Pragmatics of nano power radios

This is a brief note about high level concerns with nano power radios, solar powered without batteries.

Don’t rely on this, study it yourself, especially until I add proper links.  Some of it is just crude notes, even speculation.

Other References

A note at Mouser about ultra low power mcu design.

Context: nano power

The power supply:

  • provides low average current, around 1uA
  • has no large reserve
  • is is expected to provide zero current often (say every night)

For example:

  • solar power with a capacitor
  • no battery
  • indoor light
  • solar panel smaller than a credit card

Overview

  • radio is duty-cycled
  • a voltage monitor/power supervisor and load switch chip provides clean reset/boot
  • boot sequence must be short and monitor mcu Vcc
  • use a power budget for design
  • use synchronization algorithms
  • testing is hard
  • over voltage
  • energy harvesting

Duty-cycled radio

The radio is sleeping most of the time.  When sleeping, a low-power timer runs to wake the system.  The sleeping radio cannot wake the system when it receives.

Example: the system may sleep for a few seconds, and be awake (with radio on) for about a millisecond.  That is, the duty cycle is around 1000.

Voltage monitor/Load switch

A microprocessor (in a radio SoC) needs a fast-rising voltage to boot cleanly.  Otherwise it may enter a state where it consumes power without booting. (Fibrillating?)  It may be in that state for a long time.  The solution is to use an external voltage monitor aka power supervisor aka reset chip.  E.g. TPS3839 (ultra-low power of 150nA.)

You can’t just connect the voltage monitor to the reset line of the mcu.  Otherwise, the mcu will still consume power while its reset line is held in RESET state. (Between the time voltage is high enough for the voltage monitor to have active outputs say 0.6V and the time the voltage is high enough to run the mcu say 1.8V.)  An mcu may draw a fraction of a milliamp while held in reset.

So the voltage monitor drives a high-side load switch that switches power (Vcc or Vdd) to the mcu.  I use the TPS22860.  (You can switch ground i.e. low-side with a NMOS mosfet but it’s not so easy to design your own high-side switch.  You can’t switch the low-side of an mcu because many pins may leak to ground?)

Voltage monitor hysteresis and boot sequence

The voltage monitor asserts its Out (sometimes call Not Reset) at a certain threshold voltage but then unasserts if the voltage falls below the threshold a certain amount called the hysteresis.  While the mcu is booting, it must not use so much current that Vcc falls below the hysteresis.  The boot sequence typically does a bare minimum, then checks Vcc, and sleeps until Vcc is much beyond the the minimum.  That is, allowing time for the ‘challenged’ power supply to catch up and store a reserve.  Only then does the software proceed to use the radio, duty-cycled.

You could use a voltage monitor with higher hysteresis.  But they don’t seem to make them.  The hysteresis of the TPS3839 is only 0.05V.  You can play tricks with a diode/capacitor on the input of the voltage monitor to make it seem to have a higher hysteresis (to delay longer before un asserting.)  And there are application notes on the web about adding hysteresis to voltage monitors.  But they seem to apply to older voltage monitor designs, and don’t seem to apply to the ultra-low power TPS3839 (which samples Vcc.)

Also, you could design your own voltage monitor with more hysteresis.  For example, see the Nordic solar powered sensor beacon.  That uses a few mosfets to provide a 0.2V hysteresis (say booting at 2.4V and resetting at 2.2V).  Unfortunately, they don’t seem to have exactly documented how the design works.

Power Budget

A power budget calculates the average current of a system, given certain phases of certain durations, where each phase uses certain devices/peripherals.

Here the main phases are:

  • sleeping (say 1.5uA for 1 second)
  • radio and mcu on (say 6 mA for 1 milli second)

You can almost ignore any phases where only the mcu is active, it should be a small portion of your budget.

A discussion at Digikey.

Synchronization algorithms

These make your units wake at the same time, so they can communicate with each other.

A beacon is usually unsynchronized.  The thing that hears a beacon (e.g. a cell phone) has  enough power to listen a long time.  You also might not need to synchronize if you have a “gateway” that is always powered and listening.  (See Zigbee.)

