Patent Application
20190044237
The present invention relates generally to the field of energy harvesting and more specifically to a circuit board with a small size for use in wireless energy harvesting applications.
The wireless transmission of power has attracted considerable interest, and can be classified into two broad categories: wireless energy transfer and wireless energy harvesting. The former is used for high RF power densities (normally to transfer power from dedicated RF sources over short distances) while the latter relates to the harvesting of the much lower RF power densities that are typically encountered in the urban environment (e.g. from WiFi and mobile phone networks).
Wireless energy harvesting systems are generally designed to profit from such freely available RF transmissions by employing highly efficient RF-to-DC conversion to supply low-power devices.
As the power available for energy harvesting is typically of very low density (often 1 μW/cm2 or less), providing a circuit which is capable of harvesting such low power levels whilst having a small size is particularly difficult.
In particular, the antenna must have a good return loss, energy losses within the RF energy harvesting circuit must be minimised, and parasitic resistances, capacitances and inductances must be minimised as any parasitic resistance, capacitance or inductance can easily sap away the little energy that has been harvested.
The present invention aims to provide a circuit board for use in wireless energy harvesting applications which exhibits high gain and high efficiency that enable it to harvest energy in an environment with a low power density level of 1 μW/cm2, all whilst achieving a small size.
The present invention provides a circuit board for use in wireless energy harvesting applications. The circuit board comprises a first plane and ground plane parallel to the first plane. The ground plane has a substantially rectangular shape with a length less than 1.38λg and a width less than 0.92λg.
The first plane comprises an antenna, a feedline and a rectifier. The antenna is configured to receive an RF signal with a wavelength of λ0. The feedline is arranged to filter the received RF signal. The rectifier is arranged to generate a DC voltage from the filtered RF signal. The antenna, the feedline and the rectifier are arranged substantially co-linear along the first plane, and
where εeff is the relative permittivity of a material between the first plane and the ground plane.
Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which like reference numbers designate the same or corresponding parts, and in which:
A first embodiment of the present invention will be described with reference to
The circuit board 1 comprises a first plane 2 and a ground plane 3 substantially parallel to the first plane 2.
As will be explained further later, the first plane 2 and ground plane 3 are conveniently formed as layers on each side of a substrate 4, the substrate 4 being made of a dielectric material.
The circuit board 1 is configured to receive an RF signal with a wavelength of λ0.
The guided wavelength, λg, of an electromagnetic wave in a microstrip transmission line differs from the wavelength λ0 of the same signal in air according to the following formula:
where εeff is the effective dielectric constant of the microstrip transmission line, which, for sake of simplicity, is taken to be the relative permittivity of the material of the substrate 4 in the present disclosure. The guided wavelength may, however, alternatively be expressed in terms of an effective dielectric constant that is a function of the microstrip geometry:
where ε is the relative permittivity of the substrate 4, h is the substrate thickness, and W is the width of the conductive trace formed on the substrate. In the following, various dimensions of the circuit board 1 expressed both in terms of mm and λg. The expression of these dimensions in terms of λg allows the teachings herein to be applied in the design of circuit boards that can operate at frequencies other than those described. Provided that the relative permittivity of the substrate 4 material is known, the dimensions, in terms of λg, of various components of a circuit board 1 having the structure described herein may be deduced from measurements or simulations of how harmonics propagate in the circuit board 1, using techniques well-known to those skilled in the art.
Referring to
The first plane 2 comprises an antenna 21, a feedline 22 and a rectifier 23. The antenna 21, feedline 22 and rectifier 23, as well as the ground plane 3, are all formed from an electrically conductive material, such as copper.
The feedline 22 and the rectifier 23 may take one of many different forms known to those skilled in the art. For example, each of the feedline 22 and the rectifier 23 may be a stripline, microstrip, slotline, coplanar waveguide and a coplanar stripline transmission line, or a combination of two or more of these kinds of transmission line. However, in the present embodiment, each of the feedline 22 and the rectifier 23 takes the form of a microstrip transmission line comprising a respective conductive trace that is formed on the first plane 2, wherein a conductive layer providing the ground plane 3 common to all transmission lines is formed on an opposite side of a substrate 4.
