Power supply and distribution unit design for a scientific exobiology facility in the ISS

Universidade do Minho Escola de Engenharia Carolina Barbosa Rua Power Supply and Distribution Unit Design for a Scientific Exobiology Facility in the ISS junho de 2022 UMinho | 2022 Carolina Barbosa Rua Power Supply and Distribution Unit Design for a Scientific Exobiology Facility in the ISS Carolina Barbosa Rua Power Supply and Distribution Unit Design for a Scientific Exobiology Facility in the ISS Dissertação de Mestrado Mestrado em Engenharia Eletrónica Industrial e Computadores Instrumentação e Microssistemas Eletrónicos Trabalho efetuado sob a orientação do Professor Doutor Luís Miguel Valente Gonçalves Rodolfo Manuel Maia da Cruz Martins Universidade do Minho Escola de Engenharia junho de 2022 DIREITOS DE AUTOR E CONDIÇÕES DE UTILIZAÇÃO DO TRABALHO POR TERCEIROS Este é um trabalho académico que pode ser utilizado por terceiros desde que respeitadas as regras e boas práticas internacionalmente aceites, no que concerne aos direitos de autor e direitos conexos. Assim, o presente trabalho pode ser utilizado nos termos previstos na licença abaixo indicada. Caso o utilizador necessite de permissão para poder fazer um uso do trabalho em condições não previstas no licenciamento indicado, deverá contactar o autor, através do RepositóriUM da Universidade do Minho. Licença concedida aos utilizadores deste trabalho Atribuição-NãoComercial-SemDerivações CC BY-NC-ND https://creativecommons.org/licenses/by-nc-nd/4.0/ i Acknowledgements I would not be writing these grateful words if it were not for some very special people whom I cannot fail to mention. My heartfelt thanks go to the phenomenal family that was gave to me, for all their unconditional love, patience and for both enabling me to fly and helping me to land. To my mom, who always believes in me and pushes me to be the bravest version of myself. To my dad, for all the perseverance and lessons that make me wiser every day. To my grandma, who is always a safe haven for me no matter what, your cuddles are irreplaceable. To my brothers, who taught me to play but also to fight, without you I would not be who I am. And to my godfather, who always lets me see the brighter side of life and gives me hope in times of despair. I would also like to show my sincere gratitude to the family that I chose to take with me for life. An outspoken thanks to my best and oldest friend, Catarina, for never leaving my side no matter what and knowing me better than I do myself. To the craziest sweetest friends that college could have gifted me with, Lidl, Dimitri, Piggy, Pipi, Xico, Salomé, Merkel, Ponto, Monte, Technão, Sofia and Inês, I cannot thank you enough for all these years of fellowship, you make me feel at home wherever we are. Thank you for waking me up over and over again, both to study and to party, for never letting me give up and for sharing all you had even when you didn’t have anything. A big thanks to Professor Luís Gonçalves, not only for accepting to be my thesis supervisor but also allowing me to express my doubts and fears honestly. And to my EVOLEO supervisor, Rodolfo Martins, and my team leader, Paulo Bento, for all the knowledge transmitted, for the availability and the opportunity. Finally, I would like to thank Carlos for being the best surprise gift that college life could have given me. Thank you for all the moments of true friendship, for all the times you make me laugh and for all the times you wipe away my tears. Thank you for loving me but leaving me wild. ii STATEMENT OF INTEGRITY I hereby declare having conducted this academic work with integrity. I confirm that I have not used plagiarism or any form of undue use of information or falsification of results along the process leading to its elaboration. I further declare that I have fully acknowledged the Code of Ethical Conduct of the University of Minho. iii Resumo Bartolomeo é a primeira plataforma comercial instalada no exterior da Estação Espacial Internacional (ISS), e tem o intuito de simplificar o acesso a experiências comerciais à estação. Cada uma das empresas ou instituições que se propôs a realizar experiências nesta plataforma terá de ver os seus equipamentos alimentados por uma unidade de fornecimento e distribuição de energia que assegure performance contínuo, fiável e duradouro. Assim, este documento analisa os requisitos elétricos do sistema e a arquitetura da unidade de fornecimento e distribuição de energia que será alojada na instalação de ciência exobiológica. Esta fonte irá lidar com a proteção contra sub e sobretensões e contra correntes de inrush e também com diferentes limitações de corrente, filtragem e regulações de tensão. Também irá incluir vários sensores de temperatura, que irão permitir monitorizar os pontos de temperatura mais críticos do dispositivo, e circuitos de condicionamento de sinal para efetuar as medições da tensão e corrente na entrada. A unidade foi desenhada com o intuito de controlar o estado das saídas, efetuar as medições dos níveis de tensão e de corrente em todas os equipamentos, e da temperatura tanto no lado de fornecimento como no lado de distribuição da energia e ainda comunicar via porta série com a estação espacial para efeitos de controlo. Como é obrigatório em todas as atividades espaciais europeias, o equipamento tem de estar em conformidade com os requisitos presentes em normas internacionais bastante exigentes. Por esta razão, esta dissertação apresenta duas análises elétricas que foram efetuadas para aumentar a probabilidade de sucesso e maximizar a segurança do sistema. Inicialmente foi realizada uma Worst Case Analysis , de modo a ser possível averiguar sobre a performance do circuito em condições de operação extremas. Posteriormente, foi efetuada uma Part Stress Analysis que examina o stress máximo aplicado a cada componente. Para demonstrar que a unidade de fornecimento e distribuição de energia dimensionada cumpre com os requisitos foram simulados todos circuitos tanto individualmente como em conjunto. Os resultados obtidos permitiram preparar os próximos passos de integração, teste e validação que antecedem a produção do modelo de voo da unidade. palavras-chave: PSDU, PSU, PDU, WCA, PSA iv Abstract Bartolomeo is Europe’s first commercial platform on the exterior of the International Space Station (ISS), intended to give commercially motivated experiments quick and simplified access to the station. Any instruments that are going to be installed to perform the experiments require a power supply and distribution unit (PSDU) to ensure continuous, reliable and long lasting performance. Thus, this document analyses the electrical requirements of the system and the design architecture of the PSDU to be deployed in the scientific exobiology facility of the platform. The power supply unit deals with over and under-voltage protection, inrush current protection, current limitation, filtering, and voltage regulation. It also includes temperature sensors to monitor the most critical temperature points in the device and a signal conditioning circuit for voltage and current measurements from the input. The power distribution unit was designed to be responsible for the output state control, the voltage and current measurements at all internal instruments, and the serial communication for control purposes. As mandatory for all European space activities, the equipment must be compliant with standard requirements. For that reason, this dissertation presents two electrical analyses that were performed to increase the probability of success and maximize the safety of the system. First, a Worst Case Analysis was performed to determine the performance of the circuit under extreme operating conditions. Then, a Part Stress Analysis was produced to examine the maximum stress applied to each part. The simulations of the electrical analysis show that the PSDU meets the analysed requirements. Based on the results obtained, it was possible to propose the next steps in the analysis and preparation for the integration, testing and validation phase before the production of the Flight Model. keywords: PSDU, PSU, PDU, WCA, PSA, v Table of Contents Chapter 1: Introduction ................................................................................................. 