Design and Analysis of a Low Q Three-phase Series Resonant Converter
Renewable energy applications require highly efficient DC-DC converter stages that can work under a wide range of operating conditions. Resonant DC-DC converters have extensively been used for many applications due to their ability to operate at high switching frequencies with good efficiency. At higher power levels (e.g., greater than 1kW), three-phase resonant converters have significant advantages over single-phase ones due to reduced component stresses, smaller filter size requirements, and higher power density. This thesis presents the design and analysis of a three-phase series resonant converter for solar applications. Series resonant converters can operate with high efficiency from full load to part-load conditions but generally require a high quality factor (Q) design to maintain regulation and soft-switching at light load conditions. However, a high Q design leads to increased voltage/current stresses for the resonant components. This thesis focuses on achieving soft-switching and regulation in the entire operating range for a low Q three-phase series resonant converter. A hybrid modulation technique employing APWM and variable frequency is used to reduce the switching frequency range required for the converter operation. Due to the low Q factor, the resonant circuit waveforms are non-sinusoidal and hence, fundamental harmonic analysis cannot provide accurate results. Multiple harmonics at the input and load sides have to be taken into consideration to ensure accurate results using frequency-domain analysis. However, due to the presence of asymmetric pulse width and discontinuous conduction intervals, the voltage waveforms at the load end cannot be defined. Hence, a precise time-domain analysis technique is introduced to analyze the converter operation for varying duty cycles and switching frequencies. To maintain regulation as well as soft-switching up to extremely light load conditions, it is further proposed to operate the converter in a variable structure including three-phase, single-phase, and burst modes. The operating regions for the different modes are defined and a control strategy is proposed for the converter operation. The performance of the converter is verified using a 200V/250W experimental prototype operating with an input voltage range of 40-80V.