Energy harvesting & MCU inrush current solution - Page 1
Forum section: electronics / projects

author: Howardlong
author_rank: Super Contributor
timestamp: September 19, 2023, 01:41:05 pm
content: FolksI came up with a modified version ofthis excellent articleto deal with the situation where, during power up, a microcontroller consumes significantly more power than in its application state.It enjoys a quiescent currents under 5nA, two to three orders of magnitude less than the average minimum microcontroller current.The typical application is in small solar powered devices.But digital circuits employing CMOS like microcontrollers have a problem. During power ramp up, below standard operating voltages, the microcontroller transistors are typically operating in linear mode and thus can take significant current for a short period of time across the power bus. It is likely that in lower solar panel illuminations, the microcontroller never even manages to start up, remaining stuck in its low voltage elevated current state.Furthermore, many microcontrollers start up in a higher power state for a short period, either by design or because of side effects such as pre-clock switching states.The originally cited article works well if all you're interested in doing is dumping accumulated energy into a device for a short period of time, but it's not so good if you have a microcontroller project that needs to keep running.The problem with the original design in a low power target is that a significant amount of current is drawn by the final BJT's base emitter junction. This is of little consequence when the load is over an order of magnitude higher in terms of current draw, but it is an overwhelmingly significant overhead when our microcontroller is likely to be drawing single digit microamps (or less).So I replaced two of the BJTs with MOSFETS, and removed one of the transistors, switching the rails in the process. As I was using solderless breadboard for developing this, the choice of MOSFETs was down to what through hole parts I happened to have in stock. In a final design, SMD dual MOSFET Pch/Nch in a single package is the solution.The fundamental basis for my version of the circuit is exactly the same as the original: it's a latching circuit not dissimilar to an SCR, using positive feedback to provide a rapid turn on time, and a voltage reference (LED) to control the turn on voltage. By using MOSFETS instead of BJTs, the BE current overhead is removed.Key parts of the original design that I carried forward are (1) the use of leakage current in reverse biased diodes instead of resistors, (2) the use of a BJT as an ultra low leakage diode, and (3) the use of an LED instead of a zener as a voltage reference.Reverse biased diodes such as 1N4148 have a typical resistance of the order of 1Gohm, but be aware that glass diodes have some photovoltaic properties, so plastic packages such as SMD versions are preferred.Using a BJT as a low leakage diode offers reverse bias currents in the femtoampere range: when you're dealing with active impedances in your circuit in Gohms, this is essential.An LED is used rather than a zener because zeners typically require significant current to hit their breakdown voltage.Typically, a blue LED in this design triggers the circuit at 3.4V, and a green LED at about 2.6V.Thanks to the inherent hysteresis of the latch circuit,The turn off voltage for both is about 1.9V.Current draw of the circuit itself is about 1nA/V.Be aware of the LED's photovoltaic properties, this can significantly change the turn on rise time of the circuit.You'll note that the circuit is autonomous, i.e. there is no means to control it from the target microcontroller. This is by design, after considering the lack of use cases where this would even be needed.I've also provided a version of the circuit that you can probe with a scope and a DMM, using 100k resistors instead of reverse biased diodes, for those interested in tinkering or understanding further how the circuit works, but at the expense of if using about 10,000 times as much current!I used a Keithley 238 SMU for much of the testing: an SMU is almost essential for this kind of ultra low current design. During development, everything was built on a solderless breadboard: this avoids the need to repeatedly thoroughly clean the board of flux and other ionic contaminants every time you make a change with the soldering iron.1. My version for use with microcontrollers;2. My version with lower impedances to allow for tinkering & empirical analysis;3. Original circuit published in EDN.