If you’ve ever tried to make a really low-power circuit — especially one that runs on harvested power — you have probably fallen into at least a few of the many traps that await the unwary in this particular realm of electronic design. Well, Dave Young has been there, seen the traps, and lived to tell about it. In these territories, even “simple” systems can exhibit very complex, and sometimes downright confusing behavior when all possible operating conditions are considered. In his 2019 Hackaday Superconference talk: Scrounging, Sipping, and Seeing Power — Techniques For Planning, Implementing, And Verifying Off-Grid Power Systems, Dave discusses a number of these issues, how they interplay with low-power designs, and tricks he’s collected over the years to design and, more importantly, test these deceptively simple systems.
Dave is an electrical engineer and his company, Young Circuit Designs, has worked in the test and measurement, energy, and low-power consumer industries. We were lucky to have him share some of his 15 years of experience on the Supercon stage this past November, specifically discussing devices powered from harvested energy, be it wave energy (think oceans not RF), thermal energy, or solar. The first lesson is that in these systems, architecture is key. Digging deeper, Dave considers three aspects of the architecture, as mentioned in the talk title: scrounging, sipping, and seeing power.
Dave cautions about overly optimistic estimates of available power. For example, a 6W-rated solar panel produces an average 4.11 watt-hours per day when averaged over the entire year at his location in Rochester, NY. This is, of course, much less than you might expect. Such a discrepancy isn’t unique to solar systems, either. So much so that Dave cautions us not to overlook another possibility: eschew scrounging altogether and use primary batteries with enough capacity to run low-power systems for their design lifetime. For ultra-low-power systems, this is certainly a possibility, even in environments where “free” power is available, and may represent an easier and more reliable design alternative.
There are other issues with solar systems, too. If you’ve done solar design, you may have seen maximum power point tracking (MPPT), because solar cells yield a variable amount of power depending on the load. To extract the maximum power from the cells for a given illumination, you may have to adjust the voltage that the cells run at by changing the current drawn. There are integrated circuits designed exactly for this purpose, and for large systems, this kind of architecture is a no-brainer. However, Dave points out that these MPPT ICs can consume 5-25 mA themselves, which can easily negate their benefit for small systems. Sometimes a linear charge controller is simply more efficient because it consumes less power itself.
Besides the power source itself, a design must consider what to do with the power. In the second section of his talk, Dave starts with the premise that energy harvesting systems are always in either one of two states: they have power to burn, or are just about to run out of it entirely. While you can control the amount of time your system spends in either state with proper design, you must carefully consider how your system will behave in each. This feast-or-famine existence means you need to create a power breakdown for each state, and not just a single overall budget. This implies a careful examination of all the conditions the system may find itself in. For example, solar systems will have different power available during various times of day and year; coupled with different levels of battery charge, this creates a number of states that must be examined, and leads to a number of possible failure points.
In the final segment of his talk, Dave discusses seeing power: in other words, determining where in your system the power is going. Here again we have the benefit of some hard-learned lessons. Fist, he suggests verifying your power budget(s) at the line-item level. By turning off chips one at a time with firmware, you can verify that their actual power consumption matches the design goals. For those parts that can’t be disabled in firmware, he recommends simply removing them from the PCB as you go. The value in such an exhaustive test is that you won’t be fooled by a system that meets the overall power budget but for the wrong reasons: such a system is likely to perform incorrectly under some combination of conditions.
There are many more lessons in Dave’s video than we can cover in this brief discussion. Definitely check out the full video of Dave’s excellent talk, and if you want to follow along at home, you can find the accompanying slides here.