as) United States
`a2) Patent Application Publication co) Pub. No.: US 2009/0174263 Al
`(43) Pub. Date: Jul. 9, 2009
`
`Baarmanetal.
`
`US 20090174263A1
`
`(54)
`
`INDUCTIVE POWER SUPPLY WITH DUTY
`CYCLE CONTROL
`
`Related U.S. Application Data
`
`(75)
`
`Inventors:
`
`David W. Baarman,Fennville, MI
`(US); Scott A. Mollema, Rockford,
`MI (US); Joshua K. Schwannecke,
`Portland, MI (US); Thomas Jay
`Leppien, Grand Haven, MI (US);
`Kenneth Michael Burns, North
`Canton, OH (US)
`
`Correspondence Address:
`WARNER, NORCROSS & JUDD
`IN RE: ALTICORINC.
`INTELLECTUAL PROPERTY GROUP,
`LYON STREET, N. W. STE 900
`GRAND RAPIDS, MI 49503-2489 (US)
`
`111
`
`(73) Assignee:
`
`ACCESS BUSINESS GROUP
`INTERNATIONALLLC, Ada, MI
`(US)
`
`(21) Appl. No.:
`
`12/349,840
`
`(22)
`
`Filed:
`
`Jan. 7, 2009
`
`(60) Provisional application No. 61/019,411, filed on Jan.
`7, 2008.
`
`Publication Classification
`
`(51)
`
`Int. Cl.
`HO02J 17/00
`HO02T 7/00
`
`(2006.01)
`(2006.01)
`
`(02) US. CL. senesmessrs arn cms 307/104; 320/106
`
`(57)
`
`ABSTRACT
`
`An inductive power supply that maintains resonance and
`adjusts duty cycle based on feedback from a secondary cir-
`cuit. A controller, driver circuit and switching circuit cooper-
`ate to generate an AC signalat a selected operating frequency
`and duty cycle. The AC signalis appliedto the tank circuit to
`create an inductive field for powering the secondary. The
`secondary communicates feedback aboutthe received power
`back to the primary controller. The powertransfer efficiency
`may be optimized by maintaining the operating frequency
`substantially at resonance, and the amount of powertrans-
`ferred may be controlled by adjusting the duty cycle.
`
`124
`
`100
`
`WIRELESS
`
`RECEIVER
`
`
`
`
`
`SWITCH
`DRIVER
`
`
`TANK
`
`CIRCUIT
`
`
`SWITCH
`
`CONTROLLER
`
`
`
` DRIVER
`
`CURRENT
`
`SENSOR
`
`122
`
`ANKER 1004
`ANKER 1004
`
`1
`
`

`

`Patent Application Publication
`
`Jul. 9, 2009 Sheet 1 of 10
`
`US 2009/0174263 Al
`
`124
`
`100
`
`WIRELESS
`RECEIVER
`
`
`CONTROLLER
`
`
`
`
`
`
`
`
`SWITCH
`
`TANK
`CIRCUIT
`
`
`SWITCH
`
`J----4
`
`
`CURRENT
`
`SENSOR
`
`Fig. 1
`
`122
`
`
`
`2
`
`

`

`Patent Application Publication
`
`Jul. 9, 2009 Sheet 2 of 10
`
`US 2009/0174263 Al
`
`200
`
`226
`
`WIRELESS
`
`RECEIVER
`
`CONTROLLER
`
`SIGNAL
`RECEIVER
`
`222
`
`
`
`
`CURRENT
`
`SENSOR
`
`VOLTAGE
`
`SENSOR
`
` RECTIFIER
`SWITCH
`
`SECONDARY
`
`210
`
`216
`
`Fig. 2
`
`3
`
`

`

`Patent Application Publication
`
`Jul. 9, 2009 Sheet 3 of 10
`
`US 2009/0174263 Al
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`Patent Application Publication
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`Jul. 9, 2009 Sheet 7 of 10
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`US 2009/0174263 Al
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`US 2009/0174263 Al
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`Patent Application Publication
`
`SET INITIAL
`
`OPERATING
`
`FREQUENCY
`
`SET INITIAL
`
`DUTY CYCLE
`
`ADJUST
`
`OPERATING
`
`FREQUENCY
`
`Jul. 9, 2009 Sheet 8 of 10
`
`DUTY CYCLE
`
`DECREASE
`
`DUTY CYCLE
`
`INCREASE
`
`Fig.5
`
`9
`
`

