Low voltage MPPT boost converter IC, constant input voltage

Question

Following scenario: I have a bunch of Sunpower C50 unprocessed single solar cells. I'm trying to make a panel out of them for hiking. According to the datasheet each cell has 0.574 Vmpp an 5.65 Impp or 3.24w power. Since cutting the cells is hard I want to put 4 of them in series so that I have around 2V and 12w output... I want to utilize this as much as possible and therefore charge a single cell Li-ion battery directly with maximum power point tracking.

But I'm struggling to find an IC that fits the purpose. All energy harvesting ICs like the CJMCU-25504 with solar input are low power or not rated for 2V, and most solar battery charger ICs like a CN3791 are mostly buck converters.

I once designed a MPPT circuit myself, so I know that the output voltage of a PV panel is mostly constant, so one simple MPPT algorithm which the CN3791 for example uses is, to just keep the input voltage constant. So I was wondering if I might find a way to design a boost converter that controlls the input voltage ratehr than output? Or maybe there is just the right IC for it, I just can't find it.

Answer 1

The CN3791's MPPT pin voltage is regulated to 1.205V, so you could connect it directly to the panel through a voltage divider to set the max power point. Then drive the CN3791's VCC pin with a boost regulator to get your charging voltage.

Answer 2

The output voltage of a PV panel is dependent on the load you put on it, drawing low currents from the PV panel result in the nominal voltage. Putting a higher load with a higher current will result in the voltage dropping to near zero. Hence the MPPT part, to get the maximum power a DC to DC converter must strike a balance between the two. This also means calculating the amount of power by measuring both voltage and current from the PV cell and a multiplication operation. One MPPT algorithm perturbs the load to see if there's more or less power coming from the cell and then moves in the direction of maximum power. This means that a boost or buck converter attached to the PV cell needs to be able to change the amount of input current or load and balance the load to the PV cell.

If you're good with control systems you can actually use an microcontroller ADC to drive the feedback pin of a DC to DC converter. Microcontroller needs to know the voltage and current to be able to feed that to the mppt algorithm.