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Aspects of RF wireless power supply
2021-09-13
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Author:Frank

For many years, people have been talking about the function of long-distance wireless power supply, and have become more and more interested. The technology has been verified and has been applied in many industries such as manufacturing, building automation, and hotels. There are a variety of other short-range wireless charging technologies on the market, including Qi (inductive coupling) and magnetic resonance. However, the focus of this article will be on the various methods of pcb radio-frequency-based wireless power supplies to power devices over long distances.

Long-distance wireless power supply

Radio frequency wireless power is a technology that uses radio waves for long-distance power transmission. The transmitter uses an antenna to generate a radio frequency field and propagate to the receiver's antenna. The receiver captures part of the RF field and uses a RF-to-DC converter to generate usable DC power to power electronic devices or charge batteries. RF wireless power can be implemented in many ways, and many design decisions will affect system performance. When all variables are taken into consideration, RF wireless power networks provide a way to remove wires and batteries from many of the devices we encounter every day.

The wireless power transmission using radio frequency in the far field can be described by Friis equation.

Where PR is the received power, PT is the transmit power, GT (θT, φT) is the angle-dependent transmit antenna gain, GR (θR, φR) is the angle-dependent receive antenna gain, λ is the wavelength, r is the transmit antenna and the receive antenna The distance between ΓT is the reflection coefficient of the transmitting antenna, ΓR is the reflection coefficient of the receiving antenna, p̂T is the polarization vector of the transmitting antenna, and p̂R is the polarization vector of the receiving antenna. In general, assuming that the transmitter and receiver are matched, have the same polarization vector, and are in the main radiation beam, the equation is simplified to:

This equation shows that the received power is inversely proportional to the square of the distance, which means that if the distance is doubled, the received power is reduced by 4 times. Considering that the power is propagated on the surface of a sphere with an area of A=4πr2, this is understandable.

Another factor of radio frequency wireless power transmission is that the received power is proportional to the square of λ or inversely proportional to the square of frequency. This means that low-frequency signals will provide more received power than high-frequency signals, assuming all other variables are the same. For example, consider an amplifier that provides 1 W of RF power to a transmitting antenna with a gain of 4, or 4 W EIRP. A 915 MHz dipole antenna at a given distance will receive approximately 7 times more power than a 2.4 GHz dipole antenna.

Compared with the frequency of 5.8GHz, the power is about 40 times.

This difference in power is because as the frequency increases, the effective area of the antenna decreases. The dipole antenna is generally λ/2 long. As the frequency increases, the physical capture area of the antenna becomes smaller. However, the power density S is independent of frequency.

Equation 3 shows that the propagation of radiated power on the spherical surface is independent of frequency. The effective area of the antenna, also known as the capture area, determines the magnitude of the received power. This explains why the 5.8GHz λ/2 dipole antenna captures less energy than the 915MHz λ/2 antenna under the same conditions.

The effective area Ae of the antenna is proportional to its gain.

A higher-gain antenna can be used to increase the capture area, but a high-gain antenna comes at the cost of directivity. Depending on the application, precise antenna directivity is not always advantageous. One way to bypass this potential burden is to use multiple antennas and RF-DC converters to increase the overall capture area. However, this solution also increases the cost of the receiver due to the additional hardware. This explains why it is important to roughly determine performance and project expectations before designing a system.

The Friis equation is only valid in the far field, so it is important to determine the boundary between the near field and the far field. A commonly used method is to determine where the parallel ray approximation begins to fail, that is, the wave emitted from the transmitting antenna can be approximated as a plane wave incident on the receiving antenna. A plane wave means that the receiving antenna sees a constant amplitude and phase at its aperture (Figure 1). Generally, a phase error of π/8 or 22.5 degrees at the receiving aperture is considered an acceptable approximation of a plane wave, which creates a common boundary between the near field and the far field:

Where D is the maximum size of the transmitting or receiving antenna or array, r is the distance between the transmitting and receiving antenna, and λ is the wavelength.

Figure 1 The spherical wave approximates the far-field boundary of a plane wave.

Figure 2 Far-field focusing.

Figure 3 Near-field focusing.

Beam focus, power hot spot size

In some applications, it is advantageous to focus the RF field on the receiving antenna to maximize power throughput. This can be achieved in several ways, usually by far-field focusing (Figure 2) or near-field focusing (Figure 3) of RF power to increase power density. The far-field technology is usually called beamforming or beam steering, which is achieved by using a high-gain antenna or using an antenna array to focus at infinity to generate a directional beam. The direction of the beam is controlled by mechanically or electronically directing the signal to the receiving antenna. In the case of near-field focusing, the antenna array usually focuses each antenna element to a finite point in the near field to generate a hot spot of radio frequency power density, and the subsequent field of each antenna diverges in the far field outside the hot spot.

For far-field beamforming, it is important to understand the limitations of "focusing" RF energy. The beam size and focus area will always be larger than the physical size of the transmitting antenna. Focusing the rays from each antenna element at the infinite point in the far field means that the rays are parallel, as shown in Figure 2. However, the rays emitted from each antenna unit will propagate with distance according to the far-field beam width specifications in the commercially available antenna data sheet. The aperture of the narrow beam starts from the smallest size of the antenna and spreads as it propagates. Therefore, if the transmitting array is 1 square meter, the beam will never be smaller than 1 square meter, which is very important when transmitting RF power to a receiving antenna smaller than the transmitting antenna. Although beamforming can indeed concentrate more radio frequency power on the receiving antenna, a large part of the shaped beam may be outside the desired capture area.

