Wireless Power Transfer (WPT)
Jan 26, 2024
We are all familiar with the rechargeable toothbrush or the phone charging pad that use low power wireless power transfer, but what challenges lie ahead for high power wireless charging, for example, for an electric vehicle (EV)?
Almost all owners of plug-in electric cars rely on a heavy and expensive cable to connect the vehicle to the electric vehicle charger. The cable is expensive because it must be thick enough to carry the required peak charging current (typically from 11kW up to 100kW or more), tough enough to withstand being carelessly thrown in the back of the car or used in inclement weather and be robust enough to withstand repeated plugging and unplugging operations. Even so, the cable and connectors have a limited lifetime and will eventually become unsafe, worn or damaged by daily use. A better solution would be to dispense with the cable and connectors completely.
Fig.1: EV wireless charging concept
Figure 1 shows a concept for a wireless electric car charger. The vehicle would be simply parked over a charging coil and power would be transferred by inductive wireless power transfer to recharge its batteries. Wireless communication would ensure that power was only transferred when it is safe to do so, much as a modern mobile phone communicates with a Qi-enabled charger pad to ensure that no foreign objects are present in the charging field before power is applied.
The main difference between a mobile phone and EV wireless power charger is the power levels used. For a high-power wireless charger, efficiency must be maximized, whereas phone chargers are typically only 70% efficient. This figure is acceptable for a low-cost commodity product, but would be wasteful for an EV wireless charger, where a system efficiency figure of closer to 85% is required (AC power to DC power).
There are three ways in which the power transfer efficiency can be improved: more tightly coupled magnetic circuits, higher frequency operation and better matching, but first let us look at the basics of wireless power transfer technology.
For capacitive and magnetic WPT systems, the energy stored in a unit volume of air between the transmitter and receiver is given respectively by:
Fig. 2: Inductive wireless power transfer schematic2
Figures 2, 3 and 4 are from the RECOM AC/DC Book of Knowledge, Chapter 10.
Where E and H are the intensity of the electric and magnetic fields respectively and ε0 and μ0 are the permittivity and permeability values for free space. As μ0 is higher than ε0, about a thousand times more energy can be transferred in a coupled magnetic field than a capacitively coupled field when practical voltage and current limitations are considered. Therefore, inductive and magnetic resonance coupling lend themselves to the highest power transfer.
Essentially, inductive charging systems use a transmitter coil to generate a localized magnetic field which is coupled into a receiving coil via mutual inductance (Figure 2):
The mutual inductance, M, between the transmitter and receiver coils is given by the deceptively simple equation:
Where Lt and Lr are the winding inductances of the transmitting coil and receiving coil respectively and k is a coupling coefficient, which is dependent on the dimensions, number of turns and alignment (orientation and separation) of the coils (Figure 3):
Fig. 3: Effect of various flat coil misalignments on inductive power transfer efficiency
Fig. 4: Resonant inductive coupling using intermediary resonators
The coupling coefficient can be enhanced by inserting intermediary coils which act as ’magnetic lenses‘ to focus the magnetic flux (Figure 4). Higher power resonant inductive coupling systems may use three or more of these coils. These intermediary coils are resonant tank circuits with a capacitor in parallel with the winding which resonates at the frequency of the alternating magnetic field (Figure 5).
The resonators boost the effective magnetic field strength from the transmitting coil and concentrate the effective received field into the receiving coil, increasing the coupling efficiency significantly. Additionally, even if only part of the projected magnetic flux is intercepted by the intermediary circuits, they will still resonate, so separation distance and alignment are not so critical as with two simple flat coils.
Fig. 5: Equivalent circuit model of a WPT with intermediary resonators
The intermediary resonators do not have to be placed symmetrically as shown in Figure 4 – if the limiting factor for power transfer is sufficient magnetic flux, then paired resonators placed close to the transmitter coil will magnify the local magnetic field through the coupling factors k12 and k23 for a stronger coupling factor k34 to the more distant receiver coil.
Such intermediary coils are essential for WPT applications where the distance and alignment between the transmitting and receiving coils is not fixed, for example in an electric road that recharges a moving vehicle driving over it. Tesla, amongst other companies, have built prototype in-road charging systems where the vehicle has underbody spring-loaded metal power connectors to recharge while on-the-go, but Detroit in the USA is the first city in the US that has implemented a contactless in-road charging system based on wireless power transfer3. The system successfully demonstrated a charging rate of up to 19kW.
3 https://eu.freep.com/story/money/cars/2023/11/29/detroit-wireless-charging-road-project-electric-vehicles/71728454007/
Where the output power, Pout, is equal to the angular frequency at resonance, ω0, multiplied by the mutual inductance, M, the current in the transmitting coil It and the resulting induced current in the receiving coil, Ir. Thus, the transmitted power is directly proportional to the frequency of the alternating magnetic field. However, core eddy current and switching losses increase with higher frequency, so there is an optimum WPT operating frequency which is dependent on other system parameters for peak inductive power transfer efficiency.