`

`Patent Application Publication
`
`Jul. 9, 2009 Sheet 9 of 10
`
`US 2009/0174263 Al
`
`602
`
`INCREASE
`OPERATING
`FREQUENCY
`
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`
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`
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`
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`
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`FREQUENCY
`
`610
`
`10
`
`

`

`Patent Application Publication
`
`Jul. 9, 2009 Sheet 10 of 10
`
`US 2009/0174263 Al
`
`80
`
`70
`
`60
`
`50
`
`40
`
`30
`
`20
`
`10
`
`
`
`
`
`POWERTRANSFEREFFICIENCY
`
`°Fig. 7
`
`7F
`
`REQUENCY
`
`“INCREASE / DECREASE” SWITCH ON TIME TIMING DIAGRAM
`
`
`
`HIGH SIDE
`SWITCH “ON”
`
`HIGH SIDE
`SWITCH “OFF”
`
`HIGH SIDE
`SWITCH “ON”
`
`HIGH SIDE
`SWITCH “OFF”
`
`CURRENT ON TIME
`
`INCREASED ON TIME
`
`REDUCED ON TIME
`
`11
`
`11
`
`

`

`US 2009/0174263 Al
`
`Jul. 9, 2009
`
`INDUCTIVE POWER SUPPLY WITH DUTY
`CYCLE CONTROL
`
`[0001] This application claims the benefit of U.S. Provi-
`sional Application No. 61/019,411, filed on Jan. 7, 2008.
`[0002] The present inventionrelates to inductive power and
`moreparticularly to a system and methodfor wirelessly sup-
`plying power.
`
`BACKGROUNDOF THE INVENTION
`
`Inrecent years, wireless power supply systems have
`[0003]
`received increased attention because of someoftheir benefits
`
`over traditional wired power supply systems. Some more
`basic wireless power
`supply systems
`are specifically
`designed to charge a particular device, which can help mini-
`mize powertransfer efficiency issues. Other wireless power
`supply systems attempt to account for misalignment, charge
`different remote devices and provide different amounts of
`power. In these systems, maintaining an acceptable power
`transfer efficiency can be difficult.
`[0004]
`Somewireless power systems adjust the operating
`frequency of an AC signal across the tank circuit closer to or
`further from resonanceto increase or decrease the amount of
`
`powerdelivered to the remote device. Other wireless power
`systems adjust the resonant frequency of the tank circuit
`closer to or further from the operating frequency. One issue
`with these systems is that the power transfer efficiency
`between the inductive power supply and the remote deviceis
`a function of how close the operating frequency is to reso-
`nance. So, while adjusting the operating frequency or reso-
`nant frequency can provide some control over the amount of
`powerdelivered to the remote device, it may comeatthe cost
`of decreased powertransfer efficiency.
`[0005] Other wireless power supplies use a fixed operating
`frequency and instead adjust the rail voltage, duty cycle, or
`phase of the AC signal across the tank circuit to increase or
`decrease the amount of powerdelivered to the remote device.
`One issue with this is that in order for the power transfer
`efficiency to be acceptable, the inductive power supply and
`remote device may needto be precisely aligned andspecifi-
`cally designed to work with each other.
`
`SUMMARY OF THE INVENTION
`
`[0006] The present invention provides an inductive power
`supply that maintains resonance and adjusts duty cycle based
`on feedback from a secondary circuit. In one embodiment, the
`inductive powersupply includes a primary controller, a driver
`circuit, a switching circuit, and a tank circuit. The controller,
`driver circuit and switching circuit cooperate to generate an
`AC signalat a selected operating frequency and duty cycle.
`The AC signal is applied to the tank circuit to create an
`inductive field for powering the secondary. The secondary
`communicates feedback aboutthe received powerbackto the
`primary controller. The power transfer efficiency may be
