The linear power supply uses a feedback circuit to fix the output voltage to a constant level by means of gradual change of the gain in the circuit to compensate for the current draw by the load. A transistor is the main pass element that controls this current and this the output voltage level. The difference between the input voltage and the output voltage is dropped across this main element. At large load currents the power that goes up in heat due to the drop across the pass element can be significant.

A switching power supply on the other hand attempts to eliminate the high power dissapation on the pass element. It does this by elininating the linear operating mode of the pass element and instead operates it as a a full on/full off switch. The pulse width during which the switch is on and the frequency at which it is switching occurs are controlled by the feedback loop and the dynamic requirements of the load current variation on the supply. An output inductor and/or capacitor is used to filter the switched on/off output to an average DC output voltage.

Typical linear power supplies can operate at efficiencies (output power / input power) from like 35% to 55%. Switch mode devices can operate at efficiencies from 65% to 95%.
The heat prevention advantages of a switch mode power supply can easily be seen from these numbers.

Switch mode supplies most often see use in high current applications, low volumetric applications, low weight requirements, wide input or universal voltage input, and/or places where heat must be minimized. Their biggest disadvantage is that the switching activity can show up as quite a bit of noise on the output rails. In addition some switching designs have a minimum load requirement on the output in order for the supply to stay in regulation.

Linear power supplies are seen in most OLD electronics equipment and today are used for three main uses - i.e. low cost, low current, and in circuits where absolute minimum noise on the supply rails is required such as in analog applications. The biggest disadvantages of this type are the greater heat generated and the larger weight.

These days it is very common to see systems use a switch mode power supply for the main DC supply for the digital electronics and then see the use of low-drop-out (LDO) linear regulators to further regulate an output from the switch mode supply to a somewhat lower DC voltage(s) needed for an analog or other noise sensitive portion of the circuit.

Michael Karas