Schottky diodes in series to reduce maximum voltage at supercapacitor solar charging circuit

Question

I use the following circuit to store solar energy harvested by photodiodes into a supercapacitor. The cap is rated at maximum 2.8V. The zener diode is rated at 3V, but depending on the harvested solar current it goes up to 3.15V. The schottky diode shall prevent the supercapacitor from discharging into the photodiodes.

Problem is: the voltage drop of the schottky diode is simply too low, which means in direct sunlight the capacitor gets overcharged up to 2.9V -> bad! I do not want to take a lower rated zener diode, as the current one drives the photodiodes nicely at the MPPT in sunny conditions.

Now the question: Are there any disadvantages of using a second Schottky in series, just to get another small voltage drop of around 200mV? That would be sufficient to not overcharge the supercap. But I'm unsure about other side effects. The whole circuit is extremely low power, so I want so save every possible uA.

I have seen schottkys in series, but rather for backup reasons (in case one fails).

Answer 1

Voltage rating of the super capacitor is as important as the operating temperature specification. The higher voltage leads to significantly higher self discharge.

Adding a second diode will definitely help to keep the voltage applied to the super capacitor within its operating condition. I don't see any disadvantage because of the secondary diode addition for the presenter circuit.

One suggestion is to employ a low leakage diode instead of two Schottky if feasible. It will less discharge the capacitor when the solar voltage is not present.

Answer 2

After some testing today:

The voltage drop of shottkey diodes goes down the less current is running through them. That means it's entirely possible for the photocells to charge the capacitor up to the maximum open circuit voltage of the photocells even with the diodes in place, since their voltage drop will go to near zero at very low currents.

Answer 3

I know it's been a while and you've probably already figured something out.

1. the comment above about forgetting about the MPP voltage for a simple solution is correct. The capacitor is going to drag the voltage down anyway. For a highly efficient MPP charge, a stepdown DC-DC converter would be required and off the shelf ones will not work for this application.

2. Running diodes in series will waste power, the voltage drop times the current flowing through them will be dissipated as heat. I'd suggest using the lowest voltage drop diode available for this application, they're available down to around 0.1v.

3. OK, we cannot over volt the capacitor. Two solutions: One, reduce the number of photocells such that their maximum open circuit voltage is at or below the maximum rating of the capacitor. Two, use a zener diode rated at the maximum voltage of the capacitor to bleed off excess voltage.

I'm making a similar simple system, but with larger components (120F 16v capacitor array) and have decided to use both of the methods... making sure the panel maximum open circuit voltage is lower than the capacitor max rating AND using a zener diode in case some unpredictable environmental conditions occur that raise the PV voltage.