`optimized by maintaining the operating frequency substan-
`tially at resonance, and the amountof powertransferred may
`be controlled by adjusting the duty cycle.
`[0007]
`Inone embodiment, the secondary circuit includes a
`secondary,a rectifier, a switch, a load, a sensor, a secondary
`controller, and a communication means. A voltage and/or
`current sensor detects characteristics about the power which
`are transmitted back to the primary controller using the com-
`
`munication means. Optionally, over-voltage and over-current
`protection may be provided.Ifa fault condition is detected the
`load is disconnected using the switch.
`[0008]
`In one embodiment, a process for inductively pow-
`ering a load by maintaining substantial resonance and adjust-
`ing duty cycle is provided. Initially an operating frequency
`and duty cycle are set to an acceptable value. The initial
`operating frequency is determined by sweeping a range of
`frequencies andselecting the operating frequency which pro-
`vided the highest powertransfer efficiency. The initial duty
`cycle is set to a relatively low value, such as 20%, to ensure
`that too much poweris not delivered to the secondary. Once
`the initial values have been set, the inductive power supply
`enters a continuous process of adjusting the operating fre-
`quency to maintain substantial resonance and adjusting the
`duty cycle depending on whether the amount of poweris too
`high or too low or temperature is too high.
`[0009] The present invention provides a simple and effec-
`tive system and method for providing a selected amount of
`wireless power while maintaining a high transfer efficiency.
`Adjustment of duty cycle provides anotherlevel of control of
`wireless powertransfer, one which can be usedto fine tune the
`amount of power provided to a secondary. Additionally, the
`ability to adjust the amount of powerbeing transferred while
`maintaining substantial resonance results in fewer overall
`losses and easierfulfillment of specified power requirements.
`[0010] These and other objects, advantages, and features of
`the invention will be readily understood and appreciated by
`reference to the detailed description of the current embodi-
`ment and the drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a block diagram of an inductive power
`
`[0011]
`supply.
`FIG. 2 isa block diagram of a secondary circuit.
`[0012]
`FIGS. 3A-3D together are a circuit diagram of an
`[0013]
`inductive power supply.
`[0014]
`FIG. 4 isa circuit diagram of a secondary circuit.
`[0015]
`FIG. 5 is a flowchart of a process to maintain reso-
`nance and adjust duty cycle.
`[0016]
`FIG. 6 is a flowchart of a process to adjust the
`operating frequency to maintain resonance.
`[0017]
`FIG. 7 is an exemplary graph showing frequency
`versus powertransfer efficiency.
`[0018]
`FIG. 8 is a timing diagram showing a varying duty
`cycle.
`
`DESCRIPTION OF THE CURRENT
`EMBODIMENT
`
`I. Overview
`
`[0019] An inductive power supply or primary circuit in
`accordance with an embodiment of the present invention is
`shown in FIG. 1, and generally designated 100. The primary
`circuit 100 includes a primary controller 110, a driver circuit
`111 including a pair of drivers 112, 114, a switching circuit
`115 including a pair of switches 116, 118, a tank circuit 120
`a primary sensor 122 and an optional wireless receiver 124.
`The primary controller 110, driver circuit 111 and the switch-
`ing circuit 115 together generate an AC signal at a selected
`frequency and selected duty cycle that is applied to the tank
`circuit 120 to create an inductivefield for transferring power
`wirelessly to a secondary circuit. A secondary circuit in
`accordance with an embodiment of the present invention is
`
`12
`
`12
`
`

