TABLE OF CONTENTS In the course of the previous presentation, it was shown that the main power train has numerous dissipative elements that degrade converter efficiency. Among these dissipative factors, some are physical in nature for instance, winding resistance and cannot be eliminated. Others originating from the functional mechanism, for example, the switching losses of power switches, may be minimized or even totally eliminated.
The switching losses arise, as shown in Wu's Figure 9.10 [2], because either the switch voltage rises too quickly ahead of the decreasing current at turn-off or the switch current surges in advance of the diminishing switch voltage at turn-on. In both cases, power pulses are created where the switch voltage and current profiles cross each other. The switching losses were considered an insignificant nuisance in years past, when most switching converters operated at only tens of kilohertz. However, with the march toward higher switching frequencies of 500 KHz and beyond, the switching losses overtake other dissipations in importance and demand attention. As a result, the concepts of zero-voltage switching (ZVS) and zero-current switching (ZCS) were born.
Zero-voltage switching in general is implemented by placing a capacitor across the power switch (Figure 9.1). The motivation for taking this approach is based simply on the recognition that the capacitor's voltages are continuous. In contrast, zero-current switching is implemented by connecting an inductor in series with the power switch, Figure 9.2. In this case, the property of continuity of inductive current is employed. Furthermore, both techniques can be enhanced by taking the...
