BCC Paul - explanation re why follows shortly. > On a somewhat related topic, I've noticed that a couple of companies > (TI and Freescale, for two) have been touting switchmode power supply > ICs that are designed to operate with a single solar cell as input > (~0.5V), and I've been wondering how that works out compared to simply > splitting the same area of silicon into multiple cells that are then > connected to create a more convenient voltage ? And other companies are selling whole solutions based on proprietary single cell boost converter technology. I have looked at this with a view to using it if it was useful enough and have severe reservations about its merits. My focus is on small panels - say in the 100 mW to 1 Watt range. This is also the area where its proponents are usually targeting it - typically for cellphones and similar. I've both looked at this directly to see if it seems to make sense for use in a solar charged product, and have recently designed a circuit which gave practical insight into working with voltages of about twice what are involved in single PV cell systems. The design is a boost converter system operating from as low as 1 V input (single NimH cell) with a target of up to 2 Watts of output and "reasonable efficiency" while costing as little as possible. eg cost constraints are tight enough that use of "nice" ICs with full synchronous rectification and flyback diodes tended to not be financially viable. Attention to FET switch, inductor and wiring path had a major effect on efficiency. While making a system that "worked OK" is not hard, achieving reasonable efficiency is harder. Most specialist smps ICs targeted at this area typically claim 80% to 90% efficincy, but inspection of the datasheets usually shows that this is over only a portion of the power range and usually not at lowest Vin. Ultimately a simple flyback design and care with details gave a result that seems to be about as good as most specialist ICs claim to be. This refreshed my 'practical insight' into dealing with voltages approaching single PV cell level with reasonable efficiency. *Single PV cell systems - pros and cons:* The positives include: - Very high mechanical packing density as the single cell can occupy the whole area available with no dead space between cells - traditional cut and join methods can easily leave 10%+ dead areas, but I have also seen recent trends to minimising this so that probably only around 1% area is lost. this only matters where maximum available mounting area is constrained - such as on the body of a cellphone or PDA or charger - or on a solar charged light :-). - Good resistance to shadowing. A single solar cell will generate a certain current on short circuit *AND* will not pass a significantly greater current without a very significant voltage rise when driven externally. That is, a solar cell may be simplistically modelled as a voltage source and an optically variable resistance. In a multi-cell PV panel with all cells in series, if most cells receive say 1 sun of insolation but even only 1 cell is shadowed so as to receive say 0.1 sun of insolation then the output of the whole array will fall to about 10% of full output. The shadowing item can be small and quite inconspicuous and the shadow cast can seem extremely minor while still causing extremely major reductions in panel output. The traditional response to this problem is to add diodes rated at panel current across the cells, cathode to cell positive, such that in normal operation the diode will be reverse biased by the cell. A loaded cell at full rated power has a forward voltage typically in the in the 0.5 - 0.6 V range. When an individual cell is shadowed the current from the other series cells (which is greater than it can support) drives it into reverse bias and the parallel diode will now conduct and bypass the shadowed cell. The voltage loss due to this is the lost cell voltage (about 0.5V) plus the diode drop (depends on technology). A loss of 1V+ per shadowed cell will occur. In a 12V system which typically has 36 cells with an open circuit voltage of18 volt, selective shadowing of far less than the whole panel can lead to almost complete termination of panel output. For lightly shadowed cells a small effective voltage drop per cell will occur. With a panel made from a single cell the effect of shadowing will be to reduce the cell and thus panel output in approximate proportion to the net decrease in insolation. ie a 10% shadowed panel will reduce in output by about 10%. This effect is still likely to be approximately linear at large % shadowing. In fact the reduction in panel temperature which could occur with significant shadowing may increase panel output slightly relative to insolation received. - Reduction in manufacturing costs due to no need to LASER cut cells or to cut them at a reduced rate, reduction in panel wastage (PV material is extremely fragile and wastage happens, no need to solder interconnections or provide interconnection materials. These savings are probably relatively minimal as a proportion of total panel cost. *Disadvantages include:* - Very low voltage to work with - so high currents with attendant substantial potential increase on resistive losses. A single PV cell will typically have a 0.5V - 0.6V operating voltage at 25C and less at temperature. For eg a 12V system instead of having 36 cells there will now be one, so voltage will be 36 x lower and current 36x higher. I^2R losses would be 36^2 = 1296 x higher with the same interconnect resistances so net resistance must be MUCH lower. This is easier to achieve due to the lack of interconnects - lead resistance and termination resistance become key. However, for example, in a 500 mW 6V panel (largish PDA or torch or charger - about 2 x large cellphone panel area) effective load resistance is V^2/P = 36/0.5 = 72 ohms. To achieve say 5% power loss in leads and connections requires lead etc resistance to be 72 ohms x 5% = 3.6 ohms = easily achieved. To achieve the same loss with a single cell at 0.5V and a current of 1 amp requires a lead + connection resistance of 25 milliohm. While this is potentially achievable it places severe demands on connector integrity and contact resistance, PCB traces, lead resistances etc. This high current needs only be taken from PV panel to the necessary boost converter. - Challenging boost converter design. While making a boost converter that operates at 1 amp input is relatively trivial in a say 12V system, the same is (far far far) less true in a 0.5V system. As above, 25 milliohm = 5% losses. MOSFETs with Rdson < or even << 25 milliohm are readily available but they are premium parts. The low voltage allows use of very low Vds parts where low Rdson is more available but Vgsmaxso drops - which is not bad but leads to very delicate gates - no problem if driven suitably carefully and protected. Actual FET current will be > 1 A when switching so even 25 milliohm is not desirable. Next and inductor << 25 milliohm is also needed. When operating from say a 6V PV panel converter efficiencies of 80%-90% are achievable and if buck converting to say a LiIon battery 80% + should be attainable. For the single cell up converter to compete with say 85% efficiency requires all the above losses to be < 5% each as there are other losses not yet accounted for - overall a challenging but not impossible design. - Non auto startup. A system where V_PV_loaded > vbattery can very easily be arranged to charge a completely discharged battery and even to operate directly from the PV panel if required. However, a single solar panel cannot be used to startalmost anything - so a source of bootstrap voltage is required for startup. This point is explicitly acknowledged by at least one of the companies active in this field. A product that cannot charge its battery if its battery isn't at least partially charged could be "very annoying". ______ SO Overall this is a usable system which MAY make sense in some extreme cases, but the engineering is harder and more problematic than more conventional solutions and its hard to see that it makes sense in a majority of typical applications. Even extending to say 4 PV cells would convey many of the advantages and greatly ease the design task and make lower cost easier. Russell McMahon 2009/10/27 SME > On Tue, Oct 27, 2009 at 3:29 AM, William "Chops" Westfield > wrote: > > > > On Oct 26, 2009, at 6:53 AM, Russell McMahon wrote: > > > >> real-world PV panel performance > > > > On a somewhat related topic, I've noticed that a couple of companies > > (TI and Freescale, for two) have been touting switchmode power supply > > ICs that are designed to operate with a single solar cell as input > > (~0.5V), and I've been wondering how that works out compared to simply > > splitting the same area of silicon into multiple cells that are then > > connected to create a more convenient voltage ? > > > > BillW > > > > -- > > http://www.piclist.com PIC/SX FAQ & list archive > > View/change your membership options at > > http://mailman.mit.edu/mailman/listinfo/piclist > > > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist