As we switch to using wireless products more in our day-to-day lives, Power Electronics research is simultaneously evolving new trends in wireless charging for things like Electric Vehicles (EV). Many countries are now implementing fuel economy regulations and driving initiatives to replace gasoline vehicles with EVs; consequently, automotive manufacturers are now heavily focused on EV development. While technology advancements such as lithium-ion batteries and ultracapacitors hold great promise, the main requirement for a smoother overall transition to EVs is infrastructure and the availability of suitable and fast charging systems.
Charging systems for electric vehicles are high power conversion equipment that convert AC / DC power from the power supply source into DC power that can charge the batteries of a vehicle. The peak power currently required is on the order of 10kW to 20kW. This can go even higher depending on how much time is available to charge and the advancements in battery charging capabilities. So, there is an impetus on both the government and OEMs to develop high-power charging systems that can cater to the power needs of future EVs. This article specifically focuses on those wireless charging systems (Figure 1).
Figure 1: Block Diagram of Wireless Charging System
A wireless charging system transfers power from the source to the load without a physical connection. Common schemes available today consist of a transformer with air as the core. The power transfer takes place without any contact between the source and the load. Wireless power transfer applications start from low power mobile charging systems rated to 10’s of watts, to high power electric vehicle fast chargers rated up to 10’s of kilowatts.
Traditionally, the main issues with wireless charging systems are low efficiency and safety. Research shows various concepts have now achieved efficiencies of more than 80%, which is on par with wired power conversion systems. As the distance between the primary and the secondary coil increases, the efficiency drops exponentially; thus, the efficiency can be improved by reducing the distance between the coils and adopting different coil construction methods. Safety is taken care of by smart power control which can detect spurious power transfers and suspend power transmission immediately. Regulatory guidelines, such as SAE J2954, are being implemented so that safety is always ensured.
Wireless power transfer can be achieved in several ways, but the most common are the inductive and resonance transfer methods. Inductive power transfer is based on the transformer principle where the AC voltage on the primary side induces voltage on the secondary side thereby inducing power transfer (Figure 2). This method is highly sensitive to coupling between the primary and secondary windings – i.e. as the distance increases, the power loss becomes huge, reducing efficiency. Therefore, this method is restricted to low power applications such as mobile phone chargers.
The resonant method is based on impedance matching between the primary and the secondary side. A resonant circuit is designed to allow for the formation of a tunneling effect for the magnetic field (Figure 3). This minimizes the loss of power and results in higher efficiencies even when the coils are far apart. Therefore, this method can be used in applications where high power transfer is required. Efficiencies of more than 85% have been recorded in research with this method.
Figure 2: Inductive Power Transfer [1]
Figure 3: Resonant Power Transfer [1]
In both methods, the amount of power transferred is dependent on several parameters:
1) Air gap between the two coils
2) Values of inductance, losses in power switches, circuit parasitics, etc.
3) Frequency of power waveform
These crucial parameters are prone to huge variations in the practical system. Performing a complete system simulation helps to achieve predictable results and helps design engineers to arrive at the most efficient configurations. In the absence of an accurate simulation model, several prototypes with different components and configurations become necessary, costing time and money.
A typical wireless charging system for EV charging consists of:
1) A power supply source – This can be the power from the grid, which is AC, or it can be drawn from renewable energy sources such as Solar in DC form. So, the power conditioning unit (PCU) should consider the variations in input while transferring the required power to the load.
2) A phaseshifted resonant bridge converter – These operate at few 100s of kHz with the use switching frequencies for the PCU on input side. The device’s switching, its parasitics, losses, and so on affect the power transfer. Simulation of the topologies with accurate models can help estimate the performance at an earlier stage of the design process.
3) The primary and secondary coils and their inductances – The value of the inductance varies with position and can be viewed as variation with time. This directly impacts the system impedance and hence the losses in the system. If the inductance can be modeled and tested in a simulation platform, the most efficient configuration of the coils can be designed without the need for several prototypes. The simulation program should allow for an easy method to include equations or data driven modeling as well as provide design optimization mechanisms across a range of parameters.
4) The secondary rectifier – This converts the AC voltage to DC voltage to charge the batteries and supply other loads.
5) An effective communication protocol – This identifies whether a valid load is present on the secondary side or not. This ensures that erroneous power transfer is not initiated and ensures safety.
Considering the complexity of the system and the parameter variations of the system components, simulation and optimization of the system helps to predict performance and offers a reliable design. Simulation platforms, such as SaberRD, can implement the system to verify the performance under various conditions of spacing between coils, component variations, and so on. This is augmented with the availability of modeling tools that can create accurate models resulting in accurate simulation outputs. Figure 4 shows the implementation of the wireless mobile charging system and how the performance is checked for various conditions including change in coupling between the primary and secondary windings. For more details, an article and design example based on this type of system can be accessed in the Saber forum [2].
Figure 4: Wireless Charging System for Mobile Charging in Automobile
As mentioned earlier, the output power from a wireless charging system is highly sensitive to varying airgaps, device parameters, circuit parasitics, load and so on. In case of resonant method, the impact of parasitics introduced by the interconnects between power devices is significant. In simulation, the 3D model of the parasitic elements can be imported and a comprehensive analysis can be performed. This optimizes the design before going for the hardware prototype. In addition, analysis such as the Multivary Analysis in SaberRD, gives a high degree of freedom to analyze the design with multiple parameter variations. Hence, accurate simulation helps to determine the design’s performance precisely and reduces the number of hardware prototype iterations.
Wireless charging systems where there are no interconnecting cables to charge, enables charging on-the-go. With this in place, vehicles can have lower capacity batteries and get charged more frequently. For example, buses can charge when stopped at the bus stop as shown in Figure 5. In addition to this, the reliability of the system is improved because of the absence of mechanical connectors and cables.
Figure 5: Wireless Charging System for On-the-Go Charging
Wireless charging proves to have a huge impact on future charging systems for EVs. Reducing “range anxiety” and enabling a user experience like that of combustion engines will allow the proliferation and help fulfill the promise of EVs. Wireless charging might just turn out to be one of the key enablers.
