Matthias Kasper

Classification and Comparative Evaluation of PV Panel-Integrated DC–DC Converter Concepts

Strings of photovoltaic panels have a significantly reduced power output when mismatch between the panels, such as partial shading, occurs since integrated diodes are then partly bypassing the shaded panels. With the implementation of DC-DC converters on panel level, the maximum available power can be extracted from each panel regardless of any shading. In this paper, different concepts of PV panel integrated DC-DC converters are presented, comparative evaluation is given and the converter design process is shown for the buck-boost converter which is identified as the best suited concept. Furthermore, the results of high precision efficiency measurements of an experimental prototype are presented and compared to a commercial MIC.

Classification and comparative evaluation of PV panel integrated DC-DC converter concepts

The strings of photovoltaic panels have a significantly reduced power output when mismatch between the panels occurs, as, e.g., caused by partial shading. With mismatch, either the panel-integrated diodes are bypassing the shaded panels if the string is operated at the current level of the unshaded panels, or some power of the unshaded panels is lost if the string current is reduced to the level of the shaded panels. With the implementation of dc-dc converters on panel level, the maximum available power can be extracted from each panel regardless of any mismatch. In this paper, different concepts of PV panel-integrated dc-dc converters are presented and their suitability for panel integration is evaluated. The buck-boost converter is identified as the most promising concept and an efficiency/power density ( η- ρ) Pareto optimization of this topology is shown. Based on the optimization results, two 275 W converter prototypes with either Silicon MOSFETs with a switching frequency of 100 kHz or gallium nitride FETs with a switching frequency of 400 kHz are designed for an input voltage range of 15 to 45 V and an output voltage range of 10 to 100 V. The theoretical considerations are verified by efficiency measurements which are compared to the characteristics of a commercial panel-integrated converter.

Scaling and balancing of multi-cell converters

In this paper, the potential of the multi-cell approach for power electronic converters with efficiencies and power densities beyond the barriers of state-of-the-art systems is discussed. Based on fundamental scaling laws the benefits of splitting a system into multiple converter cells are derived in terms of lower volume and/or higher power density for a given cooling capacity. In addition, the conditions for equal current and/or voltage balancing of multi-cell systems is reviewed. The advantages of the mulit-cell systems are examined in more detail based on the example of a DC-DC boost converter realized with either parallel- or series-interleaved boost cells. It is shown, that the multi-cell systems can offer lower switching and conduction losses and/or an improved voltage spectrum depending on the choice of the switching frequency relative to a single system. Furthermore, the effects of parasitic capacitances on unwanted ground currents are investigated for both configurations.

ZVS of Power MOSFETs Revisited

Aiming for converters with high efficiency and high power density demands converter topologies with zero-voltage switching (ZVS) capabilities. This letter shows that in order to determine whether ZVS is provided at a given operating point, the stored charge within the mosfets has to be considered and the condition LI2≥2Qoss has to be fulfilled. In the case of incomplete soft switching, nonzero losses occur which are analytically derived and experimentally verified in this letter. Furthermore, the issue of nonideal soft-switching behavior of Si superjunction mosfets is addressed.

Novel high voltage conversion ratio “Rainstick” DC/DC converters

Voltage conversions with high step-down ratios are required in many medium voltage applications for supplying auxiliary electronics. In this paper a new topology for high conversion ratios with low voltage stresses of components, modular structure and simple control is presented. The operation principles are described together with analytical descriptions of the average value of the inductor currents and RMS values of the switches. In addition, the influence of the efficiency of the individual modules on the total converter efficiency is provided. Possible modifications of the “Rainstick” converter with benefits for different applications are shown. Furthermore, the measurement results of a prototype with an input voltage range of up to 2.4 kV and 30 W output power are shown.