1 1.1. Contextualization ................................................................................................................. 1 1.2. Motivation ........................................................................................................................... 3 1.3. Objectives ........................................................................................................................... 4 1.4. Contributions of this Dissertation ......................................................................................... 5 1.5. Document Structure ............................................................................................................ 6 Chapter 2: State of the Art ............................................................................................. 7 2.1. Small Spacecraft Power Systems ......................................................................................... 7 2.1.1. Power Management and Distribution ............................................................................ 8 2.2. Space Concepts .................................................................................................................. 9 2.2.1. Worst Case Analysis................................................................................................... 11 2.2.2. Parts Stress Analysis ................................................................................................. 12 Chapter 3: System Specification .................................................................................. 16 3.1. Main System Requirements ............................................................................................... 17 3.1.1. Electrical Requirements ............................................................................................. 17 3.1.2. Mechanical Requirements .......................................................................................... 21 3.1.3. Embedded Software Requirements ............................................................................ 21 3.2. Preliminary Design ............................................................................................................ 21 3.2.1. General Description ................................................................................................... 21 3.2.2. PSU Functional Description ........................................................................................ 23 3.2.3. PDU Functional Description ....................................................................................... 25 Chapter 4: Detailed Design .......................................................................................... 27 4.1. PSU Detailed Description ................................................................................................... 27 4.1.1. Electrostatic Discharge Protection .............................................................................. 27 xii PA product assurance PCB printed circuit board PDU power distribution unit PSDU power supply and distribution unit PSU power supply unit PSA part stress analysis PMAD power management and distribution system SM science module SPF single point failure SPI serial peripheral interface UART universal asynchronous receiver/transmitter VCD verification control document VDC volts direct current UVP under-voltage protection WCA worst case analysis 1 Chapter 1: Introduction The work presented in this dissertation is a small part of a much broader space mission aimed at raising the opportunity to all Member States of United Nations and developing countries to accommodate and operate payloads on the Airbus Bartolomeo external platform on the International Space Station (ISS). Where the particular focus here is on the power supply and distribution unit that will manage and deliver power to these payloads. In this first chapter, the introduction to this dissertation is presented through the contextualization, motivation and objectives of the work developed. The contextualization outlines the space mission within which the thesis is developed. Building on this, the motivations for developing this project are presented, supporting the author's choice of this topic, and giving prominence to its relevance to current paradigms. With the aim of reach a well-founded outcome, the objectives are specified and explained to structure the work and lead to the desired result. Since this project was conducted in a business context, there is a section introducing the company that set the author on this path. To guide the reader through the text, this chapter ends with the structure of the present document. 1.1. Contextualization The International Space Station consists of pressurized modules, external trusses, solar arrays, and many other components. It serves as a microgravity and space environment research laboratory in which crewmembers conduct experiments in numerous scientific disciplines: astronomy, biology, physics, and numerous microgravity research disciplines. It may also be utilized for the testing of spacecraft systems and equipment required for missions to the Moon and Mars. Since November 2000, the ISS has been continuously occupied, which is the longest continuous human presence in space. It is a joint project among five participating space agencies: NASA, State Space Corporation (Roscosmos), JAXA, ESA, and the Canadian Space Agency (CSA), as depicted in Figure 1.1 [1]. However, space is hard for most organisms, and thanks to a series of exposure facilities outside the station, researchers know how hard it is by analysing organic samples that have been exposed to solar and cosmic rays over time. Researchers have already found life that can survive space flight. Both lichens and small organisms called tardigrades 2 or 'water bears' have spent months outside the International Space Station and returned to Earth alive and well [2]. Now the European Space Agency (ESA) is looking to take exobiology, the study of life in space, to the next level with a new experimental facility, the ESA’s Exobiology Platform (EXPO). This facility is going to be the first commercial platform to be installed on the Bartolomeo payload hosting platform outside the European Columbus module on the ISS, as shown in green in Figure 1.1 and amplified in Figure 1.2. Is intended to give commercially motivated experiments quick and simplified access to the station. Researchers from a great variety of fields and small and medium-sized enterprises (SMEs) will benefit from this new platform since there are unique possibilities that cannot be achieved in any laboratory on Earth. The experiments they can do in the platform payloads will be considerably more cost-effective to conduct on Bartolomeo than it would be on satellites, as they do not require their own rocket launch but rather are accommodated on routine supply flights to the ISS. The platform offers its users an unobstructed view of Earth, direct control over experiments from the ground and the possibility of retrieving samples [3]. ESA's exobiology platform is equipped with a series of radiation experiments aimed at better understanding the evolution of organic molecules and organisms in space. Placed in a slot facing Zenith, the point in the sky directly above the observer’s head, the facility will connect two science modules (SM) to Bartolomeo. These modules will house everything needed for the experiments, including science Figure 1.1 – International Space Station expanded view [1] 3 sample containers, fluidic systems, and sensors for the experiments [4]. At the end of the three-year mission, the samples will return to Earth for in-depth study and analysis. Although, the unprotected stay in space is characterized by an intense radiation field, high vacuum, extreme temperatures, and microgravity. So, there are numerous challenges that electronic components must withstand without compromising their performance. 