`

`US 2009/0174263 Al
`
`Jul. 9, 2009
`
`shownin FIG.2, and generally designated 200. The second-
`ary circuit 200 may include a secondary 210, a rectifier 212,
`a switch 214, a load 216, a current sensor 218 or voltage
`sensor 220, a secondary controller 222, a signal resistor 224
`for communicating using reflected impedance and an
`optional wireless transmitter 226.
`[0020]
`In operation, an embodiment of the process for
`adjusting the duty cycle is shown in FIG.5, the initial oper-
`ating frequencyis set substantially at resonant frequency 504
`and the initial duty cycle is set at a relatively low value 506.
`The primary controller continuously adjusts the operating
`frequency 508 to maintain substantially resonant frequency
`and continuously determines if the amount of power being
`transferred is too high 510. If too much poweris being pro-
`vided or temperatures are above a preset threshold then the
`duty cycle is decreased 514. If too little poweris being pro-
`vided then the duty cycle is increased 512. Various conditions
`may temporarily or permanently reduce or halt the power
`transfer.
`
`II. Inductive Power Supply
`
`[0021] The present invention 1s suitable for use with a wide
`variety of inductive powersupplies. As used herein, the term
`“inductive power supply”is intended to broadly include any
`inductive power supply capable of providing power wire-
`lessly. The present invention is also suitable for use with
`“adaptive inductive powersupplies.” As used herein, the term
`“adaptive inductive power supply” is intended to broadly
`include any inductive power supply capable of providing
`powerwirelessly at a plurality of different frequencies. For
`purposesof disclosure, the present invention is described in
`connection with a particular adaptive inductive powersupply,
`shown in FIGS. 3A-3D and generally designated 300. The
`illustrated adaptive inductive power supply 300 is merely
`exemplary, however, and the present invention may be imple-
`mented with essentially any inductive power supply that can
`be modified to provide inductive power at varying duty
`cycles.
`In the illustrated embodiment, the adaptive induc-
`[0022]
`tive power supply 300 generally includes a primary controller
`310, a low voltage power supply 312, memory 314, a driver
`circuit 316, a switching circuit 318 a tank circuit 320, a
`current sensor 322, a filter 324 and optionally a wireless
`receiver 326. In operation, the primary controller 310, driver
`circuit 316 and switching circuit 318 apply powerto the tank
`circuit 320 to generate a source of electromagnetic inductive
`powerat a selected frequency anda selected duty cycle.
`[0023] The primary controller 310 of the illustrated
`embodiment includes two microcontrollers, one to control
`the frequency and one to control the duty cycle. The fre-
`quency microcontroller may be a microcontroller, such as a
`PIC24FJ32GA002, or a more general purpose microproces-
`sor. The duty cycle microcontroller may be a microcontroller,
`such as a dsPIC30F2020, or a more general purpose micro-
`processor. In alternative embodiments, the primary controller
`310 may be implemented using a single microcomputer,
`FPGA,analogor digital circuit. The driver circuit 316 may be
`discrete components, as shown in FIG. 3D [[3B]], or they may
`be incorporated into the primary controller 310. An oscillator
`(not shown) may be included within the primary controller
`310.
`
`[0024] The primary circuit 300 may also include a low
`voltage powersupply 312 for supplying low voltage powerto
`the primary controller 310, the driver circuit as well as any
`
`other components requiring low voltage powerfor operation.
`In the illustrated embodimentthe low voltage power supply
`312 provides scales the input voltage to 3.3 volts. In alterna-
`tive embodiments, a different voltage may be provided.
`[0025]
`Inthe current embodiment, the various components
`of the primary circuit 310 collectively drive the tank circuit
`320 at a frequency and duty cycle dictated by the primary
`controller 310. More specifically, the primary controller 310
`controls the timing of the driver circuit 316 and switching
`circuit 318. The timing refers to both the frequency and duty
`cycle of the signal being generated. Frequencyasit is being
`used here refers to the numberof repetitions per unit time of
`a complete waveform. Duty cycle refers to the proportion of
`time during which the waveform is high comparedto the total
`amountoftime fora complete waveform. Thus, a square wave
`as shown in FIG.8, may be describedby its frequency andits
`duty cycle. Further, the duty cycle may be adjusted while
`maintaining the same frequency and the frequency may be
`adjusted while maintaining the same duty cycle. The driver
`circuit 316 of the illustrated embodiment includes two sepa-
`rate drivers and may include additional circuit components to
`boost and filter the signal. For example,
`in the current
`embodiment, the signalis boosted to 20 volts, without effect-
`ing the timing ofthe signal.
`[0026] The switching circuit 318 includes two switches. In
`the current embodiment, the switches are implemented as
