Detailed explanation of wireless charging technology and applications

The content of this article comes from Maxim Semiconductor and the Internet.

In recent years, although wireless charging technology has received widespread attention, smartphone manufacturers began to integrate wireless charging functions into their smartphones as early as 2013. But it has not yet been truly popularized in the field of consumer electronics. Even though some Starbucks have already provided free wireless charging services and IKEA has begun to deploy wireless charging furniture, it seems that it is still a long way from people's lives. So, what kind of development process has wireless charging technology experienced and what will happen in the future? In

June 2015, the Alliance for Wireless Power (A4WP) and the Power Matters Alliance (PMA) merged to form the AirFuel Alliance. This merger has accelerated the realization of the future vision: no matter where consumers go, device charging will be interoperable and convenient. People are trying a variety of different technologies. Near-field inductive charging

Nikola Tesla demonstrated as early as the 1880s that energy can be transferred through oscillating magnetic fields, thereby enabling charging via near-field or magnetic resonance.

Magnetic resonance

The current and voltage that pass from the transmitter to the receiver must be AC. Now, the charging pad is the transmitter and plugs into the wall outlet. The AC grid voltage is stepped down and converted to DC to provide bias for the driver and controller circuits of the transmitter. The driver and controller generate switching signals and can adjust the switching frequency to convert DC back to AC for input to the primary coil. On the receiver side, the AC is rectified and then conditioned by synchronous conversion for charging the battery.

Depending on the amount of power required by the receiver, the frequency in the coil changes. The communication signal is superimposed on the power signal so both know that the device has been placed on the charging pad. Inductive charging is highly efficient but is very sensitive to coil alignment. The coupling coil needs to be adjusted slightly off the resonant frequency to optimize power transfer.

Near Field Inductive Charging System

Near-field resonance charging

Resonant charging is another form of near-field charging that works on the same principle as an electromagnetic field, but requires a resonator front end. The standard, led by AirFuel Resonant, allows for a shorter distance between the transmitter and receiver. A single 6.78 MHz transmitter can support multiple receivers without physical alignment. However, strict frequency matching is required between the receiver and transmitter to maximize the power transfer distance for a specific coil size. As the number of connected devices increases and the distance increases, the transmission power will decrease. The standard requires an independent two-way communication channel (Bluetooth) between the transmitter and each receiver.

Near Field Resonance Charging System

One of the near-field charging standards is Qi, developed by the Wireless Power Consortium (WPC), which includes more than 200 companies. AirFuel Inductive is another standard, and Powermat is a good example of a bridging technology that provides a universal ring that can be used with a charging mat to charge portable devices.

So what are the main differences between AirFuel Inductive and AirFuel Resonant? Please refer to Table 1.

Table 1 Comparison of near-field charging standards

Because of the existence of two standards, General Motors (GM) announced that its cars will support both AirFuel Inductive and Qi standards. Samsung has also decided that its mobile phones support both standards. Overview of Far Field Charging Systems

Both inductive and resonant charging require a certain distance between the transmitter and the receiver. In far-field charging (Figure 4), energy is transferred from a power hub to a specific device. Bluetooth, WiFi, ultrasound, and infrared have all been explored.

Far-field resonance charging system

Radio frequency (RF) based systems (e.g. WattUp, Cota) use one or more antennas to broadcast energy and communicate. In October 2015, Energous announced the availability of the first RF power receiver IC, which rectifies RF into a DC signal. Integration of technologies enables charging technology to be embedded in portable devices. A new technology, Cota, actually uses existing Wi-Fi and Bluetooth antennas for data communication and wireless power reception. These micro signals are then increased to the battery's charging current. The receiving device's response and specific battery charging characteristic data are wirelessly transmitted back to the power router. Once a persistent link is established, the power router will emit a beam of energy to the receiver location.

Piezoelectric Sensors

In ultrasound systems such as uBeam, a signal generator in the router generates an electrical signal that is sent to an amplifier. The amplified signal is then connected to a transmitter, which generates ultrasound waves that are focused and sent to a receiver. The ultrasound waves stress the piezoelectric sensor, which generates a charging current. The sensors used on both sides of the system need to support high efficiency and high power.

Another startup, Wi-Charge, focuses on converting line-of-sight light into energy. The transmitter uses a laser diode to accurately transmit an infrared beam to the receiver. The photovoltaic cell in the receiver then converts the light back into electrical energy. The company demonstrated the system in San Francisco in February 2015. One obvious advantage of infrared technology is that it does not radiate EMI.

Far-field technology comparison


Table 2. Examples of far-field charging methods

Design Considerations

1. Mobility
The ability to locate and track multiple mobile receivers is critical for wireless charging. In near-field charging, the relative position of the transmitter and receiver does not change, while in far-field charging, the user may be constantly moving. Users should be able to roam without losing signal.

2. Safety
Radio frequency (RF) safety depends on the amount of exposure that can be applied without harming human health. The specific absorption rate (SAR) is used to define these limits. The safety of consumer products is far more important than passing safety regulations itself. It is about building consumer security and trust, and there is no room for a bad user experience. Meanwhile, the safety of ultrasound may be a very important concern for consumers, who may feel that the room is full of radiation. One concern is whether ultrasound will affect animals. Using higher operating frequencies that even animals are not sensitive to may solve this problem.

3. System interference
Wireless charging systems are based on high-frequency switching signals, and noise sources must be predicted, including the input and output sides of the charging system.

4. Cost
Two cost factors to consider are power cost and transmission efficiency.

5. Charging a depleted battery
Can a wireless charger charge a completely depleted battery? Far-field charging systems face a challenge that near-field charging does not: the control circuit on the receiving side needs to be powered on to connect to the power router.

6. Ecosystem and Infrastructure
The desire for mobile devices to never be powered off is achievable, but the road to success will be paved with collaboration between multiple entities.

Wireless Charging Ecosystem

Cool mobile phone floating charging

Recently, a foreign company called AR Designs has developed a charging device that can float in the air - OvRcharge.



OvRcharge consists of two parts: one is the mobile phone charging case, and the other is a wooden charging base.


The device is also very simple to use. Just put the phone in the charging case and then place it on the charging base. The phone will float and start charging. When the phone is fully charged, it will automatically stop charging. But as long as you don't take it off. The phone will continue to rotate.

The working principle of OvRcharge is: there is a receiver in the phone case that can receive power from the charging base. Put a magnet on the back of the charging case, and the magnet will react with the magnetic force of the base, so that the phone will float in the air.

Currently, OvRcharge is crowdfunding on Kickstarter, with a starting price of US$185 and is expected to be shipped in December this year.

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