How to Choose High Power Wireless Charging Solution
Wireless charging has been widely used in mobile phone products, most of which are compatible with QI specifications, and their wireless charging power is 5W. Some products’ charging power extends to more than 15W, and 5W is still the main product in practical application. After the popularity of wireless charging in the mobile phone industry, if it is to be applied to other devices, such as: cleaning robots, power tools, AGV, AMR, Electric scooter, etc., a higher charging power is needed for a better use experience.
At present, the high-power wireless charging solution mainly promoted by various manufacturers is mainly centered on the 300W reference design, which can reduce the grade of parts, reduce the cost, be compatible with small power, and also strengthen the specification of parts to increase the transmission power. Product developers choosing high-power wireless charging solutions should evaluate five performance indicators: Distance & Efficiency, Safety, Communication Protocol, Load, and EMI. Cost is the key to the final decision. The following describes the performance indicators and cost evaluation contents.
Distance & Efficiency
At present, Magnetic Induction(MI) and Magnetic Resonance(MR) of high power wireless charging solutions are two technologies that send and receive energy by coils, while transmission distance and efficiency need to be evaluated at the same time. The energy sent by the coil will attenuate with the distance, and the efficiency can be improved by various compensation techniques after the distance is extended, but the attenuation of energy after the distance is extended is a physical property that cannot be overcome. The key points of the evaluation of the solution are the range of induction distance and efficiency distribution. Some solutions have a long induction distance, but they cannot operate normally at short distances, and have the limitation of the shortest induction distance. Such solutions can be damaged when operating at too short induction distances. The cost of thermal solutions and the difficulty of passing EMI detection are higher; The point of evaluation is the amount of energy lost, not the percentage of efficiency. For example, if the loss target is less than 30W, an efficiency of 70% or more is required at 100W of transmission power, and an efficiency of 90% or more is required at 300W of transmission power, that is, the higher the system design target power, the higher the efficiency will be. For example, when the Transmitter coil and the Receiver coil are both round, in order to meet the transmission efficiency of 90%, the farthest induction distance is only 1/8 of the diameter of the coil. The transmission efficiency mentioned above refers to the efficiency of transmitting DC input of the Transmitter to the DC output of the Receiver, which includes the circuit loss rather than just energy transmission between coils.
Figure 1. The relationship between coil diameter, induction distance and efficiency
*Key points of the evaluation of solution:
A1. Range of induction. The maximum induction distance is not the only concern. The solution needs to work at both the far and near range.
A2. Efficiency requirements. The power loss is calculated based on the heat dissipation capacity, and then the efficiency requirements are evaluated.
Safety
In general power systems, functions such as OVP, UVP, OCP, OPP, OTP, and SCP are standard security features; However, the most important security mechanism in wireless charging is target recognition and metal foreign object detection. The transmitter must identify the target correctly before starting power transmission. If the target is not sensed, it should be in standby state and not start power output. In wireless charging, electric energy is converted into electromagnetic energy for transmission, and the electromagnetic energy will be absorbed by the metal material and heat it. Both MI and MR technologies have problems with metal foreign objects. Different operating frequencies have different heating efficiency for the material, so as long as there is metal foreign object absorbing electromagnetic energy, it will be dangerous.
Figure 2. Metal foreign objects are the biggest threat to wireless charging
*Key points of the evaluation of solution:
B1. Target recognition. The Transmitter does not start power transmission and remains standby until it senses the Receiver. The less power the power supply end consumes during standby, the better.
B2. Metal foreign object detection. Both standby and power transmission processes require metal foreign object detection capabilities. The detection capability standard is the limit of temperature rise of the invading metal object. Small foreign bodies absorb little electromagnetic energy, so the temperature increase is small. If the temperature rise is within the limit value, it is ignored. If it is large metal foreign objects, power transmission must be stopped before the temperature rise reaches the limit value.
Communication protocol
Transmitter and Receiver need a communication mechanism to transfer data in order to complete the target recognition and power regulation function. There are two communication modes: in-band and out-of-band. In-band uses a coil that transmits power to exchange data through modulation and demodulation technology, while out-of-band uses additional communication modules, such as Bluetooth, to exchange data. In-band is one-to-one power transmission and communication under tight coupling of coils, so there is no pairing problem. As long as the power can be transmitted, the communication function can be completed. The transmission distance of out-of-band communication module is longer than that of power transmission, so there will be matching problems if there are multiple devices in the same space. Out-of -band communication module has better data transmission rate and data transmission capability.