1.2. Motivation Space and the development of technologies associated with it are now recognized by several nations as a national driver essential to the advancement of a country's social and economic progress and to international security. More than 15 years after joining the European Space Agency, Portugal is considered a success due to its rapid adaptation and integration into space programmes. The national prestige already achieved requires Portugal to position itself in the near future as a true space nation, capable of taking on the new challenges of the sector [5]. Some of the greatest scientific challenges in this area are understanding our own origins and evolution, and more reliably assessing the possibility of life outside our solar system. The intellectual impact of these discoveries is likely to be as great as that of molecular biology. The science of exobiology seeks to reconstruct the natural history of the processes and events involved in the transformation of biogenic Figure 1.2 – Bartolomeo located on the ram-side of the Columbus module at the front of the International Space Station [3] 4 elements from their origins in nucleosynthesis to their participation in Darwinian evolution in the solar system on planet Earth. This reconstruction will make it possible to develop a general theory for the evolution of living systems from inanimate matter. Despite the demanding nature, engineers operating in this field are intrinsically highly motivated, due to the nature of the task, the mission they are involved in, and the idea that their accomplishments can one day make a difference to society. The acceptance of these engineering challenges led the author to EVOLEO and consequently to the choice of this project to obtain the master’s degree. 1.3. Objectives The goal is to analyse a power supply and distribution unit that will convert, manage, and distribute power from the Bartolomeo platform to the different payloads and ensure its correct performance throughout the mission. To achieve this, there are some clear objectives to pursuit: • System specification This objective is achieved by analysing the main system requirements of the downstream and upstream sides of the PSDU to ensure that the product is developed to meet the project needs. These requirements are divided into electrical, mechanical, and embedded software, but only the ones that involve electrical concepts are analysed. To conclude the preliminary analysis, it is important to analyse the European Cooperation for Space Standardization (ECSS) standards and the ISS requirements that should be shared by management, engineering, product assurance, and sustainability teams in space projects and applications. Based on the results of the previous objective, a general overview of first top level system definition is done. The electrical design of the PSDU follows a building-block approach so that each step forms the foundation for the subsequent design step on the path of least resistance to the desired outcome. The power supply is divided into distinct blocks that are described in a modular fashion, so the result is a coherent, logical design flow where the unknows are minimized. • Detailed design In order to make a proper electrical analysis of the entire system and check that all parts fit and are correctly sized, time must be allowed in learning and understanding each block of the PSDU. To 5 summarise all the concepts learned, a detailed description of each circuit is made as well as some representative diagrams of the circuit. • Electrical analysis By considering component variability, a Worst Case Circuit Performance Analysis (WCCA) is presented to determine the performance of the circuit under extreme environmental or operating conditions. Following the above goal, to increase the probability of success of the system, a Parts Stress Analysis (PSA) is performed. This analysis involves examining the electrical circuits to determine the maximum stress on each part when all applied voltages or currents are maximised and when all variations of other parts in the circuit are set to the combination of minimum and maximum values that yields the worst-case maximum stress. Besides the execution of these analysis, the methods and requirements followed to perform them are also covered. 1.4. Contributions of this Dissertation The project was developed while working in the Portuguese branch of the company EVOLEO Technologies, Lda. (logo in Figure 1.3) for a client company. As the author has meet confidential information of high commercial value, which the company wishes to protect in order to keep the competitive advantage on its side, a non-disclosure agreement has been signed. For these reasons, the author is not allowed to publish some points of his work. In this work, the PSDU was designed by Rodolfo Martins, the CEO (Chief Executive Officer) of the company and an experienced electrical engineer, who gave the author of this work, the opportunity and responsibility to perform the full electrical analysis of the entire circuit. Figure 1.3 – EVOLEO Technologies logo 6 1.5. Document Structure The work developed in this dissertation is organized into six different chapters to facilitate the understanding and progression of the project. The Introduction of the thesis is the first chapter, here the circumstances that led to the reliability analysis of the power supply and distribution unit developed are presented, as well as the contextualization, the motivations, and the objectives. Chapter two presents the State of The Art, where an overview of Small Spacecraft Power Systems is presented, along with some theoretical concepts about Dependability Requirements for Space Systems, Worst Case Analysis and Part Stress Analysis. The System Specification is on the third chapter, it refers the Main System Requirements and the Preliminary Design of the System. Chapter four describes the Detailed Design of the PSDU and is divided into the PSU Detailed Description and the PDU Detailed Description. The Electrical Analysis performed are explained on chapter five, the Worst Case Circuit Analysis and the Part Stress Analysis. Finally, the chapter six ends this dissertation with the Conclusions of the developed work and the description of the Future Work of this project. 