This seems to still be a research topic, there is much literature to read and few open source code examples.

Testing is hard

With such a challenged, nanopower supply, testing is hard.  A bug may exhaust power so that the system brown out resets, losing information about what happened.

Most hardware debuggers make the target consume more power than the power supply can provide?  TI seems to have ultra-low power debugging tools, but I haven’t studied them.

You can implement fault/exception handlers that write to non-volatile flash so that you can subsequently connect to a debugger and read what happened.   Default handlers typically just infinite loop (which will brown out reset.)  Typical handlers will do a soft reset.  Unless your app makes a record or communicates that, you might not even know the system reset itself.

Agililent (formerly Hewlett-Packard) sells expensive instruments for monitoring power consumption.  These may tell you you when (in relation to other events) you are consuming more power than you expect, but not exactly why.

Over voltage

A solar cell is a current source, and provides a variable voltage.  Voc is voltage open circuit (when your capacitor is fully charge.)  It can exceed the Vmin of your radio (typically 3.6V.)

Voltage regulators (such as shunt regulators) that prevent that are themselves current wasters.

You can choose a solar panel whose Voc is less than the Vmin, but there are few choices in that range (Voc < 3.6V, Vope around 2.4V, for indoor light.)  Or you can require that your solar panel never be exposed to strong light.

I haven’t found a zener diode that would clamp the voltage to 3.6V, and not leak much, at such nano currents.

Energy Harvesting

This is another buzzword, but good to search on.  It often means: with a single coin cell battery.

Energy harvesting chips are available.  They solve some problems you might not have, such as over-voltage protection, or voltage boosting.

It often refers to other power sources such as heat or vibration.  Those power sources are usually even smaller than solar (light) power, but solar power is episodic (diurnal.)

Solar power in different setting differs by orders of magnitude.  Direct sun is ten times stronger than outdoor, blue-sky shade, which is ten times more than strong indoor light, which is ten timer more than  dim indoor light.

 

 

Advertisements

Some notes on Panasonic Amorton solar cells

These are just rough notes that might help someone else in their personal electronic projects.  About Amorton’s indoor solar cell products, AM1456, AM1417, etc.

These solar cells are like what you see in calculators.  They are only a few square centimeters or larger.  Typically like pieces of glass.

For low light

These indoor products are for low light.  They are characterized for as little as 50 lux, which is not much light, typical of an indoor space with average lighting.  A room with a sun facing window, on a clear or overcast day, typically has much more light.  Even typical artificial lighting provides this much light.

The power available at these lighting levels is only a few uA.  Also, the Vope (which is the operating voltage, which really means the maximum power voltage, see MPPT) is a fraction of the Voc (voltage open circuit)  in much stronger light.  For example, if the panel has four cells which each deliver a maximum of 0.6 volts, the Voc might be 2.4V but the Vope in 50 lux might be only 1.4V.

You should design your circuits to operate around Vope, since if you design to operate at the Voc, it will take strong light, and the power delivered will be smaller than you could get at Vope.

(PowerFilm does not characterize their film solar cells at such low light levels.  But they recently started selling LL3-37, which IS targeted for low light.)

Availability

Some of the glass ones are available from Digikey and Mouser.  The film versions don’t seem to be readily available in small quantities to retail buyers.

Solderability

The models that are commonly available have pre-soldered wires, AWG 30.  I have had good success in unsoldering the wires, leaving the solder ball, and soldering on a different wire (including tinned piano wire.)

Surface Mount

I also tried unsoldering the wire, cleaning the pad with flux and desoldering braid, and trying to reflow surface mount.  With very poor results (say one success in three.)  The manufacturer said this is not a supported use.  After the failure, you see a brown surface that solder won’t stick too.  Evidently the ‘interface’ between the solder and the semiconductor is very thin and its solder ability easily destroyed.

