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switch mode power supply

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Can you be a little more specific about your problem, and describe the power supply further?

In general, switching power supplies take a DC voltage and "chop-it" into a square wave. In the case of an off-line switcher, the AC is rectified, filtered and then "chopped". In a DC-DC it is just chopped directly. I this day and age, virtually all  "chopping" is done with MOSFET's. Somewhere in the power supply there will be one or more magnetic elements depending on what type (topology) of power supply it is. If you have a 10VDC voltage and switch it on and off such that it is on for 50% of the time and off for 50% of the time, rectify and filter that signal, you will have a 5V output (assuming an ideal diode).  So on-time divided by off-time plus on-time (period) gives you what is referred to as duty cycle, and duty cycle multiplied by the input voltage will give the you your output voltage. A feedback circuit is used to change the duty cycle such that the output voltage remains constant over variations in line and load. As for troubleshooting, more info about the supply is needed in order to suggest "a plan of attack".

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