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`US007211986Bl
`
`c12) United States Patent
`Flowerdew et al.
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 7,211,986 Bl
`May 1, 2007
`
`(54)
`
`INDUCTIVE CHARGING SYSTEM
`
`(75)
`
`Inventors: Peter M. Flowerdew, Brentry (GB);
`David Huddart, Westbury-on-Trym
`(GB)
`
`(73) Assignee: Plantronics, Inc., Santa Cruz, CA (US)
`
`( *) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 106 days.
`
`(21) Appl. No.: 10/882,961
`
`(22) Filed:
`
`Jul. 1, 2004
`
`(51)
`
`Int. Cl.
`H02J 7100
`(2006.01)
`(52) U.S. Cl. ...................................................... 320/108
`(58) Field of Classification Search ................ 320/107,
`320/108, 110, 112, 113, 114, 115, 116
`See application file for complete search history.
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`3,840,795 A
`3,938,018 A *
`4,873,677 A *
`5,110,793 A *
`5,396,538 A *
`5,479,486 A *
`
`10/1974 Roszyk et al.
`2/1976 Dahl .......................... 320/140
`10/ 1989 Sakamoto et al.
`.......... 368/204
`5/1992 De ............................. 320/108
`3/1995 Hong ......................... 455/573
`12/1995 Saji ............................ 455/573
`
`5,522,712 A *
`5,563,776 A *
`5,600,225 A
`5,734,254 A *
`6,134,420 A *
`6,549,379 Bl *
`6,774,603 B2 *
`6,798,173 B2 *
`6,803,744 Bl *
`2004/0145342 Al *
`
`6/1996
`10/1996
`2/1997
`3/1998
`10/2000
`4/2003
`8/2004
`9/2004
`10/2004
`7/2004
`
`Winn ......................... 417/436
`Eck ............................. 363/26
`Goto
`Stephens .................... 320/106
`Flowerdew et al.
`....... 455/41.1
`Kazmierczak et al .... 360/264.8
`Liao ........................... 320/107
`Hsu ........................... 320/134
`Sabo .......................... 320/108
`Lyon .......................... 320/108
`
`* cited by examiner
`
`Primary Examiner-Karl Eastham
`Assistant Examiner-Samuel Berhanu
`(74) Attorney, Agent, or Firm-Thomas Chuang
`
`(57)
`
`ABSTRACT
`
`An apparatus for inductive charging a battery. The apparatus
`includes a housing with a lower surface and a charging
`surface. A rechargeable device with a rechargeable battery
`may be placed on the charging surface. The apparatus further
`includes a controller for driving an oscillator, wherein the
`controller receives power from a power source. A first
`charger coil and second charger coil are disposed within the
`housing and are coupled to the oscillator. The first charger
`coil and second charger coil create a substantially horizontal
`magnetic field in the volume of space above the charging
`surface.
`
`52 Claims, 12 Drawing Sheets
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`May 1, 2007
`May 1, 2007
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`US 7,211,986 Bl
`
`1
`INDUCTIVE CHARGING SYSTEM
`
`TECHNICAL FIELD
`
`BACKGROUND
`
`10
`
`The present invention relates to the general field of 5
`charging interfaces. More specifically the invention relates
`to inductive battery chargers.
`
`2
`station contacts press into the depressions in the recharge(cid:173)
`able device contacts. However, this solution compromises
`the industrial design of the rechargeable device, and in
`addition the detent force is less than robust.
`As electronic items become smaller and the regulatory
`requirements become more stringent, the charging port
`becomes more noticeable as a relatively large unattractive
`feature of the housing, as an ESD weakness, as a relatively
`unreliable element in the system.
