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`as) United States
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`a2) Patent Application Publication 10) Pub. No.: US 2009/0121675 Al
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` HOet al. (43) Pub. Date: May 14, 2009
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`US 20090121675A1
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`(54) ELECTRONIC CONTROL METHOD FORA
`PLANAR INDUCTIVE BATTERY CHARGING
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`APPARATUS
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`($1)
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`Publication Classification
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`(2006.01)
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`Int. Cl.
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`HO2J5 7/02
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`(52) US. CD. eeececeeectesesenscneecessescneneeansenees 320/108
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`ABSTRACT
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`This invention provides an electronic control method for a
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`planar inductive battery charging apparatus on which one or
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`more electronic loads such as mobile phones, MP3players etc
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`can be placed and charged simultaneously. The powercontrol
`circuit ofthe charging pad consists oftwo powerconversion
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`stages. Depending on the nature ofthe input powersupply,the
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`first power stage is an AC-DC power converter with variable
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`output voltage control and a secondstage is a DC-AC power
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`inverter with constant current control. The combination ofthe
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`two stages provides powercontrol of the charging pad and
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`generates AC magnetic flux of ideally constant magnitude
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`over the charging areas within a group of primary windings
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`that are excited.
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`(57)
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`(75)
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`Inventors:
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`Wing Choi HO, Hong Kong (HK);
`Chi Kwan LEE,Hong Kong (HK):
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`Ron Shu Yuen HUI, Hong Kong
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`(HK); Shu Hung CHUNG,Hong
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`Kong (HK)
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`Correspondence Address:
`HESLIN ROTHENBERG FARLEY & MESITI
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`SCOLUMBIA CIRCLE
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`ALBANY, NY 12203 (US)
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`(73) Assignee:
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`(21) Appl. No.:
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`CITY UNIVERSITY OF HONG
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`KONG,Hong Kong (HK)
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`11/938,042
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`(22)
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`Filed:
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`Nov. 9, 2007
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`US 2009/0121675 Al
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`May14, 2009
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`
`
`ELECTRONIC CONTROL METHOD FORA
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`PLANAR INDUCTIVE BATTERY CHARGING
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`APPARATUS
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`FIELD OF THE INVENTION
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`[0001] This invention relates to methods and apparatus for
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`the control of a planar battery charging apparatus.
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`BACKGROUND OF THE INVENTION
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`[0002] The increasing popularity of portable consumer
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`electronic products such as mobile phones, MP3 players and
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`PDAs has prompted new concerns on the huge variety and
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`numberof battery chargers that are required and which are
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`costly, inconvenient and eventually lead to electronic waste
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`problems. Inductive or wireless charging apparatus that can
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`charge more than one electronic product have been proposed.
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`Two different approaches to the generation of AC magnetic
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`flux have been proposed, namely “horizontal flux” and “ver-
`tical flux” methods.
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`[0003]
`Inductive electronic chargers with a direct connec-
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`tion have been developed for use with sometypes of portable
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`electronic equipment such as electric toothbrushes. Inductive
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`chargers have also been proposed in a number of documents
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`suchas U.S. Pat. No. 6,356,049, U.S. Pat. No. 6,301,128, U.S.
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`Pat. No. 6,118,249. These inductive chargers, however, use
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`traditional transformer designs with windings woundaround
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`ferrite magnetic cores and the main magnetic flux between
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`the primary winding and secondary winding has to go
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`through the magnetic core materials. Other contactless charg-
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`ers proposed(e.g., Chang-Gyun Kim; Dong-HyunSeo; Jung-
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`Sik You; Jong-Hu Park; Cho, B. H., “Design of a contactless
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`battery charger for cellular phone,’ IEEE Transactions on
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`Industrial Electronics, Volume: 48, Issue: 6, December 2001
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`Page(s): 1238-1247) also use magnetic cores as the main
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`structure for the coupled transformer windings. However,
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`these battery chargers do not use a planar structure and each
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`charger is only able to charge one item of electronic equip-
`mentat a time.
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`[0004] Recent researchin the field of planar magnetics and
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`planar transformer technology has prompted the develop-
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`mentof planar contactless battery charging systems for por-
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`table electronic equipment. Among them, two proposals are
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`particularly of interest, because they allow one or more items
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`of electronic equipment to be placed and charged simulta-
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`neously on the charging surface, regardless ofthe orientation
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`of the electronic equipment.
