INDUCTIVE POWER SUPPLY WITH DUTY CYCLE CONTROL
`
`The present invention relates to inductive power and moreparticularly to a system
`
`and method for wirelessly supplying power.
`
`SUMMARY OF THE INVENTION
`
`The present invention provides an inductive power supply that maintains resonant
`
`frequency and adjusts duty cycle based on feedback from a secondary circuit.
`
`In one
`
`embodiment,
`
`the inductive power supply 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 signal at 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 about the received powerback to the primary controller. The
`
`powertransfer efficiency may be optimized by maintaining the operating frequency substantially
`
`at resonance, and the amount of power transferred may be controlled by adjusting the duty
`
`cycle.
`
`In one 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 communication means. Optionally, over-voltage and over-current protection
`
`maybe provided. If a fault condition is detected the load is disconnected using the switch.
`
`In one embodiment, a process for inductively powering a load by maintaining
`
`substantial resonance and adjusting 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 and selecting the operating frequency which provided the
`
`= ill =
`
`1
`1
`
`ANKER 1009
`ANKER1009
`
`

`

`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 beenset, the inductive power supply enters a continuous process of adjusting the operating
`
`frequency to maintain substantial resonance and adjusting the duty cycle depending on whether
`
`the amount of poweris too high or too low or temperatureis too high.
`
`The present invention provides a simple and effective system and method for
`
`providing a selected amount of wireless power while maintaining a high transfer efficiency.
`
`Adjustment of duty cycle provides another level of control of wireless powertransfer, one which
`
`can be used to fine tune the amount of power provided to a secondary. Additionally, the ability
`
`to adjust the amount of power being transferred while maintaining substantial resonance results
`
`in fewer overall losses andeasier fulfillment of specified power requirements.
`
`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 embodiment
`
`and the drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`Fig. 1
`
`is a block diagram of an inductive power supply.
`
`Fig. 2 is a block diagram of a secondary circuit.
`
`Figs. 3A and 3B together are a circuit diagram of an inductive powersupply.
`
`Fig. 4 is a circuit diagram of a secondarycircuit.
`
`Fig. 5 is a flowchart of a process to maintain resonance and adjust duty cycle.
`
`Fig. 6 is a flowchart of a process to adjust the operating frequency to maintain
`
`resonance.
`
`Fig. 7 is an exemplary graph showing frequency versus powertransfer efficiency.
`
`-2-
`
`2
`
`

`

`Fig. 8 is a timing diagram showing a varying duty cycle.
`
`DESCRIPTION OF THE CURRENT EMBODIMENT
`
`I. Overview
`
`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
`
`switching 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 inductive field for transferring power
`
`wirelessly to a secondary circuit. A secondary circuit in accordance with an embodimentof the
`
`present invention is shown in Fig. 2, and generally designated 200. The secondary circuit 200
`
`may include a secondary controller 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 impedanceand an optional wireless transmitter 226.
`
`In operation, an embodiment of the process for adjusting the duty cycle is shown
`
`in Fig. 5, the initial operating frequency is 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 powerbeing transferred is too high 510. If too much poweris being
`
`provided or temperatures are above a preset threshold then the duty cycle is decreased 514.
`
`If
`
`too little power is being provided then the duty cycle is increased 512. Various conditions may
`
`temporarily or permanently reduceor halt the powertransfer.
`
`-3-
`
`3
`
`

