Solar energy harvesting with low leakage current (tens of nA) - Page 1
Forum section: electronics / projects

author: hans
author_rank: Super Contributor
timestamp: May 07, 2020, 07:46:36 am
content: I'm investigating the use of energy harvesting in wireless sensor nodes to replace the current power system for a design I've got. The current design is based on a low IQDC/DC (TPS62740) for conversion to 1.8V, which is needed for some RF IC's (some of which are always-on, experimental chipsets), and also improves the uW/MHz of the MCU I'm using. This DC/DC has approximately 0.4uA quiescent current from the CR2032 battery (1-1.2uW). This current consumption is actually fairly dominant in my design, as I'm working with very low duty cycles of the sensor node.So I'm investigating improvements to this design. One improvement is to upgrade the DC/DC to the TPS62840, which has 6x lower IQ(60nA, 0.2uW). However, I'm also intrigued by the idea to replace the CR2032 with a supercap, and then use a solar energy harvester to charge the supercap. But so far, my exploration into solar energy harvesters is a bit disappointing. Don't get me wrong, I'm impressed by the <=0.6uA IQof these chipsets, but compared to these newer DC/DC's it's fairly high.So far I've found these EH chipsets that are tailored towards PV energy harvesting:- TI BQ25505: MPPT, 325nA from VSTOR (2.1V), does not have a down-conversion DC/DC; and needs fairly high solar power (15uW) to get started- TI BQ25570: MPPT, 488nA from VSTOR (2.1V), does have a down-conversion DC/DC that is fairly efficient (>90% at 20uA load); and needs fairly high solar power (15uW) to get started- Analog ADP5090: MPPT, 320nA from storage, does not have a down-conversion DC/DC; and needs fairly high solar power (16uW) to get started- Analog ADP5091/ADP5092: MPPT, 510nA from storage, has a down-conversion LDO (can be turned off); needs only 6uW to get started- Analog LTC3105: MPPT, very high IQof 24uA...- ST SPV1050: MPPT, 800nA from storage, has a down-conversion LDO (can be turned off); needs high power (30uA @ >0.55V) to get started- Cypress S6AE102A: no MPPT, 280nA from storage, has internal LDO but chipset basically seems like a sophisticated power mux..My question is, are people aware of alternative (better..) chipsets that can be used for (MPPT) solar energy harvesting? The hundreds of nA for these energy 'harvesting' chipsets is not really an improvement with the prospect of the TPS62840. Ironically, the chipset responsible for 'harvesting' energy is then actually wasting most of it in my prospective design.

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 07, 2020, 09:03:25 am
content: I haven't looked at all the offering, but one thing with the LTC3105 is the low start-up voltage (250 mV). The min voltage can be something to consider depending on your design, apart from IQ.

author: Marco
author_rank: Super Contributor
timestamp: May 07, 2020, 02:50:10 pm
content: Why not just straight connect the PV panel to a supercapacitor with a diode and using the TPS62840?

author: mark03
author_rank: Frequent Contributor
timestamp: May 09, 2020, 12:22:27 am
content: Is there any straightforward / simple way to add input voltage regulation to a typical buck-converter IC? True MPPT isn't that important if you can control the panel voltage at a fixed value.

author: hans
author_rank: Super Contributor
timestamp: May 11, 2020, 10:24:26 am
content: Thanks for all the suggestions. I've looked at some options this weekend. In particular, I've looked at the solar cells of Ixys with more detail, and basically split up the PV harvesting solution into 2 possibilities depending on the choice of solar panel (I'm free to choose any):1 - 2 cell solar arrayEach cell has about 400-600mV open circuit voltage, so I don't think start-up voltage is too major of a concern actually (@SiliconWizard). The 0.8 - 1.2V PV voltage (2-cell) does mean that a boost converter is needed, which probably needs MPPT for correct control of the boost converter duty cycle as you don't want to under/overload the solar cell.I've considered a case where the PV panel harvests 10uW for 12hrs/day (5uW * 86400 = 432mJ). Charging is done at 75% for most chipsets with 1V/10uA source, resulting in 324mJ added to the supercap. A boost energy harvester with 500nA IQ@ 2V then consumes 86.4mJ of energy per day. Net energy: 237.6mJ per day, or 55% energy efficiency (2.75uW load possible). That is actually more than I estimated it would be, but then again not great (45% wasted).The best chipset for this case was the ADP5090 (70%), but I would probably prefer the ADP5091 (60%) as it has no lower bound on the continuous input power. TI chipsets were comparable (55 - 62%), but need >5uW to harvest.3 - 4 cell solar arrayThe nice thing of these cells is that the open-circuit voltage potentially increases to 1.6 - 2.4V. 1.6V is a bit on the low side, but 1.7V - 2.4V is a fine range for the ICs I'm using. This means I don't need the TPS62840 at all.I've performed a simulation with a 3 and 4 cell solar panel model that outputs a day sinusoidal 0-5uA (@ 1.8-2V) output onto a BAT54 rectifier, (precharged) 1F supercap and a 750k - 1M ohm load. The 3-cell variant can power a 1M ohm load @ 1.75V (=3.1uW, 61% energy efficiency), and the 4-cell variant can do 750kR @ 1.87V (4.66uW, 95%)The nice thing is that 1.8V / 3 = 600mV, or 2V / 4=500mV cell voltages is quite close the MPPT of a Ixys solar panel anyway:https://ixapps.ixys.com/DataSheet/SM141K04LV.pdf(first graph page 2)I was quite surprised how well this works. Those energy harvesting chipsets are not cheap (5 EURs each!) and even in these very low energy loads don't seem to outperform a 5ct diode.I was a bit concerned if it would behave similar to a classic 50Hz transformer rectifier where the power factor would be very poor (e.g. huge current spike on the top of the sine wave), but probably due to the diode-like behaviour of the solar panel that's not the case, and allows for continuous harvesting (even for varying states of the supercap & solar panel).A 4-cell panel is then quite promising, but I would need to find a low-leakage Schottky diode (e.g. <200nA @ 50C) for this to work well in practice.A downside is that a 4-cell panel is rather big (although that harvests proportionally more energy..), but I think I can still fit in my application.