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`[0005]
`Thefirst type of planar battery charger modifies the
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`rotating machine conceptby flattening the “round shape”of
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`the motor
`into a “pancake shape,’
`as described in
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`GB2399225A,
`GB2398176A,
`WO2004/038888A,
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`
`
`GB2388716A, US2003-210106-Al, GB2392024A,
`and
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`GB2399230A. The magnetic flux lines flow horizontally
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`along (roughly in parallel to) the planar charging surfaces.
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`The portable electronic equipment to be charged by the charg-
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`ing device needs a secondary winding woundon preferably a
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`soft magnetic core. An AC voltage will be induced in this
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`secondary winding for charging the battery, usually via a
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`battery charging circuit.
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`[0006] A fundamental and inherent limitation of this type
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`of battery charger is that this charging device must have a
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`good electromagnetic flux guide to confine the flux along the
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`lower surface. Otherwise, if such a charging device is placed
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`on a metallic table or a conductive surface, induced current
`Page 11 of 14
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`will circulate in the metallic table or conductive surface,
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`resulting in heat generation and powerloss in the metallic
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`table or conductive surface. One imperfect way to solve this
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`problemis to place a piece of soft-magnetic material (such as
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`a layer of ferrite, iron power or amorphous soft magnetic
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`alloy) as a magnetic flux guide underthe lower surface. How-
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`ever, ifthe electromagnetic flux is large, a fairly thick layer of
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`soft-magnetic material is needed, defeating the purpose of
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`designing a “thin” charging platform and increasing the cost
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`due to the large amountof soft magnetic material required. In
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`addition, the electromagnetic shielding effect of using one
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`layer of soft magnetic material may not be sufficient for
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`electromagnetic compatibility (EMC) requirements. Some
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`flux maystill penetrate through the soft magnetic layer and
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`induce current in any conductive surface below the charging
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`platform.
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`[0007] A better solution to shield the magneticfield in the
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`lower surface is to use a combinationof a layer of soft mag-
`netic material and a conductive material as disclosed in
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`US2003-095027-A1. It is important to note that the addition
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`of a thin layer of conductive material can significantly
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`increase the shielding effectiveness as reported in US-2003-
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`095027-A1, U.S. Pat. No. 6,501,364, and Tang S.C., Hui S.Y.
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`R and Chung H., “Evaluation of the Shielding Effects on
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`Printed-Circuit-Board Transformers using Ferrite Plates and
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`Copper Sheets,” JEEE Transactions on Power Electronics,
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`Vol. 17, No. 6, November 2002, pp. 1080-1088.
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`[0008] The second approach described in WO03/105308A,
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`GB2389720A, GB2399446A, GB2389767A, GB2389767A,
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`W02007/019806is to create an AC magnetic field with the
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`flux lines flowing substantially vertically out of the planar
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`charging surfaces, i.e., in a direction substantially perpen-
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`dicular to the plane ofthe charging platform. Since the lines of
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`flux enter and leave the planar charging surface vertically, a
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`very thin secondary coil can be used to pick up the magnetic
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`flux. This results in the possibility of a slim design for the
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`secondary module that can be embeddedin the portable elec-
`tronic load.
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`SUMMARY OF THE INVENTION
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`[0009] According to the present inventionthere is provided,
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`ina first aspect, a planar battery charging system comprising
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`a primary powertransmission side formed of an array of
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`primary windings adapted to generate magnetic flux substan-
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`tially perpendicular to a charging surface, and a secondary
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`powerreceiving side comprising a secondary winding asso-
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`ciated with a battery to be charged and being adapted to
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`receive the magnetic flux when a secondary windingis placed
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`on the charging surface. A primary winding is energized by a
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`control circuit comprisinga first-stage power converter and a
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`second-stage powerinverter, the first-stage power converter
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`providing a variable DC link voltage, and the second-stage
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`powerinverter generating a fixed-frequency constant AC cur-
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`rent to the primary winding.
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`[0010]
`Thefirst-stage power converter may beeither cur-
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`rent controlled or voltage-controlled. Preferably, the current
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`fed to the primary winding is monitored to provide a feedback
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`control of the variable DC link voltage.
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`Insome embodimentsof the invention, a step-down
`[0011]
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`transformer may be provided between the output of the sec-
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`ond-stage powerinverter and the primary winding.
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`[0012] The second-stage power inverter may comprise a
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`resonant tank that sets the frequency of the AC current sup-
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`plied to the winding. In such embodiments, the second-stage
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`Page 11 of 14
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`US 2009/0121675 Al
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`May14, 2009
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`power inverter is switched at a constant frequency that is
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`equal to the frequency of the resonant tank.
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`[0013]
`Preferably, the secondary side is adapted to receive
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`poweroptimally at the excitation frequency of the primary
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`winding.
