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ASIC power management for self-powered IoT sensors

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ASICs for self-powered IoT sensor applications with integrated energy harvesting power management interfaces can help us end the need to replace batteries.

By Johan Pedersen

My IoT sensor’s battery is dead …. again ….

The Internet of Things is becoming a well-used term, and the prediction about how many millions or billions of devices that will be connected to the net can be difficult to comprehend, Gartner Says 6.4 Billion Connected “Things” Will Be in Use in 2016, Up 30 Percent from 2015, but one thing is certain – there is still major potential in installing instruments globally using wireless sensors for a wealth of applications and for creating the “internet of things”. Everything from automation and alarm systems in the home, to smart clothing and fitness trackers, to industrial monitoring and subsequent optimisation. A major barrier to realising this is the battery. If we can avoid using batteries, the rollout will be much stronger, since replacing batteries, e.g. in a wireless thermostat controlling the heat in your home, becomes a major barrier when you have 10 of them. The batteries will run out of power at different intervals, and the heating will be out of control until the batteries are replaced.

Self-powered sensors

One solution is to make the devices self-powered. An self-powered system, which charges itself with energy from its surroundings, means a service-free solution. Depending on the surroundings, energy can be harvested from many sources, such as light, movement, heat, magnetic and electrical fields. Known as energy harvesting, the subject is becoming more widely discussed, but any major distribution of batteryless and self-powered systems is something that still lies in the future.

Designed for varying energy

If you assess the available solutions and components for energy harvesting on the market, you will realise that there is a need for both deep competencies in physics and power electronics for it to get underway. It is not as simple as throwing out the battery and inserting an energy harvester, for example such as a thermoelectric generator, which harvests energy from heat. A self-powered device requires that the entire IoT sensor has been designed for energy harvesting – which includes the sensor, data processing and communication part. It must be considered that the energy source may vary and perhaps there will not always be sufficient energy, while the electrical profile of an energy-harvesting generator is very different from a battery.

Power management interfaces

The primary components for realising a self-powered solution not only require an energy harvesting generator (e.g. a solar cell, thermal generator, piezo element), they also require an interface for power management, because no energy harvesting generator has the same output characteristic as a battery. A power management interface converts current and voltage to a level that can drive the IoT sensor, which typically requires 1.8–5 V. For example, a thermal generator supplies a very low voltage output (~20–50 mV DC) at low temperature gradients, whereas a piezoelectric generator supplies high voltages (~10–100 VAC) when it harvests energy from vibrations. At the same time, the impedance of the generator must be matched, to achieve the highest efficiency, which the power management interface also deals with.

The Energy Harvesting IC market

Several IC manufacturers have components for the energy harvesting market and offer several types of power management interfaces for energy harvesting. Their input stage is typically targeted for one or two types of generator. Many solar cell ICs also cover thermoelectric generators with low gradients, since their output profiles are similar, and they therefore can be driven in the same way. However, there are compromises, and many solutions have broad specifications to ensure that there is a wide application range and high volume.

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Figure 1. A selection of energy harvesting power management ICs on the market and their distribution between solar cells, thermal and mechanical energy-harvesting generators.

Cold start at low voltages

Whether a transistor is in a known state (high/low) depends typically on whether there is voltage on its gate – in other words, on whether it is polarised or not. Until then, current out of control can flow. This typically means nothing in an electrical circuit because we are talking about very small currents, but when you have a source that supplies microwatts, it is important that there is constant control of the leakage currents. This means that many ICs require a battery as a stable energy source to be able to maintain their high efficiency properties, maintaining constant control of the circuit. If there is no stable energy store, then a large energy input will be required, before it can operate again. This is typically defined as the cold start scenario for an energy harvesting IC, which operates without a battery. The minimum input current at a cold start is typically a factor of 10 larger than when operating with a full energy storage. For example, see Fig. 2, which shows an example of this input characteristic.

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Figure 2. Example of a cold start input characteristic for energy harvesting ICs, where up to 6 times greater current is drawn when the input voltage is low, <1.5V.

