The In the last few years, soft-switching techniques  have been proposed to: 1) reduce the switching losses and 2) reduce the switching stress of switched-mode power electronics circuits in order to improve the energy efficiency and reliability of power converters. Essentially, soft-switching techniques create a zero-voltage and/or zero current conditions for the power switch to turn on or off so that the instantaneous power losses can be reduced or eliminated. Figure 1-simulated phase portrait for period2 operation.
Figure2-simulated phase portrait waveform form chaotic operation.
Figur3-Buck-boost converter with soft switching.
The soft-switched circuits are similar to their hard-switched counterparts, except that they consist of extra resonant components (Lr and Cr) and an auxiliary switch Q. The number of components of a resonant converter is approximately same as the corresponding hard-switched version because the small inductor and the small capacitor that are normally used in hard-switched circuits as turn-on and turn-off snubbers are converted into the resonant tank in soft-switched converters. Although a snubber circuit can reduce the EMI, it generally causes additional energy loss than hard-switched converters with snubber circuit. The resonant inductor Lr limits the initial current of the main switch SW in order to provide a near-zero current condition during the turn-on process of SW. Zero-voltage condition for SW can be achieved by turning on the auxiliary switch Q just before the turnoff of SW. The resonant capacitor Cr can discharge through the antiparallel diode of SW, thus clamping the voltage across SW to about 1 V for near-zero voltage turnoff of SW. One distinctive feature of the QR soft-switching technique is that the auxiliary switch Q and the freewheeling diode D are also soft-switched. A resonant switch, inductor Lr and capacitor Cr is added to the basic converter...