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`for
`[0014] The first-stage power converter may be,
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`example, a boost converter, or a buck converter, or a flyback
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`converter, or a Cuk converter or a Sepic converter. The sec-
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`ond-stage powerinverter may be, for example, a full-bridge
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`type, or a half-bridge type, or a Class D type or a Z-source
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`type inverter.
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`[0015]
`Preferably, when excited, each primary windingis
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`excited at the same frequency and generates the same AC
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`magnetic flux. The AC current supplied to the primary wind-
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`ing is preferably sinusoidal.
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`Some embodiments of the invention will now be
`[0016]
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`described by way of example and with reference to the
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`accompanying drawings, in which:
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`FIG. 1 is a schematic showing the control method-
`[0017]
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`ology according to one example of the invention,
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`[0018]
`FIG. 2 is a schematic of a boost converter,
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`[0019]
`FIG. 3 is a schematic of a flyback-type AC-DC
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`power converter,
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`[0020]
`FIG. 4is a schematic ofa full-bridge powerinverter
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`with a primary winding and a DC-blocking capacitor,
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`[0021]
`FIG. 5is aschematic ofa half-bridge powerinverter
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`with a primary winding and a DC-blocking capacitor,
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`[0022]
`FIG. 6 shows a typical winding array in a planar
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`battery charging platform,
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`FIG. 7 shows an AC-DC voltage-mode controlled
`[0023]
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`first-stage flyback converter feeding a constant current source
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`to a second-stage full-bridge fixed frequency inverter,
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`[0024]
`FIG. 8 shows an AC-DC current-mode controlled
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`first-stage flyback converter feeding a constant current source
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`to a second-stage full-bridge fixed frequency inverter,
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`[0025]
`FIG. 9 shows an AC-DC voltage-mode controlled
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`first-stage boost converter feeding a constant current source
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`to a second-stage full-bridge fixed frequency inverter,
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`[0026]
`FIG. 10 shows an AC-DC current-modecontrolled
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`first-stage boost converter feeding a constant current source
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`to a second-stage full-bridge fixed frequency inverter,
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`[0027]
`FIG. 11 shows an AC-ACcurrent-modecontrolled
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`first-stage flyback converter feeding a full-bridge inverter that
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`provides a step-down current source through a transformer,
`and
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`FIG. 12 shows an AC-AC voltage-mode controlled
`[0028]
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`first-stage flyback converter feeding a full-bridge inverter that
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`provides a step-downcurrent source through a transformer.
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`DETAILED DESCRIPTION OF PREFERRED
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`EMBODIMENTS
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`[0029]
`For planar battery charging systems, the use of a
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`standard charging pad enables charging inductively a wide
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`range of portable electronic products. In order provide such a
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`pad, preferably several conditions are met:
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`[0030]
`(1) The windingsofthe charging pad thatare excited
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`are able to generate sufficient energy for charging these
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`electronic products.
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`(2) Since the inductive charging methodrelies on
`[0031]
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`the near-field magnetic coupling of the charging pad(pri-
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`mary energy-transmitting circuit or simply primary circuit)
`Page 12 of 14
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`and the energy-receiving module (secondary circuit) inside
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`the electronic products, the charging pad should generate
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`an AC magnetic flux of a controllable magnitude that can
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`satisfy a wide range of portable electronic loads to be
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`charged.
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`(3)Sincea range ofelectronic loads may be charged
`[0032]
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`on the same charging pad, there should be a standard oper-
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`ating frequency anda set of basic secondary circuit design
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`features. In other words, the secondary circuits in a wide
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`range of electronic loads should be compatible with the
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`magnetic flux generated by the charging pad.
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`In the following, methods and apparatus that can
`[0033]
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`provide power control and AC flux control for the inductive
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`charging pad are described.In order to meetthecriteria men-
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`tioned previously, the following design and operating condi-
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`tionsare set so that the charging pad can be used to charge a
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`wide range of electronic loads and to enable the flux thatis
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`generated by the excited windingsto be controlled.
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`(1) One or more windings of the charging pad are
`[0034]
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`excited and may be termedas an “active group.” A common
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`magnitude ofAC magnetic flux maybeset for each coil on
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`the charging pad so that the AC magnetic field created by
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`each energized coil would be identical. This allows the
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`electronic products to be charged in the same mannerany-
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`where over the charging surface of the charging pad.
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`(2) Such magnetic flux should provide sufficient
`[0035]
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`energy to charge the electronic products.