7 Chapter 2: State of the Art In this chapter, several key aspects of the proposed implementation are presented, starting with more general concepts such as the power systems that have been marketed for space projects in recent years, and then move down to the heart of the problem which is the application of analysis and design methods to ensure that the reliability goals of a space project are met. 2.1. Small Spacecraft Power Systems Since 94 % of all spacecraft launched in 2020, were small spacecraft, weighing less than 600 kg, most of the current open literature on electrical power systems for space applications relates to small satellites, or SmallSat for short [6]. Yet, the overall electrical system required for such small devices has some similarities to that of a larger spacecraft. For this reason, the goal of this section it to assess and provide an overview of the state of the art in small spacecraft technologies with focus on the engineering requirements and processes for choosing a device. Although the location of the equipment is not familiar, an electrical power system (EPS) for a spacecraft encompasses the usual, electrical power generation, storage, and distribution. Being the most important and fundamental subsystem, it usually occupies a large part of the volume and mass of any spacecraft. The technologies commonly used for power generation in space include solar energy through photovoltaic cells, panels, and arrays, as would be expected, but may also involve radioisotope or other thermonuclear power generators. The generated power is typically stored in batteries; either single-use primary batteries, or rechargeable secondary batteries. Since the manage and distribution of the power is the subject of this dissertation, the focus of this section will also be the state-of-the-art technologies for this purpose. To facilitate the control of the power supplied to the loads, the power management and distribution (PMAD) system of a SmallSat is often custom designed to meet specific mission requirements. However, it is important to note that there are fundamental differences when designing it with commercial-off-theshelf components (COTS) or with space qualified parts. While military or space (MIL/QML) parts need to pass a great variety of meticulous tests, and, for example, be qualified to survive from -55°C to 125°C, 8 COTS usually go through a more flexible test package, requiring operational function for a small temperature range, -40°C to 85°C. Unlike what happens with the PSDU for the Bartolomeo module, subject of this dissertation, most of these small satellites do not need to be prepared for the harsh environments that the military parts are designed to endure. Despite of not having to pass, for example, radiation or reliability tests that the MIL/QML qualification process involves, COTS parts, typically perform better than space rated parts, being several generations in advance, more available and having a faster revision timeline. COTS make sense for SmallSats also because the missions they are involved in have more flexible qualification standards, require a lower Technology Readiness Level (TRL) and typically happen for shorter periods of time, in more favourable environmental conditions. 2.1.1. Power Management and Distribution The information described below is not intended to be exhaustive but provides an overview of current state-of-the-art technologies and their development status for a specific small satellite subsystem, the power management and distribution system. Despite sometimes being custom designed for specific power requirements, several manufacturers have introduced in the market some PMAD devices for inclusion in small spacecraft missions. Besides distributing the power from energy sources, they also condition energy, mitigating harmful transient disturbances and failures from propagating downstream and harming the connected loads. Some of the most popular PMAD systems manufacturers, that produce these systems for small spacecraft, are Pumpkin[7], AAC Clyde Space [8]–[10], GomSpace [11] and ISISPACE [12]. Although not intended to be exhaustive, a list, with some of the products these companies sell, is presented in Table 2-1 and shown in Figure 2.1. All of these companies have flight heritage, which means that they produced Table 2-1 – Description of products from Pumpkin [7] , AAC Clyde Space [8]–[10] , GomSpace [11] and ISISPACE [12] of power management and distribution systems *Available with Inquiry to Manufacturer 9 components and systems that have already successfully been employed in a low-Earth orbit (LEO) mission. When analysing the available options of PMAD in the market there are some critical metrics to consider for a good design, such as input/output voltage range, conversion efficiency, output power capabilities, and size, weight and power (SWaP). The designer has to leverage the benefits and risks to the mission when choosing PMAD, yet it must be noted that there is no single COTS solution that can meet all the requirements of a mission at hand, since most devices sacrifice specific features that can impact important metrics for SmallSats. 2.2. Space Concepts To provide a framework for the work developed, this subsection explains the dependability requirements for space systems, defined by the European Cooperation for Space Standardization (ECSS) [13] and some theoretical concepts about the electrical analyses that are presented in chapter 5. A continuous and iterative dependability assurance process must be carried out throughout the project lifecycle. Figure 2.1 – Photographs of products from Pumpkin [7], AAC Clyde Space [8]– [10], GomSpace [11] and ISISPACE [12] of power management and distribution systems 16 Chapter 3: System Specification As mentioned in the introductory chapter of this document the Exobiology facility is designed with a modular fashion and equipped with an array of instruments. The facility has a common module (CM), which includes the interfaces with the external platform and the power, data and thermal control systems, and a scientific module (SM), which contains the biological samples and the necessary scientific instruments. The scientific module operates four experiments, namely OREO cube, ExocubeBio, ExocubeChem and IceCold. The part that is the subject of this dissertation is the power supply and distribution unit, which, together with a main control unit developed by another company, forms the electronics box of the scientific module. The initial architecture of the Electrical Power System (EPS), which was made as a requirement is shown in Figure 3.1. In the first phase of this project, the project initiators elaborated the mission concept in terms of identifying and characterising the mission requirements, mentioned in this chapter, the expected performance, the reliability and safety objectives, and also the operational constraints of the mission in terms of the physical and operational environment. As a first response, EVOLEO produced a top-level design of the power supply and distribution unit, which was adapted and summarized by the author to be inserted and described later in this chapter. Accompanying the design, a general overview of the diagram was made, as well as a description of the power supply area and the power distribution area separately. Figure 3.1 – Potential electrical power system architecture 17 3.1. Main System Requirements When the project initiators provided the mission concepts, they also sent a list of applicable documents to be analysed together with the project-specific requirements. Some of the applicable documents are confidential and cannot be mentioned here. However, others, such as the European Cooperation for Space Standardization (ECSS) standards, are public and available online to reduce the risk of costly problems during development and operation; these are presented below. • Space Product Assurance: Failure modes, effects (and criticality) analysis (FMEA/FMECA) [15] • Space Project Management: Risk management [16] • Space Engineering: System engineering general requirements [17] • Space Engineering: Verification [18] • Space Engineering: Thermal control general requirements [19] • Space Engineering: Structural general requirements [20] • Space Engineering: Structural design and verification of pressurized hardware [21] • Space Engineering: Materials [22] • Space Engineering: Software [23] Due to the length of these documents, only the requirements that the client considered most important are mentioned this chapter. However, they have all been analysed to produce a compliance matrix containing the actual information on the compliance of the requirement, i.e. whether the requirement is met or not and, in case of deviations, the justification. In addition to the ADs, as mentioned earlier, a list of project-specific requirements was also created, divided into five major technical areas: electrical engineer, embedded software engineer, mechanical engineer, product assurance and systems engineer. Some of the requirements can be part of two domains at the same time, so due to the nature of this dissertation, only the requirements that relate to electronic concepts will be analysed. 