Stability Analysis of the STEREO Satellite Power System

Abstract

The Solar TErrestrial RElations Observatory (STEREO) power system is a Peak Power Tracker (PPT) topology where maximum power can be extracted from the solar arrays during heavy spacecraft load and battery charging demands. The peak power tracking modules isolate the solar array from the combined battery and power bus. The PPT modules are current mode control buck DC-to-DC converters operating at a switching frequency of 100 kHz. The duty cycle of the buck converters are controlled by the active control signal from one of the three main controllers: the battery current controller, the battery Volt/Temperature (V/T) controller, and the peak power tracking controller. The STEREO power system operates the solar arrays on all three sides of the S/A I-V curve: (1) the Isc side of the I-V curve where the solar array operates as a constant current source with high output impedance, (2) the Voc side where the solar array operates as a constant voltage source with low output impedance, and (3) at the maximum or peak power point where the solar array output impedance is somewhere in between these two extreme cases. The solar array operating point is dependent on which controller is active. The effects of the solar array operating condition and its large output impedance variation on the power stage transfer function are discussed. The stability analysis of each loop was performed and verified experimentally. I.� POWER SYSTEM OVERVIEW The Solar TErrestrial RElations Observatory (STEREO) observatories are two NASA spacecrafts that will be orbiting the sun close to Earth’s orbit with one spacecraft “ahead” of the Earth and the second “behind” the Earth for a two year mission. STEREO is a NASA Solar Terrestrial Probes program, and the two satellites will study the coronal mass ejections caused by the solar eruptions of the Sun. The STEREO satellites will be launched together in the summer of 2006 on a single Delta II 7925 launch vehicle