`In the prior art, contactless battery chargers have also
`Wireless headsets and other portable communications
`been utilized. The use of inductive coupling used for con(cid:173)
`devices are often battery powered such that a user can use
`tactless power transfer between electrical items is described
`the wireless headset or other such device without being
`in the prior art. The magnetic field generated by one coil is
`directly connected to larger power source such as an ale
`made to couple closely with that of a second coil. Changes
`outlet or automobile battery. This allows wireless headset
`15 in the field induce a voltage in the second coil hence power
`users flexibility and convenience to move about without
`transfer is possible. Inductive charging is discussed in U.S.
`being tied to a power cord. Wireless headset batteries are
`Pat. No. 3,840,795, Electric Toothbrush, U.S. Pat. No.
`generally rechargeable so that the batteries can be recharged
`3,938,018, Charger for electronic items, U.S. Pat. No. 4,873,
`and need not be discarded after use.
`677, Rechargeable watch. Basic inductive charging compo-
`Recharging of device batteries has generally achieved by 20 nents are available from companies such as Panasonic and
`a wired connection. In the prior art, devices employing
`TDK.
`rechargeable batteries typically have charging contacts so
`FIG. 1 illustrates a typical prior art arrangement to ensure
`that charging current power can be supplied to recharge the
`close coupling as disclosed in U.S. Pat. No. 5,600,225. In
`batteries without removing the batteries from the device. In
`this arrangement, mechanical coupling between the charger
`one typical setup, the portable device is inserted into a base 25
`and radiotelephone is required. The charger 1 for supplying
`charger which has spring loaded contacts that correspond to
`power for charging to the radiotelephone is installed within
`and couple with the contacts on the portable device. For
`a base case 101. A depression 102 into which the radiotele(cid:173)
`example, such a setup is used with remote handset phones
`phone may be inserted is provided on the upper surface of
`used in the home. The base charger is connected to a power
`the base case 101, and a primary coil 103 is provided in the
`source, and supplies charging current through the coupled 30
`base case 101 for producing magnetic flux which runs
`contacts to recharge the batteries located within the device.
`around the side walls of the depression 102 in a vertical
`Spring-loaded surface wiping contacts are generally used
`plane. This primary coil 103 is connected to an oscillating
`with charging bases. This is a convenience feature as users
`circuit for supplying alternating current to the coil.
`can simply drop the portable device into a cradle without
`The radiotelephone 2 is provided with a microphone 202,
`fumbling with a plug. Surface contacts can be placed on the 35
`a console keyboard 203, a display 204, a receiver 205, and
`side of a taper form headset or other portable rechargeable
`an antenna 206 mounted on a slender telephone case 201.
`device, making docking into a cradle much easier than a
`Inside the telephone case 201 is a storage battery. The
`plug.
`storage battery is connected to a secondary coil 212 by way
`However, use of surface contacts and a charging base
`of an AC-DC conversion circuit.
`station with a headset presents problems due to the smaller 40
`The base of the telephone case 201 is constructed to allow
`physical size and design of headsets. Exposed metal contacts
`insertion into the depression 102 provided in the base case
`on headsets also risk contamination by oils and moisture
`101, and in this way the radiotelephone 2 may be placed on
`from the skin of the wearer. This may cause corrosion and
`the charger 1 in an erect state. The secondary coil 212 is
`hence poor contact with the base station. Contamination also
`provided within the base portion of the case 201 of the
`may cause an electrical leakage path that may cause power 45
`radiotelephone 2.
`loss from the battery and electrolytic activity. Exposed metal
`To operate, the radiotelephone 2 is placed upon the
`contacts may also result in an allergic reaction to the user if
`charger 1 when the storage battery is to be charged. At this
`in prolonged contact with the user's skin. During the
`time, the radiotelephone 2 is held in an erect state by means
`rechargeable device docking process, the formed ends of the
`of insertion of the base portion of the telephone case 201 of
`base station charging contacts often come into contact with 50
`the radiotelephone 2 into the depression 102 provided in the
`the plastic housing of the rechargeable device and can
`base case 101 of the charger 1. An alternating current signal
`scratch the housing and pick up contamination which can
`of prescribed frequency generated in this oscillating circuit
`cause intermittent electrical contact. One potential solution
`is supplied to the primary coil 103. As a result, an alternating
`is to cut the rechargeable device housing away to fully
`magnetic field is generated by the primary coil 103 within
`expose the rechargeable device stationary contacts so that 55
`the depression 102 in the base case 101 of the charger 1. This
`the spring loaded contacts of the base station never touched
`alternating magnetic field generates an induced electromo-
`the plastic housing during docking. However, this solution
`tive force in the secondary coil 212 arranged in the base
`may compromise the rechargeable device industrial design,
`portion of the telephone case 201 of the radiotelephone 2.