3.1.1. Electrical Requirements The electrical requirements are listed below, but for ease of understanding, this list has been compiled. 1. The unit shall be able to provide an overall power of 70 W. 18 2. The unit shall be designed for a nominal input voltage of 28V (+1%/ -10%). 3. The input filter charge time shall be lower than 80% of the LCL (latching current limiter) class minimum trip off time. The upstream LCL is class 4. 4. The converter shall provide outputs compliant with the Table 3-1: 5. The output voltages shall provide power to the loads according to Table 3-2 and Table 3-3. Note: The output switches shall have an inrush/soft start control. 6. The scientific module (SM) shall not generate primary power reverse current. 7. Primary power input lines (28VDC hot and return lines) shall have an insulation resistance ≥ 1.0 MΩ between each line and between the PSDU and the common module (CM). 8. The LCL design shall be compliant with the ECSS-E-ST-20-20C – Space engineering: Electrical design and interface requirements for power supply. Table 3-1 – Output voltages and power Table 3-2 – Output current in Va Table 3-3 – Output current in Vb 19 9. The generated secondary voltages (hot and return lines) shall have an insulation resistance ≥ 1.0 MΩ between each line and 28Vdc lines provided by the common module. 10. The broadband voltage emission at the SM power inlet (28 Vdc) shall meet the requirement number 4.1.5.2 in the standard Columbus EMC & Power Quality Requirements [24]. This requirement states the following: “Equipment/assembly generated broadband voltage ripple on primary DC power buses shall not exceed 300 mVpp/A. The limit is adjustable, depending on I(A). The conversion rule for voltage emission limits in (mVpp) is: 300 m Vpp ∗ √I(A) or 2 Vpp whichever is lower.”. 11. The load transients at power inlet (28 Vdc) shall meet the requirement number 4.1.5.3 in [24] that states the following: “Both positive and negative transients on primary DC power buses shall not exceed the envelope defined in Figure 3.2 with maximum Pulse Repetition Rate P.R.R. = 10 Hz. Higher P.R.R. must be covered by limit in requirement 4.1.5.2. Time duration of the pulse is evaluated at 10% of transient amplitude. 12. The inrush current at power inlet (28 Vdc) shall meet the requirement number 4.1.5.4 in [24], that states the following: “The rate of change of current surges shall not exceed 0.1 A/µs. Note: The inrush current requirement is not fulfilled by standard power outlets (requirement: > 10 ms and capability: > 1.5 ms).” 13. The unit shall be able to sustain without incurring permanent damages the sine wave injection at primary power inlet (28 Vdc) as requested by the requirement 4.1.7.1 in [24] which states: “The equipment/assembly shall not exhibit any malfunction, degradation of Figure 3.2 – Transient Emission Limit (Equipment & Primary Power Bus) [16] 20 performance or deviation from specified parameters beyond tolerances given by the corresponding specification when a sine voltage is injected into the primary DC power lines with amplitudes as indicated in Figure 3.3.”. 14. The unit shall be able to sustain without incurring permanent damages the sine transient injection on SM1 primary power inlet (28 Vdc) as requested by the requirement 4.1.7.2 in [24], which states: “The equipment/assembly shall not exhibit any malfunction, degradation of performance or deviation from specified parameters beyond tolerances given by the corresponding specification when levels of Figure 3.4, plus 6 dB (or minus 6 dB if negative) are injected into the primary power lines. The width of the pulses, to be injected shall be at least, T = 10 µs and T = 0.15 µs 15. The unit shall comply with requirement number 4.1.9 in [24], that says: “No malfunction, degradation of performance or deviation from specified parameters beyond tolerances given by the corresponding specification shall occur when equipment and interface lines are Figure 3.4 – Peak surge current amplitude versus steady-state input current [16] Figure 3.3 – Conducted sine wave susceptibility (equipment & system) [16] 21 exposed to a repetitive electrostatic arc discharge of at least 5.6 mJ energy/ 15kV. The test must be performed with conducted and radiated ESD sources”. 16. All internal electrical wires shall be made of polyimide insulation as per European Space Components Coordination [25]. 3.1.2. Mechanical Requirements 1. The power distribution unit (PDU) shall operate and fulfil all performance requirements within a temperature range from -25 to +60°C. 2. PDU shall survive in non-operational mode within a temperature range from -30 to +80°C. 3.1.3. Embedded Software Requirements 1. The supplier shall provide the telemetry/telecommand (TM/TC) interfaces. Note: At least: Vin (Input voltage), Iin (Input current), Va, Vb and Ia, Ib (voltage and current on the single), DC-DC outlets, LCL status and trip status, status switch, inrush control, TM/TC preferred to be transmitted digitally, via RS-232 or similar. 3.2. Preliminary Design The scope of this section is to provide a brief technical description of the system solution presented of the power supply and distribution unit (PSDU) unit to be integrated into the scientific module. 3.2.1. General Description The PSDU receives primary power from the Bartolomeo facility and generates the necessary secondary voltages for the operation of the module instruments. In addition to the secondary voltage generation, the PSDU has the ability of enabling and disabling individual output lines. The block diagram of the PSDU is shown in Figure 3.5. 22 The PSDU can be split into two main blocks, the primary Power Supply Unit (PSU) and the secondary Power Distribution Unit (PDU). They are composed by the following set of features: Primary PSU: • Power input range: 24Vdc-32Vdc • Input Latched Current Limiter (LCL) • Input over-voltage protection (OVP) • Input under-voltage protection (UVP) • Over-temperature protection (OTP) • High voltage transient protection • Input soft-start • Input isolated voltage and current measurement • Sequenced and synchronized 2x 28V DCDC converter units Secondary PDU: • Load control circuit - DCDC protection • Secondary under voltage protection (UVP) • 10x outputs with +12V • 12x outputs with +5V • 2x LCL for +5V outputs • 2x LCL for +12V outputs Figure 3.5 – PSDU functional block diagram 23 • 22x switchable outputs • PSDU housekeeping functions (include primary): o 3 voltages o 7x currents o 2x temperatures • >12bit Analog-to-Digital Converter (ADC) resolution • Microcontroller Unit (MCU) • Serial universal asynchronous receiver-transmitter (UART) interface over RS-422 Safety and Protections: • 28Vdc Input Power o protected by LCL (4,5A) o discrete monitoring, command & control lines optically isolated • Secondary Output lines o galvanic isolated 28Vin DCDCs (single 12Vdc and single 5Vdc) with auto-current limitation o outputs protected by LCLs 3.2.2. PSU Functional Description The power supply unit (PSU) part of the system was designed to serve as the interface between the Bartolomeo PSU and the power distribution unit (PDU) in charge of dealing with over-voltage and undervoltage protection, inrush current protection, current limitation, filtering, and voltage regulation. It also included a temperature sensor to monitor the most critical temperature point in the device and a conditioning circuit for voltage and current measurements of the input, as is shown in Figure 3.6. More specifically, it was established that the power supply starts with a single input connector for the input power from the Bartolomeo power supply. Immediately after this connection, a transient absorption zener, was placed to protect the circuit from harmful transients and to shunt the excess current when the induced voltage exceeded the avalanche breakdown potential. Also, as a protective measure, a complex latching current limiter circuit was developed to, not only limit the current to a defined threshold, but to protect against over and under-voltages as well. In addition, as required, soft-start capacitors were added to reduce inrush current by up to 70% and smooth the current limiting. Two external signals were connected to the above circuit to make use of its latching function. One was connected to an over-temperature protection circuit that works with the temperature measured 24 at the primary side, and the other (PDU UVP in Figure 3.6) was connected to an isolated signal from the under-voltage protection circuit that uses the voltage levels at the PDU. Additionally, the measured voltage and current values from circuits that sense the input voltage and the current after the LCL were connected to an optic isolation circuit that subsequently sends them to the microcontroller unit. Upstream of the voltage regulation, an electromagnetic interference filter was connected to reduce the susceptibility of the power supply of being affected by electrical noise on the supply rails. Furthermore, it helps in reducing the propagation of conducted noise generated within the power supply back into the power rails. Before the DC-DC converters, a linear voltage regulator of 20 V was used to provide a stable power supply voltage for them, independent of the load impedance, input voltage variations and temperature. Because the DC-DC converters need to start at a well-defined order, a time-based control was used to deliver the enable signal to each converter. Then the converters take the nominal 28Vdc input voltage and convert it to the +5V and +12V that the PSDU outputs, which are galvanically isolated from the primary voltage source. Figure 3.6 – PSU Functional Block Diagram 25 3.2.3. PDU Functional Description The power distribution unit, as shown on the right side of Figure 3.5 and Figure 3.7 detailed, was designed to be responsible for the output state control, the voltage and current measurements, the connection to the external temperature sensors, and the serial communication for control purposes via the external scientific module. This unit takes the converted voltages of +12V and +5V and runs them through common mode filters to eliminate common mode noise that may have occurred on the DC-DC outputs. Since the selected DC-DC converters always require at least 10% of the power to be stable, a circuit was designed to generate a Figure 3.7 – PDU Functional Block Diagram 32 of resistors on the adjust pin and using capacitors on the output to aid in regulation stability. The representative diagram of this regulator and of the DC-DC sequence control circuit, can both be seen in Figure 4.5. 4.1.4. DC-DC Sequence Control The DC-DC converters chosen, have an inhibit terminal that is used to disable the internal switching, resulting in no output and very low quiescent input current at the 5 V converter. It was implemented a timer sequencer circuit responsible for delaying the start of the 5 V DC-DC converter. This circuit is based on a high threshold and two different RC delay elements. Two individual comparator circuits and two different but well-known RC time constants, allowed to trigger two individual start signals. 4.1.5. DC-DC Converters High voltage conversions in space applications are in majority DC-to-DC voltage converters [27]. The converters used for this project are constant frequency, pulse-width modulated switching regulators which used a quasi-square wave, single ended, forward converter design. Tight load regulation was maintained by using a wide bandwidth magnetic. Figure 4.5 – Basic representative diagram of the low-dropout regulator and the DC-DC sequence control circuit 33 For the PSU to produce the required output voltage levels, the input voltage of 28 Vdc was so stepped down by 2 DC-DC converters that, by means of switching the input power before an internal transformer, power rectifying and filtering on the output, delivered the +12 V and +5 V with the appropriate output current. The two converters are represented in Figure 4.6. The chosen converters provided short circuit and overload protection by constant current-limit feature. This protective system senses current in the converter’s secondary stage and limits it to approximately 115 % of the maximum rated output current. The switching frequency of these converters was 550 kHz, and the 12 V DC-DC provided the switching synchronization pulses to synchronize with the 5 V converter. Synchronizing the converter with the system clock allows the designer to confine switching noise to clock transitions, minimizing interference, and reducing the need for filtering. In sync mode, the converter will run at any frequency between 500 kHz and 675 kHz. The sync control operates with a quasi-TTL (transistor-transistor logic) signal at any duty cycle between 40 % and 60 %. This limited the used of pulse width modulation (PWM) to high frequencies, avoiding the Bartolomeu PSU critical frequencies between 1 and 3 kHz. 4.1.6. Voltage, Current and Temperature Measurement To account for the input power being used, there were implemented circuits to measure the voltage and current at the input of the DC-DC converters. Because these measurements are read by an ADC common to many other signals, the range of the signals had to be taken into consideration, as well as the necessary resolution error. Figure 4.6 – Basic representative diagram of the DC-DC converters circuit 34 In the case of the voltage measurement, a simple voltage divider, with the appropriate ratio was used and the range for this measurement was from 23 V to 33 V. A representative diagram of this measurement circuit is shown in Figure 4.7. For the current, the measurement was made with the shunt resistor that was also used for the LCL. Because the voltage drop on this resistor is proportional to the current that the load was using, this signal is then amplified by an instrumentation amplifier, passed through a multiplexer, and then digitized on the ADC with the range for this measurement being from 0 A to 10 A on the input. It is possible to see a representation of the instrumentation amplifier in Figure 4.8. Figure 4.7 – Basic representative diagram of the input voltage measurement circuit Figure 4.8 – Basic representative diagram of the input current measurement circuit 35 For an internal measurement of the hot spots, a temperature transducer was used, whose output is a current value proportional to the absolute temperature, with a well-known and precise temperature to current ratio. The output is afterwards converted into voltage in a precision resistor and measured on the ADC (Figure 4.9). 4.1.7. Over-temperature Protection The over-temperature protection (OTP) circuit, responsible for triggering the LCL, is based on a circuit that compares the temperature read by a temperature sensor with the specific value set by a resistor divider. Figure 4.9 – Basic representative diagram of the PSU temperature sensor circuit Figure 4.10 – Basic representative diagram of the over-temperature protection circuit 36 The thermal reference point is located between the two DC-DC converters, so that if this point reaches +95,8 °C the comparator triggers an output that trips the LCL, shutting down the PSDU. The representative diagram of the circuit can be seen in Figure 4.10. 4.1.8. Isolation Optocouplers The PSDU specifications requires the primary and the secondary powers to be isolated from each other. This is naturally accomplished at the DC-DC converters interface. However, it was necessary to perform voltage and current measurement on the primary side and pass the signal to the MCU on the PDU. To guarantee the proper isolation while maintaining these functionalities, optical isolators were used to condition the signals from the primary side (PSU) to the secondary side (PDU). The optocoupler, represented in Figure 4.11, consisted of a LED optically coupled with two photodiode detectors. The photodiode on the input side of the device acted as feedback to the LED to maintain constant LED current light output. The output photodiode of the device drove the output circuit and maintained electrical isolation. This device was hermetically sealed and maintained a linear operation in input-output optically coupling. The isolation was achieved using a unity gain amplification stage for high linearity as described in Figure 4.11. 4.2. PDU Detailed Description The power distribution board was designed to contain the processing unit for controlling the various output switches, controlling a multiplex chain and an ADC for acquiring the several measurements of voltage Figure 4.11 – Basic representative diagram of the isolation circuit 37 and current, as well as a communication interface for the external control unit. Also, integrated into the PDU are common-mode filters for each DC-DC output voltage and a load control circuit responsible of generating the minimum voltage required by the DC-DC to operate. Similarly, circuits for measuring voltage, current and temperature, as well as a protection circuit against under-voltage, have been added to the PDU. 4.2.1. Common Mode Filters The common-mode current is present on both signal and return paths, often in common phase to each other and is usually generated by imbalances in the circuit such as ripple or electrical noise at the output of the DC /DC converters. To filter this noise, two filters, were placed on secondary side connected to the DC-DC converters. Common-mode filtering involved capacitors to ground the high-frequency noise to the structure earth, a current compensated coil and a larger capacitor for energy storage and lower output impedance. 4.2.2. Load Control Circuit When the LCL turns on the power to the DC-DC converters primary side, the control unit might take some time to start pulling enough current in the output. So, to achieve a stable behaviour there is the need to provide at least 10% of the sourced output on the secondary side of the converter. Figure 4.12 – Basic representative diagram of the load control circuit 38 As so, a circuit to control the value of a resistor, like the one in Figure 4.12 was used to pull current out of the first converter. When the current being pulled by the scientific instruments is enough, this circuit is switched off. 4.2.3. Voltage, Current and Temperature Measurement Similarly, to the PSU, for the voltage measurement, a simple voltage divider with the appropriate signal conditioning was used before the multiplexer input. For the temperature measurement of the internal temperature on the PDU side, a current to voltage conversion based on a temperature sensor was used with the output on the normalized range. For the current measurement on the secondary side, a sensing resistor, and a fixed gain IC with a wide common mode voltage and external reference, for bi-directional measurement capability are used. As the Figure 4.13 – Basic representative diagram of the output current measurements 39 gain was 50, the output was selected by choosing the appropriate shunt resistor. The use of this simple integrated circuit saves board space while maintaining performance and accuracy of the measurements. The typical application circuit for this specific IC is depicted in Figure 4.13. 4.2.4. Low-dropout Voltage Regulator of 3,3 V A low-dropout voltage (LDO) regulator of 3,3 Vdc was chosen to supply the internal microcontroller unit, which is a low current load. The linear regulator was the best solution, especially because the LDO can be supplied by the existing 5 V. If a DC-DC converter was chosen instead of a LDO the risk to get problems with minimum load would be higher. 4.2.5. Voltage Reference of 2,5 V To have a common reference for all the ADCs, an external, high precision and stable 2,5 V reference IC is used. This reference voltage was also used on the signal conditioning where a shift was necessary, and enabled a reference measurement by the MCU, using a different reference of 12 V, to detect any deviation between the two references, as per the Figure 4.14. Figure 4.14 – Basic representative diagram of the voltage reference used in the PDU 40 4.2.6. Multiplexers and Analog-to Digital Converters By choosing only two analog-to-digital converters (ADC) there was the need to use multiplexers integrated on the ADC IC with 15 analog channels. The input selection is done by the same SPI communication used for data. It is possible to analise the ADCs used on Figure 4.15. To allow a lower error and high-resolution digitalization of the analog signals, a 12-bit sample and hold with a maximum of 1 MS/s was used. The data from the digitization is sent via SPI interface to the MCU that iterates the conversion data from all the ADCs on the same SPI bus. Figure 4.15 – Basic representative diagram of the multiplexer circuit with A/D converter 41 4.2.7. Signal Conditioning V/I/T This signal conditioning is used to maximize the use of the dynamic range of the ADC 1 input. As per design, the output of this signal conditioning should be as close as the range of 0 to 2,5 V. It was not always possible to amplify all signals to the full range of the ADC, but the requirements for resolution and accuracy were fulfilled. The schematic of the circuit is displayed on the Figure 4.16. For the ADC 2, depicted in Figure 4.17 responsible for the signal conditioning circuit of the current measurements, another circuit was used to buffer the signal before the ADC. The previous stage before the multiplexers should also get the use of the maximum range of the ADC 2 input of 0 to 2,5 V. Figure 4.16 – Basic representative diagram of the ADC 1 signal conditioning circuit Figure 4.17 – Basic representative diagram of the ADC 2 signal conditioning circuit 48 5.1.2. Isolated Primary Side Current Measurement The results of the simulation of the current measurement circuit in the PSU side, Figure 4.8 , connected to the opto isolation circuit, Figure 4.11 can be seen in Figure 5.7. An independent current source with a sine wave function was placed as a swinging 10 A load, its measurement can be seen in blue (I1) and an independent voltage source with a sine wave function was placed to simulate a swinging input voltage from 0 V to 32 V, represented in red. The graphic from Figure 5.7, demonstrates that the circuit is immune to voltage variations up to the point when Vin is too low to provide a stable behaviour. After the instrumentation amplifier the output voltage, pink line, vout, is acquired by the ADC so a value of 2,45 V for a 10 A load is the expected as per equation: 𝑉 𝑜 = 𝑅3 𝑅2𝑥 (2 𝑥 𝑅1 + 𝑅𝑔 𝑅𝑔 ) 𝑥 (𝑉𝑖1 − 𝑉𝑖2) = 2,45 𝑉 Figure 5.6 – Latching current limiter simulation with a steady input voltage of 32V and an over-current situation Figure 5.7 – Primary side current measurement simulation 49 5.1.3. Isolated Primary Side Voltage Measurement The primary side voltage measurement circuit, represented in Figure 4.7 was simulated together with the isolation circuit of Figure 4.11 and the