Switching power supply
A switching power supply is one that uses what are effectively digital techniques (i.e. internal signals are either fully off, or fully on) to convert input power to output, at the voltage needed.
They are generally more efficient, but are also more complex, than the linear power supplies which preceded them. They are thus now found ubiquitously in medium- to high-power applications.
In general, all switching supplies depend on turning the input current off and on quickly; voltage regulation is achieved by leaving the input on longer when the output voltage is too low, and leaving it on for a shorter time when it is too high. Passive circuit elements such as capacitors are used to filter out the ripple and noise from the switching.
The transistors usually used as switches are thus operating in either fully-on or fully-off mode; they are more efficient when used in that way, contributing to the efficiency of switching supplies.
There are a wide range of design approaches in the switching supply field, depending on factors like whether isolation is needed (i.e. from wall AC, which generally requires a transformer); the relationship between the supply voltage and the output voltage; how good the voltage regulation needs to be; cost and size goals; etc, etc.
The issue with voltage is that if a power supply takes in a given current (at the supply voltage), and wishes to produce a lower output voltage, then unless it has some way to produce more than that much output current (since otherwise the number of electrons in the current is fixed, and 'electrons out' must equal 'electrons in'), the voltage difference between the two is lost energy, lowering the efficiency.
An equivalent, but inverse problem is encountered when the output voltage needs to be higher than the input. There are a number of different design approaches that can achieve voltage conversion in an efficient manner.
Isolation, regulation and voltage/amperage conversion are different objectives, but they can be achieved either separately or in concert; e.g. use of a transformer can do the first and last in one step.
One common design is to take input AC, convert it to higher frequency, run that through a transformer (since transformers can be physically smaller and lighter at a higher frequency, for a given power throughput), and then convert the output to DC.