`aesthetics, and possibly weaken the rechargeable device
`The prior art device described in reference FIG. 1 as well
`structural integrity.
`60 as other prior art solutions require mechanical coupling
`Furthermore, the headset or other rechargeable device
`between the charger and device to be charged. To make the
`efficiency of power transfer as high as possible it is neces(cid:173)
`may not be firmly detented with the charging base, which
`sary to contain the magnetic field so that all, or most, of the
`may also cause intermittent electrical contact. One potential
`field in the first coil is linked to the second. To achieve this
`solution to
`the weak coupling between the portable
`it is typically necessary to provide some close mechanical
`rechargeable device and charging base to dish the stationary 65
`coupling such that there is a form of"plug" and "receptacle"
`contacts in the rechargeable device so that the rechargeable
`arrangement. Contactless charging has been restricted to
`device detents when the ends of the spring loaded base
`
`14
`
`

`

`US 7,211,986 Bl
`
`4
`FIGS. 3A, 3B, and 3C are an illustration of horizontal
`coupling between a pancake coil and solenoidal coil.
`FIGS. 4A, 4B, and 4C are an illustration of a potential
`configuration of the components of a secondary unit con(cid:173)
`taining a battery to be recharged.
`FIG. 5 illustrates a perspective view of an embodiment of
`the charging device of the present invention.
`FIG. 6 is a diagranimic view of the charger and the
`secondary unit.
`FIG. 7 is a diagranimic view of a further embodiment of
`the charger.
`FIGS. SA, SB, and SC illustrate controllable permeability
`of the housing top surface of the charger.
`FIG. 9 is a diagrammic view of the charger with a
`controlled permeability housing top surface and the second(cid:173)
`ary unit.
`FIG. 10 illustrate placement of the secondary unit on the
`surface of the charger.
`FIG. 11 illustrate the use of a directional logo on the
`charger.
`FIGS. 12A and 12B illustrate a charger utilizing a rotating
`horizontal field.
`FIG. 13 is a circuit schematic illustrating variable tuning
`of a drive coil.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`5
`
`3
`'mating pairs' in that the item to be charged and the charger
`are designed as a pair to achieve a closely controlled
`mechanical aligmnent of the coils in each unit, to maximize
`efficiency. This means that generally these charging methods
`are custom designed for the appliance due to non standard-
`ization of the interface and can require dexterity to use. The
`costs of the design of the charging system and the additional
`mechanical design have to be born by the individual product.
`This has restricted the adoption of contactless charging
`systems. Removing the requirement for accurate mechanical 10
`aligmnent would allow one charger design to be used across
`a range of products, allowing the development costs to be
`born by the range of products and reducing the design time
`for the introduction of a new product
`Furthermore, prior art solutions often allow charging of 15
`only one item at a time. Generally, a user has multiple
`rechargeable devices which require charging power. As a
`result, the user must transport or use a number of chargers,
`generally one for each item. As the number of devices used
`by an individual increases, the multiplicity of chargers 20
`becomes problematic.
`Thus, improved charging interfaces between charging
`base stations and rechargeable devices are needed.
`
`SUMMARY OF THE INVENTION
`
`25
`
`The present invention provides a solution to the needs
`described above through an inventive inductive battery
`charger.