Figure 3. Transmitter and Receiver Block Diagram
*Key points of the evaluation of solution:
C1. When the Transmitter is in standby state, the output is shut down. When the Receiver is close to the Transmitter, the power output can be started through the communication mechanism.
C2. The communication mechanism can complete the data transmission function under the condition of no-load, full load and dynamic load.
Load
After receiving electromagnetic energy, the Receiver coil outputs DC to the load through rectification filter, whose output mode includes Constant Voltage (CV) and Constant Current (CC), while the load can be resistive (such as light bulb), battery, mechanical and electrical device (such as motor). Most of the mechanical and electrical devices require CV power output, and the load state varies according to the operation state. This dynamic load will interfere with the communication between the coils, and the system must be capable of adjusting transmission power to maintain the load demand at the Receiver’s output.
Figure 4. Drive motor as Receiver output power
(Source: Microchip Technology https://www.youtube.com/watch?v=eRZZW1juu8s)
*Key points of the evaluation of solution:
D1. Be able to support CV or CC output and can be directly induced to start with pre-loading before induction. In some solutions, the Receiver output must be connected to the battery, and the application will be limited because CV output is not supported.
D2. Under dynamic load, it can switch from no load to full load without interruption of power supply.
EMI
EMI detection is the biggest technical barrier for the commercialization of wireless charging. It will be more difficult for high-power wireless charging to pass EMC. The reason is that the electromagnetic interference limit value of current relevant laws and regulations is formulated for low-power wireless charging products. Therefore, it is inevitable to relax the limit value for high-power operation. However, it is time-consuming to modify the regulations and noise interference can only be reduced through engineering technology to meet the requirements of regulations.
However, it takes time and time to modify the regulations, so at present, only engineering techniques can be used to reduce noise interference to meet the requirements of the regulations. Although some solutions claim to have obtained relevant EMC certification, the frequency band tested in the certification is the frequency used by the out-of-band communication module. However, the huge electromagnetic energy emanating from the main operating frequency of wireless charging is the main source of interference.
Figure 5. FCC certification https://fcc.report/FCC-ID/2AVS4-FDT-EVB-WP300
*Key points of the evaluation of solution:
E1. The main operating frequency of wireless charging is the verified frequency band of the obtained certification.
E2. Reference design of complete circuit information is provided.
About the cost
The biggest consideration for commercialization is cost. The cost of wireless charging mainly lies in the circuit and coil of Transmitter and Receiver. In the industry, a module is a finished circuit board that can be operated with coils to complete power transmission. IC solution is a complete reference design. Customers can make their own modules according to the reference design, which can reduce costs through their own procurement and production capacity.
The evaluation of the 300W Wireless Power Reference Design introduced by Fu Da Tong Technology.
Distance & Efficiency: Over 85% power transmission efficiency can be achieved within 1/4 of the coil diameter. The induction distance is determined by the size of the coil. The current circuit is capable of driving a coil with a maximum diameter of 20CM. In addition, because of the patented technology of large-size coil production, large-size coil can be operated at long distance and short distance.
Safety: Transmitter in standby state will shut down the output, which consumes less than 1W power. It does not initiate power output until it detects the proximity of the target Receiver. Before induction, it has the ability to detect metal foreign object. If there is a metal foreign object on the Transmitter coil, the power output will not be started. In the process of power transmission, it also has the ability to detect metal foreign object. If metal foreign object is inserted between Transmitter coil and Receiver coil in the process of power transmission, power transmission will be shut down to ensure safety.
Communication protocol: In-band data transmission technology is used for target identification and power control, and data transmission can be completed under dynamic load. Item D. Load: After rectification and filtering, the circuit of the Receiver can be combined with DC/DC to control the output voltage and current, and it can operate without connecting the battery. The circuit of the Receiver also does not require external power supply, and the dynamic load can cope with rapid switching from no load to full load. Item E. EMI: the Reference Design’s main operating frequency of wireless charging has been certified. As far as the critical cost is concerned, complete circuit information is provided to allow product developers to produce their own products, and the selection of parts can be adjusted according to demand. For example, the Bill of Materials for 300W Wireless Power Reference Design shall not cost more than $100.
When choosing a solution, the most important thing for product developers is to obtain Evaluation Board for self-testing to evaluate the feasibility of the solution, and complete circuit data and confirmation of the feasibility of mass production are also needed. After selecting the solution, the introduction of product needs customized coil and PCB, which can be developed by referring to the Reference Design configuration. Product requirements must be confirmed before selecting a solution: