# State-of-the-Art DC-DC Converters for Satellite Applications: A Comprehensive Review
## 1 Introduction  
Satellites have been extensively used for applications such as mobile communication, radio and television broadcasting, military intelligence, weather forecasting, and navigation. The launch of the satellites Sputnik (83 kg) and Explorer 1 (14 kg) by Russia and America, respectively, marked the beginning of the age of satellites [1, 2, 3]. Satellites are classified based on their mass into large, medium, and small satellites [1, 2]. Large and medium satellites (\(Mass>500\) kg) can carry more payload and integrate larger solar arrays to produce more power, making them suitable for communication, remote sensing, and applications with a lifetime above 10 years. Small satellites (\(Mass<500\) kg) are small and cheap, with shorter mission times than large satellites [1, 4, 5]. Hence, they are suitable for applications such as Earth observation, disaster management, space science, microgravity research, earthquake forecasting, and gravitational field measurements, to name a few [1, 6]. For many applications, two or more small satellites work together to form constellations or operate in formation flying [1, 7, 8, 9]. The classification of satellites based on their mass, their corresponding power ranges, and applications is demonstrated in Figure 1[10, 11, 12, 13, 14, 15, 16]. The mission's orbit is another parameter to be considered for satellites that are classified based on their relative distance from the Earth as Low Earth Orbit (LEO), Geosynchronous Earth Orbit (GEO), Medium Earth Orbit (MEO), and deep space missions. While LEO satellites are used for remote sensing, GEO satellites are mainly used for communication and broadcasting. MEO satellites are used for navigation, communication, and geodetic/spaceenvironment science [10; 17]. The various mission orbits and their relative distances from the Earth's surface are shown in Figure 2.  
A bus system and a payload system are the two essential components of a satellite. The elementary operations of the satellite are performed by the bus system, while the mission-specific tasks are performed by the payload system. The EPS is one of the primary and critical components of the bus, contributing to about 33% of the overall mass of the satellite [18; 19]. The EPS comprises an energy source, an energy storage unit, and a unit for controlling and distributing power, called the Power Conditioning and Distribution Unit (PCDU) [20]. The solar array is the predominantly used energy source for satellites. The energy storage unit, mostly, Li-ion batteries, ensures a continuous power supply, especially during eclipses or when the solar arrays cannot meet the peak power demand [21]. The PCDU interfaces with the PV array and storage systems or loads, ensuring effective power conditioning and transmission [22]. The EPS is designed based on the time span of the mission, orbit height from the surface of the Earth, solar panel sizing requirements, load demand, satellite mass, radiation effects, efficiency, component count, battery requirements, reliability, redundancy, and fault tolerance level [23; 24]. The propulsion system is another crucial component in a satellite, as it enables formation flying, interplanetary paths, collision avoidance, orbital maneuvering, station keeping, and orbit transfers [25; 26]. Electric propulsion (EP) has recently gained popularity, especially in small satellites, as it offers better fuel efficiency than the chemical propulsion used in large satellites [27; 28; 29]. Better fuel efficiency is guaranteed by a high specific impulse (\(I_{sp}\)), which is essentially a measure of the propellant's ability to produce thrust efficiently [30; 31]. Nevertheless, EP systems face a variety of challenges. One of the main challenges is the time taken to reach high thrust, which limits their use to specific applications. One solution is to use high-power systems, which improve the time taken to reach high thrust, providing more opportunities for timely orbit changes and interplanetary missions.