I'm designing a small solar charger with these characteristics:
Solar panel with 19% monocrystalline cells
- Voc = 7,50V
- Vmp = 6,00V (Voc -20%)
- Isc = 1,07A
- Wp = 6,42W
IC
Microchip MCP73871 linear 1A lipoly/li-on charger with VPPC capabilities (input voltage regulation loop)
Battery
Li-ion 3,7V 6600mAh 24,42Wh
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This configuration works fine, but I decided to move to SunPower cells because the solar panels fabrication, when it's needed to cut the cells, in China is made better. Mainly, due to the fact that Sunpower cells have the "back-contact" technology.
So, given that the Sunpower cells are more efficient (23%), the solar panel will output more total watts, that an IC such as MCP73871 will actually waste because of its 1A current limit.
SunPower solar panel
- Voc = 7,125V
- Vp = 6,10V
- Vmp = 5,7V
- Isc = 1,35A ◁◁◁
- Wp = ~8,25W
So, I moved my attention to another IC, this time LT3652 (rated 2A). At a first glance it looked fine but then I discovered that it didn't work with a 6V solar panel. It has a VIN_START value of at least 7,50V to start the IC, after that the actual normal nominal operation voltage can be 5,95V.
A solution to this problem could be to cut the SunPower cells smaller to get an higher voltage in the same surface, given that this IC is actually a buck-converter. But someone told me I could really lose efficiency because of the big voltage difference with the input and the battery.
Or perhaps the solution could be to use a different approach with another IC. For example, the ST SPV1040 is a very cool chip with a MPPT perturb and observe embedded algorithm and it looks simple to design. It's designed as a boost converter, so almost any low voltage solar input source is desirable.
My idea could be to use two uncut Sunpower cells to reach 1,22V. To build a panel with 2 full cells of 125mm x 125mm should be much easier to design and to fabricate, because it has no waste and there's no need to cut the cells. I will boost this voltage to charge the li-ion battery pack. However I can't understand yet how much current this IC can handle and if it's capable alone of a li-ion charging profile.
Within SPV1040's datasheet I found ILx, a value for the current limit.
Is ILx referred to the also called "switch current" of the inductor?
This project has a limited space available for the solar panel surface, so the whole design could be more complicated. I'm talking about 257mm x 175mm.
The very ultimate goal for this charger is to harvest the most possible amount of watts/time, but any solar-specialized technology such as an active MPPT should be taken into account without to fall in an excessive electric circuit complexity, because I'm not an engineer.
Any effort with this question will be very appreciated. Thank you.
Best Answer
You may be best off using 'conventional' monocrystalline silicon PV cells, arranging Vout to best suit your needand maximising packing density and area occupied on your device. Many manufacturers waste substantial area in intercell spaces (> 10% common, 20% not uncommon) and this can usually be reduced to maybe 5% of area with care.
Optimisation of front sheet optical loss can help make up losses elsewhere. Worst case should be 10% and around 1%-2% is possible if cost is not an issue.
The MCP73871, data sheet here acts as a linear regulator for charging purposes. Max Iin = 1.8A so at 4.2 V out max power = V x I = 4.2 x 1.8 = 7.6 Watts. However, unless you use a preceding MPPT converter the proposed 8V Vmp PV panel will have a maximum efficiency of 4.2/65 = 70%. LiIon cell mean voltage across charging range is closer to 3V7 so mean efficiency ~~ 3.7/6 = 62%.
The LT3652 data sheet here is a nice device if the 7,5V Vstart can be accommodated. Note that the application note tends to focus on battery chemistries which can be floated when 100% SOC (stae of charge) is reached. Lithium Ion cells MUST NEVER be "floated at end of charge. Charge voltage MUST be removed at the end of charging, that their days may be long on the face of the land. The LT3652 can accommodate this need with suitable design.
A simple buck converter / charger such as the LTC4002 may better meet your needs. This allows 5V to 22V input and uses and external MOSFET switch & external flywheel diode and external current sense resistor. fficincy of about 85% can be obtained at Vin = 6V and adding a synchronous rectifier FET in place of the flywheel diode may increase efficiency slightly.
LTC4001 is a 2 A buck regulator LiIon charger BUT Vin max of 5.5V is an annoying limitation.
Your LiIon cell will need at least 4.2V to charge fully - say 4.5V minimum available.
Sunpower cells are difficult to join when cut into fractions of a cell. Most PV panel manufacturers appear unable to do this. Using whole cells should present no difficulty.
Boosting from low voltages is usually less efficient end to end than either boosting from a voltage closer to FVout or buck converting from above Vout.
The SPV1040 is capable of 3 Watt output maximum (data sheet table page 6).
The inductor maximum maximum current (really the switch max current) is shown as 1.8A as you say BUT you must use the worst case = minimum-maximum current for design purposes.
The graphs in the SPV1040 data sheet on pages 8 & 9 show efficiency at various Vin/number of cells combinations.
At full power with 3 cells they show 80% at 4.5V out at 2 Watts without MPPT and 89% with MPPT.
Note that with 3 cells they show 2W max.
Sanity check:
3 cells ~= 1.5V full sun.
Duty cycle at 1.5V in and 4.5V out at 100% efficiency
= 4.5/(1.5+4.5) = 75%.
Max power at 100% = 1.5V x 1.8A x 75% = 2.0 Watts IN.
= what they said. Pout =~ 2W x 90% = 1.8W out MAX = far short of your desired power level.
For the SPV1040 ILX appears to be the inductor current, & the switch current & the LX pin current.