On Tue, 13 Apr 2004 16:27:29 -0700, Tim Wescott
wrote:

John Popelish wrote:


Just for efficiency reasons, I think you would want ot have enough
capacitance across the regulator input that the cell resistance drops
voltage only with respect ot the average output current, not the
switcher peak value. This can be a pretty big factor in the overall
efficiency. Using a switcher that has little ripple current on its
input (two phase boost, for instance) makes this much easier.


That's not the point. Because a switcher tends to draw a constant power
from a load it's input impedance has a negative resistive component. If
you match this with a source that has a too-high impedance it'll be
_unstable_; a big capacitor would just slow it down in this case.

While there certainly are going to be stability issues, using a
switcher with say 50 % duty cycle will draw 0 A half of the time (i.e.
the PV cell is operating in the constant voltage mode) and 2 Iave the
other half of the time (i.e. the cell would operate in the constant
current mode) and never operate at the maximum power point (here
assumed to be at Iave).

Sufficient parallel capacitances and/or series inductances or some
push-pull arrangement will keep the current constantly at Iave and
thus at the maximum power point.

Paul