Power supply design requirements for wireless charging applications
May 3, 2024
Wireless Power Transfer (WPT) applications involve the transmission of electrical power from a source to a load without physical connections. These applications often use electromagnetic fields to transfer power, typically with technologies such as inductive coupling. This blog will discuss how the power supply plays a key role in ensuring efficient and safe power transfer in WPT applications.
WPT is a popular choice in numerous applications. Its popularity spans consumer electronics, ranging from smartphones to electric toothbrushes. Moreover, in the medical field, WPT plays a critical role in powering implantable devices like pacemakers and insulin pumps. Industrial sectors also benefit from WPT technology, with its integration into power tools, automatic guided vehicles (AGVs), and beyond.
WPT requires a transmitter coil and a receiver coil, as illustrated in Figure 1. Power is transferred wirelessly using the magnetic field between the transmitter to receiver.
On the transmit side the DC input power source typically feeds a half-bridge or full-bridge topology to power an DC-to-AC power inverter. The inverter creates an alternating magnetic field using an LC series resonance tank to transmit power to the receiver. On the receiving side, series resonance components convert the incoming magnetic field to current, and a high-power rectifier converts the AC current into a DC voltage. An output regulator is used to provide a stable DC voltage to the load.
WPT requires a transmitter coil and a receiver coil, as illustrated in Figure 1. Power is transferred wirelessly using the magnetic field between the transmitter to receiver.
On the transmit side the DC input power source typically feeds a half-bridge or full-bridge topology to power an DC-to-AC power inverter. The inverter creates an alternating magnetic field using an LC series resonance tank to transmit power to the receiver. On the receiving side, series resonance components convert the incoming magnetic field to current, and a high-power rectifier converts the AC current into a DC voltage. An output regulator is used to provide a stable DC voltage to the load.
The coil configurations vary widely depending on the application and the type of power coupling; both resonant and non-resonant inductive coupling are commonly used.
Consumer WPT technology conforms to the relevant industry standard for inductive charging. The Qi (pronounced “chee”) system from the Wireless Power Consortium (WPC) is highly efficient but requires positional alignment of the transmitter and receiver coils. The competing AirFuel Alliance standard uses magnetic resonance power transfer with tuned coils that are less sensitive to misalignment and support longer distances up to a few meters.
There are other significant differences between the Qi and AirFuel systems. For example, the Qi wireless charging standard has a frequency range of 110–205kHz for low power applications (5W) and 80–300kHz for medium-power applications (up to 120W). The AirFuel specification uses a higher frequency of 6.78MHz.
The transmitter and receiver coil design, distance, and alignment play a key role in the efficiency of the WPT design. In the simplest low-power WPT applications such as an electric toothbrush the relative locations of the transmitter and receiver coils are tightly controlled, but there is greater variation in higher-power systems.
Figure 2 shows the effect on efficiency of various misalignments between transmitter and receiver coils.
Consumer WPT technology conforms to the relevant industry standard for inductive charging. The Qi (pronounced “chee”) system from the Wireless Power Consortium (WPC) is highly efficient but requires positional alignment of the transmitter and receiver coils. The competing AirFuel Alliance standard uses magnetic resonance power transfer with tuned coils that are less sensitive to misalignment and support longer distances up to a few meters.
There are other significant differences between the Qi and AirFuel systems. For example, the Qi wireless charging standard has a frequency range of 110–205kHz for low power applications (5W) and 80–300kHz for medium-power applications (up to 120W). The AirFuel specification uses a higher frequency of 6.78MHz.
The transmitter and receiver coil design, distance, and alignment play a key role in the efficiency of the WPT design. In the simplest low-power WPT applications such as an electric toothbrush the relative locations of the transmitter and receiver coils are tightly controlled, but there is greater variation in higher-power systems.
Figure 2 shows the effect on efficiency of various misalignments between transmitter and receiver coils.
Key requirements for power supplies in wireless power transfer applications
Stability and Precision. The power supply must provide a stable and precise output voltage or current to ensure consistent and efficient power transfer. Fluctuations can impact the efficiency and reliability of the wireless power transfer system.
Adaptability to Varying Loads. WPT systems may experience varying loads, especially in scenarios where multiple devices are wirelessly charged simultaneously. The power supply should be capable of adapting to these changes in load to maintain optimal performance.
