Harvesting RF Energy in 3 Steps

Contents

• Part 1: RF energy harvesting hit the wall of physics but as always, one can adapt.

• Part 2: Augmenting your RFEH collector efficiency: The role of the Harvesting chain.

• Part 3: Improving base-band conversion efficiency: The role of AEM PMIC  Family.

• Part 4: Conclusion of these series of conversations

---

Part one: RF Energy harvesting hit the wall of physics but as always, one can adapt.

In this series of 3 blogs we explore one of the most  exciting topic of the moment, Radio frequency  Energy harvesting, from the view point of e-peas,  power management IC innovator, and harvesting solutions provider.

One must admit so many publications, and patents have been published in the past 2 years   around ambient RF energy harvesting that there must be something special around. Why such an excitement?  Because our smartphones and tablets of all kinds, our Wifi router of any generation, are ubiquitous and they restlessly send data and energy in our neighbourhood. Legitimate question is “Why not harvesting a bit of it and give life to more IoT devices?”

E-peas is engaged in RF Harvesting technology for a while, and we recently announced our partnership with IGNION in order to propose a reference design to the market.

However we have to set the background of use of such RF Energy Harvesting (RFEH)  technology . We help to clarify what RFEH is capable of, what it is not and how to make best use of it. This blog describes our technology path.

Let’s consider a bit of physics first: RF Energy availability decays with distance from emitter. Considering so many consumer devices work at higher frequency (2,4GHz  or 5GHz ), the decay is big  if not gigantic. In radio technology language, the decay @ 1meter is -40dB. That correspond to one over 10000.  This is simply the application of the Friis transmission model equation.

Friis equation:

Friis equation in logarithm format:

Where Pr is the received power from transmitter Pt at a distance of d,  at a transmission wavelength of Lambda. For ease of explanation, Dt and Dr are assumed to be zero (corresponding to isotropic passive  transmit and receive antennas).

Like said, if one equipment transmits a burst at 100mW power (this is 20dBm aka the Wifi router power ) at 2,4GHz, then, 1 meter away, an object  will receive only 10micro Watts. When you think of it, this is very few energy available at 1 meter.

Likewise, consider a Bluetooth object sending “hello world” at 0dBm (1mW)  then a nearby 1meter receiver will get only 0,1micro Watt. That is not a lot to harvest, to store, to eventually energize an object.

The second fold to take into account   is that the energy sent by those familiar systems is not continuous but in burst mode. This is in order to reduce power consumption and increase spectrum resource availability for multiple users. Duty cycle ratio between activity and sleep are commonly 1/10, 1/100  1 over 1000, further reducing available energy at a given location.

So, 2,4GHz ambient energy availability is very limited in value, and seldom in appearance. Let’s close this part of the discussion  of opportunistic RF Energy harvesting from existing ambient radio protocols by saying: “ it  is practically not possible to get any valuable outcome of it”, one must walk around and adapt the concept.

How to modify the system to make it affordable then?

First thing to do is to make energy available 100% of the time. This means the use a dedicated power transmitter that is always “ON”.

Then the system architect will have to balance harvesting range vs transmit power source:

If one wants to stick to 2,4GHz then increasing the Transmit power to 1 Watt (30dBm) is a solution.

Doing so, RFEH equipment will collect 0,1mW or 100µW at 1 meter. This is close to what an organic PV cell of 5x5cm can collect at 200lux.

Another improvement is to change (reduce) frequency: If one can work at 900MHz then only 140mW will suffice. Similarly, one can also consider that jumping to 5GHz band is certainly a wrong choice, it should be considered only in very specific cases.

It is really unlikely that increasing the number of receivers would bring significant improvements: Antenna diversity is used for protocol connectivity side: they are compensating for the case of fading on one antenna. Antenna diversity is not additive but compensating each other weakness.

We, at e-peas have selected an omnidirectional antenna than can collect the maximum of energy at one location. Now it is time to  consider the Radio frequency Energy harvesting  receiver efficiency and design.

We will cover that in the next 2  parts of our blog

Part two : Augmenting your RFEH collector efficiency: The role of the Harvesting chain.

In part one of our blog we have explained that in order to have a realistic chance to harvest energy on far-end we have to:

• make the power source non intermittent

• increase the power transmitted by the source to 1watt (@2.4GHz ) 

• and consider the distance to be short at 2,4GHz ( 1 to 2 meter range )

• or possibly to reduce the frequency band to 900MHz.

