Michele Macellari

Loss Analysis of Low-Voltage TLNPC Step-Up Converters

Power losses in high-efficiency dc-dc step up converters based on the synchronous three-level neutral-point-clamped (TLNPC) configuration were investigated. TLNPC converters benefit from the reduced stress on components and from the noninsulated stacked-boost output stage to provide reduced power losses and large voltage gains. Several prototypes with increasing efficiency were produced and tested; voltage gains larger than 20× were achieved by means of hard-switched prototypes with composite switches consisting of both low-Rds(ON) and highspeed MOSFETs. At lower voltage gains conversion efficiencies exceeding 98% were demonstrated. A thorough loss analysis is reported, extended to subtle power dissipation processes, which in high efficiency converters grow in relevance after weakening of the major loss mechanisms. The related model is proven capable to accurately predict circuit performance in a wide range of operating conditions.

A techno-economical analysis of charge management in electrical vehicles

Several issues related to the prospected fast growth of the circulating Electric Vehicles are analysed. The environmental and economical topics are investigated first. Then several approaches for on-board charge management are reviewed and compared on the basis of efficiency, environmental impact and charging time. A particular effort is devoted to stress the potential benefits and limitations of Battery Swapping, and to outline the upgrades of Electric Vehicle design required to enable and promote its diffusion.

Dead time generator for synchronous boost converters with GaN transistors

An analog dynamic dead time generator for synchronous boost converters based on GaN transistors is presented. A prototype was designed to operate at switching frequencies in the MHz range, with wide variations of the output voltage, and experimentally demonstrated its capability to stabilize the dead time duration to a few nanoseconds, independent of wide variations of the switching times of the transistor in use.

On the Power System of the AMALIA moon rover

This work deals with the Electrical Power System of the Lunar Rover, developed for the AMALIA mission, intended to compete in the Google Lunar X prize. The main subsystems are described and their operation analyzed on the basis of the observed experimental behavior. Primary generation is provided by triple-junction solar cells, arranged as four sections, each connected to a dedicated Array Power Regulator with separate Maximum Power Point Tracking. A Switch Matrix allows to rearrange the inter-connections among the Solar Array sections and the APR in order to manage possible failures. During load peaks and/or with low Sun elevation the solar array is supported by a secondary generator consisting of three redundant units with Li-Ion battery cells. Two alternate configurations of the Battery Subsystem were developed: a classical 8S Li-Ion battery, for a 28V Sun-regulated bus, and a novel approach, based on a single-cell (1S) Li-Ion battery and a high-voltage-gain Battery Charge-Discharge Regulator. A specific converter was designed for this configuration, based on the non-insulated Three Levels Neutral Point Clamped switching converter topology. Its correct operation with bus voltages up to 48V and fully discharged battery cells was demonstrated.

Switching losses in low-voltage TLNPC step-up converters

High-efficiency dc-dc step up converters, based on the synchronous Three Levels Neutral Point Clamped configuration were built and investigated. The baseline circuit based on hard-switched, fast high-current MOSFETs provided high efficiency levels on a wide range of operating conditions. Several approaches to further improve performance were investigated. Very high efficiencies were experimentally demonstrated by an hard switched configuration where the switches consisted of a suitable arrangement of very-low RDSon and very fast MOSFET transistors.

Bidirectional converter for single-cell Li-ion batteries in a small space vehicle

A Battery Charge-Discharge regulator (BCDR) intended to interface a 12V bus with a single-cell Li-ion battery for power levels in the 100W range is presented. It may be useful in small space vehicles, and especially in the cases when battery sizing is driven by the need to complement the power provided by primary power sources during high peaks of load, with comparatively small energy amounts. In particular, the Bidirectional Three Level Neutral Point Clamped (TLNPC) converter configuration was selected, and a prototype was designed, built and tested. After providing a rationale of the single-battery-cell approach, and of the criteria adopted to select the TLNPC converter circuit, the paper reports the principles of operation of the converter and indicates the design criteria for the main circuit parameters. Extensive characterization of the prototype completes the work.

On the reliability of modular power conversion systems for small spacecraft

Modular power conversion systems for space application, thanks to their flexibility, can be easily adapted with minor changes to missions with different power requirements. In the perspective of an application to low-cost satellite, based on Commercial Off The Shelf components, the main design drivers are efficiency and reliability. We will focus our attention to spacecrafts with body-mounted solar cells. Thus, trade-off between the opposite requirements, efficiency and reliability, have been discussed through the analysis of several redundancy schemes. Reliability calculation of circuits will be performed on the basis of MIL-HDBK-217F. The aim of this work is to demonstrate that specific design efforts allow to increase the reliability of COTS-based power systems to levels comparable to classical systems based on spacequalified components.

A PV power generation system for missions to Mercury

A special architecture for photovoltaic generation of electricity has been studied for a mission to Mercury. This paper deals with solar panel design primary concerns, with regards to the harsh Hermian thermal environment and to the need for great amounts of power for solar electric propulsion at 1 A.U.. A novel concept of a combined system of photovoltaic generators together with Fresnel lens solar concentrators will be presented as a possible solution to the different power demands of the whole mission

Moderate accuracy relative navigation in formation flying by filtered radio measurements

Formation flying missions involve relative navigation, or the determination of the relative kinematic state of the spacecraft involved. Different options do exist, both in visible and conventional radio-frequency operations, with passive and active architectures. All the solutions need to take into account the strict requirements in terms of mass, volume and power typical to spacecraft, not to forget that they are not directly part of the payload, so that the overall impact of this avionic subsystem needs to be minimized. This paper aims to discuss a simple solution for the relative navigation of a formation, especially suitable for moderate cost, moderate performance missions (like the ones exploited by cube sats). A field strength meter onboard the chief-satellite can provide the level of received signal power, which is an observable related - even if with a poor quality - to the inter-satellite distance. An extended Kalman filter including a suitable dynamical model can exploit these measurements in order to estimate the formation kinematic state at a very limited cost. The combination of the two elements makes up for a technique which is accurate enough even in the phases immediately following the release from the launcher, where the spacecraft are in close proximity. The presented numerical simulation proves the valuable performance of such a simple system, also useful as a possible back-up solution at a limited cost in terms of spacecraft requirements.

Modular MPPT converter with series-connection for PV installations embedded in the urban environment

This Building Integrated Photo-Voltaics suffer for mismatching among the operating conditions of different regions of installations. Safety is another typical concern. In this work, we propose to face both issues by arranging the solar array in several sections provided with individual converters, capable to operate in the MPPT mode. Each converter is insulated, so that several converters are connected in series in order to reach the desired dc-link voltage, while maintaining the maximum voltage across the solar array within safety limits. The approach was tested by means of a demonstrator consisting of N=3 flyback converters. Each converter was equipped with separate MPPT. The proposed system was characterised under variable shading conditions. We verified that the dc-link voltage tends to distribute among outputs of single module converters proportionally to the respective input power. In the presence of high levels of shading it was advantageous with respect to other PV plant configurations.