Chapter 2 Pulse - Width Modulation

Pulse-width modulation (PWM) is the basis for control in power electronics. The theoretically zero rise and fall time of an ideal PWM waveform represents a preferred way of driving modern semiconductor power devices. With the exception of some resonant converters, the vast majority of power electronic circuits are controlled by PWM signals of various forms. The rapid rising and falling edges ensure that the semiconductor power devices are turned on or turned off as fast as practically possible to minimise the switching transition time and the associated switching losses. Although other considerations, such as parasitic ringing and electromagnetic interference (EMI) emission, may impose an upper limit on the turn-on and turn-off speed in practical situations, the resulting finite rise and fall time can be ignored in the analysis of PWM signals and processes in most cases. Hence only ideal PWM signals with zero rise and fall time will be considered in this chapter. Pulse-width modulation can take different forms [3]. The pulse frequency is one of the most important parameters when defining a PWM method and can be either constant or variable. A constant-frequency (CF) PWM signal can be produced simply by comparing a reference signal, r(t), with a carrier signal, c(t), as depicted in Fig. 2.1a. The binary PWM output can be mathematically written as