`The present invention provides an apparatus for inductive 30
`charging a battery. The apparatus includes a housing with a
`lower surface and a charging surface. A rechargeable device
`with a rechargeable battery may be placed on the charging
`surface. The apparatus further includes a controller for
`driving an oscillator, wherein the controller receives power 35
`from a power source. A first charger coil and second charger
`coil are disposed within the housing and are coupled to the
`oscillator. The first charger coil and second charger coil
`create a substantially horizontal magnetic field in the volume
`of space above the charging surface.
`The present invention further provides a system for induc(cid:173)
`tive charging which includes a charger. The charger includes
`a housing with a lower surface and a charging surface. A
`rechargeable device with a rechargeable battery may be
`placed on the charging surface. The charger further includes 45
`a controller for driving an oscillator, wherein the controller
`receives power from a power source. A first charger coil and
`second charger coil are disposed within the housing and are
`coupled to the oscillator. The first charger coil and second
`charger coil create a substantially horizontal magnetic field 50
`in the volume of space above the charging surface. The
`rechargeable device includes a receive coil for coupling to
`the horizontal magnetic field and producing an induced
`voltage. The rechargeable device further includes a rectifier
`for producing a rectified induced voltage to charge the 55
`battery in the rechargeable device.
`
`The present invention provides a solution to the needs
`described above through an inventive inductive battery
`charger.
`Other embodiments of the present invention will become
`apparent to those skilled in the art from the following
`detailed description, wherein is shown and described only
`the embodiments of the invention by way of illustration of
`the best modes contemplated for carrying out the invention.
`As will be realized, the invention is capable of modification
`in various obvious aspects, all without departing from the
`spirit and scope of the present invention. Accordingly, the
`40 drawings and detailed description are to be regarded as
`illustrative in nature and not restrictive.
`The present invention provides a contactless charging
`system utilizing induction which does not require a housing
`with a compartment or recess that must be mechanically
`matched to the item being charged. This creates the oppor(cid:173)
`tunity to charge a variety of battery powered electronic items
`from a single charger. Further, the charging system lends
`itself to the simultaneous charging of dissimilar items.
`In an embodiment of the invention, the charger takes the
`form of a shallow concave (herein also referred to as
`"dished" or a "dish") or similarly shaped upper charging
`surface, which whilst substantially flat and thin, develops a
`magnetic field which is substantially horizontal rather than
`perpendicular to its surface, which is typically the case if a
`coil were wound in the same plane as a plate. By developing
`an angled field that is substantially horizontal, it is possible
`to couple energy to a receiver coil comprising a long
`solenoid, lying horizontally on the upper surface of the
`charger.
`Referring to FIG. 2, a circuit diagram partly in block form
`of a charging system including a charger 302 (also referred
`to herein as a primary unit or base unit) and secondary unit
`304 (also referred to herein as a device to be charged or unit
`under charge) are shown. Circuit diagram elements are
`65 mounted on a printed circuit board disposed within charger
`302 and secondary unit 304. Secondary unit 304 includes a
`rechargeable battery 320 to be charged by charger 302.
`
`DESCRIPTION OF THE DRAWINGS
`
`The features and advantages of the apparatus and method 60
`of the present invention will be apparent from the following
`description in which:
`FIG. 1 is an illustration of a prior art induction charging
`system
`FIG. 2 is a circuit diagram partly in block form of a
`charging system in accordance with an embodiment of the
`invention.
`
`15
`
`

`

`US 7,211,986 Bl
`
`5
`Secondary unit 304 may be any small electronic device with
`a battery to be recharged. For example, secondary unit 304
`may include wireless headsets, mobile telephones, personal
`digital assistants (PDAs), cameras, or other such devices.
`Charger 302 is designed to draw power from a power
`source 306 such as a standard electrical wall outlet. In a
`further embodiment power source 306 may be an auxiliary
`power source from another piece of electronic equipment,
`such as through a USB port on a personal computer. Charger
`302 may be linked to the personal computer via the USB
`port to provide data derived from communication with one
`or more secondary units 304 to the computer for display.
