Solar charging of supercap is too slow - Page 1
Forum section: electronics / general

author: Faranight
author_rank: Unknown
timestamp: May 26, 2022, 08:23:10 am
content: Hi,I have a tiny solar panel (8 cells in series, Voc = 4.7V, Isc = 100mA) that I would like to use to charge a15F 5.5Vsupercapacitor and get a regulated 5V output on the other end. I'm using theSPV1040enegry harvesting IC that has MPPT capabilities. The issue is that this IC requires a higher voltage (2.0V) before the MPPT can kick in because lower voltages cannot reliably drive the mosfet gates on the boost converter. So, until this voltage is reached, the IC will use a passthrough which means that all current from the PV will be fed directly into the output without any control. When a load with huge energy capacity is connected at the output it will take a LOOOOOOOOOONG time to charge (with extremely low efficiency) up to 2.0V before the MPPT finally kicks in.I was wondering, if there is a way to improve this design. I've seen some boost regulators use an auxiliary tank capacitor (i.e. 4.7 uF) that is the first one to be charged to reach sufficiently high voltages so that the IC can power itself off of it to drive mosfet gates before bulk energy is finally supplied to the main output. The SPV1040 does not appear to have this ability.I'm rather new to energy harvesting IC's, so I'm not sure how to solve this. Is there an add-on circuit I can use to force the SPV1040 into MPPT mode as soon as possible to speed up the charging or should I choose a different IC? The output of my PSU must be able to supply up to 400 mA @ 5VDC (for short periods). While technically not relevant to the MPPT IC, I've seen some other energy harvesting IC's on the market that are a combination of solar charger and an output boost voltage regulator, but their maximum output current is way below the required 400 mA. This is why I'm using a separate boost converter IC that powers off the supercap to supply the regulated 5V output. So the issue really is the charging part.Thanks for any advice.

author: f4eru
author_rank: Super Contributor
timestamp: May 26, 2022, 07:55:42 pm
content: This is a boost controller, i.e. the output voltage must be higher than the input.Choose another architecture, buck for example.Also, you should probably not go up to 5.5V, as this shortens the life of the supercap

author: Faranight
author_rank: Unknown
timestamp: May 28, 2022, 02:42:34 pm
content: A boost output voltage may be higher than the input, but if you connect a low impedance load at the output then the voltage will drop down to whatever the voltage is needed to reach a current high enough to consume the incoming power. I'm not sure, if a buck would be appropriate for this task because it would only be able to charge the cap up to the voltage of the PV cells, not beyond. I will go up to 5.0V on the cap, and the cap is rated 5.5V. In the mean time I found an interesting article that describes my problem in more detail.https://www.edn.com/fast-charging-a-supercapacitor-from-energy-harvesters/This "solution" uses a small storage tank cap to power the IC and avoid the cold-start feature or at least go past it as fast as possible to force the IC into MPPT mode. Once there, a buck converter is used to transfer the excess energy into the supercap. At the same time the circuit prevents the over-discharge of he small storage cap and maintains a minimum voltage needed for the IC to operate in MPPT mode.Although, looking at the BQ25570 datasheet it would seem that this IC isn't a true MPPT charger as it has no current sense and thus cannot calculate the real PV power. One thing I noticed about a lot of these energy harvesting IC's is that they don't actually use MPPT in a sense that they would measure input voltage and current of the PV cell to calculate the power (perturb & observe). But rather they just assume that the maximum power point voltage is some percentage (i.e. 80%) of the open-circuit voltage of the PV panel. The IC's will stop switching every once in a while, sample the Voc and then adjust the switching accordingly. There is no current measurement. Why is this a thing in energy harvesting IC's?

author: Peabody
author_rank: Super Contributor
timestamp: May 28, 2022, 03:26:14 pm
content: I think it's just because if you look at the MPP at various levels of illumination, the voltage at the MPP doesn't change very much. The big change is in current. So if you set the voltage at 80% of the panel's maximum OC voltage, that will be about right for high illumination. And as illumination drops, the MPP voltage may only go down to 70% of max VOC before you get to the point that the panel isn't really producing useful power. Also, the power penalty for using 80% when it really should be 70% isn't all that great, particularly since the total power is reduced anyway. So I think it's just an approximation that's easy to implement, and which actually works pretty well. The marginal benefit of the more accurate methods is too small to justify their added costs in parts, etc.

author: f4eru
author_rank: Super Contributor
timestamp: May 29, 2022, 11:41:33 am
content: I'll recommend a buck.Solar cell output could and should be selected to be considerably higher than the max voltage of the supercap, so as to always work in a single mode.Can't achieve single mode with a cell voltage lower than the supercap voltage because the supercap starts at 0.MPPT and regulation can be achieved using a uC, outputting PWM also.... Simple and effective way.Buck can be reversible so as to work as buck/boost