as) United States
`a2) Patent Application Publication 0) Pub. No.: US 2008/0079392 Al
`(43) Pub. Date: Apr.3, 2008
`
`Baarmanet al.
`
`US 20080079392A1
`
`(54) SYSTEM AND METHOD FOR INDUCTIVELY
`CHARGING A BATTERY
`
`(22)
`
`Filed:
`
`Sep. 14, 2007
`
`Related U.S. Application Data
`
`(75)
`
`Inventors:
`
`David W. Baarman, Fennville, MI
`(US); John James Lord,
`Springfield, IL (US); Nathan P.
`Stien, West Des Moines, IA (US)
`
`(60) Provisional application No. 60/827,586, filed on Sep.
`29, 2006.
`
`Publication Classification
`
`(51)
`
`Int. Cl.
`(2006.01)
`HO2J 7/00
`(2006.01)
`HOIM 10/44
`(52) US. Ch.
`cieeeccssssssssseessssssssseeeeeesssen 320/108; 320/137
`
`111
`
`(57)
`
`ABSTRACT
`
`Correspondence Address:
`WARNER, NORCROSS & JUDD
`IN RE: ALTICOR INC.
`INTELLECTUAL PROPERTY GROUP,
`LYON STREET, N. W. STE 900
`GRAND RAPIDS, MI 49503-2489
`
`ACCESS BUSINESS GROUP
`INTERNATIONAL LLC, Ada,
`MI (US)
`
`(73) Assignee:
`
`(21) Appl. No.:
`
`11/855,710
`
`An inductive charging system for recharging a battery. The
`system includes a charger circuit and a secondary circuit.
`The secondary circuit includes a feedback mechanism to
`provide feedback to the charger circuit through the inductive
`coupling of the primary coil and the secondary coil. The
`charger circuit includes a frequency control mechanism for
`controlling the frequency of the power applied to the pri-
`mary coil at least partly in responseto the feedback from the
`feedback mechanism.
`
`
`
`111!!|1
`
`|1
`t
`
`Peak
`Detector
`
`Driver
`
`qt1 |
`
`|
`82 |
`‘||
`Vt
`!
`Il
`I
`|
`7
`!
`{|
`12
`SESEREESESSSESSEES:| 10 |
`\
`pod
`eee|
`
`
`
`|
`1
`|
`|_|
`|
`——
`+
`Inverter
`|
`Vad
`!
`|
`I-A|
`|
`Vo
`95
`III
`
`
`
`14
`
`DC Power
`Source
`
`
`FT|
`}+
`a
`%
`84
`;
`i
`i
`7
`|
`30 a
`1fT
`Overvoltage
`|
`32
`|
`detector
`
`
`
`
`
`
`
`
`
`
`
`
`
`Receiver
`
`
`¥
`
`—_—
`reli
`
`|
`|
`|
`
`|
`
`
`
`
`
`Overcurrent
`Vv
`detector
`of
`ga AOVa
`
`vy
`
`38
`
`|
`I
`|
`|
`
`!
`
`|
`
`ANKER 1012
`ANKER1012
`
`1
`
`

`

`Patent Application Publication
`
`Apr. 3, 2008 Sheet 1 of 7
`
`US 2008/0079392 Al
`
`100
`% CHARGE
`
`|
`!
`
`!
`
`DELIVERED|0
`
`CELL st eeasVOLTAGE 3 !
`
`
`
`
`
`CHARGE
`RATE
`
`Cc
`
`0
`
`0
`
`Constant
`Current
`
`|
`|
`
`1
`
`TIME (HRS)
`Constant
`Voltage
`
`2
`
`B
`
`A
`
`90
`
`80
`
`70
`60
`
`50
`
`40
`
`30
`20
`
`10
`
`>e5
`
`oO
`i=
`oO
`
`&
`c
`SG
`
`nC
`
`g
`oO
`Oo
`
`¢)
`
`Frequency
`
`FIG. 3
`
`2
`
`

`

`Patent Application Publication
`
`Apr. 3, 2008 Sheet 2 of 7
`
`US 2008/0079392 Al
`
`
`
`
`
`
`
`DC Power
`Source
`
`
`
`
`
`Receiver
`
`
`
`3
`
`

`

`Patent Application Publication
`
`Apr. 3, 2008 Sheet 3 of 7
`
`US 2008/0079392 Al
`
` 6
`
`
`
`
`)
`
`11210
`
`5+ 4
`
`do
`
`22d
`
`CONNECTION
`
`|
`
`|
`
` R18 1.5M
`R19 1K w
`
`45 R10
`SGB002JF 25MHz
`||
`. m
`
`R11. 100K
`
`J
`
`
`t
`{cit
`“FIO0pF yas4gq
`- CT
`RJ11 PROGRAMMING
`1)
`
`=
`
`AD9833
`
`NOk
`
`1/0F OUTL3__J|
`1:
`|
`3
`|
`fee
`|
`
`7
`
`C
`
`~
`
`4
`
`

`

`Patent Application Publication
`
`Apr. 3, 2008 Sheet 4 of 7
`
`US 2008/0079392 Al
`
`
`
`IRF5309L
`hoIRFZ34N/TO
`
`
`
`
`
`
`
`5
`
`

`

`Patent Application Publication
`
`Apr. 3, 2008 Sheet 5 of 7
`
`US 2008/0079392 Al
`
` ZNZNLN‘\104|<2!Ol47yso}Su|ye\AepLyOGIV}LOPATLPZ,SO==|\aMm=ile|onprtete|‘|oyaa|Nobsiz[eenSNOQ-Ost||1Poze140<6BYS||BI_-___L_____TTTV|oOwD|€dUO
`me]!WY£ry?NM&Ly!TozIS140OLY|AOLA701
`
`7?§bowenTTey-|SpMLMS2Lo)©;
`elS14GZa
`aasdvOAtAt|LOSOLLPLEN
`
`4OLZY|ag,”LMS404Z90N7|At
`lp554-----5-----------79
`
`
`
`
`
`i(oy=99
`989;|PeyOJN\|
`|\
`
`
`
`
`
`
`
`6
`
`
`
`

`

`Patent Application Publication
`
`Apr. 3, 2008 Sheet 6 of 7
`
`US 2008/0079392 Al
`
`“1.55 v
`0.105 ms!| MANUAL
`
`a i
`
`eee eee eeseee
`
`-----——-L-—-—LLdtdtdtdtdtdt
`
`a aaaOTSSSTST
`--—-——-LSLSL
`
`aaahpnepttnnpopntte
`
`a
`
`a ns as ns SOSOSO
`
`CONTRAST
`&
`
`Trig:A J
`TRIGGER
`SLOPE
`
`<I>
`
`A= 1 Vid
`
`5ms/d
`
`aAa p|B
`Vv MOVE
`
`Jd
`
`A
`
`Vv
`
`.-
`
`A M
`
`OVE
`
`FIG. 6
`
`7
`
`
`

`

`Patent Application Publication
`
`Apr. 3, 2008 Sheet 7 of 7
`
`US 2008/0079392 Al
`
`
`
`Energize primary coil at
`
`PROBE_FREQUENCY
`
`-
`
`“i
`
`S©
`
`———
`
`
`
`
`
`[Is CHARGED_FLAG set?|2s
`
`Is primary current>unloaded
`primary current?
`
`
`No
`
`
`
`Clear
`CHARGED_FLAG
`
`134
`
`
`
`No
`
`we
`
`
`MIN_CHARGE_TIME?
`
`
` Is ChRARGE_TIME<
`
`
`
`
`
`
` Lieee
`
`IIIII !|
`
`!
`
`IIIIIII
`
`
`
`
`
`
`
`
`
`
`
`
`
`122\\|OP_FREQ=OP_FREG
`
`+2*AFREQ
`
`
`
`130
`
`
`
`| “sf 52
`
`
`
`els OP_FREQ>=Max_FREQ?|
`124
`
`126.
`
`“Long charge delay|=N% Is there feedback?a aa
`
`
`8
`
`

