Automobiles have become the next wave of potential applications for wireless charging. However, as the working environment of automobiles is worse than that of consumer electronic products, the design of on-board wireless chargers also faces more stringent challenges, such as safety, environment, electromagnetic interference and whether it can be produced. Nowadays semiconductor chip manufacturers are also trying to overcome the above problems and develop multi-mode wireless charging transmitters suitable for automobiles.

In recent years, wireless charging technology has been widely used in the field of smart phones as a new technology highlight, which also correspondingly promotes the demand for on-board wireless chargers. Because of the special environment of automobiles and the special requirements of on-board equipment, the wireless charger originally designed for consumer electronics needs to solve a series of technical problems before it can be introduced into automobiles. This paper mainly introduces many technical challenges faced by the design of wireless charger on board, and gives corresponding solutions to achieve the goal of final vehicle use. Finally, a design example of a wireless charging transmitter (Tx) is shared to help readers understand and develop their own vehicle-mounted wireless charging products.

Wireless Charging Will Flourish in Two Camps

As early as the late 19th century, Nikola Tesla put forward the concept of wireless power transmission. It wasn't until the 1990s that wireless charging technology appeared in some electronic products that needed a sealed waterproof design. Since entering the 21st century, the use of wireless charging technology has grown rapidly with the application of consumer electronic products, especially smart phones and wearable devices. With the release of Samsung's Galaxy S6/S6 Edge and Apple Watch, 2015 will be a breakthrough year for wireless charging technology in market applications, which will also make wireless charging shift from industry-driven to consumer-driven in 2015. According to IHS forecast, wireless charging technology will be widely used in portable electronic products due to its convenience in the next 10 years, and its sales revenue will reach 15 billion US dollars in 2024 from 1.7 billion US dollars in 2015. Fig. 1 is the latest forecast data provided by IHS on the sales scale and annual growth rate of the global wireless charging market in the next 10 years.

Figure 1 Forecast Data Source of Future Global Wireless Charging Market: IHS

The wide application of wireless charging in portable consumer electronic products, especially smart phones, has correspondingly pushed forward the demand for vehicle-mounted wireless chargers. In recent years, almost all global car factories have announced that they will support low-power wireless chargers (5W) in future major models to meet the needs of market development.

At present, two different technical principles of magnetic induction and magnetic resonance are mainly used in the market to realize wireless charging schemes. In order to support the development and promotion of wireless charging technology in the world, three major standards alliances were established: WPC, PMA and A4WP. They all focus on the basic concepts and principles of magnetic induction and magnetic resonance to develop standards and products.

WPC's Qi standard is mainly based on magnetic induction technology, and an expanded standard based on magnetic resonance technology is currently being developed. The PMA standard is only based on magnetic induction technology. Rezence standard of A4WP only uses magnetic resonance technology. In order to compete with WPC, PMA and A4WP were merged into AirFuel Alliance in 2015. As a result, both WPC and AirFuel will support both wireless charging technologies of magnetic induction and magnetic resonance to provide users with multiple product solutions. At present, regardless of the number of members and products, WPC is in the leading position.

Although limited by the space freedom of charging and only one-to-one charging operation, the wireless charging scheme based on magnetic induction technology is dominant in the market due to its low production cost, high transmission efficiency, mass production, safety and market verification. Considering the special environment of automobiles and the special requirements of on-board equipment, on-board wireless chargers designed based on magnetic induction technology are generally accepted by the market. Besides cost and efficiency, interoperability is also one of the main considerations in the design of wireless charging products. Therefore, dual-mode wireless charging products compatible with Qi and PMA standards have become an increasing demand for on-board wireless chargers.

There are many challenges in the design of car wireless charger.

Wireless charging technology was originally applied in the field of consumer electronics. Existing technical standards and certifications do not involve vehicle-mounted use cases, which means that even products certified by wireless charging standards are not guaranteed to be directly suitable for vehicle-mounted use, because vehicle-mounted equipment requires more design considerations. Only wireless chargers that meet the certification of wireless charging technology standards and meet the special requirements of vehicle-mounted equipment can be accepted and used by car factories.

In addition to the general requirements of wireless charging products such as production cost, system efficiency and interoperability, the following technical challenges faced by the design of vehicle-mounted wireless chargers are analyzed from the perspectives of safety, environment, electromagnetic interference, standby power consumption, vehicle interface and manufacturability for the special requirements of vehicle-mounted applications.

Safety

Safety is the first element of product design. Improper use of products cannot cause harm to users. Because wireless chargers use magnetic fields for contactless transmission of electrical energy, they will heat magnetic metal materials within the effective range, which will affect the safety of use in severe cases. Therefore, it is very important to design an all-weather automatic detection system for abnormal magnetic metal objects and to prevent personal injury.