With existing high power switching technology, a resonant frequency of between 20kHz and 150kHz achieves the best results.
The final significant factor affecting the system efficiency is the matching of the supply, coil and load resistances. The maximum power transfer efficiency (PTEmax) can be derived from the following relationship (at resonance):
Where RL, Rt and Rr are the load, transmitter and receiver ohmic resistances respectively.
For best performance, the resistance of the load, receiving coil and transmitting coil should all be the same.
This creates some practical problems in the design of the WPT system. The high current power supply front end and inverter for the transmitter has a very low internal impedance, so a high frequency impedance matching transformer may be needed to get the highest coupled transmission power to the coil. Similarly, the load is a battery pack with a non-linear internal resistance characteristic which is dependent on its state of charge, so a DC/DC on board charging (OBC) unit will be required which can be impedance-tuned for optimum power reception, much like the maximum power point tracking (MPPT) circuits used in photovoltaic DC/DC converters (Figure 6).
Fig. 6: WPT power stages with anticipated conversion efficiencies
To meet the efficiency targets, the active front end (AC to DC conversion and power factor correction) will need to use a bridgeless totem pole configuration or similar (Figure 7) and the inverter will need to use a full bridge or variant of an LLC topology. Both designs will need to use several isolated transistor gate drivers, which is where RECOM can support WPT designs with standard and programmable isolated gate driver DC/DC power supplies:
Fig. 7: GaN Totem pole bridgeless rectifier example circuit
With high power switching designs, it is often difficult to balance out the power ground stray inductances in each leg, which can lead to asymmetric performance and switching instability. Isolating both the high-side and low-side gate drivers eliminates this problem (Figure 8).
RECOM offers a range of compact gate driver power supply modules with high isolation, asymmetric output voltages for optimal power transistor switching and a wide operating temperature range, making them ideal for such high-power designs, including bidirectional circuits.
Fig. 8: Full bridge gate driver example circuit
Fig. 9: RECOM’s 15kW (parallelable up to 75kW) high voltage on-board charger
In the electric vehicle itself, another active rectifier circuit will convert the AC from the receiving coil to charge an intermediary bus capacitor, CDC. This unregulated DC bus voltage can be used to supply a high-power digital DC/DC converter unit such as RECOM’s 15kW OBC design (Figure 9).
This 15kW converter design will accept a wide DC input voltage range of 25VDC up to 280VDC and boost the output voltage up to a programmable 200V-800VDC to charge a high voltage EV battery stack, with an efficiency exceeding 97%. The built-in MPPT circuit optimises the power transfer efficiency during the entire charging cycle. The CAN-bus interface allows communication with standard battery management system controllers and permits active load sharing between paralleled units.
RECOM already offers products that will allow high voltage power supplies and systems for wireless charging for electric vehicles to be built, evaluated and tested.
Fig.1: EV wireless charging concept
The main difference between a mobile phone and EV wireless power charger is the power levels used. For a high-power wireless charger, efficiency must be maximized, whereas phone chargers are typically only 70% efficient. This figure is acceptable for a low-cost commodity product, but would be wasteful for an EV wireless charger, where a system efficiency figure of closer to 85% is required (AC power to DC power).
There are three ways in which the power transfer efficiency can be improved: more tightly coupled magnetic circuits, higher frequency operation and better matching, but first let us look at the basics of wireless power transfer technology.
Wireless Power Transfer Basics
WPT technology dates back to the late 1800’s when Heinrich Hertz demonstrated high frequency spark gap wireless power transfer using two parabolic reflectors to focus RF radiation. Nikola Tesla also experimented with coupled electromagnetic resonance circuits just before the turn of the century, but there is no evidence that he succeeded in transferring meaningful amounts of electrical power. The earliest successful demonstration of Inductive power transfer was in 1910 to illuminate a light bulb held over an open transformer, but again this was not turned into a practical wireless electricity product. Despite the lack of commercial success, these early pioneers laid the groundwork for some of the main wireless power transmission technologies used today:| WPT Method | Range | Frequency | Uses |
|---|---|---|---|
| Inductive | Short | kHz-MHz | Electric toothbrushes |
| Magnetic Resonant Coupling | Mid | kHz-GHz | Phone Chargers, EV chargers |
| Capacitive Coupling | Short | kHz-MHz | Biomedical implants |
| Microwave | Long | GHz | Satellites |
| Laser | Long | THz | Drones |
For capacitive and magnetic WPT systems, the energy stored in a unit volume of air between the transmitter and receiver is given respectively by:
Fig. 2: Inductive wireless power transfer schematic2
Figures 2, 3 and 4 are from the RECOM AC/DC Book of Knowledge, Chapter 10.