`MOSfield effect transistors. In alternative embodiments,
`other circuit components may be used to implement the
`switching circuit. Additionally, depending on powerrequire-
`ments MOSFETswith different characteristics may be imple-
`mented during manufacture. In some embodiments, multiple
`sets of switches may be providedonthecircuit board, allow-
`ing one set of switchesto be solderedat the time of manufac-
`ture based onthe particular power requirements ofthat appli-
`cation.
`
`the switching circuit 115
`In one embodiment,
`[0027]
`includes two separate switches 116, 118 that are switched on
`at the same frequency, but out of phase with each other. FIG.
`8 illustrates the timing for one embodimentof such a switch-
`ing circuit. In FIG. 8, both switches have the sameduty cycle,
`but are shifted in time from each otherby half of the period of
`the switching waveform. In alternative embodiments, each
`switch may havea different duty cycle and they the switches
`may beshifted in time a different amount from each other.
`That is, half period separation and similar duty cycle are
`desirable, but unnecessary, for the switches because it may
`result in increased powertransfer efficiency from the induc-
`tive power supply to the remote device.
`[0028] The tank circuit 320 generally includes the primary
`and a capacitor. The primary of the current embodiment is an
`air-core coil inductor. A cored inductor can also be used ifthe
`
`proper considerations are madefor spatial freedom, monitor-
`ing overall power, and feedback. The capacitance of the
`capacitor may be selected to balance the impedance of the
`primary coil at anticipated operating parameters. In the cur-
`rent embodiment, although three tank capacitors are shown,
`all three capacitors need not necessarily be soldered into the
`circuit at the time of manufacture. An inductive powersupply
`maybefabricated whichat the time of soldering can have an
`appropriate capacitance value selected by soldering or
`switching different capacitors into the circuit. The tank circuit
`320 maybe either a series resonant tank circuit (as shown in
`FIG. 3D) or a parallel resonant tank circuit (not shown). The
`present invention may be incorporated into the adaptive
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`inductive power supply shown in U.S. Pat. No. 6,825,620,
`which is
`incorporated herein by reference. As another
`example, the present invention may be incorporated into the
`adaptive inductive power supply shownin U.S. Patent Appli-
`cation Publication US2004/130916A1 to Baarman, which is
`entitled “Adapted Inductive Power Supply” and was pub-
`lished on Jul. 8, 2004 (U.S. Ser. No. 10/689,499, filed on Oct.
`20, 2003), which is also incorporated herein by reference.
`Further, it may be desirable to use the present invention in
`connection with an adaptive inductive power supply capable
`of establishing wireless communications with the remote
`device, such as the adaptive inductive power supply shown in
`USS. Patent Application Publication US 2004/130915A1 to
`Baarman, whichis entitled “Adapted Inductive Power Supply
`with Communication” and was published on Jul. 8, 2004
`(USS. Ser. No. 10/689,148, filed on Oct. 20, 2003), which is
`incorporated herein by reference. Furtheryet, it may be desir-
`able to use the present invention with a printed circuit board
`coil, such as a printed circuit board coil incorporating the
`invention principles of U.S. Ser. No. 60/975,953, which is
`entitled “Printed Circuit Board Coil” and filed on Sep. 28,
`2007 by Baarmanet al, and which is incorporated herein by
`referencein its entirety. In other alternative embodiments, the
`inductor may be implemented as a multi-tap inductor and/or
`the capacitors may be implemented as a switched capacitor
`bank that may be used to dynamically, before or during use,
`alter the resonance of the primary circuit, for example, as
`described in U.S. Pat. No. 7,212,414, whichis entitled “Adap-
`tive Inductive Power Supply” and issued May 1, 2007, to
`Baarman, whichis herein incorporated by reference.
`[0029]
`In certain modes of operation, the primary control-
`ler 310 mayestablish the operating frequency as a function of
`input from the current sensor 322. The controller 310, in turn,
`operatesthe driver circuit 318at the frequency established by
`the primary controller 310. The driver circuit 316 provides the
`signals necessary to operate the switching circuit 318. As a
`result, the switching circuit 318 provides AC (alternating
`current) powerto the tank circuit 320 from a source of DC
`(direct current) power. In an alternative embodiment, the
`operating frequency is established from a separate commu-
`nication link, such as the wireless receiver 326, implemented
`in the current embodimentas an IR receiver.
`
`[0030] The primary controller 310 may also establish the
`duty cycle as a function of input from the current sensor 322.
`Planned shunting ofthe signal resistor on the secondary,
`whichwill be described in more detail below, may be used to
`provide informationto the primary using reflected impedance
`detected with the current sensor 322. Alternatively, the duty