`

`If. Inductive Power Supply
`
`The present invention is suitable for use with a wide variety of inductive power
`
`supplies. As used herein, the term “inductive power supply” is intended to broadly include any
`
`inductive power supply capable of providing power wirelessly. The present invention is also
`
`suitable for use with “adaptive inductive power supplies.” 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 purposes of disclosure, the
`
`present invention is described in connection with a particular adaptive inductive powersupply,
`
`shown in Figs. 3A and 3B and generally designated 300. The illustrated adaptive inductive
`
`power supply 300 is merely exemplary, however, and the present invention may be implemented
`
`with essentially any inductive power supply that can be modified to provide inductive powerat
`
`varying duty cycles.
`
`In the illustrated embodiment, the adaptive inductive 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 and a selected duty cycle.
`
`The primary controller 310 of
`
`the illustrated embodiment
`
`includes
`
`two
`
`microcontrollers, one to control the frequency and one to control the duty cycle. The frequency
`
`microcontroller may be a microcontroller, such as a PIC24FJ32GA002, or a more general
`
`purpose microprocessor. The duty cycle microcontroller may be a microcontroller, such as a
`
`dsPIC30F2020, or a more general purpose microprocessor.
`
`In alternative embodiments, the
`
`-4-
`
`4
`
`

`

`primary controller 310 may be implemented using a single microcomputer, FPGA, analog or
`
`digital circuit. The driver circuit 316 may be discrete components, as shown in Fig. 3B, or they
`
`may be incorporated into the primary controller 310. An oscillator (not shown) may be included
`
`within the primary controller 310.
`
`The primary circuit 300 may also include a low voltage power supply 312 for
`
`supplying low voltage powerto the primary controller 310, the driver circuit as well as any other
`
`components requiring low voltage power for operation.
`
`In the illustrated embodiment the low
`
`voltage power supply 312 provides scales the input voltage to 3.3 volts.
`
`In alternative
`
`embodiments, a different voltage may be provided.
`
`In the 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 numberofrepetitions
`
`per unit time of a complete waveform. Duty cycle refers to the proportion of time during which
`
`the waveform is high compared to the total amount of time for a complete waveform. Thus, a
`
`square wave as shownin Fig. 7, may be described by its frequency and its 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 separate drivers and may include additional circuit components to
`
`boost and filter the signal. For example, in the current embodiment, the signal is boosted to 20
`
`volts, without effecting the timing of the signal.
`
`5
`
`

`

`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
`
`power requirements MOSFETs with different characteristics may be implemented during
`
`manufacture.
`
`In some embodiments, multiple sets of switches may be provided on the circuit
`
`board, allowing one set of switches to be soldered at the time of manufacture based on the
`
`particular power requirements of that application.
`
`The tank circuit 320 generally includes the primary and a capacitor. The primary
`
`of the current embodimentis an air-core coil inductor. A cored inductor can also be used if the
`
`proper considerations are made for spatial freedom, monitoring overall power, and feedback.
`
`The capacitance of the capacitor may be selected to balance the impedanceof the primary coil at
`
`anticipated operating parameters.
`
`In the current 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 power supply may be fabricated 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 may be either a series resonant tank circuit (as shownin Fig.
`
`3B) or a parallel resonant tank circuit (not shown). The present invention may be incorporated
`
`into the adaptive inductive power supply shown in U.S. Patent 6,825,620, which is incorporated
`
`herein by reference. As another example, the present invention may be incorporated into the
`
`adaptive
`
`inductive
`
`power
`
`supply
`
`shown
`
`in U.S.
`
`Patent Application
`
`Publication
`
`US2004/130916A1 to Baarman, whichis entitled “Adapted Inductive Power Supply” and was
`
`published on July 8, 2004 (U.S. Serial No. 10/689,499, filed on October 20, 2003), which is also
`
`incorporated herein by reference. Further, it may be desirable to use the present invention in
`
`-6-
`
`6
`
`

`

`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
`
`U.S. Patent Application Publication US 2004/130915A1 to Baarman, whichis entitled “Adapted
`
`Inductive Power Supply with Communication” and was published on July 8, 2004 (U.S. Serial
`
`No. 10/689,148, filed on October 20, 2003), which is incorporated herein by reference. Further
`
`yet, it may be desirable 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. Serial No. 60/975,953,
`
`whichis entitled “Printed Circuit Board Coil” and filed on September 28, 2007 by Baarmanetal,
`
`and whichis incorporated herein by reference in 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. Patent 7,212,414,
`
`which is entitled “Adaptive Inductive Power Supply” and issued May 1, 2007,
`
`to Baarman,
`
`whichis herein incorporated by reference.
`
`In certain modes of operation,
`
`the primary controller 310 may establish the
`
`operating frequency as a function of input from the current sensor 322. The controller 310, in
`
`turn, operates the driver circuit 318 at the frequency established by the primary controller 310.
`
`The driver circuit 316 provides the signals necessary to operate the switching circuit 318. Asa
`
`result, the switching circuit 318 provides AC (alternating current) power to the tank circuit 320
`
`from a source of DC (direct current) power.
`
`In an alternative embodiment,
`
`the operating
`
`frequency is established from a separate communication link, such as the wireless receiver 326,
`
`implemented in the current embodiment as an IR receiver.
`
`7
`
`

`

`The primary controller 310 may also establish the duty cycle as a function of
`
`input from the current sensor 322. Planned shunting of the signal resistor on the secondary,
`
`which will be described in more detail below, may be used to provide information to 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 nearfield or other
`
`RF communication channels.
`
`In the illustrated embodiment, the current sensor 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 current sensor 322 includescircuitry 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 conditioning circuitry 324 is a 5K Hz 2-pole
`
`filter. 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 essentially any alternative type of sensor capable of providing information regarding
`
`reflected impedance from the secondary 400. Further, although the current sensor 322 of the
`
`illustrated embodiment is 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 readings indicative of the reflected impedance.
`
`8
`
`

`

`In the illustrated embodiment, the inductive power supply 300 further includes a
`
`memory 314 capable of storing information relating to the operating parameters ofa 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 with a 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 maximum duty cycle. The memory 314 may, however, include essentially
`
`any information that may be useful to the inductive 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 may include information regarding the wireless
`
`communication protocol of the remote device 400.
`
`ILL.
`
`Secondary Circuit
`
`The present 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.
`
`For purposes of disclosure, one embodiment of a secondary circuit 400 is shown
`
`in Fig. 4.
`
`In the embodimentof 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 converting AC power to DC), a low voltage power supply 412 that scales the
`
`received power to operate the secondary controller 428, conditioning circuitry 416, 426 to
`
`remove ripple in the signal, current sensor 418, voltage sensor 422, switch 420,
`
`load 424,
`
`_9-
`
`9
`
`

`

`s,econdary controller 428, a signal resistor 432 and an optional wireless transmitter 430.
`
`In
`
`operation, the rectifier 414 converts the AC powergenerated in the secondary 410 to DC power,
`
`whichis typically needed to power the load. Alternatively, multiple secondary coils receiving
`
`power on different phases can be used to reduce the ripple voltage. This is referenced in
`
`Application 60/976,137, entitled Multiphase Inductive Power Supply System to Baarmanet al,
`
`which is herein incorporated by reference. Multiple primary coils may be desired to transmit
`
`power on different phases in such an embodiment.
`
`In one embodiment, the load is a charging
`
`circuit (not shown) for a battery. Charging circuits are well-known and are widely used with a
`
`variety of rechargeable electronic devices.
`
`If desired, the charging circuit may be configured to
`
`both charge a battery (not shown) and/or powerthe load 424.
`
`In alternative embodiments the
`
`rectifier may be unnecessary and AC power may beconditioned to be used to powerthe load.
`
`The current sensor 418 detects the amount of current 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 knowing the voltage and/or current provided by the primary circuit, the primary
`
`controller can calculate the power transfer efficiency. By sweeping a range of operating
`
`frequencies, noting the powertransfer efficiency at each frequency, the operating frequency
`
`closest to resonance can be determined — it corresponds with the operating frequency that yields
`
`the best powertransfer efficiency.
`
`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
`
`-10-
`
`10
`10
`
`