We will focus this discussion on 2,4GHz radio. This not the most easy choice, but most of the solutions  explored here are even more efficient at lower frequency ( 900MHz ) .

As one can imagine, the receiver chain on far-end device is essential to convert this Radio frequency continuous energy flux, into continuous electrons current. Such a reception chain for harvesting does not differ a lot from a radio receiver, as shown on the right side blue shaded area below.

Let’s have a look at the so called “Wireless power Harvester circuit”.

The receiving antenna role is to collect as much as possible of radio energy. This is where e-peas solution comes into the game.

Our unique antenna technology is based upon e-peas innovative EP112 devices. One should understand this device is tightly coupled to the PCB ground plane through the use of the matching network. EP112 device is surface mounted (SMD technology), complemented with  off-the shelf SMD devices only. Such energy-collector has been sized having in mind lightweight, manufacturability, cost efficiency without compromising on performances.

A solution with SMD technology from A to Z

As such, EP112 is an SMD device that is fully compatible with standard reflow process with dimensions of 10mm x 3.2 mm x 3.2 mm. Other antenna technologies, not only differ in behaviour but also in the way they are assembled to the host PCB. They often require a special assembly step, sometime manual intervention. EP112 is really SMD and very small.

Matching Network

The companion matching network is very simple and designed to fit the non-resonant antenna with the PCB so that both PCB and EP112 cooperate in this task. The matching network determine the frequency band and the efficiency of the system. Design and simulation show 80% efficiency in band at 900MHz and 2.4GHz. The antenna system feature a max gain of 2dBi hence not sacrificing too much of the omni-directional nature of this antenna.

As one can understand there is no need for antenna diversity here. There is no privileged RX direction nor directivity in this case. The antenna is acting equally in all direction around it. Unlike directive 4 or 5 dBi elements that can only receive energy from one direction, one EP112 based design does not care of the RX antenna pointing at one specific direction: this is much more user friendly than any other solution. All together, the EP112 and matching network represent a very small part of the application PCB as shown in figure 4 (light green area). A clearance area (E) is recommended as a design precaution to preserve efficiency of the RFEH antenna.

Antenna polarization optimization

While considering the whole Harvesting system, the TX and RX antenna polarization plays also an important role here as energy transfer can be optimized by using linear polarization.

EP112 is a linear polarized antenna. When both TX and RX are aligned then the maximum of energy transfer happens. Think of polarized sunglasses this is exactly the same.

Antenna adaptation schematic is literally unique and simple:  there is no need for multiple RF energy receiver instantiations, no need for supervising equipment and control device, no dependency on the TX power control. The system harvests 100% of the time from all directions.

Summary

The list of advantages of our EP112-based solution is beneficial to the full OEM value chain:

• Designers make validation at early design phase through simulation. Reference design with AEM based award winner PMIC also come in the play to create PoCs.

• Project buyers benefit from early validation what removes headaches of BOM freezing and supply chain constraints at production ramp-up.

• Manufacturing plants manager get an antenna design that is 100% SMD without manual intervention. This is usually warmly welcome at OEMs’ and EMS’.

From Harvesting antenna to ready-useable Energy, the crucial element to evocate now is the Power Management IC ( PMIC ) from e-peas. Stay tuned to part three of this discussion!

Part three: Improving base-band conversion efficiency: The role of AEM PMIC  Family

At e-peas, we have developed a family of RFEH PMICs covering a wide range of RF sources from NFC to 5GHz.  The role of the antenna has been described in previous chapter of this blog.

In this last part of our blog,  we will focus  on the PMIC side:

E-peas’ RF energy harvesting IC solution – AEM30940 – is an integrated energy management subsystem that extracts DC power from RF Antenna signal to simultaneously store energy in a rechargeable element and supply the application electronics with two independent regulated voltages. AEM 30940 features highly efficient Boost converter – 90% efficiency and cold start at -25dBm (from 3µW @ only 380mV) and integrated LDOs and multiple storage element configuration. This allows product designers and engineers to extend battery lifetime and ultimately get rid of the disposable battery in a large range of wireless recharge, industrial monitoring, home automation, etc. AEM30940 is featured-rich member of the family of RFEH from e-peas.