`Where secondary unit 304 is a small item such as a wireless
`headset or cordless mouse, an auxiliary power source can
`provide sufficient charging power.
`Referring to FIG. 2, there is shown power source 306
`connected to a controller 308 for driving an oscillator 310.
`Controller 308 may include a rectifier. Oscillator 310 pro(cid:173)
`vides a high frequency A.C. signal to drive a charger coil.
`The frequency of the A.C. signal may vary. The lower limit
`for a practical operating frequency is determined by the
`higher field strength and/or larger coils required in the
`primary and secondary units. A higher frequency desirably
`facilitates the use of a smaller coil in the unit being charged.
`The upper limit on a practical operating frequency is deter(cid:173)
`mined by either the energy dissipated in the metallic content
`of the unit being charged, primarily the copper layers of a
`PCB, or by reaching the self-resonant frequency of any of
`the coils. These limits are therefore defined in practice by the
`volumes available for the coils, the power that must be 30
`transferred, the allowable internal temperature of the unit
`being charged and the efficiency required in the system. The
`frequency representing the best compromise between these
`different criteria is approximately in the range from 8 kHz to
`300 kHz. In an embodiment of the invention, the preferred 35
`operating range is between 10 kHz and 40 kHz. The high
`frequency signal may be raised or lower depending upon the
`specific application. A charger coil ( also referred to herein as
`a drive coil) takes the form in the particular embodiment
`illustrated of charging coil 312 and charging coil 314 40
`connected to oscillator 310. As described in further detail
`below, charging coil 312 and charging coil 314 are disposed
`at an angle to each other to direct the path of the generated
`electromagnetic field in a desired manner to enable horizon(cid:173)
`tal coupling.
`Shown in proximity to charger 302 is secondary unit 304.
`Secondary unit 304 includes a secondary unit coil 316 (also
`referred to herein as a receive coil), which may include a
`permeable material core. During charging, secondary unit
`304 is placed near charger 302 so that the magnetic flux from 50
`the magnetic field created by charging coil 312 and charging
`coil 314 passes through the secondary unit coil 316. The
`positioning of secondary unit coil 316 is such as to provide
`for maximum flux coupling of the electromagnetic field
`provided from the angular arrangement of the charging coil 55
`312 and charging coil 314. Consequently, the magnetic flux
`induces a voltage across coil 316 resulting in an induced
`current to charge battery 320. A meter may be connected
`across the secondary unit coil 316 to provide a visual
`indication of the degree of coupling.
`Within secondary unit 304, the secondary unit coil 316
`connects to a rectifier which serves as an A.C. to D.C.
`converter. Although illustrated as a rectifying diode 318, the
`rectifier may be implemented by other means. For example,
`a custom ASIC providing synchronous rectification to mini- 65
`mize voltage drops may be used. Rectifying diode 318
`provides a D.C. charging signal to battery 320, which is
`
`6
`connected in series between secondary unit coil 316 and
`rectifying diode 318. Battery 320 serves as a power source
`for secondary unit 304. Charging is accomplished with a
`constant current. A regulator circuit may be employed to
`5 charge battery 320 to a certain capacity and then convert the
`charging current to a trickle type charge. The regulator
`circuit is responsive to the temperature and voltage of
`battery 320 to limit the charging current. The description of
`charger 302 and secondary unit 304 has been described in
`10 reference to the simplified circuit diagram shown in FIG. 2
`for clarity. Other circuit elements and arrangements may be
`utilized by charger 302 in order to provide alternating
`current flow to charging coil 312 and charging coil 314.
`One feature of the invention is a charger 302 which
`15 generates an essentially horizontal magnetic field and the
`use of a long solenoidal coil 326 to receive this field. FIGS.