`

`Application No. 11/855,710 entitled System and Method for Inductively Charging a Battery to
`
`Baarmanet al, which waspreviously incorporated by reference.
`
`The secondary controller 428 may beessentially any type of microcontroller.
`
`In
`
`the
`
`illustrated embodiment,
`
`the
`
`secondary controller 428 is
`
`an ATTINY2MV-10MU
`
`microcontroller. The secondary controller 428 generally includes an analog to 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 unrelated to the frequency or duty cycle control processes.
`
`Communication of the sensed voltage and/or current in the secondary may be
`
`transmitted to the primary controller 310 in a variety of ways.
`
`In the illustrated embodiment, the
`
`information may be transmitted using the signal resistor 432 or the wireless transmitter 430.
`
`In one embodiment, signal resistor 432 may be used to send information to the
`
`primary controller 310. The use of a signal resistor 432 to provide communication from the
`
`secondary to the primary was discussed in U.S. Patent Application 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
`
`accordingly. 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. Alternatively, the signal resistor could be
`
`used solely in the previously described way as an over-voltage/over-current transmitter or it
`
`could be removedentirely.
`
`-ll-
`
`11
`11
`
`

`

`Use of a wireless transmitter or transceiver was previously described in US.
`
`Patent Application Publication US 2004/130915A1 to Baarman, which is entitled “Adapted
`
`Inductive Power Supply with Communication” that was previously 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 of these methods of transmitting data could be implemented in the present
`
`invention in order to transfer the desired data from the secondary to the primary.
`
`IV.
`
`Operation
`
`General operation of the primary circuit 100 and secondary circuit 200 is
`
`described in connection with Fig. 5.
`
`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 circuit and secondary
`
`circuit.
`
`In the current embodiment, a simple frequency sweep is used to determine whereto set
`
`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
`
`frequencies may vary, but in the current embodiment, the frequency is swept between 70k Hz
`
`and 250k Hz at steps of 100 Hz. Once the entire range of frequencies has been swept, the
`
`operating frequency that yielded the highest power transfer efficiency is selected as the initial
`
`operating frequency. The operating frequency that yielded the highest powertransfer efficiency
`
`indicates that it is the closest frequency to resonance.
`
`Further steps at a finer frequency
`
`-12-
`
`12
`12
`
`

`

`resolution can facilitate even further tuning. Other methods for determining the initial operating
`
`frequency may be usedin 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 power transfer
`
`efficiency.
`
`In yet another alternative embodiment, the sweep may be performed dynamically so
`
`that only the powertransfer efficiency value for the current frequency and the frequency with the
`
`highest powertransfer efficiency are stored. As the sweep progresses, each value is checked
`
`against the highest stored value and replacesit only if it is higher.
`
`In the embodiment described in Fig. 5, the primary circuit sets the initial duty
`
`cycle 506. The duty cycle corresponds with the amount of power transferred with each cycle.
`
`The higher the duty cycle, the more powertransferred per cycle.
`
`In the current embodiment, the
`
`initial duty cycle is set at 20%, which is considered low enoughto not risk over-powering the
`
`remote device, but is high enough such that enough poweris transferred to power the secondary
`
`circuitry.
`
`In alternative embodiments a different initial duty cycle may be set based on the
`
`application or any numberof other factors.
`
`The adjust operating frequency step 508 is a multi-step process which ensures that
`
`the operating frequency is being maintained substantially at resonance. Fig. 6 describes one
`
`embodiment of this process in more detail.
`
`In the described embodiment,
`
`the operating
`
`frequency is increased by a pre-selected amount, referred to as a step up. The adjustmentis
`
`allowed to propagate through the system and the powerefficiency is checked 604. If the power
`
`efficiency increased then the system was not substantially at resonance and the operating
`
`frequency is stepped up again. This process continues until the powerefficiency either decreases
`
`or stays the same. Once that occurs, the operating frequency is stepped down 608. The power
`
`-13-
`
`13
`13
`
`

