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Comments on Over-voltage protection for device with photovoltaic cell source

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Over-voltage protection for device with photovoltaic cell source

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I have a solar panel rated for 6V open-circuit and up to 150mA. I would like to connect this to an SPV1040 or similar buck-boost converter with MPPT functionality to be able to charge a 3.6V battery under cloudy conditions.

The problem is that the SPV1040 has a maximum input voltage of 5.5V, while the panel is rated for 6V and I have measured up to 7.5V in sunny conditions. Of course, 6V is an open-circuit specification and the 7.5V measurement was open-circuit as well. I read that the maximum power point is usually around 75% of the open-circuit rating, which would give 4.5V which is below the 5.5V maximum rating of the SPV1040.

Would it still be necessary or sensible to place a zener diode voltage regulator in front of the SPV1040 to make sure the input voltage will not exceed 5.5V? If so, is it necessary to have a resistor in series with the zener diode to limit the current? I can imagine that since the power source is current limited anyway this may not be needed, and I don't want to waste power in the resistor in the common case that the zener diode is not dissipating any power. So, given the 6V open-circuit and a 5.5V zener diode that can handle 150mA, do I need the resistor in this circuit?

Example circuit: a DC power source, resistor, zener diode, and DC/DC-converter. The resistor and zener diode form a typical voltage regulator for the DC/DC-converter, to limit the

Or am I going about this wrong and is another type of over-voltage protection preferable?

(I have also looked for other buck-boost converters with MPPT functionality and a wider input voltage range, but without success. There is the LTC3119, but it is far more complex (and expensive). Other chips with a higher maximum input voltage typically also have a higher minimum input voltage, which is not ideal; or they are only available in packages that I wouldn't be able to solder by hand.)

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This IC is not well suited for the task.

MPPT (Maximum Power Point Tracking) relies on matching impedance according to the maximum power transfer theorem. However, in the case of a photovoltaic array (PV), it functions as a current source with a voltage limit (Voc) and a short circuit limit (Isc). The load line representing Voc/Isc=Rmpp, which is the effective incremental resistance at the maximum power point (MPP), is transferred due to the MPT theorem. However, unlike in 50 ohm systems with 50% loss, this is a current source, resulting in a different scenario.

The incremental resistance is not Vmpp/Impp, but rather the derivative or slope of the V vs I curve at MPP.

This information is not commonly taught, but is based on personal experience. It is worth noting for professors.

If you refer to the quadratic points on the MPT curve on Wikipedia, you will find that at full sun, Vmpp/Voc is approximately 82%, and this drops to 72% at half sun. Therefore, if Voc=7.2V, 82% of this is 5.9V, which is close to the MPP voltage for full sun. Ideally, to transfer this current to a battery, a 5 to 6V battery without an MPP is desired.

There are better plots but this shows the Voc, Ioc and MPP quadratic set of points. Take your Voc values and scale them to choose the % of Voc for the MPT point or MPP.

You can use a suitable 6V battery to store enough energy for a few days then a MOSFET LDO with a much less than 0.5V min. dropout.

Image alt text

What this means is you average solar current, voltage or power must exceed the average load of your application to sustain 5V. There is no lossy tracking of MPPT but rather using the best 5 to 6.5V battery to store linear charging and then never draw more average energy than being supplied. The LDO ought to have a heatsink if you need a power burst that raises it's temperature and use your Thermodynamic thermal resistance rules.

For a 150 mA PV array, I think this is a sensible open loop design.

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I need this especially during poor conditions when the panel won't yield 6V by far (3 comments)
I need this especially during poor conditions when the panel won't yield 6V by far
Keelan‭ wrote 10 months ago

If I understand this correctly, you are proposing to take out the SPV1040 and connect a 6V battery directly to the solar panel. Perhaps I should have noted that the 7.5V I measured is in full sun, and that in shadow the panel easily goes down to 3V or lower. I especially need to harvest energy during early spring and late fall when sunlight is sparse. This is why I started to look for a solution that would harvest enough energy in these poor conditions as well, rather than relying on a large enough battery that can fill up when conditions are good.

TonyStewart‭ wrote 10 months ago · edited 10 months ago

Yes If 6V exceeds the reverse voltage rating on the PV then a diode is needed. When the voltage drops below 6V the energy is getting quite small % so harvesting under these conditions requires high impedance loads and there are custom IC's to do this with great effort for small reward.

Perhaps you can start by measuring the estimated MPT impedance or Zmpt = Voc/Isc then the the maximum power Pmpt for those conditions you wish to harvest. Then you can define the load to store or use during those conditions. so what is the range for each value? 3V will be almost useless other than to indicate how dim it is outside.

The panel has no pupils like your eyes so brightness is a function of power Pmpt

You know is P=I^2R so if you have a load R=Zmpt you can then transfer maximum power if you know Zmpt. Perhaps some reversed LED's or Photo diode can be used to match the array Voc with some resistor like 1M then you can use this to regulate the impedance of your load.

TonyStewart‭ wrote 10 months ago

Otherwise using PWM pulsed energy to store power into a suitable inductor with CCM continuous drain you can match the average impedance with a circuit that does this with clamp protection where the % of drained current determines the reactive impedance as a voltage booster to a small stored capacitor.

These are just hand-waving suggestions to get you looking for harvesting solutions that exist.