Voltage Stacking with energy harvesting STM32WB55

[methene]

Hello, I am attempting in voltage stacking of 2 STM32WB55 board, but unlike arduino I could not connect Top board's GND to bottom board's 3.3 V pin. I would like to request for help or insights for alternative method I could make it work. I would really appreciate the help. 

Problem

1. When I connect the Top STM32 GND to the VDD of Btm STM32, it discharged the voltage in Btm capacitor.

2. On one attempt I rearranged the circuit in similar stacked configuration, it did not discharged the Btm capacitor voltage. Instead the system goes haywire.

Thank you.

 

 

[Andrew Neil]

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@methene wrote:

I am attempting in voltage stacking .

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Not sure what you mean by that?

 

[methene]

Sorry for being unclear, I had attached a image that I had drawn for better explanation of my issue.

[Andrew Neil]

Thanks for adding the diagram - these things are so often better described by a diagram than in words.

But some more detail is required:

• How, exactly, did you connect the boards together?
  Please provide a schematic.
   
• What problem(s), exactly, did you encounter?

 

Also, what is the purpose of connecting the boards like that anyhow?

Why not just have them separate, or in parallel?

[methene]

Ouh sorry, what I meant was they are not "exactly" the same but very similar, as you mentioned by definition it should be the same. And this can be observed when I did my arduino testing experiment of voltage stacking.

 

Currently my problem faced would be that how can I connect the pins of STM32WB55 board with the capacitor to achieve similar result that I had obtained with arduino voltage stacking. 

[Andrew Neil]

Again, see:

https://community.st.com/t5/stm32-mcus-boards-and-hardware/supply-power-from-nucleo-stm32g4/m-p/743011/highlight/true#M22616

The boards are simply not designed to work in this way.

If you really want to pursue this, you're going to have to do a custom board design which is specifically designed for this. Even that is likely a dead-end, as the chips themselves have not been designed for this.

If you're interested in maximising energy harvesting, I would suggest that you look into switched-mode power supplies, and Maximum Power Point Tracking (MPPT)...

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[methene]

Yes I am fully aware the boards are not meant for the purpose of this, is just that my topic are related to voltage stacking of microcontrollers with energy harvesting capabilities. I appreciate any input on how can I make it functionable, but if it is impossible I might have to switch my approach to this.

 

Thank you very much for you knowledge. Would appreciate if there any other method I could achieve similar result.

[unsigned_char_array]

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@methene wrote:
my topic are related to voltage stacking of microcontrollers with energy harvesting capabilities"

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What are "microcontrollers with energy harvesting capabilities"?
As far as I know this MCU does not suck energy out of the environment. It's not an energy harvesting MCU. You need to feed it power the old fashioned way. Which could be an energy harvesting circuit. An energy harvesting circuit usually takes a very low power power source (microwatts or lower) and stores this energy in a small battery or super capacitor and periodically wakes up an MCU which does it's thing (take a measurement, or send some data wirelessly) and then goes back to sleep. Here you seem to be connecting the solar cells directly to the MCUs. You need a battery.

You need to go back to first principles. An MCU consumes less current at a lower voltage. So if you give it the lowest voltage it needs you can run at a lower power (voltage*current). You can save much more power by lowering the clock frequency, using sleep modes or even powering down. A very low voltage might make it harder to interface the MCU with other chips, unless you use a different voltage for the GPIO(sometimes this is possible). You pick a voltage (or voltage range) and you stick with it.
You probably need a voltage regulator to achieve a voltage within the operating conditions of the MCU unless your power source happens to have just the right voltage (and stable). A voltage regulator is not perfectly efficient. Especially at low currents where quiescent current (no-load leakage current) is probably dominant.

Connecting two MCUs in series won't work as they require a specific constant voltage, not a constant current. They would need a shunt regulator similar to Lithium ion cell balancing circuits to keep the voltage equally distributed.
Voltage stacking in ICs is very complicated as it requires multiple voltage domains and therefore cross voltage domain interfaces in order to allow communication between different domains. In addition you would need some logic that would equalize the current consumption of both domains as to not have to waste some current using shunt regulators. Voltage stacking has been proposed for GPUs. That would make sense as parallel tasks could be synchronized to consume similar current. The GPU would then require double the voltage and half the current. I think current GPUs need about 1V and can draw hundreds of Amps! I see no point in doing that for low power MCUs.

Maybe you were thinking that if the solar cells produce more than twice the voltage an MCU needs you could lower the current when connecting the MCUs in series instead of in parallel. But a switched capacitor voltage regulator does that too internally (it charges capacitors in series and discharges them in parallel to half the voltage), so you could use that too. Or use a buck converter. Or connect the solar cells in parallel.

 

 

 

[methene]

You are correct, my phrasing is incorrect, I am connecting an energy harvesting circuit which compose of PV and capacitor to power STM32WB controller. The PV will charge the capacitor and once it reaches a certain threshold it carries out an action and then enter back a low power mode that consumes less power to charge again and the process repeats. 

 

Since in energy harvesting, the voltage are not stable instead it charges "increase" and discharged "decrease", does that meant that it is impossible to do voltage stacking for energy harvesting system.

 

I am running same tasks for the top and bottom domains, to make sure they consume similar voltage and current. My goals is to find out if it is possible for voltage stacking to work on a energy harvesting system.

[unsigned_char_array]

My advice is to optimize the voltage regulators for lowest quiescent and lowest shut down current. That's the bottleneck.
And measure current consumption (this is hard to do for very low currents, but there are devices that do this).