MPPT using an inverting boost converter

I'm reminded of the old experiment:

- Hook up a battery to a voltage divider, which uses a CdS photocell in one leg.

- Watch voltage on scope.

- Hold photocell near scope trace. See deflection.

If you have the trace bright enough and fast enough (faster than the recombination rate of the CdS, usually < 1ms/div), the DC level gets pushed around, and under sensitive enough conditions (vert gain, supply voltage, resistor ratio, trace brightness, etc.), it can be deflected by several photocell widths. Go just a little too far, boop, it slips away.

If you do it on a slow sweep rate (~10ms/div?), you can see the dynamic response (and "drool" over time) as the beam sweeps towards and away from the photocell. Obviously, it won't "catch" if it's much more than a photocell diameter away from baseline.

What's interesting is it demonstrates positive feedback (catching then losing tracking corresponds to hysteresis), yet it's doing it with negative feedback (the signal remains stable while tracking).

Doing it with PV cells is interesting, because in principle, you can use a sufficiently small, extremely geared, motor to push around a fairly large panel for not much extra panel. Limited ultimately by required slew rate and positional accuracy, mechanical losses, and rotation of the Earth.

Tim

How about an IP camera out in th emiddle of the yard, pointing up? With that, the problem is reduced to a SMOP.

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-Lasse

That is my idea, and why I asked. Maybe I should have added some more information. Yes I need a uP to control it, but the advantage of this device is that I can convert up and down - hence use a wider span of the solar panel.

My panel will be fixed anyway. I know that I can adjust it to be straight at the sun, but I dont want to go that far. Secondly, winter is not an issue for a summer house, here I rather think of heating to keep the battery alive.

A "normal" charger is a buck/step down, as the panel my go to 19-21V to charge a 12V battery. At some point the Buck will not work anymore, and here is the idea to boost what it left. This buck/boost can do the trick. Measuring the battery can be a but tricky, but that it not hard to do

I ask directly: I will get more power also using the panel when it is below "buck level", and I have experience with this in a simple test, that they can charge even by moonlight (but I did not have enough time to investigate how much)

The advantage is also that I can use almost any panel for such a charger

WBR Sonnich

On Sunday, May 31, 2015 at 7:16:01 AM UTC-7, Sonnich Jensen wrote: ....

ow "buck level", and I have experience with this in a simple test, that the y can charge even by moonlight (but I did not have enough time to investiga te how much)

The optimum output voltage of the panel does not change much with the light intensity. The boost capability will probably not be needed.

If you do a Google search for "pv voltage vs irradiance" you can find many graphs. For a normal 36 cell unit the voltage will probably only vary from 16-22V with the highest being in cold conditions.

A PV cell can modeled as a current source proportional to irradiance in par allel with a diode. The output voltage varies as the log of the current (i .e. light intensity) so doesn't change a great deal. It also varies with t emperature so at high intensity conditions in the summer the two effects te nd to compensate as the high temperature will reduce the voltage.

kevin

elow "buck level", and I have experience with this in a simple test, that t hey can charge even by moonlight (but I did not have enough time to investi gate how much)

ht intensity. The boost capability will probably not be needed.

y graphs. For a normal 36 cell unit the voltage will probably only vary fr om 16-22V with the highest being in cold conditions.

arallel with a diode.

Or, more specifically, for a silicon PV, one big silicon diode paralleling each PV cell. The sun causes the diodes to be forward-biased, so you want to tap off the juice before the bucket gets that full.

y) so doesn't change a great deal. It also varies with temperature so at h igh intensity conditions in the summer the two effects tend to compensate a s the high temperature will reduce the voltage.

Cheers, James Arthur