`cycle may be established using a separate communication
`link, such as the wireless receiver 326, implemented in the
`current embodiment as an IR receiver. This could also be near
`field or other RF communication channels.
`
`In the illustrated embodiment, the current sensor
`[0031]
`322 is a current transformer having a primary coil connected
`to the tank circuit and a secondary coil connected to the
`primary controller 310. In the current embodiment, the cur-
`rent sensor 322 includes circuitry to adjust the gain of the
`output of the current sensor to accommodate the ranges
`accepted by the primary controller 310. Further, the amount
`of gain may be adjusted by the primary controller 310 by
`applying a signal to the switch. The inductive power supply
`300 may include conditioning circuitry 324 for conditioning
`the current transformer output before it is supplied to the
`primary controller 310. In the current embodiment, the con-
`
`ditioning circuitry 324 is a 5 K Hz 2-polefilter. Although the
`illustrated embodiment includes a current transformer for
`
`sensing the reflected impedance of the secondary or remote
`device, the inductive power supply 300 may include essen-
`tially any alternative type of sensor capable of providing
`information regarding reflected impedance from the second-
`ary 400. Further, although the current sensor 322 ofthe illus-
`trated embodimentis connected directly to the tank circuit,
`the current sensor(or other reflected impedance sensor) can
`be located in essentially any location where it is capable of
`providing readingsindicative of the reflected impedance.
`[0032]
`In the illustrated embodiment, the inductive power
`supply 300 further includes a memory 314 capable of storing
`informationrelating to the operating parametersof a plurality
`of secondaries 400. The stored information may be used to
`permit the inductive power supply 300 to more efficiently
`power the secondary 400 and more readily recognize fault
`conditions. In someapplications, the inductive power supply
`300 may be intended for use witha specific set of secondaries
`400. In these applications, the memory 314 includes the
`unique resonant frequency (or pattern of frequencies) for
`each secondary 400, along with the desired collection of
`associated information, such as maximum and minimum
`operating frequencies, current usage and minimum and maxi-
`mum duty cycle. The memory 314 may, however, include
`essentially any information that may be useful to the induc-
`tive power supply 300 in operating the secondary 400. For
`example, in applications where it is desirable to establish
`wireless communications with the secondary 400,
`the
`memory 314 mayinclude information regarding the wireless
`communication protocol of the remote device 400.
`
`Il. Secondary Circuit
`
`[0033] Thepresent invention is intended for use with a wide
`variety of remote devices or secondaries of varying designs
`and constructions. It is anticipated that these various remote
`devices will require powerat varying frequency and will have
`different power requirements.
`[0034]
`For purposes of disclosure, one embodimentof a
`secondary circuit 400 is shown in FIG.4. In the embodiment
`of FIG. 4, the secondary circuit 400 generally includes a
`secondary 410 for receiving power from the inductive power
`supply 300, a rectifier 414 (or other components for convert-
`ing AC power to DC), a low voltage power supply 412 that
`scales the received powerto operate the secondary controller
`428, conditioning circuitry 416, 426 to removeripple in the
`signal, current sensor 418, voltage sensor 422, switch 420,
`load 424, secondary controller 428, a signal resistor 432 and
`an optional wireless transmitter 430. In operation,the rectifier
`414 converts the AC power generated in the secondary 410 to
`DC power, which is typically needed to power the load.
`Alternatively, multiple secondary coils receiving power on
`different phases can be usedto reducetheripple voltage. This
`is referenced in Application Ser. No. 60/976,137, entitled
`Multiphase Inductive Power Supply System to Baarmanetal,
`whichis herein incorporated by reference. Multiple primary
`coils may be desired to transmit poweron different phases in
`such an embodiment. In one embodiment, the loadis a charg-
`ing circuit (not shown) for a battery. Charging circuits are
`well-known and are widely used with a variety of recharge-
`able electronic devices. Ifdesired, the charging circuit may be
`configured to both charge a battery (not shown) and/or power
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`the load 424. In alternative embodimentsthe rectifier may be
`unnecessary and AC powermaybe conditionedto be used to
`powerthe load.
`[0035] Thecurrentsensor 418 detects the amountofcurrent
`in the received power and provides that information to the
`secondary controller 428. The voltage sensor 422 detects the
`amount of voltage in the received power and provides that
`information to the secondary controller 428. Although the
`illustrated embodiment includes both a voltage sensor 422
`and a current sensor 418, only one is necessary. By sensing