Environment

The use environment of vehicle-mounted products is worse than that of consumer products: wider working temperature range, risks of moisture and oil spill, thermal shock, etc.

electromagnetic interference

More and more electronic products are used in modern automobiles, which not only brings convenient driving experience, but also makes the electromagnetic compatibility (EMC) environment of complete automobiles very harsh. In general, AM/FM (amplitude modulation/frequency modulation) radio, anti-theft device, capacitive switch, intelligent key and NFC equipment will be equipped in the car. They are especially sensitive to external electromagnetic interference (EMI) when working. Therefore, the introduction of the car-mounted wireless charger cannot affect the normal operation of the existing equipment. At the same time, its own design also needs good electromagnetic interference resistance (EMS) to ensure normal operation in harsh electromagnetic interference environment.

Standby power consumption

Standby power consumption is a key indicator of charger products. Standby power consumption is particularly important for vehicle-mounted wireless chargers, especially when only battery power is available, so some innovative designs are needed to achieve as low standby power consumption as possible.

Auto interface

Due to the particularity of the automobile environment, in order to truly integrate the vehicle wireless charger into the whole vehicle system and provide a good user experience, it is necessary to design interfaces that conform to the automobile specifications, such as input voltage range, communication interface, etc. To ensure all-weather wireless charging experience and convenient update and maintenance.

productibility

Automobiles are highly complex systems and require high reliability, so the production test, diagnosis and soft repair of on-board equipment also require high requirements. Therefore, the on-board wireless charger needs to have automatic testing and diagnosis, and convenient updating and maintenance capabilities to meet the requirements of automobile manufacturability.

Solutions to Overcome Vehicle Technical Challenges

In view of the above technical challenges faced by the design of vehicle-mounted wireless chargers, this paper proposes some feasible corresponding solutions to help developers and even end users to design vehicle-mounted wireless charging products that meet the technical standards of wireless charging.

Solutions to Security Problems

For the protection of external abnormal magnetic metal objects, an external foreign body detection algorithm (FOD) can be used. There are usually four measures that can be used:

1. Detect the temperature of the wireless charging transmitter coil and charging surface, and terminate power transmission when the temperature exceeds the default limit.

2. Detect the voltage or current signal at the coil end of the wireless charging transmitter, and terminate power transmission when its amplitude exceeds a preset limit.

3. Power loss method, which detects the power transmitted by the wireless charging transmitter and the power received by the charging receiver, and terminates power transmission when the power difference (i.e. power loss) exceeds a preset limit.

4. Resonance shift method: before the system establishes power transmission, in the free resonance phase of the resonance network of the wireless charging transmitter, the amplitude or frequency of the coil terminal voltage or current signal is detected. When the amplitude or frequency is offset from the amplitude or frequency of the free resonance of the self-resonance network of the wireless charging transmitter, and the offset value exceeds a preset limit value, an external object is determined to be placed in the effective charging area of the wireless charging transmitter. If the external object is determined to be an illegal charging receiver in the following identification phase, the wireless charging transmitter determines that the external object is a foreign object and does not establish power transmission.

Among them, measures 1, 2 and 3 can identify external foreign matter only after the system establishes power transmission, while measure 4 is an effective external foreign matter identification method before the system establishes power transmission.

In order to ensure a reliable and all-weather FOD function, it is necessary to execute an effective FOD algorithm before and after power transmission is established in the system. Considering the cost, these four measures can be used in combination.

Solutions to Environmental Problems

Facing the harsh vehicle-mounted use environment, wireless chargers need to be designed with component materials with car gauge grade or AEC-Q100 or above certification to meet the requirements of reliable operation under wide temperature range, humidity, thermal shock and various stresses. In addition, vibration and bumping are inevitable when the vehicle is running, which is a great challenge to the design of wireless charger based on tightly coupled magnetic induction technology. In addition to using special devices to fix the wireless charging transmitter and mobile electronic equipment integrating the charging receiver on the mechanical structure, the following auxiliary measures should also be considered in the design of wireless charger to ensure tight coupling and reliable operation of the system:

1. Use multi-coil or single large-size coil design to increase effective charging area and achieve the purpose of free positioning and alignment.

2. Use printed circuit board (PCB) type coils to improve the service life of coils in vibration environment.

3. It is necessary to improve the vertical distance (Z-gap) for reliable charging, usually 68 mm, preferably 10 mm or more. Finally, perfect system fault protection function is also essential, such as voltage, current and temperature protection, to ensure reliable operation of wireless charger in harsh vehicle environment.

Solution to Electromagnetic Interference Problem

Vehicle applications have strict requirements on EMC standards, but wireless chargers are electromagnetic emission sources themselves, so it is particularly difficult to design vehicle wireless chargers that meet vehicle regulations. First, it is recommended to use a fixed operating frequency wireless charging transmitter topology design to control power transmission, which can reduce the frequency band of external emission and simplify the design of EMI filter. At the same time, fixed working frequency is beneficial to design to avoid interference to other radio communication equipment, such as AM/FM radio, smart key, etc. Secondly, it can support the online jump of the working frequency of the wireless charging transmitter according to whether the system receives the access of the electronic equipment of the specific radio wave frequency band, so as to reduce the influence of the wireless charger on the radio equipment of the specific frequency band when working. Finally, some experimental test results show that wireless charging and NFC integrated in a system cannot work at the same time due to mutual interference. Therefore, it is necessary to develop a system control scheme for coexistence of wireless charging and NFC, and coordinate the work using the principle of time-sharing to solve the problem of coexistence of wireless charging and NFC.