`

`US 2008/0079392 Al
`
`Apr. 3, 2008
`
`SYSTEM AND METHOD FOR INDUCTIVELY
`CHARGING A BATTERY
`
`BACKGROUND OF THE INVENTION
`
`[0001] Charging of batteries with an inductive power
`supply is well-known. Inductive charging of batteries for
`electric automobiles as well as the charging of smallelectric
`appliance batteries such as those for toothbrushes has met
`some amount of success. Because inductive charging does
`not require a physical connection between the battery and
`the charger, the charging is considerably more convenient.
`However, not all batteries are easily charged inductively.
`Lithium-ion batteries (Li-Ion) are one such type of battery.
`[0002] Recharging Li-lon batteries is not as straightfor-
`ward as that of other batteries. Li-Ion batteries are unable to
`
`absorb an overcharge. If a constant current is applied to a
`fully charged Li-Ion battery, metallic lithium plating may
`develop which could lead to failure of the battery. Thus, care
`should be taken not to overcharge the battery.
`[0003] Conversely, charging a Li-lon battery to full capac-
`ity presents some difficulty. The maximum voltage of a
`Li-Ion battery can be attained relatively quickly during
`recharging by applying a constant current to the battery.
`However, when the Li-Ion battery reaches a maximum
`voltage, the Li-Ion battery may notbe fully charged. Without
`further charging, the battery will only be approximately 65%
`charged. If a constant current is continually applied to the
`battery after the battery has reached its maximum voltage,
`then the battery could be overcharged, which could lead to
`premature battery failure.
`[0004] Conventional battery chargers have been devel-
`oped to fully charge a Li-Ion battery. Generally, the battery
`charger uses a constant current, constant voltage schema to
`charge the battery. A discharged battery is first charged at a
`constant current level in the range of 0.1 C to 1 C amperes,
`where C is the battery capacity in amp-hours, until
`the
`battery reaches the desired voltage of about 4.2 volts. At this
`point, the battery charger switches to a constant voltage
`mode, providing the sufficient power to maintain the battery
`at this final voltage while providing additional charging to
`the battery.
`[0005] The charging profile for a typical Li-Ion battery is
`shown in FIG. 1. A constant current is applied for a prede-
`termined period. During this phase,
`the charging of the
`Li-Ion battery is generally constant. For a typical battery,
`this phase lasts somewhat less than one hour. The Li-Ion
`battery eventually exhibits a constant voltage near a pre-
`ferred voltage prior to attaining a full charge. A constant
`voltage is then applied to the Li-Ion battery. After approxi-
`mately an hour of charging with a constant voltage, the
`battery has typically attained its maximum charge.
`[0006]
`Ifthe charging of a Li-Ion battery does not follow
`the charging profile shown in FIG.1, then there is a risk that
`the battery will not be fully chargedorthat the charging will
`damagethe battery.
`[0007] The charging of a Li-Ion battery is further compli-
`cated because the battery is often not fully discharged before
`charging. If some residual charge remains on the battery,
`then optimal charging may require some amountof constant
`current charging followed by constant voltage charging, or,
`alternatively, the optimal charging may require only constant
`voltage charging. For better performance,
`the battery
`charger should provide a mechanism for compensating for
`the charge state of the battery.
`
`[0008] Charging Li-Ion batteries is especially problematic
`where inductive charging is used. In an inductive battery
`charger, a primary coil located in the charger provides power
`to an inductive secondary located in the battery. The voltage
`across the secondary is then rectified and applied to the
`battery to recharge the battery. There is no direct physical
`connection between the battery and the battery charger.
`Because there is no physical connection betweenthe battery
`and the battery charger, information regarding the state of
`the battery is not readily available to the battery charger.
`[0009] At the same time, portable devices need to be
`lightweight. Thus, complex circuitry to monitor the charge
`state of the battery and relay that information to the induc-
`tive charger increases the cost, size and weight of the
`portable device.
`[0010] An inductive system capable of charging a battery
`having a unique charge cycle while using a relatively simple
`circuit directly coupled to the battery is highly desirable.
`
`SUMMARY OF THE INVENTION
`
`[0011] An inductive system for recharging a battery, such
`as a Li-Ion battery, having a unique charging cycle generally
`includes a charger circuit having a primary coil for induc-
`tively supplying charging power and a secondary circuit for
`inductively receiving the charging powerand applying that
`powerto a battery. The secondary circuit includes a feed-
`back mechanism to provide feedback to the charger circuit
`through the inductive coupling of the primary coil and the
`secondary coil. The charger circuit includes a frequency
`control mechanism for controlling the frequency of the
`powerapplied to the primary coil at least partly in response
`to the feedback from the feedback mechanism.
`
`the feedback mechanism
`In one embodiment,
`[0012]
`includes a subcircuit for varying the reflected impedance of
`the secondary circuit. In this embodiment, the charger circuit
`may include a feedback detector for monitoring a charac-
`teristic of the powerin the charger circuit that varies at least
`in part in response to changesin the reflected impedance of
`the secondary circuit. In this embodiment,
`the feedback
`detector may be coupled to the primary coil to allow the
`controller to monitor the current through the primary coil.
`[0013]
`In one embodiment,
`the feedback mechanism
`includes an over-voltage detector or an over-current detec-
`tor, or both. In this embodiment,
`the detectors may be
`arranged so as to control one or more switches, such as
`transistors. If either an over-voltage condition or an over-
`current condition is detected in the secondary circuit, then
`the switch is turned on and the current from the secondary
`coil is shunted from the battery through a resistor. In this
`way, the battery is protected from significant exposure to
`over-voltage or over-current conditions. The feedback
`mechanism maybe directly coupled to the battery.
`[0014]
`In one embodiment,
`the feedback detector is a
`current sensor coupled to the primary tank circuit. In this
`embodiment, whenthe current is shunted through the feed-
`back signaling resistor in the secondary, the current through
`the secondary coil
`increases, which varies the reflected
`impedance of the secondary circuit resulting in increased
`current through the primary coil. The increase in current
`through the primary coil is detected by the current sensor in
`the primary circuit, which could include a peak detector,
`thereby providing to the controller a feedback signal for
`detecting whether the battery is in an over-voltage or over-
`current state. In one embodiment,
`the frequency control
`
`9
`
`

`

`US 2008/0079392 Al
`
`Apr. 3, 2008
`
`mechanism makes appropriate adjustments to the frequency
`to correct the over-voltage or over-current state by reducing
`the power supplied to the secondary coil.
`[0015]
`In one embodiment, the charger circuit includes an
`inverter and a tank circuit. In this embodiment, the operating
`frequency of the inverter is reduced to move the frequency
`of the power applied to the primary coil closer to the
`resonant frequency of the tank circuit, while the operating
`frequency of the inverter is increased in order to move the
`frequency of the power applied to the primary coil away
`from the resonant frequencyofthe tank circuit. It would be
`equally possible to arrange the system such that an increase
`in the inverter frequency would movethe power applied to
`the primary coil closer to resonance and thereby increase
`powertransfer while a decrease in inverter frequency would
`movethe powerapplied to the primary coil farther from the
`resonant frequency of the tank circuit and thereby decrease
`the powertransfer.
`[0016]
`In another aspect, the present invention also pro-
`vides a method for operating an inductive charging system
`having a charger circuit with a primary coil and a secondary
`circuit with a secondary coil. The method generally includes
`the steps of detecting whether a battery is present in the
`secondary circuit and charging the battery by way of one or
`more charging cycles. The charging step generally includes
`the steps of: applying powerto a primary coil at a frequency,
`evaluating feedback from the secondary circuit through the
`primary coil and secondary coil
`inductive coupling and
`adjusting the frequency of the powerapplied to the primary
`coil as a function of the feedback from the secondary circuit.
`In this way,
`the frequency of the power applied to the
`primary coil is changed in order to optimize the charging for
`the battery. Several charging cycles may be necessary to
`fully charge the battery.
`[0017]
`In one embodiment, the methodis utilized with a
`charger circuit having a tank circuit. In this embodiment, the
`charging cycle may include the alternative steps of moving
`the power applied to the primary coil closer to the resonant
`frequency of the tank circuit or moving the powerapplied to
`the primary coil farther from the resonant frequency of the
`tank circuit. The charger circuit may include an inverter. In
`such embodiments, the steps of moving the frequency of the
`powerapplied to the primary coil may be further defined as
`moving the operating frequency of the inverter.
`[0018]
`In one embodiment, the charging step generally
`includes the steps of: applying powerto the primary coil at
`a specific frequency; determining whether a feedback signal
`from the charger circuit is recetved by the charger circuit;
`and varying the frequency of the power applied to the
`primary coil as a function of the feedback signal to either
`increase or decrease the power delivered to the secondary
`circuit. In one embodiment, the step of varying the fre-
`quencyis further defined as including the steps of: moving
`the frequency of the power applied to the primary coil
`farther from resonance if a feedback signal from the feed-
`back mechanism is received or moving the frequency of the
`powerapplied to the primary coil closer to resonance if a
`feedback signal
`from the feedback mechanism is not
`received.
`
`In one embodiment, the charging step generally
`[0019]
`includes the steps of incrementally moving the frequency of
`the power applied to the primary coil closer to resonance
`until a feedback signal is received; after a feedback signalis
`received, incrementally moving the frequency of the power
`
`applied to primary close farther from resonance until a
`feedback signal is no longer received; and applying powerto
`the primary coil at the frequency for a charging period. The
`process may be repeated.
`the step of determining
`[0020]
`In one embodiment,
`whether a feedback signal is received includes the steps of
`sensing the current in the charger circuit and comparing the
`sensed current with a predetermined threshold.
`[0021]
`In one embodiment, the method further includes
`the step of terminating the charging cycle whenthe time for
`completing one charging cycle is less then a minimum
`charging cycle time. The method may further include the
`step of terminating the charging cycle whenthe frequency of
`the powerapplied to the primary coil meets an upper and/or
`lower threshold.
`the detecting step further
`[0022]
`In one embodiment,
`includes the steps of: applying a pulse of power to the
`primary coilat a predetermined probe frequency, sensing the
`reflected impedance, and determining whether a battery is
`present as a function of the sensed reflected impedance.
`[0023] The present invention provides a simple andeffec-
`tive inductive charging circuit that permits nonlinear charg-
`ing profiles to be implemented in an inductive system with
`a small number of components. The over-current and over-
`voltage detectors not only provide feedback usedto drive the
`frequency of the charging power, but also protect the battery
`from potential harmful power conditions. The charging
`profile can be easily changed by varying a numberofstored
`values that dictate operation of the system. The present
`invention is well-suited for use in charging portable elec-
`tronic devices, such as cell phones, personal digital assis-
`tants, handheld gaming devices, personal media players and
`other similar devices. In this context, the secondary circuit
`may be incorporated into the portable electronic device so
`that the device can be placed in close proximity to the
`chargercircuit for charging, thereby eliminating the need to
`plug the device into a charger.
`[0024] These and other objects, advantages, and features
`of the invention will be more fully understood and appre-
`ciated by reference to the description of the current embodi-
`ment and the drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 shows the charging profile for a typical
`[0025]
`Li-Ion battery;
`[0026]
`FIG. 2 shows a system for inductive charging of a
`battery;
`FIG. 3 is a powertransfer curve between a primary
`[0027]
`coil and a secondary coil;
`[0028]
`FIG. 4 showsa circuit diagram corresponding to
`the block diagram of FIG. 2 for the charger circuit;
`[0029]
`FIG. 5 showsa circuit diagram corresponding to
`the block diagram of FIG. 2 for the battery side;
`[0030]
`FIG. 6 showsthe outputof the peak detector caused
`by the increased current through the secondary coil; and
`[0031]
`FIG. 7 is a flow chart for a method of operating a
`battery charger.
`
`DESCRIPTION OF THE CURRENT
`EMBODIMENT
`
`[0032] An inductive charging system in accordance with
`an embodimentof the present invention in shownin FIG.2.
`The inductive charging system 4 is configured to inductively
`
`10
`
`10
`
`

