Overview

Identifying and minimizing the losses within a switching power supply is one of the biggest challenges facing the power supply designer. The losses are primarily centered within the power semiconductors, with smaller losses occurring within the magnetics. Appreciating their nature can help one to select the optimum MOSFETs and rectifiers as well as the optimum frequency of operation. One can even predict on paper the amount of loss within each component and determine the best heatsinking methods.

There are three major types of losses within the power supply. Conduction losses are those voltage and current products that occur when a MOSFET (or power switch) or a rectifier is conducting current. This is duty-cycle dependent. Switching losses occur when the power switch or rectifier is transitioning between the ON state to the OFF state and vice versa. These are frequency dependent since there are more transitions per second at higher frequencies. Finally, we have gate drive losses, where there is some conduction loss in the MOSFET driver's output stage with that current exiting back through the input stage and not going to the output as output power.

To observe and calculate the losses within the actual switching power supply, one needs some fairly specialized tools: A voltage oscilloscope probe, an oscilloscope current probe and a contact thermal sensor. The waveforms can then be multiplied either graphically or by an intelligent oscilloscope with graphical math functions. Plus, it is always a good idea to isolate the earth ground connections...