In this dissertation, four kinds of novel soft switching PWM inverters using parallel resonant DC link topologies are proposed to resolve both the high voltage stresses and restricted PWM capability of the conventional soft switching inverters with various resonant DC link topologies simultaneously. These have the minimum voltage stresses (1.0 p.u.) for all of the switching devices and highly improved PWM capability due to not only variable link pulse width but also variable pulse position.
At first, a novel soft switching inverter is proposed by using a modified parallel resonant DC-link (PRDCL). This topology has most of the advantages of the conventional RDCL inverter such as high switching frequency, low switching loss, reduced acoustic noise and EMI and so forth. As it has also the above mentioned distinctive merits, the proposed PRDCL inverter can be applied to high power application. Furthermore, the design and control of the PRDCL circuit are fairly simple. Operational principle, detailed analysis and design procedures are described. A 7.5kVA prototype PRDCL inverter is implemented and tested with 20kHz switching frequency using power transistor modules. Experimental results verifying the principle of operation are presented.
Another approach for a soft switching PWM inverter with low voltage stress and improved PWM capability is presented having only one additional switch. This circuit needs no snubber circuit owing to either ZVS or ZCS at switching instant and can operate at relatively high switching frequency by introducing new shoot-through pole concept. It has also simple structure and flexible PWM capability due to the convenient choices of switching conditions. Based on the operational principle, analyses and design procedures are described and verified by simulations.
A soft switching quasi-parallel resonant DC link (QPRDCL) inverter with enhanced PWM capability is suggested using two additional switches. As mentioned earlier, it has also minimum voltage stress and provides the flexibility of selecting the on/off instants of the resonant link resulting in improved PWM capability. The operational principles and the detailed analysis of the QPRDCL inverter is described for the resonant components design and the inverter control. A space vector PWM (SVPWM) with optimal vector sequence suitable for the QPRDCL inverter is also presented through the comparisons among five different modified SVPWM techniques classified by the voltage vector sequences. The operation of the QPRDCL circuit and the performance of the selected optimal SVPWM is verified by the experimental results.
Finally, A low loss quasi-parallel resonant DC link ($L^2$QPRDCL) circuit with advanced PWM capability is proposed for soft switching three phase PWM inverter. This circuit has also all of the aforementioned advantages like as other topologies. Owing to being scarcely any freewheeling current in the $L^2$QPRDCL during the zero voltage period, low loss operation in the resonant circuit can be obtained. As a result, the proposed $L^2$QPRDCL inverter is applicable to high power range. Operational principles and detailed analyses are presented. By adopting the Non-Nearest Four Vector ($N^2$FV) selection law in the SVPWM technique and carrying out some modification suitable for the $L^2QPDCL inverter, the narrow pulse problem originating from the link operation, which should be considered in the inverters having resonant DC links, can be eliminated. Experimental results are also presented to verified the link operation and the performance of the selected SVPWM method.
