WO2017078543A1

WO2017078543A1 - Inductive power receiver

Info

Publication number: WO2017078543A1
Application number: PCT/NZ2016/050174
Authority: WO
Inventors: Ron Rafer FLORESCA; Arunim KUMAR
Original assignee: Powerbyproxi Limited
Priority date: 2015-11-02
Filing date: 2016-11-02
Publication date: 2017-05-11

Abstract

An inductive power receiver comprising a power receiving coil; a switch mode regulator configured to regulate the power from the power receiving coil to a load depending on a control signal; and a controller configured to determine the control signal and to communicate a power request to an inductive power transmitter based on the control signal.

Description

INDUCTIVE POWER RECEIVER

FIELD

This invention relates generally to an inductive power receiver. BACKGROUND

Electrical converters are found in many different types of electrical systems. Generally speaking, a converter converts a supply of a first type to an output of a second type. Such conversion can include DC-DC, AC- AC, and DC-AC and AC-DC electrical conversions. In some configurations a converter may have any number of DC and AC 'parts', for example a DC-DC converter might incorporate an AC-AC converter stage in the form of a transformer.

One example of the use of converters is in inductive power transfer (IPT) systems. IPT systems are a well-known area of established technology (for example, wireless charging of electric toothbrushes) and developing technology (for example, wireless charging of handheld devices).

IPT systems will typically include an inductive power transmitter and an inductive power receiver. The inductive power transmitter includes a transmitting coil or coils, which are driven by a suitable transmitting circuit to generate an alternating magnetic field. The alternating magnetic field will induce a current in a receiving coil or coils of the inductive power receiver. The received power may then be used to charge a battery, or power a device or some other load associated with the inductive power receiver. Further, the transmitting coil and/or the receiving coil may be connected to a resonant capacitor to create a resonant circuit. A resonant circuit may increase power throughput and efficiency at the corresponding resonant frequency.

There is increasing interest in inductively coupled power transfer systems in which the power transmitter includes an array of transmitter coils beneath a charging surface (commonly referred to as "charging mats"). These charging mats may be used to charge handheld devices, for example.

Ordinarily, the transmitting coils are driven by a converter. The characteristics of the driving current (such as frequency, phase and magnitude) will be related to the particular IPT system. In some instances, it may be desirable for the driving frequency of the converter to match the resonant frequency of the resonant transmitting coil and/or the resonant receiving coil.

Some inductive power systems use a backscatter communications channel or some other form of communication to allow an inductive power receiver to communicate load requirements to the primary side. For example, changes to the magnitude of the power transmitter coil current, its voltage and/or its frequency may be requested by a power receiver in order to correspond with the load requirements of that power receiver. This is known as primary side regulation and can help to regulate the output voltage or current from the receiver. The Qi standard by the Wireless Power Consortium (WPC) is an example primary side regulation.

A problem with existing IPT systems that are based on primary side regulation is that the control loop which regulates the output voltage from the inductive power receiver may be slow to respond, resulting in poor system performance under certain conditions such as load step, load dump and sudden changes in the coupling between the inductive power transmitter and the inductive power receiver.

A further problem with existing IPT systems that are based on primary side regulation is that in cases where there are more than one inductive power receiver being charged by a single inductive power transmitter, such as a charging mat, the transmitter must be able to vary the magnetic field supplied to each inductive power receiver independently. This is so that the power requirements of each inductive power receiver can be individually met. For example, it is possible that each inductive power receiver will have a different alignment, coil size, casing thickness and power requirement compared with the other inductive power receivers. Therefore a different magnetic field strength output may be required for each inductive power receiver. Supplying a range of individualized magnetic fields can result a complex and inefficient inductive power transmitter.

The present invention may provide an improved inductive power transfer system, or may at least provides the public with a useful choice.

SUMMARY

According to one example embodiment there is provided an inductive power receiver comprising: a power receiving coil; a switch mode regulator configured to regulate the power from the power receiving coil to a load depending on a control signal; and a controller configured to determine the control signal and to communicate a power request signal to an inductive power transmitter based on the control signal. According to another example embodiment there is provided an inductive power transfer system comprising: an inductive power transmitter configured to generate a magnetic field; and an inductive power receiver including a power receiving coil coupled to the magnetic field and a switch mode regulator configured to regulate the power from the power receiving coil to a load depending on a control signal; wherein the inductive power transmitter is configured to change the magnetic field in response to a change in the control signal.