DC/DC replaces LDO

An LDO is often used to deliver the correct voltage, if the IoT sensor’s MCU operates at a lower voltage than the power source. But the higher the voltage difference between the source and consumer, typically the higher the loss in the LDO. A typical lithium, battery supplies 3 V or 3.7 V, whereas an MCU perhaps ideally runs on 2 V. In many cases, it therefore makes sense to utilise a DC/DC converter, which ensures that this loss is kept to a minimum, which can be up to 40 % less in a DC/DC converter compared to an LDO.

One-size does not fit all

Since many ICs attempt to cover a wide range of energy harvesting generator technologies, this means it is easier to find an IC solution that can function, but optimisation is difficult. Typically, the energy available is sparse, and a high efficiency is therefore important. However, the price of an IC is even more important. Extra functionality in an IC, which is not used, is a waste of silicon, resulting in a price higher than necessary. This means that many IoT sensor developers hit a price wall, when the Bill of Materials is made for a self-powered solution. If the generator costs a couple of dollars, and the energy harvesting IC costs even more, there is suddenly a large price difference compared to the less than one dollar price of a coin cell battery.

ASIC – Custom chip for self-powered IoT sensor

At high volumes, >100,000 IoT sensors, it makes sense to evaluate custom made ICs (ASIC) for integration of discrete solutions. An ASIC can include energy harvesting power management interface, sensor interface, processor and communication interface on the same chip, in other words, a system-on-a-chip (SoC). This optimises the use of silicon and thus the unit price. You can keep the leakage current to a minimum and operate with high efficiency, and you can miniaturise your IoT sensor, with far fewer components. By building a system on a single chip, you can also ensure energy-smart operation. In other words, based on how much energy there is present, the IoT sensor can adjust how much it does.

In scenarios where the complete self-powered IoT sensor solution comes down in price to a couple of dollars and takes up less space than a matchstick, it becomes relevant to talk about the wide distribution of IoT sensors. Levels, such as temperature, humidity, vibration on e.g. a production line can suddenly be measuring per metre, alarm and safety systems can monitor all doors and windows and the indoor climate in a building can be controlled through a matrix of measurement points rather than using a single sensor. There are many possibilities, and it is easy to come up with the first ideas for potential self-powered concepts. Once the IoT has become free of the battery, there is ‘almost’ no limits to what can be optimised in our daily lives.

DELTA (from 2017 a part of FORCE Technology) works every day with IoT sensors and energy harvesting in many different industries and develops ASIC IP for energy harvesting. DELTA runs the project “Batteryless and self-powered electronics”, which is part of a GTS performance contract with the Danish Agency for Science, Technology and Innovation. The activities are supervised by representatives from Danish industry and their aim is to promote Danish companies’ exploitation of the technological and commercial potential of energy harvesting.

Contact

For more information contact Johan Pedersen, Specialist, Energy Harvesting & IoT at DELTA, tel. +45 72 19 43 23, jep@delta.dk.

This is a translation of an article published in Ektuel Elektronik, No. 13, 2016.


DANISH VERSION

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ASIC power management til selvforsynende IoT-sensorer

ASICs til selvforsynende IoT-sensorapplikationer med integreret energy harvesting power management interfaces kan hjælpe os af med batteriskift.

Af Johan Pedersen

Min IoT-sensors batteri er løbet tør… igen…

Internet of Things er ved at være et godt brugt begreb, og spådommene omkring hvor mange millioner eller milliarder enheder, der kommer på nettet, kan være svære at forholde sig til Gartner Says 6.4 Billion Connected “Things” Will Be in Use in 2016, Up 30 Percent from 2015, men ét er sikkert – at der ligger et stadigt stort potentiale i at instrumentere omverdenen med trådløse sensorer til et væld af applikationer og få ”ting på nettet”. Alt fra home automation og alarmsystemer, intelligent tøj og fitness trackere til industriel monitorering og herigennem optimering. En stor barriere for at realisere dette er batterier. Hvis vi kan undgå batterier, vil udrulningen gå meget stærkere, da et batteriskift fx i en trådløs termostat bliver en stor barriere, når man har 10 af dem. De løber tør på skift, og varmen buldrer derudaf ude af kontrol, indtil man får batterierne skiftet.