As explained previously, the first plane 2 and ground plane 3 are conveniently formed as layers on each side of the substrate 4. The substrate 4 is made from a dielectric material and provides a suitable mechanical support to hold the first plane 2 and the ground plane 3 in a spaced-apart configuration substantially parallel to each other. It will be understood by the skilled person that “parallel” does not mean that the angle between the first plane 2 and the ground plane 3 is strictly zero degrees, but that variations in the angle up to ±2.5 degrees are encompassed, as such variations will not significantly degrade the performance of the circuit board 1. It will be further understood that the substrate 4 is not an essential component and that any suitable mechanical structure can be provided to hold the first plane 2 and the ground plane 3 in their respective planes.
One example of a material that can be used for the circuit board 1 is Rogers 4003C. Using Rogers 4003C circuit board material provides a total thickness of the first plane 2, substrate 4 and ground plane 3 of substantially 0.0234λg, equivalent to 1.524 mm at 2.45 GHz in a substrate 4 with a relative dielectric permittivity of 3.55. The skilled person will understand that this dimension is not critical and a variation of ±10% can be encompassed, as such variations will not significantly degrade the performance of the circuit board 1.
The circuit board 1 will exhibit a relative dielectric permittivity which may affect any circuitry placed on the first plane 2. The present inventors have found that a suitable circuit board has a relative dielectric permittivity of between 3.5 and 3.6, preferably 3.55, which is achieved using Rogers 4003C. It will, of course, be appreciated that this choice of circuit board material is given by way of example only, and that other substrate materials (e.g. IS680-345 produced by Isola Corp.™, which has a relative permittivity of 3.45 or a RO3000® series high-frequency laminate) may alternatively be used. The relative permittivity of the substrate material is preferably between 2.17 and 10.2, and more preferably 3.55, as in the present embodiment.
Referring again to
According to the first embodiment, the antenna 21, feedline 22 and rectifier 23 are arranged substantially co-linear along the first plane 2, as the inventors have found that this reduces energy losses and reduces parasitic resistances, capacitances and inductances. From this, the skilled person will understand that the antenna 21, feedline 22 and rectifier 23 are formed in a line on the first plane 2.
The first embodiment is a single band, co-planar RF energy harvester.
The antenna 21 is configured to receive an RF signal. By way of non-limiting example, such an antenna 21 could be used to receive signals (or energy) in the waveband of Wi-Fi (operating around 2.4 GHz). In particular, the antenna in
The antenna 21 in the first embodiment is a patch antenna which is provided on the first plane, although it need not be so configured. As shown in
The inventors have found that the antenna 21 provides good performance if it is configured so that each side of the antenna 21 has a length between 0.48λg and 0.50λg, preferably 0.488λg, equivalent to a length between 31.2 mm and 32.2 mm, preferably 31.7 mm at 2.45 GHz in a substrate 4 with a relative dielectric permittivity of 3.55. These dimensions were found to exhibit the maximum energy reception of an RF signal with a frequency of 2.45 GHz.
Turning now to the feedline 22, the feedline 22 is arranged to filter the RF signal.
The feedline 22 has an input impedance which substantially matches that of the antenna 21 to ensure a minimal loss of energy at the interface between the antenna 21 and the feedline 22. Furthermore, the feedline 22 has an output impedance which substantially matches that of the rectifier 23 to also ensure a minimal loss of energy at the interface between the feedline 22 and the rectifier 23. Therefore, at the frequency of the received signal, a good match is achieved between the antenna 21 and the rectifier 23 to minimise any reflections at the input side of the rectifier 23. Therefore, the feedline 22 may be arranged to match the impedance between the antenna 21 and the rectifier 23.