It is acknowledged that the terms "comprise", "comprises" and "comprising" may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, these terms are intended to have an inclusive meaning - i.e., they will be taken to mean an inclusion of the listed components which the use directly references, and possibly also of other non-specified components or elements.

Reference to any document in this specification does not constitute an admission that it is prior art or that it forms part of the common general knowledge.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings which are incorporated in and constitute part of the specification, illustrate embodiments of the invention and, together with the general description of the invention given above, and the detailed description of embodiments given below, serve to explain the principles of the invention.

Figure 1 is a schematic diagram of an inductive power transfer system;

Figure 2 is a receiver capable of backscatter communications and with a linear regulator for power flow control;

Figure 3 is a graph of a relationship between load current and rectified voltage;

Figure 4 is receiver with a switch mode regulator for power flow control;

Figure 5 is a circuit diagram of a receiver that has a buck switch mode converter;

Figure 6 is a flow diagram of a first method for a receiver to set the desired control signal;

Figure 7 is a circuit diagram of a receiver that has a boost switch mode converter;

Figure 8 is a circuit diagram of a receiver that has a coupled coil switch mode converter; and

Figure 9 is a flow diagram of a second method for a receiver to set the desired control signal.

DETAILED DESCRIPTION

An inductive power transfer (IPT) system 1 is shown generally in Figure 1 . The IPT system includes an inductive power transmitter 2 and an inductive power receiver 3. The inductive power transmitter 2 is connected to an appropriate power supply 4 (such as mains power or a battery). The inductive power transmitter 2 may include transmitter circuitry having one or more of a converter 5, e.g., an AC-DC converter (depending on the type of power supply used) and an inverter 6, e.g., connected to the converter 5 (if present). The inverter 6 supplies a transmitting coil or coils 7 with an AC signal so that the transmitting coil or coils 7 generate an alternating magnetic field. In some configurations, the transmitting coil(s) 7 may also be considered to be separate from the inverter 5. The transmitting coil or coils 7 may be connected to capacitors (not shown) either in parallel or series to create a resonant circuit.

A controller 8 may be connected to each part of the inductive power transmitter 2. The controller 8 may be adapted to receive inputs from each part of the inductive power transmitter 2 and produce outputs that control the operation of each part. The controller 8 may be implemented as a single unit or separate units, configured to control various aspects of the inductive power transmitter 2 depending on its capabilities, including for example: power flow, tuning, selectively energising transmitting coils, inductive power receiver detection and/or communications. There may also be a separate communications module.

The inductive power receiver 3 includes a receiving coil or coils 9 connected to receiver circuitry which may include power conditioning circuitry 10 that in turn supplies power to a load 1 1 . When the coils of the inductive power transmitter 2 and the inductive power receiver 3 are suitably coupled, the alternating magnetic field generated by the transmitting coil or coils 7 induces an alternating current in the receiving coil or coils 9. The power conditioning circuitry 10 is configured to convert the induced current into a form that is appropriate for the load 1 1 , and may include for example a power rectifier, a power regulation circuit, or a combination of both. The receiving coil or coils 9 may be connected to capacitors (not shown) either in parallel or series to create a resonant circuit. In some inductive power receivers, the receiver may include a controller 12 which may control tuning of the receiving coil or coils 9, operation of the power conditioning circuitry 10 and/or communications. There may also be a separate communications module.

It is understood that the use of the term "coil" herein is meant to designate inductive "coils" in which electrically conductive wire is wound into three dimensional coil shapes or two dimensional planar coil shapes, electrically conductive material fabricated using printed circuit board (PCB) techniques into three dimensional coil shapes over plural PCB 'layers', and other coil-like shapes. The use of the term "coil", in either singular or plural, is not meant to be restrictive in this sense.

Messages from the inductive power receiver 3 to the inductive power transmitter 2 can be sent using a backscatter communications channel. In an example embodiment the voltage across and the current through the receiving coil or coils 9 is amplitude modulated by, or under control of, the controller 12 or a communications module, in accordance with a data stream. This modulation is then observed as voltage or current amplitude variation in transmitting coil or coils 7 and can be demodulated by the inductive power transmitter 2, so that the original data stream can be recovered. Among other uses, this backscatter communications channel can be used to enable primary side regulation without the need for dedicated radio transceivers. Primary side regulation mediated by a backscatter communications channel is sometimes necessary in order to meet wireless power interoperability standards. Primary side regulation can be very efficient because a significant source of loss in any IPT system 1 is due to losses in the transmitting coil or coils 7 and the receiving coil or coils 9 and primary side regulation can allow these losses to be minimized.