`3A, 3B, and 3C are an illustration of horizontal coupling
`between a pancake coil 322 and solenoidal coil 326. The
`EMF induced in a coil depends strongly on the angle that it
`20 makes to the magnetic field. Referring to FIG. 3A, the
`maximum possible induction ( coupling) occurs when the
`field passes through the coil. This occurs when the direction
`of the field is orthogonal to the plane of the coil. Illustrated
`in FIG. 3Ais maximum induction for a pancake coil 322 and
`25 a solenoidal coil 322 from a horizontal field 324. This
`arrangement forms the basic geometry of a charger 302
`utilizing a horizontal field.
`Referring to FIG. 3B, if the pancake coil 322 or solenoidal
`coil 322 is rotated by 90° around a vertical axis, there is no
`flux through the coil, and no induced EMF. With a pancake
`coil 322 the decrease from full coupling to zero coupling
`follows a sinusoidal curve. The change in coupling for a
`cored solenoidal coil 326 is less well defined but tends to be
`more linear. A 30° rotation from maximum coupling results
`in approximately 14% reduction in induced EMF for a short
`coil and 30% reduction in the output of a winding on a long
`thin core. Referring to FIG. 3C, the induced voltage as a
`function of angle to the field for a pancake coil 328 and long
`solenoid coil 330 is shown.
`FIGS. 4A, 4B, and 4C are an illustration of a potential
`configuration of the components of a secondary unit con(cid:173)
`taining a battery to be recharged. The configuration is
`advantageously arranged to allow for easy placement of the
`secondary unit on charger 302 while still providing for the
`45 desired horizontal field coupling.
`Referring to FIG. 4A, hand held secondary units having a
`housing 340 with length 334, width 332 and thickness 336
`would normally be placed on a flat surface, such as a desk,
`with the dimension indicated as thickness 336 in the vertical
`plane. For ergonomic reasons it is usual to arrange a battery
`320 in such a secondary unit such that the battery major axis
`is in the horizontal plane and a PCB 338 carrying the
`electronic circuits would also generally be in the horizontal
`plane. The inventive system advantageously utilizes a coil
`wound on a long, thin permeable core ( a solenoidal winding)
`as the inductive element to receive energy from a horizontal
`field in the secondary unit being charged. Use of a solenoidal
`winding is possible because the charger generates a substan(cid:173)
`tially horizontal field. With some shapes of housing there
`60 may be advantage to rotating the coil in the horizontal plane
`and/or translating it vertically.
`FIG. 4B illustrates the geometry associated with charging
`with a singular direction horizontal field 346 whilst FIG. 4C
`illustrates the implications of using a singular direction
`vertical field 348. Coil windings are placed under the battery
`in an embodiment of the invention. Referring to FIG. 4B,
`when charging is conducted with a horizontal field 346, a
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`US 7,211,986 Bl
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`7
`solenoidal winding on a high permeability core 342 may be
`utilized. The use of a high permeability core advantageously
`allows a high induced voltage in the solenoidal coil. The
`effective permeability of a magnetic core is a direct function
`of the ratio of its length to its diameter, so a long thin core
`couples more effectively to a field than a short flat one. Also
`advantageously, battery 320 and PCB 338 are in a separate
`magnetic flux path, thereby minimizing the effect on the coil
`Q. However, in a singular direction horizontal field 346, a
`rotation of solenoidal winding on permeable core 342 in the
`horizontal plane will produce a coupling null.
`Referring to FIG. 4C, when charging is conducted with a
`singular direction vertical field 348, a low permeability core
`is utilized resulting disadvantageously in a low induced
`voltage. Furthermore, battery 320 and PCB 338 are in the 15
`same magnetic flux path, producing the potential for high
`loss. Advantageously, when a vertical field 348 is used, there
`is no coupling null with rotation of the coil in the horizontal
`plane.