`

`efficiency is checked 608.
`
`If the power efficiency increases then the operating frequency is
`
`stepped downagain, until the powerefficiency stays the same or decreases. The final step is to
`
`step up the operating frequency 610 to get back to the operating frequency with the peak power
`
`efficiency. This is merely one embodiment of a process to maintain the operating frequency
`
`substantially at resonance. Any other process could be used to maintain the operating frequency
`
`substantially at resonance.
`
`One reason that the operating frequency is stepped up and stepped down can be
`
`explained by looking at an exemplary graph of operating frequency vs. power efficiency, shown
`
`in Figure 7. As can be seen, there are several peaks of powerefficiency over the range of
`
`operating frequencies shown. The initial sweep of frequencies sets the operating frequency to
`
`the resonant frequency, i.e. the highest peak on Figure 7. Each time the adjustment comes,
`
`although the operating frequency has not changed,
`
`the power efficiency values may have
`
`changed as a result
`
`in any number of factors, most notably movement of the secondary.
`
`Typically, the change in the graph is merely a slight shift, meaning that the optimum operating
`
`frequency may be a few steps in either direction. This is why the current embodimentsteps up
`
`and steps down.
`
`If the first step up leads to a decrease in power efficiency transfer, the process
`
`immediately steps down until.
`
`If stepping down also leads to a decrease in powerefficiency
`
`transfer then it is evident that no adjustment is necessary and the operating frequency was
`
`already at resonant frequency.
`
`In an alternative embodimentan analog circuit could be used to
`
`directly determine howfar off resonance the system is, causing the controller to react directly to
`
`the proper frequency. A phase comparatoris one suchcircuit.
`
`In the current embodiment,
`
`the operating frequency is adjusted with each
`
`iteration, however, in alternative embodiments, the operating frequency may be adjusted less
`
`-14-
`
`14
`14
`
`

`

`frequently or only when an event triggers that it should be adjusted. For example, if a motion
`
`detector on the secondary indicates movementor a change in orientation of the secondary. Or,
`
`for example, if there is a sharp decrease or increase in the amount of power provided to the
`
`secondary.
`
`The next step is to determine if the amount of power being received by the
`
`secondaryis too high 510. If the amount of power being received is too high then the duty cycle
`
`of the power being transferred is reduced 514.
`
`If the amount of power being received is not too
`
`high then the duty cycle of the power being transferred is increased 512.
`
`In the current
`
`embodiment, the duty cycle should not exceed approximately 49% in order to reduce the risk of
`
`causing a short circuit.
`
`In the current embodiment, after the duty cycle is adjusted, up or down,
`
`the operating frequency is re-adjusted 508. As explained above, duty cycle refers to the “switch
`
`on time” or the proportion of time during which the waveform is high compared to the total
`
`amount of time for a complete waveform. An exemplary graph illustrating a signal with a
`
`varying duty cycle is shown in Fig. 8. The graph depicts a graph of time vs. current. The solid
`
`line represents the waveform generated by the primary circuit with the current duty cycle. The
`
`dashed line represents what a waveform would look like with an increased duty cycle. The dash-
`
`dotted line represents what a waveform would look like with a decreased duty cycle. Note that
`
`because the duty cycle is being increased symetrically and decreased symetrically, the frequency
`
`of the waveform does not change with the adjustment in duty cycle.
`
`It is worth noting that in
`
`some embodiments, during operation,
`
`the frequency may not be adjusted, while duty cycle
`
`adjustments cotninue to take place.
`
`Duty cycle may be stepped up or downbya pre-selected amount.
`
`In the current
`
`embodiment, the step up and step down amounts are static and equal. However, in alternative
`
`-15-
`
`15
`15
`
`

