N.D. Ghatpande

Digitally controlled autonomous Li-ion active cell balancing, using isolated resonant reset forward converter, for space applications

The Energy storage is required to support spacecraft loads during eclipse and also when the demand is more than power generation at any time. The most widely used energy storage technology is the battery, which stores energy in an electrochemical form. There is a need for highly efficient and long life battery systems for spacecrafts at various power levels. Li-ion based battery technology has better performance over traditional battery technologies. The battery system comprises of protection, management and balancing. Of these, balancing is most important that ensures the better performance and life of the battery system. A novel energy converter based active/non dissipative cell equalization method is implemented. This method comprises of digitally controlled resonant reset forward converter for individual Li-ion cells connected in series. This converter is used to transfer the energy from the higher voltage cell to the lower voltage cell. The main objective is to maintain cells in a battery at the same state of charge (SOC), and the voltage. The most innovative and attractive aspects of the system are efficient converter, filter and the discrete digital controller. This paper mainly concentrates on various control techniques that can be implemented to achieve effective cell balancing and to compensate the temperature, component aging and radiation effects for space applications. The performance of the system is inferred from the analysis of the simulation and experimental results.

Effect of solar cell capacitance and interface cable inductance on the shunt switch current transients in spacecraft power conditioning unit

Spacecraft Bus is regulated with switching shunt voltage regulator having photovoltaic cells as the primary source of power. This source switches between the bus loads and the shunt switch for fine control of spacecraft bus voltage. The solar cell capacitance along with inductance and resistance of the interface cable between solar cells and power conditioning unit, generates damped sinusoidal currents superimposed on the short circuit current of solar cell when shunted through switch. The peak currents of sinusoidal transients have to be considered in the selection of shunt switch in power conditioning unit. Solar Panel is a combination of series and parallel solar cells forming many strings. Hardware simulation by illuminating solar panel interfaced to a power condition unit with actual spacecraft cable length is a difficult approach. In view of this a scaled down version of solar panel (having single parallel) is illuminated with known interface cable length. This hardware simulation provides sufficient information to generate solar cell model to carry out software simulation and analyze current transients of shunt switch for the actual solar string rating.