**Ordinate** - Current

**Abscissa** - Voltage

**Blue Line** - Plot of solar array output impedance. Starts
out as a horizontal line (current source) and then gradually curves into a
vertical line (voltage source).

**Red Line** - Plot of input impedance of a switching-mode
power supply, a load that gets its power from the solar array. The plot starts
out as a resistive straight line showing current increases as voltage
increases, then when switching action starts, turns into a constant power
hyperbola with current decreasing as power increases.

**Point A** - Point A labeled where resistive part of regulator
input impedance intercepts constant current portion of solar array output
impedance. This is a stable but undesired operating point, undesired in that
the switching-mode power supply load latches up as a resistive load before
switching action starts.

**Point B** - Point B labeled where constant power hyperbola of
regulator input impedance intercepts the constant current portion of the solar
array output impedance. A black vertical dotted line is drawn through this
point indicating a separatrix in the phase plane.

**Point R** - Point R is labeled on the solar array output
impedance at the inflection point between a constant current source and a
constant voltage source. This is the unstable solar array maximum power point
that the shunt regulator (not shown on the graph) regulates the solar power
system bus voltage.

**Point C** - Point C is labeled where the solar array and
switching-mode power supply load would operated if the solar array shunt
regulator failed open. This would be a voltage higher than the desired
regulated bus voltage.