Conduction and Switching Losses
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 depend-
ent 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 con-
tact 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 on the AC power plug of all test equipment used to probe the power supply.
Be very suspicious of the plots displayed on the oscilloscope. Radiated noise is frequently
picked up by the loop created between the ground clip and the probe tip. It is better to use
coaxial cable with less than 3 cm of lead length at one end, which is soldered directly to the ter-
minals of the component being examined.
Sanjaya Maniktala presents a very thorough examination of the losses within a typical switch-
ing power supply and how to calculate the resulting losses.
— Marty Brown
As switching frequencies increase, it becomes of paramount importance to reduce the
switching losses in the converter. These are the losses associated with the transition of
the switch from its on-state to off-state, and back. The higher the switching frequency,
the greater the number of times the switch changes state per second. Therefore, these