Scaling nominal solar cell impedances for array design

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A methodology for estimating the AC impedance of solar arrays and in particular the Mars Pathfinder (MPF) spacecraft cruise stage and lander solar arrays is presented. During an early stage in the spacecraft design, the MPF mission, cost and mass constraints resulted in a proposal to use a gallium arsenide on germanium (GaAs/Ge) cell solar array for the cruise stage and a silicon, back surface field/reflector (Si BSFR) cell solar array for the Mars lander. Previous work had shown that the AC impedance of Si BSFR cells was very different from that of GaAs/Ge cells. There was concern over how the difference in AC impedance would influence the design of the power bus voltage control circuitry and the stability of the power bus in general. The results presented in this paper are based on AC impedance measurements made on single cells and small solar cell arrays. Those measurements were then scaled to the larger spacecraft arrays. In each case, a generic AC model was used to construct impedance equations which were then correlated to test results to find the resistive and capacitive components of the cell impedance between 100 Hz and 100 kHz. The data was obtained using dark forward biasing to simulate different load points and intensities. Cell impedance tests show a large difference between Si BSFR and either Si nonBSF or GaAs/Ge cells, mainly because of the inherently high effective capacitance of the BSFR cells.