TWO-SWITCH, SOFT-SWITCHED CONVERTERS WITH ASYMMETRICAL PWM CONTROL

Abstract

In order to build a high efficiency, high density power supply, the internal switching frequency of the converter must be increased. However, the switching loss increases correspondingly. Many of the soft-switched converters proposed in the 1980's faced problems such as increased current levels, increased MOSFET voltage stress, wide switching frequency range, minimum load requirement, increased component count and increased complexity of the circuit. Thus, the overall aim of the present work was to investigate soft-switching dc/dc converter topologies suitable for high density power supply applications. The project work can be divided into two parts. In the first part, two specific soft-switched converters with asymmetrical Pulse Width Modulation (PWM) control were studied. In the second part, the problem was generalized and the relationships among a class of two-switch, soft-switched converters with asymmetrical PWM control were explored. The converters proposed m the first part of the work achieve lossless switching without paying the penalties mentioned earlier. By using the asymmetical PWM method, constant frequency operation of basic soft-switching half-bridge converter topology is achieved without additional components or complexity. Two main variations are derived from this basic topology by implementing different output filtering circuits (inductive or capacitive). The converters' steady-state de characteristics and design procedures are established. Experimental results obtained from two 50W converters (one for each variation) are utilized to verify the theoretical waveforms and the de characteristics. Due to the inherent advantages of the proposed converter/control scheme, such as low MOSFET voltage stress, low component current stress and constant frequency operation, the converters are found to be capable of achieving high efficiencies even under partial load conditions. It may be pointed out that the current work on the inductive filter variation (buck-reset-TRL3) was carried out independently of those reported in [ 14, 15]. In the second part of the thesis, the inter-relationships among two-switch, soft-switched, PWM converters are explored. Even though the circuit configurations may appear different, many of the two-switch converters proposed by several researchers were found to be related to one another. They can be viewed as having evolved from a generalized circuit which operates under an asymmetrical PWM scheme. Starting from a generalized two-switch PWM converter, the non-isolated converters are first conceptually derived. Isolated versions with output inductive and capacitive filtering are then introduced. Different specific converters are obtained and classified according to the type of core reset mechanism and output filter circuit. The modes of operation of the basic isolated soft-switched converter with output inductive and capacitive filtering are discussed in detail. The operations of the converters have been verified using SPICE. The steady-state de analysis of all the converters are performed and the results compared. Experimental results for buck-boost-reset TRC3, which is perhaps a new converter, are presented to verify the theoretical predictions. Though many of the converters have been described elsewhere, this work integrates them under a common qualitative and theoretical framework.