Operating Frequency Selection for Loosely Coupled Wireless Power Transfer Systems with Respect to RF Emissions and RF Exposure Requirements Jagadish Nadakuduti, Lin Lu, Paul Guckian Qualcomm Technologies, Inc.
Introduction Loosely Coupled Wireless Power Transfer (LC WPT) systems provides spatial freedom while maintaining efficient power transfer, thus it gives industrial designer greater flexibility in embedding WPT capability into work surfaces, home furniture, and automotive environments. LC WPT is commonly implemented in the frequency range between 100 khz and 10 MHz. In this paper, we propose specific frequencies to operate LC WPT that is commercially viable to demonstrate compliance with existing regulatory framework requirements of RF emissions and RF exposure.
Radiated Emissions Qualcomm Contribution to CEA R6.3 WG4 16-May-2012 Meeting (Dallas, TX)
CISPR 11 Requirements CISPR 11 Group 2, Class B is used to assess for harmful interference from LC WPT. LC WPT systems are required to meet free-space E- and H- field levels to conform with CISPR 11 limits on wanted emissions at the operating frequency and its harmonics, and also on unwanted (spurious) emissions up to 18 GHz. CISPR 11 places no limits on RF emissions from ISM equipment when operating at ISM band frequencies.
Wireless Power Transfer Systems Radiated emissions test has been performed on 468 khz and 6.78 MHz prototypes (shown below) with comparable form factors and load conditions Both systems use similar Tx and Rx coil structure. Hence, the 468 khz system requires higher operating Ampere-Turns (AT) to achieve the desired induced voltage at Rx coil. Results are presented in next slide 468 khz wireless charging pad 6.78 MHz wireless charging pad
Radiated Emissions Assessment per CISPR 11 International Limits for ISM H-field emissions at 3m for 468 khz system is 60.5 dbua/m, which is 29.3 db over the limit. H-field emission from 6.78 MHz system is 54.1 dbua/m. 6.78 MHz is an ISM frequency, which has no restriction on emissions. In order to meet CISPR 11 limit, the AT in the 468 khz Tx coil should be reduced by ~ 30dB, which results in low induced voltage at Rx input that is insufficient to meet the requirements for WPT spatial freedom in commercial electronics application.
Dependency of LCWPT Power Transfer The power transfer dependency can be shown using the equivalent circuit of LCWPT system R Src R 1 L 1 Tx M L 2 Rx R 2 R L Note: The coupled coils subsystem consist of Tx coil (Tx) and Rx coil (Rx) V src V in C 1 C 2 Source Z in = R in + jx in (X in = 0 at resonance) Mutual inductance (M), coil losses (R 1, R 2 ) and frequency (ω) are fundamental variables controlling the power transfer. At resonance, the load (R L ) represented at Tx coil, Z in, can be expressed as Z in = R in = R 1 + (ωm)2 R 2 +R L For a given M, the lower the operating frequency, the higher is the input current required for targeted power transfer, which in turn results in higher emissions at the fundamental frequency.
RF Exposure Design Strategy Qualcomm Contribution to CEA R6.3 WG4 16-May-2012 Meeting (Dallas, TX)
Human Exposure Limits FCC limit on 1g SAR is 1.6 W/kg to prevent tissue heating for f > 100 khz ICNIRP 1998 and 2010 standards have limits on induced current density (J) and induced electric field (E) between 1 Hz and 10 MHz to prevent nerve stimulation in both central and peripheral nervous systems (CNS and PNS) 2010 standard specifies the E limits for both CNS and PNS 1998 standard was based on effects seen in CNS from biological studies but specifies the induced J limits for all tissues in head and trunk regions As of today, ICNIRP 2010 standard has not been adopted by regulatory bodies. Hence, human exposure should be qualified for all exposure quantities in 100 khz to 10 MHz frequency range: RF Exposure Limits for General Population (100 khz 10 MHz) SAR [W/kg] (Whole Body Average) SAR [W/kg] (Head/Trunk) SAR [W/kg] (Limbs) Induced E [V/m] (CNS and PNS) (2x2x2 mm 3 -avg) Induced J [ma/m 2 ] (Head/Trunk) (1 cm 2 - avg) ICNIRP 1998 0.08 2 (10-g) 4 (10-g) -- f/500 ICNIRP 2010 0.08 2 (10-g) 4 (10-g) 1.35 x 10-4 f -- FCC 0.08 1.6 (1-g) 4 (10-g) -- -- Note: f is in Hz
Analytic Expressions for Exposure Quantities For a homogeneous tissue medium in close proximity to a current carrying loop, induced electro-motive force (EMF) inside the human body is given by: EMF = dφ dt Where, φ is magnetic flux coupling into the body from a N-turns loop which is carrying coil current I ; dl is the path of induced currents inside the homogeneous tissue medium; M is the mutual inductance between induced currents loop and 1-turn of the source loop (alternatively, NM can be treated as summation of mutual inductance from each individual turn of the source coil). Here, assuming induced E- field is uniform along the path dl. Thus, induced J, induced E and SAR at any location in the tissue are given by: E = MωNI dl ; = M d(ni) dt ; EMF = E dl = E dl J = σ E = σmωni ; SAR = dl σ E 2 where, ω = 2πf (f is frequency), σ is the conductivity of the tissue, ρ is density of the homogeneous tissue medium, and NI is Ampere-Turns (AT). ρ = σ ρ MωNI dl 2
System Metric for RF Exposure SAR compliance depends on the Ampere-Turns (AT) as well as f. Induced E or J as a percentage of exposure limits is proportional to AT, and is independent of f Induced E and J (dotted lines) are always parallel to the limit but not SAR. Depending on AT, the dotted lines move only vertically but the difference between induced E (or J) and the limit remains constant over f as illustrated below. Assessment AT is a common factor that controls the levels for all exposure quantities in terms of their respective limits. Therefore, Ampere-Turns is the fundamental metric for all exposure analysis.