Some of the product variations for AM1456, AM1417 have no pre-soldered wire, but only conductive paste, and they are available only in large quantities (AFAIK.)  I don’t think these are intended for reflow soldering either.

Amorton recently started selling model AM1606, which IS intended for surface mount (SMD.)  Available from Mouser.

Durability

They seem relatively robust.  I have dropped them from desktop height onto concrete and they don’t seem to break.  I have had a few, small, conchoidal chips out of the edge, seeming cosmetic, not affecting the power out.

Under extreme mechanical stress, the soldered pads occasionally detach at the ‘interface’.  See above re surface mount.

Safety

The glass edges are not sharp.  I have never cut myself on the edges.  I suppose the manufacturing process somehow rounds the edges a little, even though they appear quite square.  However, I suppose the edges are intended to be enclosed in a frame.

But since they are small and glass, they ARE a hazard for small children, and if they should break into pieces.

 

Fundamentals of mobile, indoor energy harvesting

This is a quick list of essential facts, without links or references.  I learned these working on my application: solar powered Calder mobiles (trademarked Solabiles.)

Energy harvesting:

  • not connected to the power grid
  • years, or forever, between any battery changes
  • harvests ambient or waste energy
  • harvests light, heat, or vibration

Light has the most energy, for mobile or ‘anywhere’ harvesting.  Only constrained points, such as on a hot surface (a body?) or on a vibrating motor, might have more harvestable energy.

Energy harvested is proportional to the light level.

Light levels indoors are exponentially smaller than outdoors.  Roughly:

  • full sun: 100k lux
  • shade but with full sky exposure: 10k lux
  • bright indoor: 1k lux
  • average home lighting: 100 lux

The lighting within a room can vary exponentially:

  • near a south facing window, overcast winter day: 1k lux
  • other side of the room: <100 lux

Lighting in a room can depend on the direction windows face:

  • near a south facing window, overcast winter day: 1k lux
  • near a north facing window, overcast winter day: 100 lux
  • other side of the north room: 10 lux

The human eye and brain hides the above facts.  It doesn’t seem that the lighting is so different in a room, but a luxometer tells the story.

Solar cells produce energy proportional to their area.

Solar cell technology probably is not going to get much better soon.  They might double in efficiency, but they probably won’t reach 100% efficiency.  You must make improvements elsewhere.

At low light levels (indoors), amorphous silicon solar cells produce more energy than crystalline silicon solar cells.

Solar cells and panels produce the most power at their maximum power point (see MPPT.)  In other words, when they ‘see into a load’ of the voltage of their MPP.  (I don’t really understand why, and the best water analogy I can come up with is:  a very thin sheet of water is a lot of water but has little pressure, and can’t do much useful work.)

You don’t necessarily need an “energy harvesting” chip.  They provide:

  • voltage boosting
  • MPPT
  • battery management (using a battery might bring its own set of problems to a design.)

If you don’t need either of those, you do your own MPPT: insure the voltage the solar cell sees remains near the MPP (whenever conditions allow.)

Ordinary capacitors, supercapacitors, and batteries use different technologies that each bring their own design problems.

At very low light levels, say <50 lux, the leakage of circuits is important to a design.  See my next post.

Cutting Powerfilm thin-film solar cells

Experimental result: you can cut Powerfilm thin-film solar cells.  I wrote this blog because whenever I search for that topic, the usual results are obscure references to “follow the instructions” and the manufacturer’s website doesn’t seem to discuss it.   Maybe this is not an important use case.  But I wanted a solar cell (a panel really) smaller than the smallest one made by Powerfilm (model SP3-37, which is about 4×6 cm.

Cutting down a thin-film solar cell

Simply take a sharp scissors and cut in on a line along the direction of the fingers (the T-shapes that seem to gather charge from the top of one row of cells and carry it in series to the bottom of the next row of cells.)  In other words, each piece retains a portion of the silvery contact bars at opposite sides of the panel.

I was able to cut on SP3-37 into thirds very nicely (each piece about 1.2 cm by 6 cm.)  For some reason, there is a slight voltage drop on the pieces.  Under a certain light condition, the whole panel generated 3V open circuit, a two-thirds piece generated about 2.8V and a one-third piece generated about 2.6V.