A problem with relying on primary side regulation alone to regulate the output from an inductive power receiver 3 is that primary side regulation can be slow to respond when the IPT system 1 begins to fall out of regulation. This is particularly a problem when sudden changes occur in the IPT system 1 such as loading transients or changes in the coupling factor between the transmitting coil or coils 7 and the receiving coil or coils 9, and could result in the load 1 1 being either over-voltage or under- voltage.

As a partial solution to these problems, in addition to using primary side regulation facilitated by a backscatter communications channel, an IPT system 1 may use a simple linear regulator on the inductive power receiver 3 to provide additional, fast-acting regulation of the power supplied to load 1 1 . In this way, a combination of the efficiency of primary side regulation and the speed and precision of secondary side regulation can be achieved.

Figure 2 is a block diagram of an inductive power receiver capable of backscatter communications and with a linear regulator for power flow control. Inductive power transmitter 2 transmits power to receiving coil or coils 9. The receiving coil or coils 9 are connected to a rectifier 201 , which can for example be a synchronous rectifier, and can be connected in full bridge and in half bridge configurations. Rectifier 201 is connected to linear regulator 202, which regulates the received DC voltage so that it is suitable for load 1 1 . An AC-side modulator 203 and a DC-side modulator 204 are used to amplitude modulate the current in or voltage across receiving coil or coils 9, in order to send backscatter messages to the inductive power transmitter 2. Sometimes, only one of these two modulator circuits are necessary for the backscatter communications channel to function, and their placement within the inductive power receiver 3 may vary. Current sensing circuit 205 is used to monitor the current drawn by the load 1 1 . Vrect 206 is the voltage output from the rectifier 201 and may also include any voltage drop caused by current sensing circuit 205 though this voltage drop is often negligible. In some cases more than one current sensor may be used. Vioad 207 is the voltage which is supplied to the load 1 1 . Each part of the inductive power receiver 3 can be monitored and/or controlled by controller 12.

The exact position of the current sensing circuit 205 within the inductive power receiver 3 is not critical to the operation of the inductive power receiver 3 because the controller 12 can be calibrated to account for measurement biases or errors, for example by using an offset. Other possible positions for the current sensing circuit 205 could be either before or after the linear regulator 202 or in series with the load 1 1 .

The inductive power transmitter 1 will vary its frequency, amplitude or duty cycle, or any combination of these, in order to help regulate the voltage or current received by receiving coil or coils 9. Communication from the inductive power transmitter 2 to an inductive power receiver 3 may for example be achieved using frequency modulation of the switching frequency of the inductive power transmitter 2. In an example embodiment, the primary side regulation scheme allows an inductive power receiver 3 to send messages to an inductive power transmitter 2 using a backscatter communications channel. Amongst other information and commands, these messages contain requests that the inductive power transmitter 2 provide either "more power", "less power" or "the same power". The inductive power transmitter 2 can deliver the requested change in power by changing its output frequency, output current, output voltage or some combination thereof. The "more power" requests can also request a specific percentage increase in power, for example, between a 0% and a 30% increase. Similarly, the "less power" requests can request a specific percentage decrease in power, for example, between a 0% and a 30% decrease. In some control schemes, a request for "more power" corresponds to a "positive control error", a request for "less power" corresponds to a "negative control error" and a "control error" of zero corresponds to requesting "the same power".

The receiving coil or coils 9 may be part of a dual-resonant circuit, that is, a circuit tuned to resonate at two different frequencies. One of these frequencies is designed to be the IPT frequency eg: 1 10kHz and the other frequency is so that inductive power receivers 3 may be detected, which may be required by some inductive power transmitters 2 eg: 1 MHz. Typically these two frequencies should be sufficiently separated. The dual resonant circuit of the inductive power receiver 3 may comprise a receiving coil 9 and two resonant capacitances (not shown). The purpose of the first resonant capacitance is to enhance the power transfer efficiency. The purpose of the second resonant capacitance is to enable a resonant detection method.

The linear regulator 202 may be a low drop-out ("LDO") type. In some cases, linear regulator 202 can function as, may include or be replaced by a load disconnect switch. A load disconnect switch may be necessary according to some wireless power interoperability standards for inductive power receivers 3. The inductive power receiver 3 may be designed to draw enough power that backscatter communications is still possible even when the load 1 1 is disconnected.