Hardware verification of a hyper-efficient (98%) and super-compact (2.2kW/dm 3 ) isolated AC/DC telecom power supply module based on multi-cell converter approach

Due to the increasing electricity demand of data centers driven by the emergence of cloud computing and big data, the focus on the development of telecom and data center power supplies is shifted towards high efficiencies. In this paper, a multi-cell converter approach for a telecom rectifier module breaking through the efficiency and power density barriers of traditional single-cell converter systems is shown. The comprehensive optimization of the entire system with respect to efficiency and volume is described and the applied component loss models are explained. Furthermore, the design of a hardware demonstrator based on the optimization results is presented and several important design aspects are explained in detail.

Hyper-efficient (98%) and super-compact (3.3kW/dm 3 ) isolated AC/DC telecom power supply module based on multi-cell converter approach

In this paper, a multi-cell converter approach for a telecom rectifier module breaking through the efficiency and power density barriers of traditional single-cell converter systems is shown. The potential of the multi-cell approach for high efficiency is derived from fundamental scaling laws of the system performances, such as the losses generated by the semiconductors and the harmonic spectrum, in dependence of the number of converter cells. Furthermore, a comprehensive optimization of the entire system with respect to efficiency and volume has been performed and the applied component loss models are described in detail. The achievable performance of the system is compared to a leading edge state-of-the-art single-cell converter system which currently sets the benchmark in terms of efficiency and power-density. In addition, the degrees of freedom of multi-cell converter systems in terms of converter operation are outlined and optimum control schemes are derived.

Impact of PV string shading conditions on panel voltage equalizing converters and optimization of a single converter system with overcurrent protection

Unequal irradiance of series connected PV panels in a string strongly decreases the output power of the PV system, as either PV panels are bypassed or operated below their MPP. By connecting balancing converters around each pair of adjacent PV panels, all panels can maintain the operation close to their MPP regardless of any mismatched operating condition. In this paper a balancing converter concept with ZVS is introduced and the operation of a string of PV panels equipped with balancing converters is analytically described. Based on this analytic framework, different examples of shading scenarios of a PV string are examined regarding the average inductor current values that occur within the balancing modules, in order to derive the required specifications of a balancing converter. Moreover, a two-level overcurrent protection concept is introduced which enables to keep all balancing converters within the safe operating area at all times, even at shading conditions with high mismatch. Furthermore, the design optimization of a highly efficient PV voltage balancing converter with a wide input voltage range is explained in detail and finally the concept is verified with measurement results of a demonstrator system.

Analysis of capacitive power transfer GaN ISOP multi-cell DC/DC converter systems for single-phase telecom power supply modules

The Input Series Output Parallel (ISOP) multi-cell converter approach allows breaking the performance barriers of conventional single-cell telecom rectifier systems by leveraging the advantages of using multiple interleaved low-voltage and/or low-current converter cells. The ISOP interconnection in the DC/DC converter part of the cells, however, requires the employment of some kind of isolation in each cell, which is typically provided by transformers. An analysis of the losses and of the volume of the entire multi-cell system reveals that these transformers contribute a major part to the system losses and are responsible for a significant share of the total volume. However, as the transformers are mainly required for providing galvanic isolation in the ISOP structure and not for voltage conversion, series capacitors represent an alternative to decouple the series connected input terminals of the cells from the parallel connected output terminals. Compared to conventional solutions with transformers, the resulting capacitive power transfer ISOP multi-cell DC/DC converter (CPT-ISOP-MCC) system features lower losses and a smaller volume. In this paper, the benefits as well as the limitations in the design and operation of CPT-ISOP-MCC systems are analyzed in detail. In order to comprehensively evaluate the CPT against the magnetically isolated concept, i.e. inductive power transfer (IPT) converter topology, a multi-objective optimization is performed with respect to the achievable efficiency and power density for both types of converters. Based on the optimization result, a prototype of a CPT GaN DC/DC converter is realized and compared to its IPT GaN DC/DC converter counterpart, along with measurement results. Furthermore, a complete single-phase 3.3kWAC/DC converter system with a high power density of ρ = 3.92kW/dm3 and an efficiency of η = 97% is presented, incorporating the CPT-DC/DC converter stages in ISOP topology.