ProblemThe solar-array power system lockups in the battery-discharge mode when there is sufficient energy to charge the battery. The problem occurs in a Direct Energy Transfer (DET) system consisting of a solar array directly powering the system voltage bus. System voltage regulation is provided by a shunt regulator working against the solar-array impedance. A battery provides power when there is insufficient energy from the solar array and a battery charger charges the battery when the solar-array output exceeds that required by the load.
Lockup can occur during a dark-to-light transition of the solar-array power source. In space systems, this can occur during initial deployment at launch or each time there is an orbital transition from eclipse to sunlight. It can also occur on the transition back to solar-array power if a transient increase in load requires transient energy from the battery.  D
Figure 1: DET System I-V Characteristics Figure 1 shows the state-plane of the system where I is the current in the inductance of the cable from the solar array (approximates the solar-array current) and V is the voltage across the bus capacitance (approximates bus voltage). Superimposed on the I-V state-plane are the input characteristic of a switching-mode power supply (red) and the output characteristic of a solar-array (blue). (Notice other discussions in this hypertext sometimes plot similar characteristics in the V-I plane instead of the I-V plane as shown here.) A family of system trajectories can be plotted on this plane for practical values of system inductor current and capacitor voltage. The trajectories and isoclines are not plotted here, but are in the references. What these trajectories show is that point B is an unstable operating point on a separatrix that passes between operating points A and B (dotted black line). Point A is stable and a node (real) or a focus (complex), but not a desired operating point since it is on the turn-on resistive part of the load line before the switching-mode power supply load begins operation. The stability of point C, which is the voltage the system goes to if the regulating shunt regulator fails open, depends on the values of L and C. For practical values of L and C it is usually stable. The normal end-of-life full-load operating point, point R, is usually selected to be the peak power point, near the inflection point of the solar-array characteristic. This point is unstable but is stabilized by the shunt regulator which regulates to a voltage near the point R voltage. Also not plotted in Figure 1 are the composite load lines in the various operating modes -- battery charging, shunt regulation, etc, and plots of the solar-array characteristics at various illumination levels. If these were added to Figure 1, then the trajectories from sun to eclipse and eclipse to sun could be plotted along these load lines. When this is done (see references), there is no latchup possible on the sun to eclipse trajectories. However, when the battery is powering the load, the operating point is on the left-side of the separatrix (dotted black line through point B) and under some conditions can not get back to the desired right-side of the separatrix. This presents a controllability problem and results in latchup. The load continues to be partially powered by the battery, even if there is sufficient energy from the solar-array to power the load and charge the battery. The discharge is usually small in tightly regulated bus systems but can be very large in semi-regulated bus systems. Do not use this information for design without independent verification of the information. Editor: Jerrold Foutz |
