When solar cells are electrically connected to form solar arrays, they are organized into strings. Each string represents a specific number of cells connected in series to produce a specific voltage. The strings are then connected in parallel to add their currents to meet the array power requirement. This requires that the strings have the same voltage. Blocking diodes are used to take out strings with voltage that is too low, resulting in loss of power. When the arrays are mounted to a non-coplanar surface such as a spacecraft body or inflatable structure, many strings will have voltages lower than the rated voltage. This regulator manages the voltage of each string individually so that its power may be used, regardless of its voltage. It does this by converting each string’s energy into a series of high-voltage pulses that charges a reservoir capacitor to one of a set of common voltages used by the spacecraft bus. This allows for use of all of the illuminated strings in producing well-regulated power at pre-programmed voltages.
The two methods previously used are the use of blocking diodes to simply “disconnect” strings that are not producing the required voltage, and a DCDC converter that converts each coplanar surface to a common voltage. Then the power for each surface is connected in parallel.
A power management system for deployable or body-mounted solar arrays integrates the following functions into a single package:
- Array shunt regulation capable of combining solar cell strings of varying illumination and voltage.
- Real-time programmable voltage conversion and regulation for two different power busses. Voltage of each bus may be set remotely at any time.
- Battery charge control. This too may be controlled remotely and modified for specific flight operations.
Each string is individually connected to the power management circuit so that it charges a capacitor when illuminated. As the capacitor is charged, it stores the energy produced by the string, and as it does, its voltage increases to the voltage of the string charging it. The circuit samples each capacitor in turn, and converts the energy stored in it to a high-voltage pulse using an inductor. The pulse is then transmitted to a collection circuit. After it samples all of the strings, it returns to the first string and begins another cycle. Thus each capacitor for each string is sampled and converted on each cycle. The cycle time is always the same, and is a fraction of a second. The capacitor for each string is sized to be able to hold all of the energy produced by the string in a single cycle time.
As the pulses arrive at the collection circuit, they are used to charge one of several reservoir capacitors. There is a reservoir capacitor for each bus voltage specified for the spacecraft: one for the battery charge circuit, and another that is connected to a shunt resistor that dissipates its energy into space as heat. The collection circuit routes pulses to each capacitor, charging it up to its required voltage by varying the width of the charge pulses. When all reservoir capacitors are charged to required voltage, the pulses are sent to the shunt to be dissipated.