Editing Wireless power transfer (section)

=== Capacitive coupling ===
{{Main|Capacitive coupling}}

[[Capacitive coupling]] also referred to as electric coupling, makes use of electric fields for the transmission of power between two [[electrode]]s (an [[anode]] and [[cathode]]) forming a [[capacitance]] for the transfer of power.<ref>{{Cite web |url=https://www.wipo-wirelesspower.com/technology/resonant-capacitive-coupling |title=Resonant Capacitive Coupling |last=Webmaster |website=wipo-wirelesspower.com |access-date=2018-11-30}}</ref> In capacitive coupling ([[electrostatic induction]]), the conjugate of [[inductive coupling]], energy is transmitted by electric fields<ref name="ECN2011"/><ref name="Gopinath" /><ref name="Trancutaneous Capacitive Wireless Power Transfer"/><ref name="Capacitive Wireless Power Transfer to biomedical implants"/> between electrodes<ref name="Capacitive Elements for Wireless Power Transfer to biomedical implants"/> such as metal plates. The transmitter and receiver electrodes form a [[capacitor]], with the intervening space as the [[dielectric]].<ref name="Capacitive Elements for Wireless Power Transfer to biomedical implants"/><ref name="Gopinath" /><ref name="Sazonov" /><ref name="Valtchev" /><ref name="Puers">{{cite book |last1=Puers |first1=R. |title=Omnidirectional Inductive Powering for Biomedical Implants |publisher=Springer Science & Business Media |date=2008 |pages=4–5 |url=https://books.google.com/books?id=SKW6BrWWnNgC&q=%22wireless+power%22+capacitive&pg=PA4 |isbn=978-1402090752}}</ref><ref name="Huschens">{{cite journal |last1=Huschens |first1=Markus |title=Various techniques for wireless charging |journal=EETimes-Asia |year=2012 |url=http://m.eetasia.com/STATIC/PDF/201206/EEOL_2012JUN01_RFD_POW_TA_01.pdf?SOURCES=DOWNLOAD |access-date=16 January 2015}}</ref> An alternating voltage generated by the transmitter is applied to the transmitting plate, and the oscillating [[electric field]] induces an alternating [[electric potential|potential]] on the receiver plate by electrostatic induction,<ref name="Gopinath" /><ref name="Huschens" /> which causes an alternating current to flow in the load circuit.  The amount of power transferred increases with the [[frequency]]<ref name="Huschens" /> the square of the voltage, and the [[capacitance]] between the plates, which is proportional to the area of the smaller plate and (for short distances) inversely proportional to the separation.<ref name="Gopinath" />

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| image1   = Wireless power system - capacitive bipolar.svg
| caption1 = Bipolar coupling
| image2   = Wireless power - capacitive charge sink.svg
| caption2 = Monopolar coupling
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Capacitive coupling has only been used practically in a few low power applications, because the very high voltages on the electrodes required to transmit significant power can be hazardous,<ref name="Sazonov" /><ref name="Valtchev" /> and can cause unpleasant side effects such as noxious [[ozone]] production. In addition, in contrast to magnetic fields,<ref name="Karalis" /> electric fields interact strongly with most materials, including the human body, due to [[dielectric polarization]].<ref name="Puers" /> Intervening materials between or near the electrodes can absorb the energy, in the case of humans possibly causing excessive electromagnetic field exposure.<ref name="Sazonov" /> However capacitive coupling has a few advantages over inductive coupling.  The field is largely confined between the capacitor plates, reducing interference, which in inductive coupling requires heavy ferrite "flux confinement" cores.<ref name="Gopinath" /><ref name="Puers" /> Also, alignment requirements between the transmitter and receiver are less critical.<ref name="Gopinath" /><ref name="Sazonov" /><ref name="Huschens" /> Capacitive coupling has recently been applied to charging battery powered portable devices<ref name="ECN2011"/> as well as charging or continuous wireless power transfer in biomedical implants,<ref name="Trancutaneous Capacitive Wireless Power Transfer"/><ref name="Capacitive Elements for Wireless Power Transfer to biomedical implants"/><ref name="Capacitive Wireless Power Transfer to biomedical implants"/> and is being considered as a means of transferring power between substrate layers in integrated circuits.<ref name="Meindl">{{cite book |last1=Meindl |first1=James D. |title=Integrated Interconnect Technologies for 3D Nanoelectronic Systems |publisher=Artech House |date=2008 |pages=475–477 |url=https://books.google.com/books?id=OtY-66XCMuYC&q=%22wireless+power%22+%22capacitive+coupling%22&pg=PA475 |isbn=978-1596932470}}</ref>

Two types of circuit have been used:

* Transverse (bipolar) design:<ref name="Trancutaneous Capacitive Wireless Power Transfer"/><ref name="Capacitive Wireless Power Transfer to biomedical implants"/><ref name="Harakawa">{{cite web |last=Harakawa |first=Kenichi |title=Wireless power transmission at rotating and sliding elements by using the capacitive coupling technology |website=2014 ANSYS Electronic Simulation Expo October 9–10, 2014, Tokyo |publisher=ExH Corporation |date=2014 |url=http://www.ansys.com/staticassets/ANSYS/staticassets/resourcelibrary/presentation/aese2014-wireless-power-transmission.pdf |access-date=5 May 2015 |url-status=dead |archive-url=https://web.archive.org/web/20150925111819/http://www.ansys.com/staticassets/ANSYS/staticassets/resourcelibrary/presentation/aese2014-wireless-power-transmission.pdf |archive-date=25 September 2015}}</ref><ref name=":0">{{cite web |url=http://www.pro-physik.de/details/articlePdf/1102293/issue.html |title=Coupling games in metamaterials |year=2010 |access-date=18 January 2016 |last=Liu |first=Na |archive-date=11 October 2016 |archive-url=https://web.archive.org/web/20161011200343/http://www.pro-physik.de/details/articlePdf/1102293/issue.html |url-status=dead}}</ref> In this type of circuit, there are two transmitter plates and two receiver plates.  Each transmitter plate is coupled to a receiver plate. The transmitter [[electronic oscillator|oscillator]] drives the transmitter plates in opposite phase (180° phase difference) by a high alternating voltage, and the load is connected between the two receiver plates.  The alternating electric fields induce opposite phase alternating potentials in the receiver plates, and this "push-pull" action causes current to flow back and forth between the plates through the load.  A disadvantage of this configuration for wireless charging is that the two plates in the receiving device must be aligned face to face with the charger plates for the device to work.<ref name="X. Lu" /> 
* Longitudinal (unipolar) design:<ref name="Gopinath" /><ref name="Huschens" /><ref name=":0" /> In this type of circuit, the transmitter and receiver have only one active electrode, and either the [[ground (electricity)|ground]] or a large passive electrode serves as the return path for the current.  The transmitter oscillator is connected between an active and a passive electrode. The load is also connected between an active and a passive electrode. The electric field produced by the transmitter induces alternating charge displacement in the load dipole through [[electrostatic induction]].<ref>{{cite web |url=https://www.google.ch/patents/US20090206675 |title=Device for transporting energy by partial influence through a dielectric medium |date=2006 |access-date=18 January 2016 |website=Google.ch/Patents |publisher=TMMS Co. |last1=Camurati |first1=Patrick |last2=Bondar |first2=Henri}}</ref>

Resonance can also be used with capacitive coupling to extend the range.  At the turn of the 20th century, [[Nikola Tesla]] did the first experiments with both resonant inductive and capacitive coupling.