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`(3)The AC magnetic flux generated by the charging
`[0036]
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`padis set at a specific frequency. The secondary energy-
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`receiving circuits of all electronic loads are designed to
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`pick upthe flux at the same frequency. This ensuresthat the
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`secondary circuits are compatible with the charging pad.
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`(4) Since the magnetic flux is a function of the
`[0037]
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`excitation current in the windings of an active group, the
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`powerinverter that drives the windings in the charging pad
`should be current controlled.
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`(5) In order to control the power delivered to the
`[0038]
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`inverter and hence the windingsofthe active group, the
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`front-stage power converter should have a controllable out-
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`put voltage. Since the second-stage powerinverter is con-
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`trolled to inject a constant AC current into the windings of
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`the active group, the control of the output voltage of the
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`first-stage power converter can be used to control the power
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`of the active group.
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`[0039]
`(6) In order to reduce harmonic losses in the wind-
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`ings and EMIradiation,the current in the primary winding
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`is sinusoidal. Therefore, a capacitor should be addedto the
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`primary winding to form a resonant tank. The second-stage
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`inverter should be operated at this resonant frequency to
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`ensure that the current in the windingis sinusoidal.
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`FIG. 1 shows an embodimentof an electronic con-
`[0040]
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`trol system for an inductive charging pad (primary system)
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`and comprising the two power stages that drive a primary
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`winding that forms part (or all) of an active group of the
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`charging pad. The front or first-stage power converter
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`dependsonthe nature of the input powersupply. If the power
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`supply is an AC mains,this first-stage power converter should
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`be an AC-DC powerconverter. If there is a DC powersupply,
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`then the first-stage power converter should be a DC-DC
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`powerconverter. In either case, the output voltage (i.e., DC-
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`link voltage V,, in FIG. 1) should be a controllable variable.
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`[0041]
`Ifthe input powersupply is a DC voltage source,the
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`front-stage DC-DC power converter can be, but
`is not
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`restricted to, a flyback converter, boost converter, buck con-
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`Page 12 of 14
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`

`

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`US 2009/0121675 Al
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`May14, 2009
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`verter, Cuk converter and Sepic converter. An example of
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`boost converter is given in FIG.2. Ifthe input powersupplyis
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`an AC voltage source (such as the AC mains), an AC-DC
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`powerconverter with or without powerfactor correction can
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`be used as the front-stage power converter. An AC-DC con-
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`verter typically consists of a diode rectifier and a DC-DC
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`converter. An example of a flyback type AC-DC converteris
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`shown in FIG. 3. The second-stage powerinverter can be, for
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`example, a full-bridge inverter (FIG.4), a half-bridge inverter
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`(FIG.5) or a Z-source inverter.
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`[0042] The primary winding(s) of the charging pad may
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`consist of a single coil or an array of coils 1 connected in
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`series as shown in FIG. 6 where hexagonal coils are shown as
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`an example. The array of coils can be of single-layer or
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`multi-layer structures. The array of coils may be re-config-
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`urable into groups in orderto achieve localized charging such
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`that, if necessary, only those coils through which energy
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`transfer is required are energized.
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`[0043] The control principle is now explained with refer-
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`ence to FIG. 1. The front-stage power converter controls its
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`output voltage (Vdc) by varying the duty-cycle of the con-
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`verter switch. Typically, a pulse-width modulation (PWM)
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`technique is used to control the duty-cycle of the converter
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`switch, which in turn, controls the output voltage ofthe power
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`converter. The duty-cycle function is denoted as M(d) in FIG.
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`1. Thefirst-stage power converter that provides a controllable
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`DC-link voltage (Vdc) can be controlled either in “voltage
`control” modeor in “current control” mode.
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`[0044] The second-stage powerinverter adopts a fixedfre-
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`quency control. Generally, the diagonal pairs of switches in
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`the full-bridge inverter are switched together and the two
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`pairs are switched in a complementary mannerso that an AC
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`voltage can be generated at the output of the inverter. If
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`necessary, a small dead time can be introducedin the switch-
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`ing instants of the two switches in the same inverter leg in
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`order to achieve soft switching, thus reducing the switching
`loss and EMIradiation. The inverter is switched at a constant
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`frequency, which should preferably be the sameasthe reso-
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`nant frequency of the resonant tank in the primary circuit.
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`Since the current fed from the first-stage power converter is
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`kept constant, the second-stage inverter generates a fixed-
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`frequency constant AC current into the winding(s) of an
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`active group. The fixed-frequency operation is important
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`because the secondary circuits of the electronic loads will be
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`designed to receive power transfer at this frequency. This
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`frequency should preferably be chosen so that the radiated
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`electromagnetic interference (EMI) should notviolate inter-
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`national electromagnetic compatibility (EMC) requirements.
