Control Based Soft Switching Three-phase Micro-inverter: Efficiency And Power Density Optimization

In the field of renewable energy, solar photovoltaic is growing exponentially. Grid-tied PV micro-inverters have become the trend for future PV system development because of their remarkable advantages such as enhanced energy production due to MPPT implementation for each PV panel, high reliability due to redundant and distributed system architecture, and simple design, installation, and management due to its plug-and-play feature. Conventional approaches for the PV micro-inverters are mainly in the form of single-phase grid connected and they aim at the residential and commercial rooftop applications. It would be advantageous to extend the micro-inverter concept to large size PV installations such as MW-class solar farms where threephase AC connections are used. The relatively high cost of the three-phase micro-inverter is the biggest barrier to its large scale deployment. Increasing the switching frequency may be the best way to reduce cost by shrinking the size of reactive components and heat-sink. However, this approach could cause conversion efficiency to drop dramatically without employing soft switching techniques or using costly new devices. This dissertation presents a new zero voltage switching control method that is suitable for low power applications such as three-phase micro-inverters. The proposed hybrid boundary conduction mode (BCM) current control method increases the efficiency and power density of the micro-inverters and features both reduced number of components and easy digital implementation. Zero voltage switching is achieved by controlling the inductor current bidirectional in every switching cycle and results in lower switching losses, higher operating frequency, and reduced size and cost of passive components, especially magnetic cores. Some iv practical aspects of hybrid control implementation such as dead-time insertion can degrade the performance of the micro-inverter. A dead-time compensation method that improves the performance of hybrid BCM current control by decreasing the output current THD and reducing the zero crossing distortion is presented. Different BCM ZVS current control modulation schemes are compared based on power losses breakdown, switching frequency range, and current quality. Compared to continuous conduction mode (CCM) current control, BCM ZVS control decreases MOSFET switching losses and filter inductor conduction losses but increases MOSFET conduction losses and inductor core losses. Based on the loss analysis, a dual-mode current modulation method combining ZVS and zero current switching (ZCS) schemes is proposed to improve the efficiency of the micro-inverter. Finally, a method of maintaining high power conversion efficiency across the entire load range of the three-phase micro-inverter is proposed. The proposed control method substantially increases the conversion efficiency at light loads by minimizing switching losses of semiconductor devices as well as core losses of magnetic components. This is accomplished by entering a phase skipping operating mode wherein two phases of an inverter are disabled and three inverters are combined to form a new three-phase system with minimal grid imbalance. A 400W prototype of a three-phase micro-inverter and its hybrid control system have been designed and tested under different conditions to verify the effectiveness of the proposed controller, current modulation scheme, and light load efficiency enhancement method

Soft-Switching GaN-Based Isolated Power Conversion System for Small Satellites with Wide Input Voltage Range

As we pursue the advancement of small satellites for space missions with more capabilities, there is a significant need for cutting-edge, modularly configurable, high density power converters. This article proposes a fixed switching frequency, high efficiency, compact isolated converter for sensitive loads such as radar, communication systems, or other instruments on small satellites

A GaN-Based Four-Switch Buck-Boost Converter Using Ripple Correlation Control for Maximum Power Point Tracking in Dynamic Deep Space Environments

As the demand for high-performance power conversion in spacecraft continues to grow and spacecraft mass and volume budgets become increasingly tight, it is essential to design DC-DC converters with higher efficiency and power density. Although photovoltaic (PV) efficiency has increased over time, solar irradiance and temperatures can fluctuate dramatically in deep space. This causes significant variations in the maximum power point (MPP) of the PV array, which can decrease the overall system efficiency unless accounted for. Thus, it is imperative to track the MPP of the PV panels to maintain optimal efficiency. This paper presents the experimental development of a four-switch, GaN-based buck-boost converter with an implementation of the Ripple Correlation Control (RCC) MPPT algorithm for dynamic deep space environments. Due to the use of GaN HEMTs, the experimental system achieves better efficiency and power density compared to the previous state of the art implementations. A simulation of the prototype buck-boost converter was implemented in SaberRD (Synopsis), and a digital design of the RCC-based MPPT controller utilizing the StateAMS tool is presented. The simulation results show that this controller swiftly and precisely converged to the MPP of the source PV panels in a dynamic solar irradiance condition

GaN-Based, Ultra-Compact Power Conversion System for the PUFFER Autonomous Mobility Platform

In the pursuit for the development of small rovers for planetary science missions, there is a distinct need for the development of an advanced, autonomously controlled, power subsystem. Existing bus management systems used in large spacecraft missions are not suitable for small spacecraft missions, as they are massive, relatively inefficient, and expensive. For extremely compact rover mission concept, newly developed high-density, high-efficiency, lightweight, and low-cost electronics are required. This paper presents a radiation-hardened power subsystem for the Pop-Up Flat-Folding Explorer Robot (PUFFER) mission concept, utilizing GaN-based converters for solar array conversion, battery management, and point of load applications to provide an extremely compact power subsystem

Zero Voltage Switching Forward-Flyback Converter With Efficient Active Lc Snubber Circuit