`the voltage and/or current in the secondary circuit and know-
`ing the voltage and/or current provided by the primary circuit,
`the primary controller can calculate the powertransfer effi-
`ciency. By sweeping a range of operating frequencies, noting
`the powertransfer efficiency at each frequency, the operating
`frequency closest to resonance can be determined—ttcorre-
`sponds with the operating frequency that yields the best
`powertransferefficiency. In addition, the voltage and current
`sensors 418, 422 can be used in conjunction with a protection
`algorithm in the secondary controller 428 to disconnect the
`load 424 if a fault condition is detected. This concept is
`described in more detail in U.S. patent application Ser. No.
`11/855,710 entitled System and Method for Inductively
`Charging a Battery to Baarmanet al, which was previously
`incorporated by reference.
`[0036] The secondary controller 428 may be essentially
`any type of microcontroller. In the illustrated embodiment,
`the secondary controller 428 is an ATTINY24V-10MU
`microcontroller. The secondary controller 428 generally
`includes an analogto digital converter, and is programmed to
`process the voltage and/or current readings and transmit them
`to the primary controller 310 of the inductive power supply
`300. The microprocessor may also include other code unre-
`lated to the frequency or duty cycle control processes.
`[0037] Communication of the sensed voltage and/or cur-
`rent in the secondary may betransmittedto the primary con-
`troller 310 in a variety ofways. In the illustrated embodiment,
`the information may be transmitted using the signal resistor
`432 or the wireless transmitter 430.
`
`Inone embodiment, signal resistor 432 may be used
`[0038]
`to send information to the primary controller 310. The use of
`a signal resistor 432 to provide communication from the
`secondary to the primary wasdiscussed in U.S. patent appli-
`cation Ser. No. 11/855,710 entitled System and Method for
`Inductively Charging a Battery to Baarman et al, which is
`herein incorporated by reference. The signal resistor 432,
`when shunted, sends a communication signal that signifies an
`over-current or over-voltage state. When the resistor is
`shunted, the peak detector on the primary circuit is able to
`sense the over-voltage/over-current condition and act accord-
`ingly. The signal resistor 432 of the present invention may be
`shunted systematically to communicate additional data to the
`primary controller 310. For example, a stream of data could
`represent the sensed current and/or sensed voltage. Alterna-
`tively, the signal resistor could be used solely in the previ-
`ously described way as an over-voltage/over-current trans-
`mitter or it could be removedentirely.
`[0039] Use ofa wireless transmitter or transceiver waspre-
`viously described in U.S. Patent Application Publication US
`2004/130915A1 to Baarman, which is entitled “Adapted
`Inductive Power Supply with Communication”that was pre-
`viously incorporated by reference. Specifically, the use of
`WIFI, infrared, blue tooth, cellular or RFID were previously
`discussed as ways to wirelessly transmit data from a remote
`
`device to an inductive power supply. Further, communication
`using the induction coils and a power line communication
`protocol was discussed. Any ofthese methodsoftransmitting
`data could be implementedin the present invention in order to
`transfer the desired data from the secondary to the primary.
`
`IV. Operation
`
`[0040] General operation of the primary circuit 100 and
`secondary circuit 200 is described in connection with FIG.5.
`[0041]
`In this embodiment, the primary circuit determines
`and sets the initial operating frequency 504. Typically, the
`goal of setting the initial operating frequencyis to set it as
`close to the resonant frequency as possible, which varies
`depending on manydifferent factors including, among other
`things, the orientation and distance between the primary cir-
`cuit and secondary circuit. In the current embodiment, a
`simple frequency sweepis used to determine where toset the
`initial operation frequency. Specifically, in this embodiment,
`the range of valid frequencies is swept and the powertransfer
`efficiency at each frequency is noted. The step between fre-
`quencies may vary, but in the current embodiment, the fre-
`quencyis swept between 70 k Hz and 250k Hz atsteps of 100
`Hz. Oncethe entire range of frequencies has been swept, the
`operating frequency that yielded the highest powertransfer
`efficiency is selected as the initial operating frequency. The
`operating frequency that yielded the highest power transfer
`efficiency indicates that it is the closest frequency to reso-
`nance. Furtherstepsat a finer frequency resolution can facili-
`tate even further tuning. Other methods for determining the
`initial operating frequency may be used in alternative
`embodiments. For example, an initial operating frequency
`may be selected based on known primary and secondary
`component. Further, modifications to the sweeping process
`may include dynamic step adjustment proportional to the
`powertransferefficiency. In yet another alte