Essentially, inductive charging systems use a transmitter coil to generate a localized magnetic field which is coupled into a receiving coil via mutual inductance (Figure 2):
The mutual inductance, M, between the transmitter and receiver coils is given by the deceptively simple equation:
Where Lt and Lr are the winding inductances of the transmitting coil and receiving coil respectively and k is a coupling coefficient, which is dependent on the dimensions, number of turns and alignment (orientation and separation) of the coils (Figure 3):
Fig. 3: Effect of various flat coil misalignments on inductive power transfer efficiency
Fig. 4: Resonant inductive coupling using intermediary resonators
The resonators boost the effective magnetic field strength from the transmitting coil and concentrate the effective received field into the receiving coil, increasing the coupling efficiency significantly. Additionally, even if only part of the projected magnetic flux is intercepted by the intermediary circuits, they will still resonate, so separation distance and alignment are not so critical as with two simple flat coils.
Fig. 5: Equivalent circuit model of a WPT with intermediary resonators
Such intermediary coils are essential for WPT applications where the distance and alignment between the transmitting and receiving coils is not fixed, for example in an electric road that recharges a moving vehicle driving over it. Tesla, amongst other companies, have built prototype in-road charging systems where the vehicle has underbody spring-loaded metal power connectors to recharge while on-the-go, but Detroit in the USA is the first city in the US that has implemented a contactless in-road charging system based on wireless power transfer3. The system successfully demonstrated a charging rate of up to 19kW.
3 https://eu.freep.com/story/money/cars/2023/11/29/detroit-wireless-charging-road-project-electric-vehicles/71728454007/
High Frequency Wireless Power Transfer
It would be possible to carry out charging by induction by using the low frequency 50/60 Hz alternating current available from the mains supply, but this would be inefficient for higher powers. The higher the transmission frequency, the more power can be transferred according to:
Where the output power, Pout, is equal to the angular frequency at resonance, ω0, multiplied by the mutual inductance, M, the current in the transmitting coil It and the resulting induced current in the receiving coil, Ir. Thus, the transmitted power is directly proportional to the frequency of the alternating magnetic field. However, core eddy current and switching losses increase with higher frequency, so there is an optimum WPT operating frequency which is dependent on other system parameters for peak inductive power transfer efficiency.
With existing high power switching technology, a resonant frequency of between 20kHz and 150kHz achieves the best results.
The final significant factor affecting the system efficiency is the matching of the supply, coil and load resistances. The maximum power transfer efficiency (PTEmax) can be derived from the following relationship (at resonance):
Where RL, Rt and Rr are the load, transmitter and receiver ohmic resistances respectively.
For best performance, the resistance of the load, receiving coil and transmitting coil should all be the same.
This creates some practical problems in the design of the WPT system. The high current power supply front end and inverter for the transmitter has a very low internal impedance, so a high frequency impedance matching transformer may be needed to get the highest coupled transmission power to the coil. Similarly, the load is a battery pack with a non-linear internal resistance characteristic which is dependent on its state of charge, so a DC/DC on board charging (OBC) unit will be required which can be impedance-tuned for optimum power reception, much like the maximum power point tracking (MPPT) circuits used in photovoltaic DC/DC converters (Figure 6).
Fig. 6: WPT power stages with anticipated conversion efficiencies
To meet the efficiency targets, the active front end (AC to DC conversion and power factor correction) will need to use a bridgeless totem pole configuration or similar (Figure 7) and the inverter will need to use a full bridge or variant of an LLC topology. Both designs will need to use several isolated transistor gate drivers, which is where RECOM can support WPT designs with standard and programmable isolated gate driver DC/DC power supplies:
Fig. 7: GaN Totem pole bridgeless rectifier example circuit
With high power switching designs, it is often difficult to balance out the power ground stray inductances in each leg, which can lead to asymmetric performance and switching instability. Isolating both the high-side and low-side gate drivers eliminates this problem (Figure 8).
RECOM offers a range of compact gate driver power supply modules with high isolation, asymmetric output voltages for optimal power transistor switching and a wide operating temperature range, making them ideal for such high-power designs, including bidirectional circuits.
Fig. 8: Full bridge gate driver example circuit
Fig. 9: RECOM’s 15kW (parallelable up to 75kW) high voltage on-board charger
This 15kW converter design will accept a wide DC input voltage range of 25VDC up to 280VDC and boost the output voltage up to a programmable 200V-800VDC to charge a high voltage EV battery stack, with an efficiency exceeding 97%. The built-in MPPT circuit optimises the power transfer efficiency during the entire charging cycle. The CAN-bus interface allows communication with standard battery management system controllers and permits active load sharing between paralleled units.
Conclusion
Wireless power transfer is a viable alternative to wired electric vehicle charging systems in terms of technology, even if it is not mainstream yet due to the higher cost. As EVs become the norm rather than the exception, the ease-of-use and convenience of simply driving up to a parking slot and starting to charge the battery wirelessly will make WPT more attractive, especially as the technology already exists for the vehicle to move and park itself. Ultimately, on-the-go WPT charging using electric roads will eliminate the “range anxiety” of using EVs, enabling the battery to be fully charged at the end of the journey, not just at the start.RECOM already offers products that will allow high voltage power supplies and systems for wireless charging for electric vehicles to be built, evaluated and tested.
Applications