This paper describes a Boundary Mode Forward-Flyback Converter (BMFFC) with zero-voltage switching that is able to process power efficiently. The theoretical analysis and operating principle of the BMFFC are presented in detail. A non-dissipative LC snubber that recycles energy to the input source is employed in order to suppress the voltage spike caused by the leakage inductance of the transformer. The relatively large snubber capacitor also significantly reduces turn-off loss. Following a detailed design procedure, a 200W prototype with a 25-50VDC input and 230VDC output was constructed and tested in order to evaluate the performance of the BMFFC. © 2014 IEEE

Zero voltage switching Forward-Flyback Converter with efficient active LC snubber circuit

This paper describes a Boundary Mode Forward-Flyback Converter (BMFFC) with zero-voltage switching that is able to process power efficiently. The theoretical analysis and operating principle of the BMFFC are presented in detail. A non-dissipative LC snubber that recycles energy to the input source is employed in order to suppress the voltage spike caused by the leakage inductance of the transformer. The relatively large snubber capacitor also significantly reduces turn-off loss. Following a detailed design procedure, a 200W prototype with a 25-50VDC input and 230VDC output was constructed and tested in order to evaluate the performance of the BMFFC. © 2014 IEEE

Design And Implementation Of Three-Phase Gridconnected Two-Stage Module Integrated Converter

This paper presents a high efficiency module integrated inverter (MIC) with both stages ZVS operation for three phase grid tied photovoltaic system. LLC resonant dc-dc converter is employed in the first stage to track the maximum power of PV panel. A novel and simple soft switching scheme without adding auxiliary components is proposed for three phase for the second stage. Meanwhile, modeling and control strategy of three phase four-wire DC/AC converter are also discussed. In addition, a dedicated Center Points Iteration (CPI) MPPT algorithm for LLC resonant topology is applied to quickly tracking the maximum power. In addition, the capacitance of DC-link is also investigated because DC-link capacitor plays an important role for dual-stage MIC. Finally, a 400 watts prototype has been built and tested. The peak efficiency of the whole system prototype has been measured, which is up to 96%, 98.2% in the first stage, and 98.3% in the second stage, respectively. © 2013 IEEE

Modeling and analysis of DC-link voltage for three-phase four-wire two-stage micro-inverter

DC-link voltage control plays a key role for two-stage micro-inverter application. Modeling &analysis of dc-link voltage is investigated in this paper. The first-stage circuit employs transient maximum power point tracking method to realize fast MPP tracking without causing output distortion and dc-link variation. In order to keep dc-link voltage constant, injecting current to grid varies with the output of a positive feedback of dc-link voltage regulator at steady situation. Two disturbances to cause dc-link constant are investigated and small signal model of dc-link voltage is also built while considering dynamic response. Followed by the small signal model of dc-link voltage, the dc-link capacitance is also calculated. The performance of dc-link voltage control is verified by experimental results based on a 400 watt two-stage three-phase four-wire prototype with grid-connected, the input voltage from 35V to 60V, dc-link voltage 400Vdc and AC output voltage 110V RMS. © 2014 IEEE

Modeling And Analysis Of Dc-Link Voltage For Three-Phase Four-Wire Two-Stage Micro-Inverter

DC-link voltage control plays a key role for two-stage micro-inverter application. Modeling &analysis of dc-link voltage is investigated in this paper. The first-stage circuit employs transient maximum power point tracking method to realize fast MPP tracking without causing output distortion and dc-link variation. In order to keep dc-link voltage constant, injecting current to grid varies with the output of a positive feedback of dc-link voltage regulator at steady situation. Two disturbances to cause dc-link constant are investigated and small signal model of dc-link voltage is also built while considering dynamic response. Followed by the small signal model of dc-link voltage, the dc-link capacitance is also calculated. The performance of dc-link voltage control is verified by experimental results based on a 400 watt two-stage three-phase four-wire prototype with grid-connected, the input voltage from 35V to 60V, dc-link voltage 400Vdc and AC output voltage 110V RMS. © 2014 IEEE

Phase Skipping Control To Improve Light Load Efficiency Of Three Phase Micro-Inverters

Incident solar power on a PV panel is highly variable due its dependence on weather, time of the day and shading which forces the micro-inverter to operate at light loads for a significant period of its operation time. Poor light load efficiency of power converters further reduces the effective power available. This paper proposes a control technique - Phase Skipping Control, to improve light load efficiency of DC-AC stage of Grid Tied-Three Phase Micro-Inverters. This control technique is independent of the underlying topology; however, to establish its proof of concept, Phase Skipping Control, is implemented on a 400W Three Phase Half Bridge PWM Inverter. Improvement in light load efficiency with Phase Skipping Control is calculated theoretically using a proposed loss model for Half Bridge PWM Inverter. Furthermore, a prototype for 400W Three Phase Half Bridge PWM Inverter is developed for experimental verification of light load efficiency improvement using Phase Skipping Control. Experimental results shows 5.43% increase in efficiency at 10% load and 0.46% increase in CEC efficiency of the Three Phase Half Bridge PWM Inverter. © 2014 IEEE