RF Power Harvesting Circuits

Key Takeaways

•  RF power harvesting, the IMN is crucial for optimal power transfer between the source and the load, reducing power reflection and enhancing system efficiency.

• Energy harvesting rectifiers Rectifiers and voltage multiplier circuits, like the Cockcroft–Walton and Dickson multipliers, are key in converting and amplifying RF signals into usable DC power, with each design catering to specific voltage and efficiency needs.

• RF power harvesting systems can tap into both static (e.g., broadcast radio, mobile base stations) and dynamic (e.g., Wi-Fi access points, police radios) ambient RF sources, requiring sophisticated approaches for continuous and efficient energy supply in IoT applications.

RF power harvesting circuit components

RF power harvesting circuits are designed to work with minimal voltages and currents, utilizing a variety of circuit techniques. RF waves emanate from satellite stations, wireless internet, radio stations, and digital multimedia broadcasting. A radio frequency power harvesting system is capable of capturing this electromagnetic energy and converting it into a usable direct current (DC) voltage. Although the environmental power density of RF waves is low, their efficiency can be significantly enhanced by adding an intentional source for better power transmission. Additionally, a custom-designed boosting-RF power harvesting circuit can be implemented to meet the specific demands of the load application.

RF Power Harvesting Circuits Stage Breakdown

RF Power Harvesting Circuit Components

Impedance Matching Circuit 

In electrical systems with low power consumption, power leakage during transmission can result in energy loss. To address this issue, integrating an Impedance Matching Network (IMN) circuit is essential to ensure optimal power transfer between the RF source and the load. In WPH applications, the receiving antenna is regarded as the source, while the rectifier or voltage multiplier is considered the load. In DC circuits, power transfer is most efficient when the resistances of the source and load are equal. However, in RF circuits, impedance takes the place of resistance in this consideration. A mismatch in impedance between the source and load can lead to power reflection within the circuit, reducing overall system efficiency. 

The role of the IMN is to align the impedance of the source and load by introducing reactive components, thereby enhancing the efficiency of the power transfer process. Some examples of IMN circuits include:

• L networks
• Reversed L networks
• T networks 
• Pi networks
• Multiband Matching Network

Energy Harvesting Rectifier

The efficiency of an Energy Harvesting (EH) circuit is influenced by its rectifier component. In the context of power harvesting, the RF signal captured by the antenna typically has a sinusoidal waveform. After processing through the IMN, this signal is rectified and later boosted to satisfy the power requirements of the specific application. Typically, they involve configurations like single diode rectifiers, voltage doublers, and more complex structure discussed below, each made to optimize AC to DC. 

Voltage Multiplier Circuits

The voltage multiplier is a specialized type of rectifier circuit designed to convert AC input and also amplify into DC output. In scenarios where the rectified power falls short for the intended application, it becomes necessary to enhance the DC output. This is achieved by arranging single rectifiers in series, thereby forming a voltage multiplier.

• The most basic configuration of this is the Cockcroft–Walton voltage multiplier. This circuit operates on a principle similar to the full-wave rectifier but includes additional stages for achieving higher voltage gain. 

• Another variant is the Dickson multiplier: modifies the Cockcroft–Walton design, featuring stage capacitors that are shunted to mitigate parasitic effects. This makes the Dickson multiplier more suitable for applications requiring small voltages. However, achieving high power conversion efficiency (PCE) with this setup can be challenging. The high threshold voltage across diodes in the circuit can lead to leakage current, diminishing overall efficiency. Moreover, in the presence of high resistance loads, there can be a significant drop in output voltage, resulting in a reduced current supply to the load.

• A Dickson charge pump typically employs a series of diode-coupled stages to efficiently transfer charge and boost the voltage. It's efficient for applications where only a modest increase in voltage is needed and provides a relatively stable output. 

• Differential drive voltage multipliers are able to achieve high voltage multiplication and can be more efficient, but are rather complex.

Left: Cockcroft–Walton voltage multiplier. Right: Dickson multiplier

Antenna Design Notes

In antenna or rectenna design, key performance parameters include gain, resonance frequency, and bandwidth. Under the assumption of an unobstructed space and an isotropic transmitting source, the dispersion of waves is uniform in all directions. However, it's crucial to recognize that antennas don't always distribute power in a spherical (isotropic) pattern. Depending on their design, antennas can direct energy more specifically in certain directions. 

RF Power Fundamentals

Power loss in space is typically described by free space path loss (FSPL), which refers to the reduction of signal power as it travels through open space. To calculate FSPL, it's necessary to have data on antenna gain, the frequency of the transmitting wave, and the distance between the transmitter and the receiver. The properties of electromagnetic waves vary depending on their distance from the transmitting antenna. These variations in behavior are divided into two distinct categories: the far-field and the near-field.

Near and Far Field

In the far field, the pattern of electromagnetic waves tends to be relatively uniform. However, in the near-field, the electric and magnetic components are significantly stronger and independent, to the extent that one component may dominate the other. The near-field region is defined as the space within the Fraunhofer's distance, whereas the far-field region extends beyond the Fraunhofer's distance. 

The Fraunhofer's distance is a key parameter in defining the boundary between near-field and far-field regions. It is calculated based on the maximum dimension of the radiator (D) and the wavelength (λ) of the electromagnetic wave.

Although the Fraunhofer distance establishes a boundary, the actual transition between the near-field and far-field regions is not sharply defined. 

• Within the near-field, the area extending from the antenna up to a certain distance is known as the non-radiative/reactive near-field region. Here, the electric (E) and magnetic (H) fields are not in phase, leading to energy distortion. 

• As one moves further from the antenna within this near-field region, towards the far-field, they enter the radiative near-field or Fresnel region. In this zone, the reactive nature of the electromagnetic waves becomes less dominant, but the phases of E and H fields still vary with distance.

The spatial distribution of the near and far field regions is depicted in the figure.

RF Energy Free-Space Power

RF energy harvested from free space typically has low power density, as the electric field power density diminishes at a rate proportional to 1/d², where d represents the distance from the RF source. Thefore, a power amplifier circuit is needed to generate sufficient DC energy from the electromagnetic waves to power the loads and applications. This scenario leads to two possible outcomes: 

• If the power consumption of the load is less than the average power harvested, the electronic devices at the load can operate continuously.

• If the load consumes more energy than what the power harvesting circuit generates, the devices cannot function continuously.

RF Energy Harvesting From Ambient RF Sources

Static sources, despite being stable-power transmitters, require a more complex approach to power supply for sensor devices. This often involves modulating the signal, such as altering the frequency and transmitted power. Examples of static sources include ambient entities like broadcast radio, mobile base stations, and television, which are commonly harnessed in power harvesting scenarios.

On the other hand, dynamic sources are transmitters that emit signals in an uncontrolled manner, not specifically monitored by Internet of Things (IoT) systems. To effectively harness energy from these sources, an intelligent Wireless Energy Harvesting (WEH) system is essential. This system must continually scan the channel to identify potential harvesting opportunities. Examples of such unmonitored ambient sources include Wi-Fi access points, microwave radio links, and police radios. These dynamic sources present a unique set of challenges and opportunities for energy harvesting in various IoT applications.

Revolutionize Your RF Power Harvesting Circuits with Cadence AWR Software. Elevate the efficiency and performance of your designs through advanced simulation tools. Click here to transform your RF harvesting projects with Cadence AWR now.

Leading electronics providers rely on Cadence products to optimize power, space, and energy needs for a wide variety of market applications. To learn more about our innovative solutions, talk to our team of experts or subscribe to our YouTube channel.