Selvforsynende sensorer

Én løsning er at gøre enhederne selvforsynende med energi. Et energi-autonomt system, der lader sig selv op med energi fra omgivelserne, betyder en servicefri løsning. Afhængigt af omgivelserne kan det dreje sig om energi fra mange kilder såsom lys, bevægelse, varme, magnetiske og elektriske felter. Emnet kaldes energy harvesting og er på flere og fleres læber, men større udbredelse af batteriløse og selvforsynende systemer lader stadig vente på sig.

Designet til varierende energi

Hvis man vurderer de tilgængelige løsninger og komponenter på energy harvesting markedet, vil man opleve, at der er behov for dybe kompetencer både indenfor fysik og effektelektronik for at komme i gang. Man kan ikke blot smide batteriet ud og indsætte en energihøster som fx en termoelektrisk generator til at høste energi fra varme. En selvforsynende enhed kræver, at hele IoT-sensorenheden inkl. sensor-, dataprocesserings- og kommunikationsdelen er designet til energy harvesting. Der skal nemlig tages højde for, at energikilden kan variere, og der muligvis ikke altid er energi nok. Samtidig er den elektriske profil af en energy harvesting generator er meget forskellig fra et batteri.

Power management interfaces

De primære komponenter, det kræver at realisere en selvforsynende løsning, er ikke kun en energy harvesting generator (som fx solcelle, termogenerator, piezoelement), men også et interface til power management, da ingen energy harvesting generator har samme output karakteristik som et batteri. Et power management interface sørger for at konvertere strøm og spænding til et niveau, der kan drive IoT-sensoren, som typisk har et behov, der ligger indenfor for 1,8-5 V. Fx leverer en termogenerator en meget lav udgangsspænding (~20-50 mV DC) ved lave temperaturgradienter, hvorimod en piezoelektrisk generator leverer høje spændinger (~10-100 VAC), når det høster energi fra vibrationer. Samtidig skal impedansen af generatoren matches for at opnå højeste effektivitet, hvilket power management interfacet også tager sig af.

Energy harvesting IC-markedet

Flere IC-producenter har komponenter til energy harvesting markedet og udbyder flere typer power management interfaces til energy harvesting. Disse består af et indgangstrin, som typisk er målrettet en eller to typer generatorer. Mange solcelle IC’er dækker fx også termoelektriske generatorer ved lave gradienter, da deres impedans-match er lignende, og de derfor kan drives på samme måde. Dog er der kompromiser, og mange løsninger har brede specifikationer for at sikre bred anvendelse og høj volumen.

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Figur 1. Udsnit af energy harvesting power management IC’er på markedet og deres fordeling mellem solceller, termiske og mekaniske energy harvesting generatorer.

Kold start ved lave spændinger

Om en transistor er i et kendt stadie (høj/lav) afhænger typisk af, om der er spænding på dens gate – altså om den er polariseret. Indtil da kan der sagtens løbe strømme, som er ude af kontrol. Dette gør typisk ikke noget i et elektrisk kredsløb, da det drejer sig om små strømme, men når man har en kilde, der fx kun leverer mikroWatt, så er det vigtigt, at der konstant er styr på lækstrømmene. Derfor kræver mange IC’er et batteri som stabil energikilde for at kunne opretholde deres højeffektive egenskaber, så der konstant er styr på kredsløbet. Hvis der ikke er et konstant energilager, skal der et stort energi-input til, før den igen kan operere. Dette er typisk defineret som et koldstartsscenarie for en energy harvesting IC’er, der opererer uden batteri. Minimum input-strøm ved koldstart er typisk en faktor 10 større end ved operation med fyldt energilager. Se fx figur 2, hvor et eksempel på denne input-karakteristik er vist.

