# How to let a capacitor be fully charged before being discharged by a load?

I am having a problem getting a supercap to be fully charged before being discharged by a 0.9 V to 5 V booster.

Here is my situation: I am using TEG to charge a capacitor to 2.3 V. I am able to do that if I disconnect the 0.9 V to 5 V booster. However, when I connect the booster, it will discharge my supercap when it is charged to 0.9 V.

How do I allow my supercap to be fully charged to 2.3 V before being discharged completely? Based on the diagram, I want my microcontroller to be active for around 10 seconds to complete a task. If the supercap is always being discharged at 0.9 V, my microcontroller is only active for 1 second and then goes back to off mode.

Does anyone have an idea? I'm doing energy harvesting, I don't want to use any external power sources like batteries.

• Does your boost converter have an ENABLE input? Commented Mar 21, 2017 at 17:11
• I think as long the input to the 2nd boost converter has more than 0.9v it will be ON Commented Mar 22, 2017 at 2:38
• Plz show datasheet of your boost reg... Commented Mar 22, 2017 at 11:21
• im guessing the cap goes to gnd, not in series as in the picture? The second booster will come on when the input voltage is above 0.9V (check its leakage current also). If you want it to come on only when the cap is charged to >2V add in a diode or similar with a known voltage drop and very little on resistance to maintain current delivery. (I.e. the cap would need to charge to 0.9V plus the extra voltage drop of whatever part you choose to turn the 2nd stage on) Commented Feb 13, 2019 at 14:15

Something like the following should work. The comparator compares the input to a reference voltage, and when its >2.5V it turns on the PMOS transistor. Once the comparator output is high the sense threshold changes and the comparator will not switch off until the output drops to 0.9V. I leave it to you to find the resistor values and an appropriate reference chip.

simulate this circuit – Schematic created using CircuitLab

The TLV3691IDCKR comparator chip can work with a supply voltage down to 0.9V and consumes 200nA of current.

The SI1013X-T1-GE3 PMOS transistor has a threshold of 0.45V and can be turned on by the comparator even with a 0.9V supply.

https://www.vishay.com/docs/71167/71167.pdf

The resistors should all be chosen to minimize current draw. Choosing R2 > 1MEG is a good start.

• How much current does your zener need? Commented Jun 30, 2021 at 12:59
• @ElliotAlderson A regular zener would probably draw several mA, which is probably too much. I didn't have a specific reference chip in mind so I put it there as a generic example of a reference. If someone is just building one of these in the lab one could just use a regular silicon diode forward biased with 1uA to get a reference in the 0.4V ~ 0.5V range. Otherwise one could use a micro-power voltage reference powered from a 3.3V coin cell battery (which would probably last several years). Commented Jun 30, 2021 at 18:15

Use comparison circuit and a FET to block VOUT (after the capacitor) until a threshold is met.

Set some hysteresis to shut back off after it drops below 0.9V.

Also...I wonder if you could use VSTORE and Vout2 instead. Have your bulk capacitance on VSTORE and enable Vout2 with a comparison circuit.

• I was considering #2 also, but I think the OP wants it to stay on till the cap voltage pretty much drains to the limit the 2nd stage can handle. That gets a little trickier. Commented Mar 21, 2017 at 19:01
• Good point, I've edited.
– scld
Commented Mar 21, 2017 at 19:12
• Yeah the connection draw is wrong. But using any scr and fet surely we need Vdd right ? Which also means I need batteries already ?? Commented Mar 22, 2017 at 2:37

I am assuming that cap is actually shown incorrectly in that picture and is actually tied to ground between on one side and attached between the 3108 and the 2nd booster.

You need a voltage sensing circuit across the capacitor that turns on the 2nd part when then cap is charged to some level with sufficient hysterics to keep it alive down to some other level.

Perhaps a you can find a suitable SCR that turns on at the appropriate capacitor voltage and stays alive with whatever current your downstream circuitry is consuming until the voltage drops so low it can't supply that current any more.