`

`US 2008/0079392 Al
`
`Apr. 3, 2008
`
`charge a battery having a nonlinear charging profile, such as
`a Li-Ion battery. The system 4 generally includes a charger
`circuit 6 and secondary circuit 8. The charger circuit 6
`generally includes a primary coil 15, a frequency controller
`80 for applying power to the primary coil at a desired
`frequency and a feedback detector 82 for receiving feedback
`from the secondary circuit 8. The secondary circuit 8 gen-
`erally includes a secondary coil 30 for receiving inductive
`powerfrom the charger circuit 6 and a feedback mechanism
`84 for providing feedback to the charger circuit 6 indicative
`of the voltage or current in the secondary circuit 8. The
`frequency controller 80 varies the frequency of the power
`applied to the primary coil 15 as a function of the feedback
`from the secondary circuit 8. Although described in connec-
`tion with the charging of a conventional Li-Ion battery, the
`present invention is well suited for use in charging other
`types of batteries, including batteries having different charg-
`ing profiles.
`[0033] As noted above, the charger circuit 6 generally
`includes a frequencycontroller 80, a primary coil 15, and a
`feedback detector 82. In the illustrated embodiment,
`the
`frequency controller 80 includes a controller 20, an oscilla-
`tor 18, a driver 16 and an inverter 10. In this embodiment,
`these components collectively drive a tank circuit 12. More
`specifically, the inverter 10 provides AC (alternating cur-
`rent) powerto the tank circuit 12 from a source of DC (direct
`current) power 14. The tank circuit 12 includes the primary
`coil 15. The tank circuit 12 may beeither a series resonant
`tank circuit or a parallel resonant
`tank circuit.
`In this
`embodiment, the driver 16 provides the signals necessary to
`operate the switches within the inverter 10. The driver 16 in
`turn operates at a frequency set by the oscillator 18. The
`oscillator 18 is, in turn, controlled by the controller 20. The
`controller 20 could be a microcontroller,
`such as
`a
`PIC18LF1320, or a more general purpose microprocessor.
`Although shown asessentially discrete devices in the illus-
`trated embodiment, the driver 16, oscillator 18 and could
`alternatively be integrated and could be implemented as
`modules within the controller 20.
`
`In the illustrated embodiment, the feedback detec-
`[0034]
`tor 82 detects the current in the tank circuit 12. In operation,
`the controller 20 uses signals from the peak detector 22 to
`assist in determining the frequency of operation for the
`oscillator 18, and thereby the frequency of the inverter 10.
`Although the feedback detector 82 ofthe illustrated embodi-
`mentdetects current in the tank circuit 12, other character-
`istics of the powerin the charger circuit 6 may be evaluated
`to provide charge information regarding the battery.
`[0035] The secondary circuit 8 generally includes a sec-
`ondary coil 30, a rectifier 32, and a feedback mechanism 84.
`The secondary coil 30 inductively receives power from the
`primary coil 15. The rectifier 32 provides DC powerto
`charge the battery 34. In this embodiment, the feedback
`mechanism 84 is configured to provide feedback when the
`current applied to the battery 34 exceeds a threshold value
`or when the voltage applied to the battery 34 exceeds a
`threshold value. As shown, the feedback mechanismofthis
`embodiment generally includes an over-voltage detector 36,
`an over-current detector 40, an OR gate 38, a switch 42, and
`a resistor 44. The output of an over-voltage detector 36
`indicates whether the voltage across the battery 34 1s above
`a predetermined level. Similarly,
`the output of an over-
`current detector 40 indicates whether the current to the
`