`Neither the singular direction vertical field 348 nor the 20
`singular direction horizontal field 346 delivers the ideal set
`of characteristics. However, since efficient coupling is the
`most important requirement in a charger, a horizontal field
`is preferred. Furthermore, as described below, charger 302
`generates a horizontal field and advantageously is designed 25
`with a rotating horizontal field so that a coupling null does
`not result with horizontal rotation of the secondary unit. The
`charging system of the present invention advantageously
`provides for a high permeability core so high induced
`voltage in charging coil, no null with rotation in the hori- 30
`zontal plane, and a battery and PCB in a separate flux path
`so there is a low effect on coil Q.
`FIG. 5 is a perspective view of charger 302 showing a
`housing 356 with a lower surface 359 and a charging surface
`357 on which secondary units are placed for charging. 35
`Although illustrated in a block housing in FIG. 5, lower
`surface 359 and charging surface 357 may be incorporated
`into a variety of housing shapes, including a configuration
`with raised sides as illustrated in FIG. 7. Lower surface 359
`is designed to act as a base when the unit is placed on a
`horizontal surface. Charging surface 357 is designed to
`receive items which will receive power from charger 302. In
`an embodiment of the invention, charging surface 357 is a
`shallow concave surface in the vertical dimension 363.
`Charging surface 357 is concave along the length dimension
`367 and along the width dimension 365, forming a dish or
`bowl like structure with a base surface parallel to the lower
`surface 359. The depth of the dish is smaller than the
`dimensions of length dimension 359 or width dimension
`365. In further embodiments, charging surface 357 is con(cid:173)
`cave in only the length dimension 367 or width dimension
`365 or flat. Charging surface 357 may include a plurality of
`dished recesses to permit the secure and defined location of
`a plurality of items placed on the surface. The dished
`recesses may be optimized for the alignment of some subset
`of items that are placed on the surface to receive power from
`charger 302. In a further embodiment of the invention,
`charging surface 357 possesses one or more markers indi(cid:173)
`cating a preferred alignment and/or orientation for items that
`may placed on that part of the surface to receive power from
`charger 302.
`A feature of the invention is the provision of a shaped
`electromagnetic field to optimize coupling between charger
`302 and secondary unit 304. As shown in FIG. 6, in an
`embodiment of the invention, a horizontal field is generated 65
`using angled paired coils embodied in charging coil 312 and
`charging coil 314. By utilizing a concave surface in the
`
`8
`vertical dimension, discrete charging coils 312 and 314 may
`be positioned adjacent to the concave surface at varying
`inward angles with respect to a vertical axis to generate the
`desired horizontal field. Raising and angling charging coil
`5 312 and charging coil 314 places the coil center-lines closer
`to the axis of the solenoid, thereby providing for improved
`coupling.
`FIG. 6 is a diagrammic view of a charger 302 and
`secondary unit 304. FIG. 6 illustrates an exemplary physical
`10 arrangement of a charger 302 of the inductive charging
`system. Charger 302 includes a housing structure 356 defin(cid:173)
`ing a charging surface 357 advantageously shaped to receive
`a secondary unit 304 incorporating a rechargeable battery
`320. Controller 308 and oscillator 310 described in reference
`to FIG. 2 are disposed within housing structure 356 (not
`shown).
`Charger 302 may also contain additional components
`typical of battery charging devices. For example, charger
`302 may always be in a power on status, or alternatively may
`include a manually operated on/off button for turning the
`charger power on and off. Charger 302 may also include an
`indicator light such as a light emitting diode which serves as
`an indictor of the power status. Secondary unit 304 may also
`include an indicator light coupled to a secondary unit 304
`control circuit which indicates charging status.
`To operate, the secondary unit 304 is placed upon charger
`302 when the battery 320 in secondary unit 304 is in need
`of charging. The secondary unit 304 is placed on charging
`surface 357 provided on charger 302. Charger 302 is then
`connected by way of a power cord to an external power
`source 306. When charger 302 is in a power on state, power
`con