Roy Lewallen wrote:

In my limited experience, you have to be a little careful using a
switching, or even a series pass, regulator with a solar panel. Most are
designed to regulate voltage coming from a relatively stiff source, and
some become unstable when hooked to a high impedance source like a solar
panel. This can often be overcome by putting a big capacitor across the
panel, and it can of course be overcome by designing the regulator to
function properly with the high impedance source in the first place. And
quite a few regulators work just fine without modification. But it's
something to keep in mind when using a regulator designed for more
conventional applications.

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.

--
John Popelish

John Popelish wrote:

Roy Lewallen wrote:

In my limited experience, you have to be a little careful using a
switching, or even a series pass, regulator with a solar panel. Most are
designed to regulate voltage coming from a relatively stiff source, and
some become unstable when hooked to a high impedance source like a solar
panel. This can often be overcome by putting a big capacitor across the
panel, and it can of course be overcome by designing the regulator to
function properly with the high impedance source in the first place. And
quite a few regulators work just fine without modification. But it's
something to keep in mind when using a regulator designed for more
conventional applications.


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.

Presumably what you need is a controller that detects when the supply
voltage gets down to some threshold, then regulates the supply-side
current rather than the load-side voltage.

Come to think of it that'd be a fun thing to design...

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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.

Presumably what you need is a controller that detects when the supply
voltage gets down to some threshold, then regulates the supply-side
current rather than the load-side voltage.

Come to think of it that'd be a fun thing to design...

Very few switchers draw an instantaneously constant power from the
unregulated source. Almost all can draw an average constant power
(over the switching period). The difference means a lot when you
consider what the variations do to the total losses in the solar
cells. You missed my point, completely.

--
John Popelish

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

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.

Presumably what you need is a controller that detects when the supply
voltage gets down to some threshold, then regulates the supply-side
current rather than the load-side voltage.

Come to think of it that'd be a fun thing to design...

Very few switchers draw an instantaneously constant power from the
unregulated source. Almost all can draw an average constant power
(over the switching period). The difference means a lot when you
consider what the variations do to the total losses in the solar
cells. You missed my point, completely.

--
John Popelish

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

John Popelish wrote:

Roy Lewallen wrote:

In my limited experience, you have to be a little careful using a
switching, or even a series pass, regulator with a solar panel. Most are
designed to regulate voltage coming from a relatively stiff source, and
some become unstable when hooked to a high impedance source like a solar
panel. This can often be overcome by putting a big capacitor across the
panel, and it can of course be overcome by designing the regulator to
function properly with the high impedance source in the first place. And
quite a few regulators work just fine without modification. But it's
something to keep in mind when using a regulator designed for more
conventional applications.


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.

Presumably what you need is a controller that detects when the supply
voltage gets down to some threshold, then regulates the supply-side
current rather than the load-side voltage.

Come to think of it that'd be a fun thing to design...

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com