`battery 34 is above a predetermined amount. The output of
`
`the current detector 40 as well as the output of the voltage
`detector 36 are coupled to the inputs of an OR gate 38. The
`OR gate 38 could be a discrete circuit, or it could be a
`connection between the outputs of the detectors. The output
`of the OR gate 38 is coupled to the switch 42. The switch 42
`is controlled by the output of the OR gate 38 and is
`connected in series between the rectifier 32 and the resistor
`44. The switch 42 could be any suitable switch such as a
`bipolar transistor, a field effect transistor, or an insulated gate
`bipolar transistor. The resistor 44 is connected in series
`between the switch 42 and ground.
`[0036]
`In operation,
`if the output of the over-voltage
`detector 36 or the output of the over-current detector 40
`indicate an over-voltage or an over-current situation, then
`the output of the OR gate 38 turns on the switch 42. When
`the switch 42 is on, current from the rectifier 32 flows
`through the resistor 44 to ground.
`[0037] Because the impedance ofthe resistor 44 is much
`less than the impedance of the battery 34, a current surge
`occurs through the resistor 44, thereby causing a current
`surge through the secondary coil 30. The diode 64 prevents
`the battery 34 from supplying any current when the switch
`42 is turned on. The current surge through the secondary coil
`30 creates a similar current surge in the charger circuit 6
`through the primary coil 15. The current surge is detected by
`the peak detector 22. The controller 20 then changes the
`frequency of the oscillator 18.
`[0038]
`In the illustrated embodiment, the primary coil 15
`and the secondary coil 30 are loosely coupled. Because the
`twoare loosely coupled in this embodiment, the slope of the
`powertransfer curve about the resonant frequency is not as
`steep as if the coils 15, 30 were tightly coupled. An exem-
`plary power transfer curve for the coils 15, 30 is shown in
`FIG. 3. In this embodiment, the power transfer is highest
`when the inverter 10 is operating at resonance. However,
`even if the inverter 10 is not operating at resonance, sig-
`nificant power transfer can occur when the inverter is
`operating off-resonance. Generally, the inverter 10 is oper-
`ated between frequency A and frequency B. Frequency B is
`somewhat less than the resonant frequency. Between fre-
`quency A and frequency B, the powertransfer curve can be
`piece-wise linearized by a software lookup table located in
`the controller 20. Thus, a decrease in the operating fre-
`quency ofthe inverter 10 will result in a generally predict-
`able increase in the powertransferred from the primary coil
`15 to the secondary coil 30. As can be seen by the graph,it
`would be equally effective to use an operating frequencyless
`than the resonant frequency. If so, then an increase in the
`operating frequency would lead to an increase in the power
`transfer, and vice-versa.
`[0039]
`FIG. 4 shows a circuit diagram corresponding to
`the block diagram of FIG. 2 for the charger circuit 6 of the
`system. The peak detector 22 (comprised of 22A and 22B)
`is connected in series with the primary coil 15 and provides
`a signal by way ofthe transformer 50 that is proportional to
`the current
`through the primary coil 15. The signal
`is
`rectified by a diode 52 and then used to charge capacitor 54.
`The operational amplifiers 56, 58 are used to smooth the
`signal for sampling by the controller 20. This particular
`circuit diagram is exemplary andis not intendedto limit the
`scope of the invention to a specific circuit design.
`[0040]
`FIG. 5 showsa circuit diagram for the secondary
`circuit 8 of the system. As with the charger circuit diagram
`of FIG. 4,
`the secondary circuit diagram of FIG. 5 is
`
`11
`
`11
`
`