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Figur 2. Eksempel på koldstart input-karakteristik for energy harvesting IC’er, hvor der ved lav input-spænding trækkes op til >6 gange større strøm.

DC/DC erstatter LDO

Hvis IoT-sensorens MCU opererer ved en lavere spænding end strømkilden, benyttes ofte en LDO til at levere den rette spænding. Men jo højere spændingsforskellen mellem kilde og forbruger er, jo højere er tabet i LDO’en. Et typisk lithiumbatteri leverer 3 V eller 3,7 V, hvor en MCU typiske forsynes af 2 V.  Derfor giver det i mange tilfælde mening at benytte en DC/DC-converter, der sørger for at minimere dette tab, hvilket kan være op til 40 % mindre end i en LDO.

One-size passer ikke alle

At mange IC’er forsøger at dække bredt over flere energy harvesting generatorteknologier, gør det lettere at finde en IC, der kan fungere, men svært at optimere. Der er typisk sparsomt med energi tilstede, og en høj effektivitet er derfor vigtig. Endnu vigtigere er dog prisen på en IC. Ekstra funktionalitet, der ikke bruges i en IC, er spild af silicium, hvilket resulterer i en højere pris end nødvendigt. Derfor løber mange IoT-sensorudviklere hovedet mod en prismur, når Bill of Materials skal stilles op for en selvforsynende løsning. Hvis generatoren koster et par dollar, og energy harvesting IC’en oveni koster endnu mere, er der lige pludselig langt til den ene dollar, som fx et knapcellebatteri koster.

Skræddersyet chip til selvforsynende IoT-sensor

I høj volumen, >100.000 IoT-sensorer, giver det mening at evaluere skræddersyede IC’er (ASIC) til at integrere ellers diskrete løsninger. En ASIC kan favne både energy harvesting power management interface, sensor interface, processor og kommunikationsinterface i samme chip, hvilket realiserer et såkaldt System-on-Chip. Herved optimerer man brugen af silicium og derved stykprisen. Man kan holde lækstrømmene i bund og operere højeffektivt, og man kan miniaturisere sin IoT-sensor til meget færre komponenter. Ved at samle systemet i én chip kan man også sikre energibevidst operation. Det vil sige, at på baggrund af hvor meget energi der er tilstede, kan IoT-sensoren justere, hvor meget den laver.

I scenarier, hvor den samlede selvforsynende IoT-sensor løsning kommer ned i en pris af et par dollar og fylder mindre end en tændstikæske, bliver det relevant at tale om den store udbredelse af IoT-sensorer. Niveauer såsom temperatur, fugtighed, vibration på fx en produktionslinje kan pludselig monitoreres pr. meter, alarm og sikkerhedssystemer kan monitorere alle døre og vinduer, og indeklimaet i en bygning kan blive styret gennem en matrix af målepunkter i stedet for en enkelt føler, der sidder lige ved siden af radiatoren. Mulighederne er mange, og det er let at komme på de første ideer til potentielle selvforsynende koncepter. Når først IoT-sensoren bliver fri af batteriet, er der ”næsten” ingen grænser for, hvor den kan optimere vores hverdag.

DELTA (fra 2017 en del af FORCE Technology) arbejder til dagligt med IoT-sensorer og energy harvesting i mange forskellige industrier og udvikler ASIC IP til energy harvesting. DELTA driver projektet ”Batteriløs og selvforsynende elektronik”, som er en del af GTS-resultatkontrakten med Styrelsen for Forskning og Innovation. Aktiviteterne vejledes af repræsentanter fra den danske industri og har til formål at fremme danske virksomheders udnyttelse af de teknologiske og forretningsmæssige muligheder i energy harvesting.

Kontakt

For mere information kontakt Johan Pedersen, Specialist, Energy Harvesting & IoT hos DELTA, tlf. +45 72 19 43 23, jep@delta.dk.

Denne artikel er bragt i Aktuel Elektronik nr. 13, 2016.

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