results are shown in Figure 5.8. The same independent voltage source with a sine wave function was placed to simulate a swinging input voltage from 0 V to 32 V, also represented in red. The output voltage obtained before the optic isolation, Vout1 and after the optic isolation Vout2 of 3,77 V for 32 V is expected as per the equation: 𝑉 𝑜 = 𝑅2 𝑅2 + 𝑅1 𝑥 32 𝑉 = 3,77 𝑉 5.1.4. DC-DC Sequence control The simulation of the DC-DC sequence control circuit of Figure 4.5 was done together with the 20V voltage regulator to confirm that the delay at the start of the 5V DC-DC was accomplished with a value of approximately 35 ms. As can be seen in Figure 5.9, after 34.33 ms the voltages at the inputs of the operational amplifier becomes equal (vin+ = vin- = 17.80V) and so the output voltage (vout) drops to 0V enabling the DCDC. Figure 5.8 – Primary side voltage measurement simulation Figure 5.9 – DC-DC sequence control circuit simulation 50 5.1.5. Load Control Circuit In order to protect the PSDU at power ON, from unstable DC-DCs due to less than minimum load current, a circuit, shown in Figure 4.12 was implemented to ensure a minimum of 10 % of the 12 V DC-DC actual sourced output. The minimum load the DC-DC will see at power on is approximately 1 A, accounting for the control unit and the module fans. As 10 % of 1 A is approximately 100 mA, the load control circuit accomplished the requirement, adding 100 % margin to the output load. As depicted in Figure 5.10, the circuit pulls approximately 210 mA, blue line, I(Dlr1) until the current being pulled by the load, I(Rs9) is greater than this value. Once the load reaches the 210 mA, the resistor DLR1 is gradually placed off-line by a transistor. The action of the transistor implied the dissipation of heat on it, while in transition from fully ON to OFF. So, to evaluate this, the measurement of the power dissipation on the transistor (pink line) and on the resistor DLR1 (yellow line) was performed and can be seen in Figure 5.11. The worst scenario yields to 0,631 W in the transistor and about 2,48 W in DLR1. The duration depends on how fast the loads were turned ON to reduce the need for DLR1. Figure 5.10 – Load Control Circuit simulation with minimum load current Figure 5.11 – Load Control Circuit simulation of the power dissipation in DLR and transistor 51 5.1.6. Secondary Undervoltage Protection The results in Figure 5.12 evidence the moments where the output of the under-voltage protection circuit detects that one of the DC-DC is not supplying the correct voltage level, sending a high-level signal to the LCL. The simulation was performed with two independent voltage sources providing 5V (red line) and 12V (blue line) to simulate de DCDC outputs. Before the 4 s mark only the 5V DC-DC is working so so a siand after the 13 s mark only the 12 V DCDC (blue line) is supplying, so the circuit sends a signal (green line) to switch OFF the PSDU on both moments. Because both DC-DC are working between approximately the 7 to 8.5 second mark, the voltage level at the output of circuit went down and no signal was sent to the LCL, so the PSDU was not shutdown. As it is demonstrated in Figure 5.13, at power ON, the circuit waits approximately 332 ms before implementing the combinatory table, leaving enough time for all DC-DCs to come to a stable state. This circuit was simulated together with the LCL to be possible to witness the signal being received by the LCL and its trip-off time capacitor starting to charge, and the load being switched off. The result of this simulation is shown in Figure 5.13. Figure 5.12 – Combinations of operation for the Secondary Under Voltage protection Figure 5.13 – Under-voltage protection output voltage delay time 52 5.2. Part Stress Analysis The scope of this section is to explain how the analysis of the stress applied to some of the electrical parts of the Power Supply and Distribution Unit for the module was performed. The goal was to verify that no component was over stressed even under worst case situations. For confidentiality reasons, the modifications made to the circuits to reach this stage, as well the results of the complete analyses cannot be shown in here. Some of the results of the analysis performed are shown in Table 5-1. On the first line of the first table, for example, all instances of a specific capacitor were analyzed and the one in worst conditions was selected to perform the PSA. The capacitor has a rating voltage of 50 V, and this parameter must be derated to 60% of its value, so the maximum allowed voltage that can appear at its terminals is 60% of 50 V, i.e., 30 V. By analyzing all circuits where this capacitor is present, it is possible to note that maximum possible voltage at its terminals is 20 V. Since the derated stress is the Max Stress / Max permit. i.e., 66.7% and this is less than 100%, the component is compliant. Table 5-1 – Part stress analysis of the capacitors 53 The following tables, Table 5-2, Table 5-3 and Table 5-4, show the same analysis applied to some of the inductors, resistors, and diodes of the circuit, respectively. Table 5-2 – Part stress analysis of the Inductors 54 Table 5-3 – Part stress analysis of the Resistors 55 56 With this analysis, some components have been found to be over the parameter value after derating and needed a re-selection, in order to keep the design and capability to be within the limits. Table 5-4 – Part stress analysis of the Diodes 57 Chapter 6: Conclusions The work presented in this dissertation concerns the analysis of a custom power supply and distribution unit for scientific exobiology facility in the International Space Station (ISS). As an application-specific, custom-engineered PSDU it will deliver exactly what is expected and did not involve redesigning the system to fit the range of what a COTS unit could supply. Since this unit will have the lives of the crew that work on the ISS and operational goals that depend upon its reliable performance, the benefits gained from this custom design far outweigh any financial or time-related constraints. However, building electronic systems for high-reliability space applications requires diligent application of the best practices, use of the highest-reliability parts and materials, proven manufacturing processes and rigorous testing under environmental conditions that bracket the expected conditions in orbit. Worst Case Analysis was how it was concluded that the circuitry will work as intended given that each constituent part will be subject to such variations over life. WCA, proved that, even if all parameters of all parts were to change simultaneously to their most unfavourable values, the circuit will still have a very high probability of meeting its performance requirements over the mission life. At last, to prove the design robustness, the Parts Stress Analysis (PSA) to all components of the circuit determined the optimal operating range for each component using derating guidelines and allowed to verify that all selected components will work below their normal operating limit. This will reduce the deterioration rate of the component and minimize failures attributed to extreme operating conditions. Thus concluding, that on the one hand the average strength of the circuit has been increased as much as possible and that on the other hand the average stress has been reduced to its lowest. 6.1. Future Work To assess the failure modes and identify the causes of potential failures while also analyzing their impacts on the system, a Failure Mode, Effects, and Criticality analysis (FMECA) must be performed. With this analysis it will be possible to infer if any of subsystems presents Single Point Failures (SPF) that could be catastrophic, causing loss of life, be life-threatening or permanently disabling someone. It will also show