Efficiency and Power Factor. Efficiency is critical in WPT applications to minimize energy losses during power transfer. Power supplies with high-efficiency help in maximizing the overall performance of the wireless power transfer system.
Safety Considerations. WPT systems often require isolation between the power supply and the wireless power transfer components for safety reasons. Isolated power supplies prevent electrical hazards and ensure compliance with safety standards.
Foreign Object Detection (FOD). A metal object, such as a coin, a key, or a nail, may be inadvertently located near the magnetic fields generated by the WPT system, creating a potential safety hazard and affecting the electrical characteristics of the WPT system. High magnetic fields may also produce adverse symptoms in living objects located between the transmitter and receiver. Identifying such unintended objects and avoiding transmitting power to them is known as foreign object detection (FOD). FOD is typically accomplished by calculating the power received at the receiver and communicating it back to the transmitter. A mismatch in the two values after correction factors have been applied is assumed to be due to a foreign object absorbing power and triggers a shutdown.
Temperature Monitoring and Protection. Power supplies should incorporate temperature monitoring mechanisms and overtemperature protection (OTP), especially in wireless charging pads or modules. Standard protection features should also include short circuit protection (SCP), and overvoltage protection (OVP).
Frequency Matching. In resonant inductive coupling-based WPT systems, the power supply frequency must be compatible with the resonant frequency of the system for efficient power transfer. Frequency matching enhances overall WPT performance.
Communication and Control. WPT systems typically use communication interfaces to exchange information between the transmitter and receiver such as load requirements or system status. This facilitates intelligent power management and control. Qi receivers use amplitude shift keying (ASK) to communicate power level requests and for FOD detection; Qi transmitters use frequency shift keying (FSK). Resonant WPT systems meeting the AirFuel standard use a protocol derived from the popular Bluetooth Low Energy (BLE) standard.
In addition to wireless communication between the transmitter and receiver, the power supply should also include a wired digital interface such as PMBus. This feature allows the control system to optimize the power supply voltage for peak performance, and the power supply can transmit status information, error codes, and other parameters.
Electromagnetic Compatibility (EMC). The power supply should comply with electromagnetic compatibility standards to prevent interference with other electronic devices and ensure the reliability of the overall WPT system.
In addition to wireless communication between the transmitter and receiver, the power supply should also include a wired digital interface such as PMBus. This feature allows the control system to optimize the power supply voltage for peak performance, and the power supply can transmit status information, error codes, and other parameters.
Electromagnetic Compatibility (EMC). The power supply should comply with electromagnetic compatibility standards to prevent interference with other electronic devices and ensure the reliability of the overall WPT system.
RECOM products suitable for wireless power transfer applications
As mentioned above, an ideal power supply for WPT use should be well regulated, both for line and load variations, efficient, isolated, protected (SCP, OVP, and OTP), and meet the relevant EMC requirements.
RECOM offers many AC/DC power supplies that meet these requirements. The RACM600-L series, for example, can provide 450W continuous power (up to 800W peak) and offers a PMBus interface. The RACM1200-V supplies up to 1000W fan-less power with 1200W boost power available for up to 10 seconds, again with a PMBus option. Custom variants are also available.
RECOM offers many AC/DC power supplies that meet these requirements. The RACM600-L series, for example, can provide 450W continuous power (up to 800W peak) and offers a PMBus interface. The RACM1200-V supplies up to 1000W fan-less power with 1200W boost power available for up to 10 seconds, again with a PMBus option. Custom variants are also available.
Conclusion
Power supplies in wireless power transfer applications need to meet a combination of stability, efficiency, safety, and compatibility requirements to ensure seamless and reliable power transfer in diverse scenarios ranging from consumer electronics to industrial applications. The design considerations for WPT power supplies are crucial in advancing the adoption of wireless charging technologies.
RECOM has some valuable training material for those wanting to learn more about this fast-changing field. Our AC/DC Book of Knowledge contains much information about some of the basic physics and practical design techniques applicable to WPT. And RECOM design engineers are always available to discuss custom design projects.
RECOM has some valuable training material for those wanting to learn more about this fast-changing field. Our AC/DC Book of Knowledge contains much information about some of the basic physics and practical design techniques applicable to WPT. And RECOM design engineers are always available to discuss custom design projects.
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RACM600-L
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