The Resonant Soft-Switching PWM Power Converter And Its Nonlinear Analysis

With the advancement of power electronics, computer technology and controltheory, variable speed theory and technique for ac motor have been made a bigprogress. However if a conventional PWM-type structure inverter with hardswitching mode is used, the switching loss in an IGBT increases dramatically anddv/dt stress on the inverse bridge is very high. As a result, the maximum operatingswitching frequency is limited to a relatively low value, the output waveform of theinverter is not in good quality, and conversion efficiency is low. Therefore, studyingon the resonant soft switching power converter is presently one of the main researchdirections of the power electronics. Higher switching frequencies of the converterhave the concomitant advantages of higher current regulator bandwidth, smallerreactive component size, higher conversion efficiency and power density, fastersystem dynamic response and smaller acoustic noise.In this dissertation, We mainly investigate the resonant soft switching PWMconverter and its nonlinear analysis. The main contributions are as follows:1. In voltage-type SPWM inverter, time delay is inserted in switching signals toprevent a short circuit in DC link and protect power devices. This causes time delayeffect, which is detrimental to the performance of the inverter. Through analysis ofthe time delay effect in the SPWM inverter, presentations of the fundamentalcomponent and low order harmonics of output voltage of the SPWM inverteraffected by the time delay effect are derived. The effect of the time delay on torqueand velocity of ac load are studied. Simulation results show that the time delay effectresults in a decrease of the fundamental component and an increase in the low orderharmonics in the output voltage of the inverter. Moreover, the time delay effect willbe more serious as the carrier frequency goes up, and output torque decreasesdramatically as the output frequency gets lower. Then a specially designed circuit forcompensating the time delay effect is proposed. Experimental Results show thatparts of output voltages corresponding to low order harmonics are almost completelysuppressed. The performance of the inverter is greatly improved. 2. The principle of the series resonant DC link inverter is analyzed. Themathematical models of the inverter system are established. The minimal initialcondition for making resonant DC bus voltage return to zero volt during oneoscillation period is derived, i.e. the resonant inductor has the minimal initial currentI_{L min}(0). The minimal conducting time t_{t min}, of the switching device T for obtainingthe minimal initial current I(L min)(0) in the resonant inductor is found. The highfrequency soft switching inverter—AC induction motor drive systems are analyzed.The system oscillation characters under variable load conditions are studied. Theselection and calculation of the resonant frequency and parameters are given.Simulation results given in Fig 3—12 show that a relatively large value ofinductance L should be selected for resonant inductor L in the series resonant DClink so that the resonant peak voltage V(C max) can be decreased and the efficiency ofthe soft switching converter can be increased.3. In order to overcome the disadvantage of high peak voltage and high peakcurrent stresses on the switching devices of the resonant DC link inverter (RDCLI)and parallel resonant DC link inverter (PRDCLI), a three-phase DC-AC series-parallelresonant converter with zero-voltage soft switching conversion concept is proposed. Adetailed analysis of its operating principle is presented. The mathematical model ofthe proposed converter is derived. The computer simulations of the converter undervarious working conditions are given. Compared with RDCLI and PRDCLI, theproposed novel DC-AC series-parallel resonant conversion topology has manyadvantages, which enables controlled oscillating peak voltage, simplified controlcircuit, fewer switching devices needed by the resonant circuit and various PWMregulating strategy available.With the use of MCS—8098 single chip computer as a controller, IGBT aspower devices, SLE4520 as a PWM wave generator, a three—phase DC/AC seriesparallel resonant converter is fabricated. The design method of the converter ispresented. The computation and selection of the resonant frequency and resonantparameters are given. The control method and circuit are discussed. The simulationand experimental results show that operating principle of the converter system iscorrect. The system design is reasonable.4. Because the resonant frequency, PWM switching frequency in inverse bridgeand output frequency exist concurrently in the resonant DC/AC converter, it is very difficult to do nonlinear analysis for the resonant DC/AC power converter. The newconcept and method of stage separation modeling for analyzing it are proposed. Asymbolic analysis algorithm is presented. The principle of the algorithm is describedin detail. First, the modeling and nonlinear analysis for the resonant DC link circuitis studied. The Computing formulae of the voltage in Capacitor and current ininductor of the resonant DC link circuit are derived. Second, nonlinear modeling andsimulation of PWM inverse bridge are studied. Finally we combine nonlinear modelof the resonant DC link circuit and model of PWM inverse bridge into one wholenonlinear system. The simulation is done by the demand of system output. Thesteady state character is obtained. The simulation results show that the method foranalyzing the resonant converter is validity.In final chapter the main work is concluded and prospective research direction isdiscussed.