Switching PSU Theory of Operation / Repair Tips & Techniques

In HVSMPS (High voltage switchmode power supplies) the input voltage is converted to DC at some high voltage and stored in capacitor prior to being converted to AC (via chopper) and so on... The reason for high voltage is that twofold. Same thing can be applied to 240/480 supplies. For powersupplies that must (or can) run off 115V/230V you can use voltage doubled 115VAC or halfwave(or fullwave) rectifed 230VAC to get nearly the same voltage in the capacitor bank making multivoltage operation convenient. Also for a given volume of capacitor higher voltage means lower current for a given power. Lower current means less loss to internal (and external) resistances in the circuit, hence higher effientcy. Once you have the high voltage DC you than chop it into square waves using various possible circuits so that it can be applied to a transformer for conversion to lower AC voltages. One of the hallmarks of HVSMPS units is small size and this is accomplised by useing higher frequencies that nominal 50/60HZ. Typically frequencies from 5khz (old designs) to as high as 250 or even 500khz (very latest designs) are used to allow using very compact transformer cores. A Ferrite core at 100khz for 400 watts is far smaller than the common EI core for 50/60hz, lighter too! I hope this gives some insight to SMPS. I've not mentioned how regulation is achieved but the common way is to vary the width of the pulse appled to the transformer, The narrower the pulse the less power it contains. The circuit monitors the rectified and filtered DC from the output side of the transformer (torid, cup core or other form of ferrite core transformer) output side and uses that information (voltage and current limits) to modulate the chopper and therfor by servo action regulate the output. In the quote that likely ment the switching chopper circuit, commonly used in higher powered is a 4 (or more) transistor(or HEXfet) bridged circuit. Allison