Power Primer

A project log for SoLoRa - Solar Powered always-on IoT sensor node

Unique, compact power conditioning enables always-on sensor monitoring and Long-Range radio (LoRaWAN) link for IoT applications.

Joe MillerJoe Miller 07/15/2018 at 07:260 Comments

The challenges of always-on power with intermittent energy harvesting

When energy from the main power source is available, current (A1) feeds the energy storage cell and the main circuit with power. Ideally, while power is being generated, it is desirable for A1 = A2, which means there is no power loss through the power management circuit.  Also, when the power source is not available and the power storage cell supplies power to the main circuit (path A3) it is ideal that none of the main storage cell's energy is lost due to reverse leakage back to the power management circuit (-A2). 

In the end, the average current over time (Ah) provided by the power source (A1) must be greater than the time-use percentage of power consumed by the the Main Circuit (A3) plus the time-use percentage of reverse leakage back to the regulator when power is not available at the main source plus any A1-A2 loss (Power management leakage). 

Power Management Design Trade-offs

These trade-offs directly affect the always-on design margins when used with intermittent power sources. 

ComponentFactors that improve design marginsTradeoffs
Solar CellLarger or more efficient panelCost
Power ManagementEfficiency
Integrated vs discrete solution
Cost, size, time
Main Circuit (software)Lower Duty CyclesLower data rates
Main Circuit (microcontroller)Lower core Voltage
Peripheral management, sleep modes
Lower process speed
Complexity (dev time)
Main Circuit (Sensors)Lower power, faster integration periodPrice, quality
Storage CellSize is dependent on multiple factors
given above
dependent on other
design parameters

Another factor that affects always-on power margins are the site dependent equivalent solar hours per day. This site dependent variable is used in calculation as a weighting factor for the above design trade-offs.

Of course all these factors are to consider the overall design priorities and specifications. For instance a priority on my design includes easy to assemble, low cost and flexibility. This has shaped my design, most considerably in the Power Management circuit. I could not find an off-the shelf solution that satisfied these three goals so I design my own solution, a hybrid of an adjustable linear regulator with a shutoff circuit to prevent leakage back into the regulator.  The idea of using a linear regulator is not the first topology one would think to use in a circuit with high efficiency ambitions, but the number work out. I will show my calculation and justifications in an upcoming log.