Overview

A converter basically consists of an array of on-off electronic switches that use power semiconductor devices. If the switches are considered ideal or lossless (zero conduction drop, zero leakage current, and instantaneous turn-on and turn-off times), the instantaneous and average power will balance at input and output of the converter. Switching mode operation makes the converter nonlinear, thus generating source and load harmonics and also EMI problems. The discrete time switching characteristics cause a delay in signal propagation. Of course, a high switching frequency reduces the propagation delay. A converter can be single stage, or multiple conversions may be involved in a cascaded converter system. Several types of commutation (transferring current from the outgoing device to the incoming device) can be used. Thyristor converters are characterized by line (or natural), load, or forced commutation. Line-commutated converters are used extensively in utility systems, and these will be discussed in this chapter. Force-commutated thyristor converters that require auxiliary transient circuits are practically obsolete. Converters that use devices such as power MOSFETs, GTOs, IGBTs, and IGCTs are characterized by self-commutation. Again, a converter can be based on hard switching (as in Figure 2.19) or soft-switching. In a soft-switched converter, dv