For those of us who, in our daily commute, spend an inordinate amount of time in stop-and-go traffic (or completely parked on those things called Interstate Highways), know that this idle time can be used for many other purposes, and that the automobile has always been a key market for technologies that assist us in being able to do some of those other “tasks,” such as phone conversations, texting and e-mailing, online shopping and surfing, movie downloading and video streaming, game playing, and more. If you look left or right while in traffic, you can see that a majority of these tasks evolve around the use of the handset. To supplement this high usage, one of those newer implemented technologies is in-vehicle wireless charging functionality built into the center console area or some other easily accessible location. The purpose? To remove all those plug-in cables and to have the handset sitting in a known location while being charged.
Within the past three years, the wireless power technology “wars” were resolved with “Qi” or Wireless Power Consortium (WPC) being the winner, and now the low power de-facto standard. This was further validated with the implementation of the Qi technology by all leading worldwide handset makers. Prior to this, automakers did implement wireless charging within their vehicles, but there was always concern as to whether a sale would be lost solely because the buyer’s phone was not compatible with the embedded charging technology.
Car model implementation numbers, with wireless charging available, have grown from 40+ in early 2016, to 100+ models (currently), which equates to over 12 million (2.4M OEM installed, 9.7M after market) vehicles having Qi based in-vehicle systems installed in 2018 alone. The majority of those systems were compliant with the Qi Basic Power Profile (BPP) and for 5W (watts). The new direction is for faster charging and higher power. A majority of new designs are targeting compliance with the Qi Extended Power Profile (EPP) or 15W capability. This added convenience of being able to charge faster comes with additional technical hurdles that must be overcome. The three main issues being EMI compliance, efficiency and thermal limitations.
The 15W System
Within the WPC standard, there are sub-categories (e.g. MP-A8, MP-A9, MP-A13) that specify various aspects of the wireless power system and the configuration of the transmit (Tx) coil that is placed within the center console area. Done for interoperability purposes, the standard defines: input DC voltage, Tx coil size and shape, electrical parameters, frequency control (fixed versus variable), power level, and power control (voltage/frequency/phase/duty cycle). The input voltage, using the vehicle’s main battery, is typically 12V into the transmitter’s circuitry and thus has an elevated voltage, creating a stronger electrical field (E) than the 5V input voltage associated with many desktop wireless chargers. Due to the resonance mode of operation within the system, the actual voltages on the coils (resonators/antennas) can be around 100V, meaning the radiated noise will be stronger for the 15W system than the 5W system.
EMI Issues and Solutions
On newer vehicles, there are numerous RF systems, all needing to co-exist to ensure that what they are doing does not affect anything else. Some of these being: AM/FM radio, GPS, ADAS systems, multiple cellular bands, Blue Tooth, WiFi, asset tracking, short wave radios, key fobs, police scanners, telematics, etc., and maybe, even a few CB radios for all those 10-4 buddies out there.
A few of these RF systems operate within the 87-205 KHz (can be up to 300 KHz) fundamental frequency range of the Qi EPP wireless power system and/or through the low harmonics. AM radio, 525 KHz to 1705 KHz (in the Americas), is required to be EMI free as it is used as part of the Emergency Broadcast System. New remote keyless entry systems (RKE) operate at 125 KHz as do some Tire Pressure Monitoring Systems (TPMS) which use this frequency to drive the initiator L-C coil circuit.
Automotive applications have very stringent requirements for EMI. CISPR 25 (Comité International Spécial des Perturbations Radioélectriques) is a non-regulatory engineering automotive standard that sets conducted and radiated emission limits that must be met for the protection of other on-board receivers. It defines these limits over a frequency range of 150 kHz to 2500 MHz which could possibly be conducted by other vehicle mounted antennas.
Within CISPR 25, there are classes that define the level of permitted conducted and radiated noise emissions limits, with radiated noise being the real concern. The Class emission [radiated] limits versus bands are given in Table 1 for Peak, Quasi-peak and Average measured voltage up through the FM radio band.
click for full-size image
Table 1: CISPR 25 Radiated Limits by Class
With the increased Qi EPP power levels, meeting Class 4 has been a challenge, and no Class 5 system is available in the market yet. For in-vehicle wireless charging, the AM frequencies up to 1.8 MHz are the most sensitive but certification testing does go up past 1 GHz. Actual CISPR 25 Class 5 measurement data is provided in Figure 1.