Solution to Standby Power Consumption Problem

When the illegal charging receiver is supplied or the charging operation has been completed, the wireless charging transmitter should be in the standby state, and constantly detect whether there is an external object in the effective charging area. If the object is a legal charging receiver, the wireless charging transmitter will exit the standby state. Therefore, for the special working mechanism of the wireless charger, it is necessary to design a low-power system standby state in combination with external object detection methods. Generally, the following methods can be recommended.

The first method is to use an independent capacitive or proximity Touch Sensor to sense whether an external object is placed in the effective charging area, and if so, power is supplied to the wireless charging function circuit, thus ensuring that only the touch sensor supplies power and consumes power in the standby state and ensuring the lowest system standby power consumption.

The second method is to use the general input/output port (GPIO) of the wireless charging controller to simulate the capacitive type contact sensor if you do not want to use an independent contact sensor to reduce the system cost. At this time, the wireless charging controller must supply power, but it can be in a low power consumption deep sleep mode for a long time in the standby state, and wake up periodically through its internal timer, and then execute the Simulation algorithm of the contact sensor to detect whether external objects are put into the effective charging area. Compared with the first method, this method sacrifices standby power consumption to save cost.

The third method is to use analog PING (short pulse mode) to detect whether any external object is placed in the effective charging area. This method requires the entire wireless charging power circuit to work during the PING, but it can periodically trigger analog PING operation using a timer inside the wireless charging controller, so that during non-analog PING, the wireless charging power circuit can be powered down and the wireless charging controller can be in a low power consumption deep sleep mode.

Compared with the first and second methods, the third method is the simplest to implement but has the largest standby power consumption. Both the second and third methods reduce the average standby power consumption by lengthening the timing interval of the timer, but the response time for external object detection will be correspondingly slower.

Designing an Interface Suitable for Vehicle Application

The car wireless charger needs to consider the special environment of the car and design the interface conforming to the car. Usually these interfaces include:

1. Input power supply

Because different power supply occasions will cause changes in the input voltage, such as battery and engine power supply, its input voltage range is 916 volts (V); However, the input voltage can be reduced to about 6V at the moment of ignition of the engine, so the input voltage of the vehicle-mounted wireless charger needs to support a wide range from 616 V ..

2. Communication interface

CAN network is usually used in automobiles. The on-board wireless charger supporting CAN interface CAN connect itself as a node of CAN network to the whole vehicle system, which is convenient for effective management, monitoring, diagnosis and on-line software maintenance.

3. For the purpose of better user experience and battery protection, it is recommended to add an engine ignition detection interface so as to optimize the working mode of the wireless charger according to the working state of the engine and achieve the best charging experience.

Solutions to Productive Problems

Vehicle wireless charger needs to have automatic test and diagnosis, convenient update and maintenance capabilities to meet the requirements of vehicle system productivity. The solution of wireless charger based on software function is the best choice. Through software technology and network communication interface, automatic test and diagnosis in the production and maintenance process, online maintenance and update of software can be conveniently realized to ensure high reliability of the system.

Solutions to Interoperability Problems

Obtaining the certification of wireless charging technology standard can only ensure that all products under the same standard can be paired with each other, but the reality is that there are currently two major standard alliances and charging receiver products are also used in the market. In order to further improve the interoperability of on-board wireless chargers, dual-mode wireless chargers compatible with Qi and PMA are the better solutions. At the same time, in order to share the hardware platform and save production costs, it is necessary to comply with the technical specifications of Qi and PMA respectively. for example, currently Qi's A13 and PMA's PMA-6 wireless charging transmitter will be a better choice.

WPC/PMA compatible dual-mode vehicle wireless charging Tx design example

Fig. 2 shows a design example of a vehicle-mounted wireless charging transmitter compatible with Qi A13 and PMA PMA-6 specifications. The functions of this example include compliance with Qi low power v1.2 specification and PMA SR1E specification; Support multi-coil free positioning alignment design; Supporting perfect FOD algorithm, external foreign matter detection can be carried out before and after power transmission is established; Support on-line operating frequency hopping to avoid interference to specific radio frequency bands; Low standby power consumption design supporting contact sensors and analog PING, etc.

Fig. 2 physical diagram of vehicle-mounted wireless charging transmitter

Based on wireless charging technology and market analysis, combined with the special requirements of on-board equipment, this paper introduces the technical challenges faced in designing on-board wireless chargers, and gives feasible solutions to help readers design their own on-board wireless charging products.