The present inventors have found that an impedance of the antenna 21 and the feedline 22 that can be effective to minimise energy loss is substantially 100Ω. More particularly, the present inventors found that, with an impedance of 100Ω, a surprising effect could be achieved because this impedance permits the downsizing of the circuit board 1 without adversely affecting its performance. In particular, the selection of an impedance of substantially 100Ω enables a reduction in size of the antenna 21 and rectifier 22. For example, the present inventors found that the circuit board 1 could perform all the required functions when the impedance was substantially 100Ω and the dimensions of the ground plane 3 were substantially 1.32λg by 0.831λg which is equivalent to 85.7 mm by 54 mm at 2.45 GHz in a substrate 4 with a relative dielectric permittivity of 3.55, approximately the size of a typical credit card. Moreover, a reduction of the insertion loss, which is the power loss due to the insertion of devices on the transmission line (e.g. the feedline 22), was found.
In one configuration, the feedline 22 may achieve the filtering of the received RF signal by reflecting RF harmonics generated by the rectifier 23 back towards the rectifier 23. This can be useful because the energy harvested is at a very low level so it is beneficial to keep as much energy as possible within the circuit board 1. Therefore, the feedline 22 is configured to reflect harmonics back towards the rectifier to thereby keep energy within the circuit board 1 which would otherwise be reradiated by the antenna 21. The harmonics are reflected back towards the rectifier 23 so that some of their power can be converted by the rectifier 23 to DC, improving the efficiency of the rectification.
By way of example, the feedline 22 may comprise a number of different structures to reflect the harmonics generated by the rectifier 23. The feedline 22 may comprise a first part 221 and a second part 222. As depicted in
The feedline 22 may be arranged to reflect the second and third harmonics generated by the rectifier 23. More particularly, the first stub 2211 may be configured to reflect the second harmonic and the second stub 2212 may be configured to reflect the third harmonic. Of course, other harmonics may be optionally reflected instead or as well.
The feedline 22 may be configured as described in UK patent application number 1516280.3 titled “RF-to-DC converter” and filed on 14 Sep. 2015, the full contents of which are incorporated herein by cross-reference.
The rectifier 23 is configured to generate a DC voltage from the received signal. The rectifier 23 may be implemented in a number of different ways. The present inventors found that using a diode 231, a second feedline 232 and a capacitor 233 to form the rectifier 23 is particularly effective.
In particular, the rectifier 23 is arranged to rectify the received RF signal and thereby generate a DC signal. In the rectification of the received RF signal, the rectifier 23 will generate harmonic RF signals on both the input and output sides of the rectifier 23. Therefore, the total energy in the circuit board 1 comprises a mix of DC, fundamental frequency, second harmonic, third harmonic and higher harmonic signals of the received RF signal, in addition to the received RF signal itself.
However, due to good matching of the antenna 21 to the rectifier 23, the reflection generated by the rectifier 23 at the fundamental frequency of the received RF signal will be diminished. This good matching is achieved by way of the feedline 22, as described previously.
The present inventors have also considered further components that may be formed in the first plane 2 to achieve further advantages.
In particular, the first plane 2 may further comprise a low pass filter 24. The low pass filter 24 is arranged to output the DC voltage generated by the rectifier 23. The low pass filter 24 may comprise a third feedline 241 and a second inductor 242.
Alternatively or in addition to the low pass filter 24, the first plane 2 may also comprise a power management module 25. The power management module 25 is arranged to store the DC voltage generated by the rectifier 23 which may have been output by the low pass filter 24. In situations such as energy harvesting, the collected energy at any instant in time is extremely low because the energy density is low. Accordingly, to make use of the collected energy, the energy must be stored and accumulated before it can be utilised. A number of options exist to provide this functionality and the present inventors have found that a power management module 25 is one effective way to store and accumulate the energy generated.
However, the input impedance of the power management module 25 is high and therefore harmonic RF energy generated by the rectifier 23 may be lost. To prevent this, the low pass filter 24 may be configured to reflect the RF harmonics back towards the rectifier 23, to thereby keep the harmonic RF energy in the circuit board 1. Therefore, the low pass filter 24 is configured to output substantially only the DC voltage generated by the rectifier 23.
