For operations with a constant voltage load, both two input
powers increase linearly as the output power increases. When
the power converter is supplied from two sources with a same
voltage of 12 V, the primary source contributes a slightly
higher power than that the secondary one does. The primary
source supplies much more power in the case that its voltage is
higher than that of the secondary one.
On the other hand, the primary source supplies a constant
power for a constant current as indicated by (6). As the output
power becomes higher, the secondary source has to be
increased to fulfill the load power requirement.