Frequency Characteristic for Let E norm, J norm and SAR norm represent percentage of exposure limits for induced E, induced J, and SAR, respectively. Compliance (1 of 4) E norm is independent of f, J norm is weakly dependent on f due to σ s frequency dependency and SAR norm is proportional to f 2. Frequency at which human exposure reaches limits for both induced E (or J) and SAR is defined as frequency crossover point as depicted in the plot. When the operating frequency is below the cross-over point, induced E (or J) is more stringent; above the cross-over point, SAR is more restrictive. E norm = J norm = SAR norm = E = 2πfMNI 1 E limit dl 1.35 10 4 f J = σ2πfmni 1 J limit dl f 500 10 3 SAR SAR limit = σ ρ 2πfMNI dl 2 1 1.6
Frequency Characteristic for Compliance (2 of 4) To comply with SAR and induced E Lower bound of the frequency cross-over point for SAR and induced E in homogeneous muscle phantom reaches minimum when M E = M SAR E norm = 1 M E 2πfNI max dl 1 1.35 10 4 f = 1 dl NI max = 1.35 10 4 M E 2π SAR norm = 1 σ ρ M SAR 2πfNI max dl 2 1 1.6 = 1 f min = 1.6 ρ σ 1 1.35 10 4 450 khz where, M E and M SAR represents the equivalent mutual inductance between 1-turn of the source loop and the induced current loop that is formed by magnetic flux coupling into the body over specified volumes for averaging E and SAR, respectively. σ (= 0.44 S/m @ 500 khz) is the conductivity and ρ (=1090 kg/m 3 ) is the density of muscle tissue.
Frequency Characteristic for Compliance (3 of 4) Upper bound of the cross-over point occurs when narrow muscle tissue region (2x2x2 mm 3 ) is surrounded by low conductivity (here, assuming zero conductivity for a theoretical extreme case) tissue of 1g volume (Ω =10x10x10 mm 3 ). Then, SAR = 1 Ω Ω σ ρ E 2 dω SAR max = SAR limit = 1 Ω Ω σ ρ E max 2 dω = 1.6 E max = E limit = 1.35 10 4 f max f max 4. 5 MHz where, σ (= 0.59 S/m @ 5 MHz) is the conductivity and ρ (=1090 kg/m 3 ) is the density of muscle tissue. The frequency cross-over point of SAR and induced E varies between 450 khz and 4.5 MHz depending on anatomical structure. Therefore, there is no optimal operating frequency.
Frequency Characteristic for Compliance (4 of 4) To comply with SAR and induced J (head & trunk regions) The frequency cross-over point of SAR and induced J in homogeneous muscle phantom is greater than 10 MHz. In heterogeneous structures, the cross-over point is also most likely greater than 10 MHz. Therefore, lower bound of frequency cross-over point for SAR and induced J is greater than 10 MHz when the induced J limit applies to all tissues in head and trunk. J is more restrictive between 100 khz and 10 MHz, where both SAR and J limits apply.
Importance of Human Body Model Detail Not only do heterogeneous structure impact the induced fields but also the posture of human body could heavily influence the fields induced inside the body. For example, loops formed by hands and trunk of the body will result in completely different induced fields even in homogeneous muscle model. Hand touching the thigh (forming a body loop) vs. hand separated from the body (using air blocks). Exposure results from the hand touching case went up by a factor of 5 for induced J, 2.5 for induced E and 3 for SAR comparing to hand separated case. 2-D distribution of induced J shown in orthogonal planes for: (a) hands forming body loop, and (b) hands separated from body (a) (b)
Relation Between Exposure and Power Transfer Given operating frequency, Ampere-Turns determines RF exposure levels, and the power transferred depends on system implementation. For example, below shows the equivalent circuit representation of LC WPT system, wherein, for the same operating ATs in Tx coil, power delivered can be altered based on load parameters. R Src V src R 1 L 1 M V C 1 C 2 in L 2 R 2 R L At resonant R Src Tx Rx Tx V src V in R 1 Rin Source Z in = R in + jx in (X in = 0 at resonance) Source R in = R1 + (Mω)2 R 2 + R L Power transferred relates to the coil current and load (R in ), where as exposure is determined by Ampere-Turns. The challenge in meeting both requirements is design or implementation specific IEEE Wireless Power Transfer Conference, May 15-16, 2013 Peragie, Italy
Conclusions RF Exposure Qualcomm Contribution to CEA R6.3 WG4 16-May-2012 Meeting (Dallas, TX)
Executive Summary Assessment of RF emissions and RF exposure for LC WPT is necessary in order to demonstrate compliance with regulatory requirements. The ISM band frequencies (say, 6.78 MHz) is an attractive option for commercial viability, as there are no restrictions on RF emission at fundamental. There is no optimal operating frequency for RF exposure compliance as it varies with the inhomogeneity of anatomical tissue structure. Power delivered depends on the implementation of a LCWPT system and is not directly related to RF exposure. Usage scenario and detail of exposed anatomical structure have a significant influence on the exposure evaluation. A comprehensive RF exposure assessment methodology is required for wireless power technology. Selection of operating frequency for wireless power technology needs to be determined from all aspects IEEE Wireless Power Transfer Conference, May 15-16, 2013 Peragie, Italy
Thank You Discussions & Summary of demo system Qualcomm Contribution to CEA R6.3 WG4 16-May-2012 Meeting (Dallas, TX)