Cutting in in thirds avoids cutting into the fingers, and gives three pieces that look about the same (with the finger down the middle.)  I don’t see why you couldn’t cut along the fingers and get six pieces.  I would guess that if you cut so that some piece has no fingers, that piece would have much reduced function.  But I haven’t tried it.

Cutting in this way leaves the pieces with edges that are not sealed by the outer covering plastic laminate of the entire panel.  I can’t say what the effect is on the lifetime, or warranty.

When I first contemplated cutting a thin-film solar cell, I worried that the shearing action would somehow short the layers of the solar cells.  Evidently not, at least in the short term.

Trimming the silvery contact bars

In my opinion, the silvery contact bars are much larger than they need to be.  Probably the manufacturer contemplates they need to be that large for ease and reliability of soldering to them, and that they will always be hidden by a bezel of the enclosing product.

Some applications, such as in RC planes, for which the manufacturer makes a lightweight model, you would also want to trim off excess silvery contact bars.

I haven’t yet tried to trim the silvery contact bars.

Simple trimming of excess

On the stock part, you can see where the outer plastic laminate seals the edges, away from any functional layers.  In other words, you can see the channels where the manufacturer has cut a strip of the parts into parts.  The stock part seems to have some excess, especially along the edge having a silvery contact bar.  Even if I don’t want a smaller panel, I usually trim this excess away just for neatness and to save weight.

Cutting non-rectangular shapes

Suppose you wanted to cut a thin-film solar panel into a decorative shape.  I suppose you could do that as long as you retained a portion of each silvery contact bar.

Cutting down the number of cells in series

I don’t think you can do that.  You are stuck with the lowest voltage panel that the manufacturer makes (3V: 5 cells of 0.6V each, in series.)  In my opinion, there will be demand for 1.5V panels.  Many electronics nowdays work at only 1.5V.

But it might be possible to cut the thin-film solar cell orthogonally to the fingers, and fabricate a new silvery contact bar to replace the one you cut away.

BQ25504 programming with one resistor net

This is a report of an experiment, a circuit board design using the BQ25504 , an energy harvesting chip.  The chip is intended for tiny, free-standing devices that harvest power from their environment, so they can run indefinitely without new batteries.

It is programmable using resistor nets. Unfortunately, some of the programming is for the battery management functions:

  • the undervoltage condtion, UV, when the chip should stop drawing from a battery
  • the overvoltage condition, OV, when the chip should stop charging a battery

If you are not using a battery, but say a super cap, you might not care about UV and OV.  But the chip’s pins that do the programming must still be connected.

That datasheet example shows three resistor nets, one resistor net for each of the above functions, and one resistor net for the battery OK condition. (And another resistor net for MPPT.)

My question is: can the three functions be programmed with one resistor net?  The answer is yes.

How I did it:

  • keep only the resistor net for OK programming (ROK3, ROK2, and ROK1)
  • connect pin 6 (VBAT_OV) to pin 10 (OK_PROG)
  • connect pin 8 (VBAT_UV) to pin 9 (OK_HYST)

On the evaluation board for the chip, with:

  • ROK3 = 1.4Mohms
  • ROK2 = 4.2Mohms
  • ROK1 = 4.4Mohms

This yields programmed voltages of:

  • UV = 2.2V
  • OK_PROG = 2.4V
  • OK_HYST = 2.8V
  • OV = 3.1V

(Which meets the requirements for ordering, specified in the datasheet.)

To actually modify the evaluation board requires unsoldering three resistors, cutting a trace, and soldering in two jumpers.

In my test, it worked.  ( But I didn’t actually test how the chip behaved in the OV conditions.  I didn’t use an oscilloscope to verify the waveforms.)

The three resistor nets have seven resistors total.  Four extra SMD resistors is no big deal, but if the intended use is in tiny devices, why not eliminate them? (If your application doesn’t need battery management.)