From the plot, this design does not fully pass Class 5 certification, though it does meet Class 4 requirements. EMI noise mitigation starts with the system’s electrical design and the following sections address some key areas used in the design to meet CISPR 25 requirements.
The first area of mitigating EMI noise is implementing a fixed frequency system. Within the Qi standard, there are means to allow for variable frequency to better “tune” the two sides for improved performance. However, to meet the stringent EMI noise levels associated with in-vehicle power systems, a changing frequency would make complying with these even more problematic. Also, European automakers have restrictions above 145 KHz so the fixed operating frequency of current solutions is set at around 127 KHz.
The next technique is to remove square wave currents though the Tx coil and have these currents as close to sinusoidal as possible. This approach reduces the noise “spikes” that could be otherwise generated. This can be achieved by using an inductor as this passive device smooths out the square wave current created by the turning ON/OFF of the switches (MOSFETs), and helps to ensure that the switching scheme is “clean” and noise free.
Further EMI suppression can be realized with the addition of a common mode filter (CMF) placed on the power lines in series with the Tx coil windings. Currents through the coils are 100% alternating current (AC) and have no direct current (DC) content like many power supplies that involve DC current and some allowable ripple current. The coil’s current can be thought of as being 100% ripple current. Therefore, selection of the ferrite material used for this CMF is important and AC core loss must be at an absolute minimum at the 127 KHz fixed frequency.
Another EMI noise suppression technique is to add EMI noise suppression magnetic sheets to absorb operating frequency, harmonics and spurious noise generation that may be transmitted out of the backside of the main Tx shield. The magnetic sheets remove EMI noise via two methods. First, the permeability (µ’) of these materials enables these shields to contain [absorb] the EMI noise magnetic flux (φ) and keep it from being radiated. Next, the resistive properties (µ”) of these shields create a resistive path for the unwanted frequencies’ flux field and attenuate the EMI noise and remove it from the environment in the form of heat. This relationship is given in Equation 1.
µ = µ’ – jµ” [Eq. 1]
For EMI suppression applications, higher µ’ yields better shielding performance through magnetic flux containment and higher µ” yields better noise suppression through material core losses. Having too high of a µ’ value can decrease performance. Due to a phenomenon called magnetic coupling (K), having an additional magnetic sheet can shift the inductance value of the Tx coil and de-tune the circuit through mutual coupling (M or Lm) and move it away from the desired fixed frequency.
Lastly, if EMI suppression sheets do cause fixed frequency problems, there are non-magnetic materials that also suppress EMI noise. The challenge is to obtain a material that can absorb some level of noise energy, yet is not too metalized as to simply reflect the EMI noise rather than removing it and not suppress the desired H field. Silver alloy-based films with low surface resistances (~4 ohms/square) have been used and demonstrate improved EMI noise suppression up to 1 MHz and dampening of problematic harmonics. These non-magnetic sheets, placed on the top side of the windings, tend to better suppress voltage/E field-based harmonics rather than current/H field-based harmonics.
The Tx coil comes with its own magnetic shield which contains magnetic flux generated by the sinusoidal electrical current going through the winding. For the fundamental operating frequency (127 KHz), the shielding material is selected to have higher µ’ and very low µ” as to not attenuate the desired magnetic flux field. This shield contains the wanted magnetic flux at the operating frequency for improved performance and some of the harmonic flux, thus becoming part of the of the overall EMI compliance solution.
>> Continue reading about “Efficiency – Wireless Power System Factors” in this article originally published on our sister site, Power Electronics News.
|Chris T. Burket has been with TDK for over 25 years and occupies a desk in the Los Angeles office. He has held various positions within TDK and is currently a Product Marketing Engineer. He drives a 1968 Camaro which did not come with an integrated wireless charging system option when new.|