EMC Question of the Week: April 23, 2018
Decreasing (speeding up) the transition time in a power inverter's switching components,
- reduces the losses in the switch
- generally reduces the electric-field coupling from the switching circuit
- generally reduces the magnetic-field coupling from the switching circuit
- all of the above
Answer
The correct answer is "a." Switching devices, such as MOSFETs and IGBTs, dissipate the most power when they are neither fully on or fully off. The more quickly they transition between on and off, the less power they dissipate internally.
The other choices are incorrect, because faster switching generally increases the electric and magnetic field coupling. Electric-field coupling is usually proportional to the rate of change of the voltage, dV/dt, in the source circuit. Magnetic-field coupling is usually proportional to the rate of change of the current, dI/dt, in the source circuit.
A decade ago, slowing a power switch's transition time to reduce field coupling was not an option. Small increases in switching time usually had a measurable effect on the efficiency of power inverters. However, today's switching components are capable of switching so fast, that it is sometimes necessary to slow the switching slightly in order to control conducted and radiated emissions at very high harmonics of the switching frequency.
Have a comment or question regarding this solution? We'd like to hear from you. Email us at