Boost-based MPPT for the MTM PCDU of the Bepicolombo mission

Abstract

BepiColombo is an ESA mission to Mercury to be launched in 2013. A better knowledge of the origin and evolution of the planet, of its structure and vestigial atmosphere, of its magnetosphere, and of the origin of its magnetic field are the main objectives for the program. The journey to Mercury will last for approximately 6 years, and will be based on the gravity of the Earth, Venus and Mercury, and on the use of Solar Electric Propulsion. For the last, the use of the MPPT concept is essential for the mission. A mission power demand of up to 14kW is foreseen in the cruise phase for the Mercury Transfer Module (MTM) PCDU, being the power subsystem based on a 100V bus. Under this scenario, the use of a classical step-down regulator for the implementation of the MPPT power cell would require to keep the worst case minimum solar array voltage over the bus for any mission operating condition. Then, the maximum solar array voltage would become as high as to overpass the insulating capability of the isolation layer between the solar array cells and the substrate, under the high temperature environment experienced by the spacecraft near Mercury. As a result, the development of a step-up MPPT Array Power Regulator (APR) becomes a critical issue for the mission feasibility. Moreover, due to the hard environment that the solar array will be exposed to, the segregation of the solar array power is a very desirable feature. Furthermore, apart from the two classical operating modes of the APR – conductance or MPPT, depending on the spacecraft user loads demand and the available solar array power – the APR will have to operate in S3R mode for solar array voltages over the bus, with a fully autonomous transition between the three operating modes. This paper covers all the aspects related with the design of the APR MPPT concept and its implementation: APR power cell topology, control scheme, control strategy, protections. The implications on the design of the MTM PCDU MEA will be also addressed. Finally, they will be presented the results of the test carried out over an 1/10 scaled-down engineering model of the BepiColombo PCU - including 3 APRs - in front of the real operating conditions foreseen for the MTM PCDU, including all the relevant issues related to the behaviour of the Electric Propulsion load like beam-out events and load transients.