A power supply is a buffer circuit that provides power with the characteristics required by the load from a primary power source with characteristics incompatible with the load. It makes the load compatible with its power source.

Example: A power source might be the 60 Hz, single phase, 120 V ac power found in a home in the United States or the 50 Hz, single phase, 220 V ac found in the United Kingdom. The load might be a logic circuit in a personal computer that requires regulated 5 V dc power. The power supply is the circuit that makes the 120 V ac or 220 V ac power source and 5 V dc load compatible.

Question: Can you recommend some good articles on current mode DC-DC converters?

Answer: Current-mode control, also called current-programmed control and current-injected control, has existed since at least 1978 (earlier in U.S. Government reports). I have annotated some papers with abstracts that might be of interest.

For the best way to measure the loop gain of a switching power supply while the circuit is running closed-loop with a regulated output, see the page on Loop Gain Measurement Setup. You will need a Frequency Response Analyzer to do this, as it cannot be done with a signal generator.

Question: Is a minimum load necessary on a switching-mode power supply?

Original Question: Recently I have been trying to use an SMPS (a widely available PC power supply) for my hobby work. I find that the SMPS starts up, only if I apply a minimum load of about 0.5 ampere on the 5 V output. Is this the normal operation of an SMPS? Is it necessary to ensure a minimum load, just to ensure that the supply kick starts?

Answer: When a switching-mode power supply with an output LC filter operates with sufficient load to keep the instantaneous inductor current above zero, the filter averages the switching voltage waveform on the filter input. When it operates at lighter load, the voltage tends to rise to the peak value of the switching waveform. (See the waveform in the tutorial on a heater, where the average value is Vin*D and the peak value is Vin.) The feedback loop may be able to compensate for some of this but usually not all of it, especially if multiple levels are controlled by one feedback loop. It is customary to include overvoltage sensing or protection on all outputs and if the voltage rises on any output, these may be triggered. Also, it may be more difficult to keep the power supply stable at light loads.