`

`US 2008/0079392 Al
`
`Apr. 3, 2008
`
`exemplary and not intended to limit the scope of the inven-
`tion to a specific circuit design. Power from the secondary
`coil 30 is used to charge capacitor 60, which in turn is used
`as the powersupply for the circuitry connectedto the battery
`34. A rectifier 32 produces a DC current from the AC current
`supplied by secondary coil 30. A capacitor 62 is charged to
`provide a DC power source for charging the battery 34. A
`blocking diode 64 prevents the battery 34 from discharging
`whenthe secondary coil 30 is not receiving power or when
`the feedback mechanism is signaling an over-voltage or
`over-current condition.
`
`If either the over-voltage detector 36 or the over-
`[0041]
`current detector 40 determines that too much voltage or too
`much current is applied to the battery 34, then the transistor
`42 is turned on,thereby discharging the capacitor 62 through
`the resistor 44, resulting in a lower voltage across the battery
`34. In this embodiment, the secondary circuit 8 includes a
`blocking diode 64 that prevents current from flowing into
`the capacitor 62 from the battery 34.
`[0042] When the current flows through the resistor 44,
`additional current
`is drawn from the secondary coil 30,
`which in turn causes an increase in current through the
`primary coil 15.
`[0043] Because the voltage drops across the capacitor 62,
`the voltage across the battery 34 drops as does the current
`through battery 34. Thus, the over-voltage condition or the
`over-current condition is corrected. The detectors 36, 40 are
`cleared, thereby causing the transistor 42 to turn off. The
`period the transistor 42 is turned on due to an over-current
`or an over-voltage condition to the time the transistor is
`turned off due to a correction of the over-current or over-
`
`voltage condition is the signal time.
`[0044]
`In this illustrated circuit design, the duration ofthe
`signal time is controlled by two RC circuits 66, 68 within the
`detector circuits 36, 40. In this embodiment, the voltage
`detector 36 is configured to have a hysteresis of about 80 mV
`to reduce oscillation when the battery 34 voltage is near an
`over-voltage condition.
`[0045] As stated, when the transistor 42 is turned on,
`increased current flows through the secondary coil 30,
`causing increased current flow through the primary coil 15.
`This increase in current is detected by the peak detector 22.
`The output of the peak detector caused by the increased
`current through the primary coil is shown in FIG. 6. The
`output of the peak detector in the illustrated embodiment
`increases by about 1.55V for about 10 ms. The character-
`istics of this signal may vary from application to application
`depending on the characteristics of the circuit components.
`For example, the magnitudeofthe increase and the length of
`the increased signal may be controlled as desired.
`[0046] The controller 20 continuously samples the output
`of the peak detector 22. When a sudden increase is detected,
`an internalflag referred to as FB_flag is set. When a decrease
`is detected, FB_flag is cleared. However, a copy of FB_flag
`referred to hereinafter as FB_latch is also set. FB_latch is
`not cleared when a decrease is detected. It can only be
`cleared by the controller 20. FB_latch therefore can be
`checked periodically by controller 20 to determine whether
`an over-voltage condition or an over-current condition
`occurred during a given period of time. Thus, the system
`provides a feedback mechanism to the controller.
`[0047] Auser of a portable device may remove the device
`from the charger circuit 6 before it is fully charged. Addi-
`tionally, the user may place the device in the charger before
`
`the battery is full discharged. In order to optimally charge
