Wireless Power Transfer
Some History 1899 - Tesla 1963 - Schuder 1964 - Brown from Garnica et al. (2013) from Schuder et al. (1963) from Brown (1964)
Commercialization 1990s onward: mobile device charging Now: commercialization & standardization greencarreports.com - October 25th techradar.com - October 18th techradar.com - October 16th consumerreports.org - October 7th
Nearfield Coupling: Inductive and Resonant Coils Goals maximum power transfer high energy efficiency range & freedom of movement
Wireless Charging Example 15 mm 12mm chip with complex dispersive impedance frequency: 13.56 MHz (λ = 22.5m) Simulation issue: electrically small with small details frequency domain technique receiver 50 Ω port d (= 10 mm) transmitter 0.2 mm
Equivalent Circuit Extraction Why an equivalent circuit model? Fundamental description of geometry Can intuitively understand energy transfer mechanism (inductive vs. capacitive) Quick what-if analyses by circuit simulation Final detailed analysis & design in full-wave 3D simulation tool!
Equivalent Circuit Extraction 1. Construct equivalent circuit model topology each coil: self inductance L self capacitance C self resistance R between coils: mutual inductance M mutual capacitance C 21
Equivalent Circuit Extraction 2. 3D sim. results estimate main component values Z-parameter phase phases ~ 90º inductive L1 = L2 400 nh M 26 nh k 0.065
Equivalent Circuit Extraction 3. optimise equivalent circuit (goal: match 3D results) original optimised original optimised L 400 nh 400.5 nh M 26 nh 26.1 nh C s 0 0.27 pf C 12 0 0.13 pf R 0 0.55 Ω
Matching No matching = no coupling Matching = coupling
Matching No matching = no coupling Matching = coupling
Matching Options CST DESIGN STUDIO full circuit simulation tool (including harmonic balance) tight link with 3D EM field results very flexible optimisation and project construction general multiport matching broadband and multiband matching optimisation using real components bidirectional link to CST STUDIO SUITE
Matching in Optenni CST DS various matching circuit options
Matching in CST DESIGN STUDIO real components (e.g. from Optenni) TOUCHSTONE import or circuit elements (also non-linear) 3D CST MWS model
Matching in CST DESIGN STUDIO Impedances! Port 1: 50 Ω Port 2: complex and frequency dependent real components (e.g. from Optenni) TOUCHSTONE import or circuit elements (also non-linear)
Matching in CST DESIGN STUDIO input output efficiency = S 21 2 S-parameters Z port1 = 50 Ω Z port2 variable
Matching in CST DESIGN STUDIO 1 V input output power = V I AC Task Z port1 = 0 Ω Z port2 variable P out /P in 0.87 P out = 17 mw
Matching in CST DESIGN STUDIO 1 V input output power = V I AC Task Z port1 = 50 Ω Z port2 variable P out /P in 0.85 P out = 4 mw
Matching in CST DESIGN STUDIO 1V input
Coil Separation: 2 to 20 mm matching circuit designed for 10 mm separation
Coil Separation matching circuit designed for 2 mm separation
Coil Separation System Assembly Modelling: optimise matching circuit for each separation distance matching circuit adjusted for each separation (e.g. Ricketts et al. (2013) or Beh et al. (2013)) sweep distance optimise matching goal: maximise S 21
Horizontal Offset & Rotation Lateral offset causes large drop off in coupling x = 0 d = 10 mm x = ±5 mm x = ±10 mm x = 0 axially aligned
Horizontal Offset & Rotation Rotation between coils has very small effect d = 10 mm θ 0º < θ < 180º coils axially aligned
Horizontal Offset & Rotation Lateral offset causes large drop off in coupling x = 0 d = 2 mm x = ±6 mm x = ±12 mm x = 0 axially aligned
Spiral Coil Design H-field magnitude at 2 mm above coil plane 43 mm based on Casanova et al. (2009)
Spiral Coil Coupling x = 0 x = ±12 mm x = 0 axially aligned
Resonant Coupling
Resonant Coil Transfer Example systems of coupled resonators Integral Equation Solver good for metal structures d source d coils drive loop load loop Source: Sample et al. (2011) TX coil RX coil
Tuning the Coil 1. Add capacitor to resonate drive loop 2. Modify coil geometry for self-resonance 60 cm drive loop coil
Coupling to Second Coil strong coupling but at different frequencies! d = 30 cm
Coupling to Second Coil even and odd mode coupling frequency splitting even mode: 7.1 MHz odd mode: 7.9 MHz
Frequency Splitting good coupling possible but tuning required Source: Sample et al. (2011)
Critical Coupling Distance strong coupling (S 21 = -2.5 db) even at 110 cm
Rotation of Coils S 21 coupling insensitive to ±40º rotation of receiving coil coils 110 cm apart axially aligned rotation angle
Multiple Coils S 21 of -5.8 db at 4.4 m coil separation! 4.4 m
Multiple Coils Around a Corner S 21 of -5.9 db for curved path
Shielding High Power Two coils 10 cm apart 30 turns 1000 A Coils modelled in simplified fashion in Magnetostatic solver in CST EM STUDIO Aluminium housing and ferrite shielding (μ r = 1000)
Shielding High Power No shielding Ferrite shield lower coupling (184 μh) higher field leakage higher coupling (385 μh) much lower field leakage
Conclusion Wireless power transfer is a field of active research and an increasing number of commercial applications Simulation is an important tool in designing wireless power transfer systems, both nearfield and farfield CST STUDIO SUITE provides tools for addressing all aspects of design, from circuit to 3D EM to system level