To achieve this, the low pass filter 24 may comprise a third feedline 241 and a second inductor 242. The second inductor 242 is configured to perform a ‘low-pass’ function in that it allows DC energy to flow but blocks the flow of RF energy and reflects the RF energy back towards the rectifier 23. The harmonics are reflected back towards the rectifier 23 so that some of their power can be converted by the rectifier 23 to DC, improving the efficiency of the rectification.
The present inventors have also found that the positioning of the power management module 25 is important. In particular, the inventors found that positioning the power management module 25 such that it was further than four times the dielectric thickness away from any part of the antenna 21, feedline 22 or rectifier 23 minimised parasitic effects to less than 1%. The dielectric thickness is the distance between the first plane 2 and the ground plane 3. In the first embodiment, therefore, as described previously and depicted in
The present inventors also found that, in addition to a power management module 25, the first plane 2 may comprise a load 26. The load 26 may be arranged to be driven by the power management module 25. The load 26 may be implemented in a number of different ways, for example, a resistor is a typical load 26 that would utilise harvested RF energy to cause a current to flow through the load 26.
In particular, the antenna 21′ differs in its formation on the circuit board 1. The antenna 21′ of the second embodiment is substantially square, with two diagonally opposed corners 52 (as shown in
The inventors found that an optimum length for each connecting straight line 54 was between 0.063λg and 0.078λg, preferably, 0.07λg. This is equivalent to between 4.1 mm and 5.1 mm, preferably 4.6 mm at 2.45 GHz in a substrate 4 with a relative dielectric permittivity of 3.55.
From this description, the skilled person will understand that, in plan form, the antenna 21′ has six sides, with four sides having substantially the same length and the other two sides having a different length. In other words, the antenna 21′ looks like the substantially square antenna 21 of the first embodiment but with triangular corner sections removed from two diagonally opposite corners 52 of the antenna 21. That is, the 0.488λg by 0.488λg square antenna 21 of the first embodiment is modified to remove an isosceles triangle from two diagonally opposite corners 52. That is equivalent to 31.7 mm by 31.7 mm square antenna 21 at 2.45 GHz in a substrate 4 with a relative dielectric permittivity of 3.55. Each triangle has a base length of 0.05λg, so that each connecting straight line 54 has a length of 0.07λg. That is equivalent to a triangle with a base length of 3.25 mm, so that each connecting straight line 54 has a length of 4.6 mm at 2.45 GHz in a substrate 4 with a relative dielectric permittivity of 3.55.
The antenna 21′ of the second embodiment has the advantageous effect of capturing circularly polarized RF signals which ensures that the maximum amount of RF energy is harvested irrespective of the orientation of the circuit board 1.
The present inventors have found that the gain of the antenna 21′ is greater than 5 dBi (relative to an isotropic antenna) and the farfield inverse axial ratio is less than 2 dB (0 dB is the ideal for circularly polarised fields).
The present inventors also investigated a number of dimensions to be considered when constructing the circuit board 1 depicted in
As will be understood by the skilled person, the dimensions depicted in
Firstly, the ground plane 3 may have a length of 1.32λg (85.7 mm) and a width of 0.831λg (54 mm). This is roughly the size of a credit card. As explained previously, the circuit board 1 itself may have the same dimensions as the ground plane 3 or have a large size, preferably the circuit board 1 will have the same size as the ground plane 3.
Regarding the feedline 22, this comprises a first part 221 and a second part 222, arranged co-linearly. The first part may be 0.308λg (20.03 mm) long and 0.011λg (0.7 mm) wide. The second part 222 may be 0.193λg (12.52 mm) long and 0.028λg (1.8 mm wide).
The first part 221 comprises a first stub 2211, a second stub 2212 and a first inductor 2213. The first stub 2211 may have a length of 0.157λg (10.22 mm) and a width of 0.0115λg (0.75 mm). The first stub 2211 may be positioned 0.017λg (1.11 mm) from the second part 222 of the feedline. The second stub 2212 may have a length of 0.105λg (6.84 mm) and a width of 0.0115λg (0.75 mm). The second stub 2212 may be positioned 0.091λg (5.92 mm) from the first stub 2211. The first inductor 2213 has one end connected to the first part 221 of the feedline and its other end connected to ground. The first inductor 2213 may have a value of 10 μH. The first inductor 2213 provides a return path via ground for DC energy on the input side of the rectifier 23, thereby forming a DC loop and making DC energy available at the output side of the rectifier 23. In particular, the first inductor 2213 performs a ‘low-pass’ function in that it allows DC energy to flow but blocks the flow of RF energy. The first inductor 2213 may be placed 0.162λg (10.5 mm) from the meeting point of the antenna 21 and the first part of the feedline 221.
The second part 222 further comprises a capacitor fan 2221. The capacitor fan 2221 may have a radius of 0.133λg (8.64 mm) and a chord length of 0.161λg (10.46 mm). These dimensions equate to an inside arc angle of substantially 74.5 degrees, which is the angle between the two walls of the capacitor fan 2221.
The rectifier 23 comprises a diode 231, a second feedline 232 and a capacitor 233. The second feedline 232 may have a length between 0.1363λg (8.86 mm) and 0.1369λg (8.90 mm) and a width between 0.026λg (1.7 mm) and 0.029λg (1.9 mm). The second feedline 232 may have a length of 0.1366λg (8.88 mm) and a width of 0.028λg (1.8 mm). The capacitor 233 may have a value of 10 pF. The capacitor 233 helps to ensure that the primary harmonic, f0, is well matched into the next stage.
The optional low pass filter 24 comprises a third feedline 241 and a second inductor 242. The second inductor 242 may have a value of 10 μH. The third feedline 241 may have a length between 0.045λg (2.9 mm) and 0.048λg (3.1 mm) and a width between 0.0031λg (0.2 mm) and 0.0062λg (0.4 mm). The third feedline 241 may have a length of 0.046λg (3 mm) and a width of 0.0046 (0.3 mm).
The present inventors modelled the expected gain from the circuit board according to the second embodiment.
Similarly,
From both
From both
The distance d3 (shown in
To confirm this result, the present inventors performed the same simulation for the circuit board 1 according to the second embodiment but varied the distance d3 between the edge of the antenna 21′ and the nearest edge of the first plane 2.
As can be seen, the gain has reduced as compared to the simulation shown in
To summarise, the following table details the results:
By way of the further comparison, the present inventors performed simulations of the first embodiment of the present invention so that the performance of the first and second embodiments could be compared. In the simulations of the first embodiment, the inventors found that, with a distance d3 of 0.097λg, equivalent to 6.3 mm at 2.45 GHz in a substrate 4 with a relative dielectric permittivity of 3.55, from the nearest edge of the first plane 2 to the antenna 21, at 2.45 GHz and a theta value of 0 degrees an average gain of 2.8 dB was achieved and an farfield inverse axial ratio of 130.
More particularly,
Many modifications and variations can be made to the embodiments described above. For example, in the embodiments described above, the antenna 21, 21′ and feedline 22 had impedances of substantially 100Ω. However, acceptable performance can still be achieved when other impedances, such as the standard 50Ω, are used.
In another example, the first inductor 2213 could be replaced by a connection to the ground plane, preferably being formed by a “via”.
Moreover, the present inventors found that locating the antenna 21, 21′, feedline 22 and rectifier 23 co-linear along a centreline 51 of the first plane 2 can further reduce energy losses and parasitic resistances, capacitances and inductances.
The foregoing description of embodiments of the invention has been presented for the purpose of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations can be made without departing from the spirit and scope of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1602358.2 | Feb 2016 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2017/050319 | 2/8/2017 | WO | 00 |