`the battery, the inductive battery charger may detect the
`presence of the battery as well as compensate for the unique
`charging profile for a battery.
`[0048]
`FIG.7 is a flow chart showing one embodimentof
`the process for detecting whether the secondary circuit 8 is
`proximalto the charger circuit 6 and for optimally charging
`the battery if the secondary circuit 8 is present.
`[0049] The process starts. Step 100. The probing process
`99 is commenced. The controller 20 waits for a predeter-
`mined time period of PROBE_INTERVAL. Step 102. After
`PROBE_INTERVALhas elapsed the controller 20 causes
`the inverter 10 to produce a low frequency current at
`PROBEFREQUENCYthrough the primary coil 15. Step
`104. The current through the primary coil 15 is detected.
`Step 106.
`[0050]
`Ifthe secondary circuit 8 is present, then the probe
`by the charger circuit 6 will induce a probe current in the
`secondary coil 30. The battery 34 will not be damaged even
`if it is fully chargedat the time of the probe. First, the probe
`is of a short duration on the order of 10 to 20 milliseconds,
`in this embodiment, while the quiescent period is usually
`several seconds long. Additionally, the over-voltage detector
`36 and the over-current detector 40 by way of transistor 42
`will shunt excessive probe current through the resistor 44
`rather than through the battery 34.
`[0051]
`In this embodiment, the amount of current through
`the primary coil at the PROBE_FREQUENCYhas been
`previously determined experimentally and saved into the
`memory ofthe controller. If the current through the primary
`coil 15 is approximately equal
`to the predetermined
`unloaded primary current (Step 108), then the secondary
`circuit 8 is not present
`in the secondary circuit 8. The
`CHARGED_FLAGis cleared. Step 109. The system then
`waits for another PROBE_INTERVALbefore starting the
`process again.
`[0052]
`Ifthe current flowing through the primary coil 15
`is not approximately equal to the predetermined unloaded
`primary current, then the secondary circuit 8 is present.
`[0053] The CHARGED_FLAGis then checked. Step 111.
`The CHARGED_FLAG indicates whether the battery is
`fully charged. If the CHARGED_FLAG1s notset, then the
`charging process begins.
`[0054] The frequency of the inverter is set by the control-
`ler 20 to FREQSTART. Step 110. The system then delays
`for a predetermined period of time to eliminate any tran-
`sients. Step 112.
`[0055] The controller 20 then determines whether a feed-
`back signal, discussed previously, has been received.If not,
`then the frequency is decreased by AFREQ. Step 116. In this
`embodiment, a decrease in the frequency moves the system
`toward resonance, and therefore increases the powertransfer
`from the charger circuit 6 to the battery 34.
`[0056] AFREQ could be a constant, or it could be deter-
`minedby obtaining the value from a lookup table indexed by
`the operating frequency of the inverter at the particular time
`AFREQ is used. The values selected for AFREQ may be
`frequency dependent and chosen so that if the operating
`frequency is reduced or increased by AFREQ,
`then the
`corresponding increase or decrease in current is the approxi-
`mately the same for AFREQsfor all operating frequencies.
`For example, if the charger circuit 6 is operating near the
`resonant frequency of the tank circuit 12, then a decrease in
`the operating frequency by 100 Hz will
`substantially
`
`12
`
`12
`
`

`

`US 2008/0079392 Al
`
`Apr. 3, 2008
`
`increase the current through the tank circuit 12. If, on the
`other hand, the charger circuit 6 is operating relatively far
`from resonance, then a change of 100 Hz will not result in
`a substantial increase in the current through the primary.
`AFREQ maytherefore be chosen to cause approximately the
`same change in primary c