Analysis, Design and Implementation of Efficient High Frequency Power Converters for PV Applications using Generalized Steady-state Modeling

• Author / Creator

  Daryaei, Mohammad

• Efficiency and power density of Power Electronics converters has been constantly increased for decades and is projected to further improve. As an example, the efficiency for converters used in telecommunication and PV applications has increased from around 90% in 1990s to about 98% in recent years which has also contributed to the increase of converter power density. Efficiency and power density improvements can be achieved by proper design of power converters. However, existing approaches for converter design are mainly based on inaccurate or numerical analysis methods which fail to render a systematic and optimal design tool. Generally, design of a power converter includes suitable topology adoption, circuit parameters selection, component selection, etc., which are imposed by the Steady-state model of the converter. Steady-state models of power converters that provide accurate closed-form expressions for converter waveforms are extremely valuable for converter design. An obstacle in the development of such models is the inherent non-linearity of switching power converters. This thesis presents a systematic procedure to model a broad class of power converters using ordinary differential equations (ODEs) with periodic and discontinuous inputs, and provides an approach to determine closed-form expressions for their steady-state waveforms. The presented Laplace Based Steady-state Modeling (LBSM), serves as an effective analysis and design tool for power converters. The value of LBSM is demonstrated by using it to obtain closed-form expressions for the steady-state waveforms of different types of converters. In particular, two commonly used topologies for PV micro-inverters – the series resonant converter and the phase-shift converter – are analyzed and compared using LBSM and their optimum operating conditions and applications are discussed. The converter waveforms, soft switching ranges and other characteristics obtained using LBSM are also validated through simulations and experiments.
  A specific category of power converters are the PV converters which require improved system level energy yield in addition to converter level efficiency and power density. The system level energy yield considerations imposes the use of PV Module Integrated Converters (MIC) with high efficiency around their unity conversion ratio. Partial Power Processing (PPP) concept is a potential solution for MICs as it improve the efficiency around the unity conversion ratio however, partial power converters inherently have limited Maximum Power Point Tracking (MPPT) range. In this thesis, the challenge of limited MPPT range for partial power converters has been overcome by utilizing a proposed topology and Pulse Density Modulation (PDM) technique which modulates converter's mode of operation. The converter operation is toggled among three highly efficient conversion modes called as Pass-through, Bypass and Process modes. The proposed topology achieves high efficiencies for Pass-through and Bypass modes by limiting the switching actions while it gets high efficiencies even under Process mode with appropriate utilization of converter parasitic elements to achieve soft-switching.
  The proposed MIC is analyzed using the LBSM technique to optimize the converter design. As the proposed MIC uses center tapped transformer, a new model for the center tapped transformer is proposed in this thesis which simplifies the analysis of the converter and provides better insight into different parasitic effects of the transformer. Leveraging the LBSM and based on the single cycle analysis of the converter, an equivalent circuit is proposed for the MIC which contains both high and low frequency effects of the PDM control. The LBSM is then used to obtain accurate and closed form equations for the converter waveforms and design parameters which are then used to propose a new PDM approach. A 1 MHz prototype converter for a 220 W sample PV module has been developed and experimentally tested. The soft-switching operation, PDM regulation of the output and efficiency improvements have been validated experimentally. It has been shown that the proposed converter can reach 99.6% to 96% efficiency for the power mismatches in the PV module ranging from 0 to 50% of the maximum module power generation capability, respectively. The efficiency drop is shown to be linear with power mismatch level without any abrupt reductions that is commonly observed in conventional PV module integrated converters.

• Subjects / Keywords

  • PV converter
  • Module integrated converter
  • Planar transformer
  • High frequency
  • Steady state analysis
  • Laplace transform

• Graduation date

  Fall 2023

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-mreg-5a59

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.