Similarly, the AEM30330 is an integrated energy management circuit that extracts DC power from an ambient energy harvesting source to simultaneously supply an application and store energy in a storage element. AEM30330 has more preconfigured storage management, cold start @-25dBm (3µW at 275mV). The AEM30330 bring a value proposition in a large range of applications, such as asset tracking/monitoring, industrial applications, retail ESL/smart sensors, Smart home/building.

The AEM30300 is the smallest member of the family derived from AEM30330, it has no embedded LDO (all others members do have), he shares  the same Maximum Power Point Tracking system and ultra low energy coldstart capability, with the others.

Key parameters of the PMIC  for our RFEH application include:

Ultra-low-power start-up

Cold start from -19 dBm ( Rectifier included )

• Very efficient energy extraction

• Open-circuit voltage sensing for Maximum Power Point Tracking (MPPT)

• Selectable open-circuit voltage ratios from 35% to 80% or fixed impedance

• Programmable MPPT sensing period

• MPPT voltage operation range from 100 mV to 4.5 V

• Adaptive and smart energy management.

• Switches automatically between boost, buck-boost and buck operation, to maximize energy transfer from its input to the output.

Let’s see why  e-peas PMIC input sensitivity plays a critical role here  specifically in conditions of 2,4GHz radio. (Of course, this is valid for 900MHz and it brings the same product competitive advantage to developers).

One can see that whether the matching network has -19dBm input sensitivity and the system can start working, or it has not the sufficient sensitivity, and the system does not work. The e-peas PMIC itself is capable of cold starting with its -25dBm sensitivity (3 µW cold start).

Let’s see how to store this precious harvested energy then.

Supporting a variety of Storage element:

Once our selected AEM30xxx family member starts harvesting, it will store this energy into a storage element of your choice. You have two options here:

Either  define 100% of the Over discharge / Charge Ready / Over Charge levels  from a customer setting mode or simple use one of the Presets (there are 14 in AEM30330), be it a super cap a dual cell super cap, Li Ion battery, a LiPO battery, etc., etc.

An Ultra low quiescent current  PMIC:

From inception, AEM family is designed to prevent hacking energy for the storage element. All 3 devices work from less than 1µA from the Storage element.

As an illustration the tiny AEM30300 is the most efficient. It will not steel a lot for it as shown on the data sheet page 9:

AEM Evaluation kits for RF harvesting applications:

In the second part of this blog, we have introduced our RF antenna EP112 evaluation kit.

This device is essentially a piece of “empty PCB “ populated with the Harvester SMD part, including EP112 and the impedance matching network. One can see them at the bottom left of the figure. In the configuration shown, it collects GHz radio energy and pass it through an SMA connector.

To evaluate the performance of the Receive antenna, developers need to down-convert this radio energy to base-band. This is done by a PMIC Evaluation kit from e-peas   that is specifically designed for that purpose.

This EVK will include the rectifier section and impedance adaptation to the rectifier network to be connected to the PMIC.

This task of energy conversion requires a bit of expertise that our staff has combined in a kit that is commercially available.

https://www.mouser.fr/ProductDetail/e-peas/2AAEM30940C0411?qs=eP2BKZSCXI4iVtUwpco9ng%3D%3D

Developer can simply interconnect both EVKs, select a storage element among the variety of options , switch on the Power emitter and see energy flowing into it.

Part four: Conclusion of these series of conversations

RFEH is not anymore rocket science.  It can be reasonably considered for harvesting far-field radio energy at 900MHz and 2,4GHz. But developers have to consider the capabilities and limitations of such technology before to engage in a product development in order to be successful as fast as possible. At e-peas we have covered this pre-work and have exposed to major conclusions to you.

Having a dedicated radio transmit power source is recommended, and careful design of the receiver antenna, plus smart selection of the Power management IC have been discussed.

We also provided recommendations of implementation and warned about pitfalls that may pop up while using this technology.

@e-peas we want to provide developers with a set of solutions that are flexible enough to cope with applications like: Electronic Shelf Labels, Remote control unit and remote sensor nodes and even more. Our AEM-based EVKS and EP112 evaluation kits are made for you.

The RFEH solution from e-peas is made of one EP112 SMD device, a handful of passive components (less than 10) and one AEM30xxx PMIC.

Article by Bruno Damien – Ecosystem Marketing Director at e–peas S.A.

---

■ For inquiries please contact the following.

e-peas SA

Rue Fond Cattelain 1 b. 4
1435 Mont-Saint-Guibert, Belgium

T: +32 10 77 12 30
E : sales@e-peas.com