In the case of near-field focusing, the rays emitted by each antenna converge at a certain point in the near-field to form a local hot spot with high radio pcb frequency power density, as shown in Figure 3. The -3dB (half power) size of the hot spot can be as small as slightly less than λ/2. Depending on the size of the receiving antenna, the size of the hot spot can be comparable to the size of the receiving antenna. If the sizes of the two are similar, more effective coupling can be achieved between the transmitter and the receiver. However, due to the tight coupling of this scheme, the system should be simulated and designed as a whole, that is, the transmitting antenna and the receiving antenna. Since the antennas are very close, their impedance will change, and the amplitude and phase of the field passing through the aperture of the receiving antenna are likely to be uneven. Although the design of the far-field antenna has a consistent amplitude and phase in its capture area (that is, it is assumed to be a plane wave), typical antenna design practices may not be suitable for near-field operation, so system simulation is important for optimizing near-field wireless power solutions. Performance is critical.

Both far-field and near-field focusing can provide higher radio frequency wireless power throughput. However, achieving this brings complexity, which tends to increase costs. Beam focusing solutions may include mechanical or electronic guidance, such as motors or amplitude and phase adjustment circuits. This increase in cost makes it difficult to prove wireless benefits. Since transmitters with a single antenna and amplifier are much smaller and cost less than beam focusing solutions, this method is more feasible for high-volume applications.

Building materials

Since the radio frequency wireless power is transmitted through various dielectric materials, the antenna can be embedded inside the product, because no line of sight is required between the transmitter and the receiver. This also means that wirelessly powered sensors can be permanently embedded in building materials and placed behind walls. Typical indoor building materials (such as gypsum board) are "RF-friendly", as we know it from the popularity of Wi-Fi.

Considering the influence of walls on radio frequency wireless power transmission, there are several characteristics that affect power transmission. All dielectric materials have dielectric constant (ie, relative dielectric constant) and loss tangent. Generally, a dielectric material is characterized by its loss or how it attenuates the radio frequency signal propagating through it. This loss is related to the loss tangent of the material. For materials like gypsum board, the loss tangent may be quite low, while for masonry materials like bricks and concrete, the loss tangent will be larger. Since the dielectric constant of the material is greater than the dielectric constant of indoor air, this difference creates an interface between the media, resulting in the refraction and reflection of waves on the surface of the material.
The reflected power and the angle of reflection depend on the polarization of the wave with respect to the incident surface and are described by the Fresnel equation. For simplicity, the following equation assumes a lossless, non-magnetic medium.
Among them, RS is the power reflection coefficient of vertical polarization, RP is the power reflection coefficient of parallel polarization, θi is the angle of the incident wave, θt is the angle of the refracted wave, and ε1 and ε2 are the dielectric constants of the two media.
These equations show the reflected and transmitted power at the interface (Figure 4). When the incident angle is less than 60 degrees, 80% or more of the radio frequency wireless power can be transmitted to the wall. Interestingly, in the case of parallel polarization, 100% of the radio frequency wireless power can be transmitted to the wall under Brewster's angle.
Because the pcb board is not lossless and two interfaces are created: the room enters the gypsum board and the gypsum board to the air behind, using Ansys HFSS simulation helps to visualize how the gypsum board affects the spread. The scheme consists of 12.8 mm thick plasterboard, εr=2.19, tanδ=0.0111, and a 915MHz transmitting dipole antenna is located 0.5 meters away from the wall. The amplitude of the electric field (E field) of a 4×2 m vertical polarization plane of incidence is plotted. To facilitate comparison, delete the wall and repeat the simulation. These figures show a top-down view of the incident plane.
The simulation without walls shows a smooth, uniform E-field loop. In Figure 5a, the part of the ring where the incident angle is close to zero (that is, directly down from the dipole) shows results similar to the example without walls, because the incident angle is small and the gypsum board reflects little. At steeper angles—on the far right and left of the dipole—the reflected E field is higher, causing more distortion. The reflected wave creates constructive and destructive interference to the main E field from the dipole. Examining these two images, since the dielectric constant of the gypsum board is relatively low, there is very little RF reflection, so the two simulations have similar E-fields. The simulation confirmed that the radio frequency wireless power supply can be realized in a non-line-of-sight situation. Even if a wall is used to separate the transmitting and receiving antennas, power can also be transmitted, relatively unaffected by obstacles.
in conclusion
The radio frequency wireless power supply can be realized in many ways. Due to the complexity of each environment, various system parameters can be adjusted to meet the needs of individual applications. Generally speaking, low frequency signals have greater radio frequency power throughput. The size of the receiving product usually determines the maximum antenna size, which determines the lowest frequency for power transmission. Although electrically small antennas can be used, the bandwidth of these antennas is very narrow, making them unsuitable for mass production because manufacturing tolerances can cause changes in the resonance frequency.
Concentrating radio frequencies in the near or far field provides an additional method of increasing throughput. However, incorporating multiple antennas into an array with auxiliary electronics will double the deployment cost, so a transmitter with a single antenna and amplifier may be more advantageous for high-volume applications. Standard indoor building materials have little effect on the RF field, so multi-room RF wireless power systems are possible.
Considering the design options, the pcb radio frequency wireless power system can be designed to meet the different needs of many applications in many vertical markets. Radio frequency wireless power is not a future technology, but a technology currently being deployed, which will be rapidly expanded and adopted on a large scale in the near future.