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`[0045] Under voltage-control mode,the first-stage power
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`converter feeds DC-link voltage to the second-stage power
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`inverter. Under current-control mode, the first-stage power
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`converter feeds a DC current to the second-stage power
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`inverter. In both operating modes, the objective is to maintain
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`a constant current feeding the second-stage powerinverter.
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`That is, under voltage-control mode, the power converter will
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`vary its output voltage (Vdc) in order to keep constant the
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`current feeding the second-stage inverter. Under the current-
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`control mode, the power converter is controlled to provide a
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`current of a desired value to the second-stage inverter.
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`[0046] Thecurrent feeding the second-stage powerinverter
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`is monitored by a current sensor which can be a small resistor
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`or a hall-effect current sensor. If the power absorbed by the
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`secondary load increases (decreases), this current may be
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`reduced (increased). A feedback current is compared with a
`Page 13 of 14
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`preset reference (Vref) that represents the desired current
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`feeding the primary winding, which can be set by users, or
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`according to somestandardor to flux, poweror other require-
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`ments communicated back from the load. Theerror signal is
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`then amplified by amplifier A and compared by a comparator
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`C with a triangular carrier reference of a fixed frequency that
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`determines the switching frequency ofthe first-stage power
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`converter. The duty cycle will be dynamically adjusted to
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`control the DC-link voltage (Vdc) so as to keep the current
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`feeding the second-stage inverterto the desired current value
`within a small tolerance.
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`[0047]
`FIG. 7 shows one example of the invention where
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`the charging pad is powered by an AC mains. An AC-DC
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`flyback converter is used to feed an DC-AC inverter that
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`drives the primary winding(s) inside an active group of the
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`charging pad. An AC-DC flyback converter comprising (di-
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`ode bridge 2 and gate driver 3) operating under a voltage-
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`control modeis illustrated here. Sensing resistor Rsen is used
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`to monitor the current feeding the inverter. This sensed cur-
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`rent signal is filtered by a resistive-capacitivefilter and then
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`compared with a reference value (Vref—whichis arbitrarily
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`set at 2.5V and represents the desired current value in the
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`inverter) in an error amplifier 4, which generates an error
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`signal in its output. This currenterror signal (Ve) is then fed to
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`a comparator 5 and compared with a sawtooth reference
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`signal (Vsw)that is set at a specific frequency which deter-
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`mines the switching frequency ofthe first-stage AC-DC fly-
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`back converter. The output of this comparator provides the
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`PWMsignal for switching the power converter. The duty
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`cycle of this PWM signal controls the output voltage of the
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`flyback converter (i.e. the DC-link voltage Vdc). Vdc is con-
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`trolled in a manner that keeps the current flowing into the
`inverter to the desired value within a certain small tolerance.
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`The second-stage inverter is simply driven at constant fre-
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`quency(at the resonant frequency of the resonant tank in the
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`primary circuit) to inject an AC current ofconstant magnitude
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`into the primary circuit ofthe charging pad. The resonant tank
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`of the primary circuit ensures that current in the primary
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`winding(s) of an active group is sinusoidalin order to reduce
`harmonic losses and EMIradiation.
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`[0048]
`FIG. 8 shows an example implementation of the
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`invention, wherethe first-stage power converter is operated
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`under current-control mode. In this implementation, the cur-
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`rent sensor Rsenis used to sense the current feeding into the
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`inverter. The sensed current signalis filtered and then com-
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`pared with a desired current reference (Vref—arbitrarily set
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`at 2.5V) in an error amplifier 14. The currenterror signal (Ve)
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`is the outputofthis error amplifier. Another current sensor Rs
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`is used to monitor the current in the power switch of the
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`flyback converter. This switch current is the same as the
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`current in the primary winding of the coupled inductor in the
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`flyback converter. The sensed switch current (Vipk)is filtered
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`and then compared with Ve at comparator 15 in order to
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`generate the PWMsignalfordriving the switch ofthe flyback
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`converter. The objective is to for the first-stage converter to
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`feed a desired level of current into the second-stage inverter.
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`The second-stage inverter is simply driven at constant fre-
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`quency(at the resonant frequency of the resonant tank in the
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`primary circuit) to inject an AC current ofconstant magnitude
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`into the primary circuit ofthe charging pad. The resonant tank
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`of the primary circuit ensures that current in the primary
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`windingis sinusoidal in order to reduce harmonic losses and
`EMIradiation.
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`Page 13 of 14
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`US 2009/0121675 Al