The function of an inverter is to change a DC input voltage and/or current to a symmetrical AC output voltage and/or current of desired magnitude and frequency. The output voltage and/or current waveforms of ideal inverters should be sinusoidal. However, the waveforms of practical inverters are nonsinusoidal and contain certain harmonics. The harmonic contents of the output voltage and/or current can be minimized or reduced significantly by high switching frequencies. The desire for high switching frequencies also stems from the reduction of volume and weight of smoothing devices and transformers in power circuits, and the reduction of acoustic noise by running power circuits at ultrasonic frequencies. But the increase of the inverter switching frequency continue to exact a heavy penalty in terms of system efficiency. In this thesis, the techniques which can increase the inverter switching frequency without the sacrifice of system efficiency are proposed.
First, a regenerative snubber for three-level high-power GTO inverter having simple construction and high efficiency is proposed. It consists of series inductor and shunt capacitor snubbers with DC/DC converter for energy recovery trapped in the snubbers. In particular, the new DC /DC converter as an essential component of regenerative snubber has very low switching loss due to resonant operation with zero voltage switching and has small size due to high switching frequency over 10KHz using power transistors. The proposed regenerative snubber is also thought to be applicable for other multi-level power GTO inverters.
Second, two new zero voltage switching partial-resonant link AC/AC converters are proposed which overcome the shortcomings of conventional resonant ac and dc link AC/AC schemes and consist of only 12 unidirectional switches. The proposed converters synthesize output ac waveforms with integrals of high-frequency current pulses like series-resonant converters. But switching operations occur at zero voltage instants instead of at zero current instants. The partial resonant link normally does not resonate during entire operation interval but does only for the duration of switching transients. In particular, the proposed converters can easily and exactly control individual current pulse amplitude in each switching cycle. Symmetrical configuration of the power scheme also provides fully regenerative operation with buck-boost capability. The advantages of the proposed schemes are shown through analyses and simulation results.
Third, A zero voltage switching (ZVS) DC/DC converter being capable of constant frequency PWM control of output voltage and having wide linearity is proposed. ZVS operations are achieved not only for the primary switches but also for the secondary rectifier diodes to reduce the switching stresses and losses. The auxiliary circuit recovers the energy stored in the resonant inductor during reverse recovery interval of output diode whereby elimination the serious reverse recovery problem. The proposed converter also overcomes the other shortcomings of the conventional resonant DC/DC converters such as high VA ratings of devices and passive components, load dependent DC characteristics, etc.