`

`embodiments, the step amounts may be dynamic and different. For example, in battery charging
`
`applications it may be beneficial to decrease duty cycle in large steps and increase duty cycle in
`
`small steps. Various batteries require different charging algorithms and the duty cycle control
`
`may be used to provide the correct battery charging profile.
`
`In another example, the duty cycle
`
`may be stepped up or down proportional to the amount of power demanded by the secondary.
`
`The amount of power demanded by the secondary can be determined by reading the current
`
`and/or voltage sensor. Where there is a small change in the readings, a small change in duty
`
`cycle may be implemented and where there is a large change in the readings, a large change in
`
`duty cycle may be implmemented.
`
`In one embodiment, there are built-in delays between the changes in operating
`
`freqeuncy and changes in duty cycle. These delays can account for any phase issues that may
`
`arrise because of the speed at which the operating frequency or duty cycle is being changed.
`
`This process continues as desired or until the power supply is turned off, the
`
`secondary is removed,or in the case of charging a battery, when the battery is fully charged.
`
`The primary circuit may adjust the duty cycle depending on the demandsof the
`
`secondary. For example, in one embodiment, one goal may be to maintain a certain amount of
`
`voltage or current in the secondary. Using feedback from the secondary, such as the sensed
`
`voltage and/or current,
`
`the operating frequency may be adjusted to ensure optimum power
`
`transfer efficiency by ensuring operation at substantially resonant frequency and the duty cycle
`
`may be adjusted to provide additional or less power to meetthe desired goal.
`
`The above description is that of the current embodimentof the invention. Various
`
`alterations and changes can be made without departing from the spirit and broader aspects of the
`
`invention.
`
`-16-
`
`16
`16
`
`

`

`tll
`
`[omMnsty
`
`SSOP,
`
`JoyJonUED
`
`IOATa00y
`
`17
`17
`
`

`

`
`
`JO\STISOYTeUsIS
`
`ps
`
`neeSSOTOTIAA
`
`IOSUISIBEITOA,
`
`7ONsIy
`
`Jo}TUISUeIL,
`JgT[oUOCD
`oresmin
`ja|
`
`Ott
`
`We
`
`7
`
`Arepuosas
`
`18
`18
`
`
`
`
`

`

`moa3
`
`vodoval,
`
`a,3
`
`PHTwe:
`Verne
`38
`
`OFOZOEdSP
`
`an
`
`
`OP]smanaevinooruiie
`oy]CMENOMLYEODTad
`
`RongLSSEON
`
`SMTINGLEOE
`
`Hele
`
`PSWSNIETAMOHORN
`
`
`
`TOMLINCeeOOTOnNETOSE
`
`EVENSTSOORIMETLISS
`
`SPRLNOATAMOOTANOTNY|eba
`PRONOWEMOTE
`
`
`
`th
`\
`
`ESA
`
`rcaSSRASEMEENEORSEE
`
`CASOENONLAYce38828cy]SeusedNdeLNO'SHVORAY
`tereSEMATROAWOREOGL=HauTONAtINOISReauNEDL
`
`60a$9,LianOMSENONwe,
`
`
`
`20aFe]CHEWSNOORRENO
`
`Homge|WHYHONDATELODOOR
`
`Boge)PRUEROEPHIROS
`enTge|DRRENS$8
`SrySezoowozersrzoid
`
`Seu
`FORbENWGtd
`PURERZINEERATNY
`SUEPANDeR
`
`ELGUAGRTARTINGIELSWINE
`
`TIRUMORAIMNOTERET
`ZERENOTVORTAENY
`PSaUNDmIOSHRY
`TATEFONEGEGeyLEM
`BViRNdFRIN
`IYRENDAHASRAGNY
`‘AYfodaga'ysenWOE
`
`
`
`
`NORE
`
`BONES
`
`
`
`19
`19
`
`
`
`
`

`

`
`
`
`
`
`
`agoinsty
`
`20
`20
`
`

`

`Se
`c