Guide to specifying a power supply from SL Power Electronics

Linear power supplies were the first to be made available and are still the choice for low noise requirements on the output voltage rail – but the output power of these are generally limited to no more than 300 Watts.



In Linear supplies, a transformer is designed to change the voltage levels of the AC voltage and then rectify this to give a DC level. Further regulation could be made using a series element (transistor) to dissipate the unwanted voltage variation into a stable voltage level. The advantage was that the output ripple and noise was in the low frequency range (AC mains frequency) and thus very noise quiet. The disadvantage is that they are big, bulky and inefficient for outputs of low voltage. Linears cannot accept a wide input mains voltage range.



Switched mode power supplies are easily the most popular power sources. Their main advantage is their small size and the fact that output power levels can be in the Kilowatt levels.



The advantage of the switched mode design is to reduce the size of the transformer by passing a square wave switching waveform through the transformer at a higher frequency, thus reducing its size and weight. Regulation is achieved by controlling the pulse width into the transformer. This has enabled high efficiency, cost reductions and power factor control. Thus the term ' Switched Mode Power Supply ' was born (SMPS).



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