching components.
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