A practical inductive power receiver 3 may need to provide a fixed output voltage to the load 1 1 even during sudden changes in system parameters. These changes can include a load step, i.e., a sudden increase in the current drawn by the load 1 1 , a load dump i.e., a sudden decrease in the current drawn by the load 1 1 and a sudden change in the coupling coefficient between transmitting coil or coils 7 and receiving coil or coils 9, such as might be caused by vibration or bumping an inductive power receiver 3 onto or away from a transmitting coil or coils 7. These system parameter changes can cause the voltage delivered to the load 1 1 to sag below regulation or to rise above regulation, which is undesirable.

Using primary side regulation alone it may not be possible or practical to keep the load voltage Vioad 207 within regulation during sudden system parameter changes, due to the slow reaction speed of the primary side regulation feedback loop. For example, in order to ensure a certain maximum steady state output voltage ripple in an inductive power receiver 3 with an operating frequency of 1 10kHz and an output resistance of 1 Ω, the output capacitor across the load 1 1 may be chosen to have a time constant that is 100 times greater than the period of the operating frequency. In this example that would mean an output capacitance that was 909 F. However, after a step-change in the current drawn by the load 1 1 , the time taken for an inductive power receiver 3 to send the required number of "more power" or "less power" messages using a backscatter communications channel with a 1 10kHz carrier may be 50ms. To maintain the same output voltage ripple as under steady state conditions would then require 5F of capacitance; for many applications an impractically large value.

The regulation problem caused by the slow reaction speed of the primary side regulation feedback loop can be addressed using the linear regulator 202. Given an input voltage Vrect 206 that is higher than the voltage desired at the load 1 1 , the linear regulator 202 can step down the voltage so that Vioad 207 stays within regulation even when Vrect 206 sags or rises a little. However due to the inherent properties of linear regulators, given an input voltage Vrect 206 that is lower than the voltage desired at the load 1 1 , the linear regulator 202 cannot increase the voltage in order to keep Vioad within regulation. Therefore, it may be beneficial for the inductive power receiver 3 to request "more power" from the inductive power transmitter 2 until voltage at Vrect is somewhat higher than the desired output voltage at Vioad. Then, if the load 1 1 suddenly draws more current, the linear regulator 202 can react quickly to maintain a regulated voltage across the load 1 1 , provided that Vrect 206 does not sag below the desired value for Vioad (ignoring the saturation voltage across the linear regulator 202). For this reason a measure of how close the linear regulator 202 is to its maximum output voltage is useful.

The difference between the input voltage and output voltage of the linear regulator 202 can be used to estimate how close the present output voltage of the linear regulator 202 is to its maximum output voltage. For example, for the inductive power receiver 3 in Figure 2, the minimum voltage drop across the linear regulator 202 may be known and may be approximately zero volts if a low drop-out (LDO) regulator may be used. Therefore Vrect 206 minus Vioad 207 gives a measure of how close the present output voltage of the linear regulator 202 is to its maximum output voltage. The voltage difference between the present input voltage of a linear regulator and input voltage that would cause the output voltage to sag is sometimes known as the voltage regulation headroom.

In a different approach, for some types of inductive power receiver 3 one or more regulator control signals can be used to estimate how close the present output voltage or current of a regulator is to the regulator's maximum output voltage or current. A regulator control signal can also be used to estimate how much more power the load 1 1 can draw before the voltage or current supplied to the load 1 1 falls out of regulation. They way that the regulator control signal relates to how close the regulator is to falling out of regulation depends on the several factors which may include the type of regulator, the type of control signal and the operating conditions of the inductive power receiver 3 circuit. A drawback of using a linear regulator 202 for secondary side regulation is that as current is drawn by the load 1 1 any voltage drop across the linear regulator 202 will contribute to power loss in the linear regulator 202 which can lead to heating and inefficiency. In order to minimize this power loss, it is possible to specify a desired amount voltage drop across the linear regulator 202 for each value of current supplied to the load 1 1 or for each value of current supplied by the rectifier 201 . This desired voltage drop across the linear regulator 202 should strike a balance between minimizing power losses in the IPT system 1 and ensuring that the chance of a voltage dropout or a voltage spike at the load 1 1 is acceptably low. By requesting "more power" or "less power" as required, the inductive power receiver 3 can adjust the voltage drop across the linear regulator 202 to the desired value for a given load 1 1 . Instead of a desired voltage drop across the linear regulator 202, for certain regulator types it may be more appropriate to specify a desired bypass current through a regulator 202 that is connected in parallel with the load 1 1 .

The way that the desired voltage drop across the regulator 202 changes with respect to the current drawn by the load 1 1 is shown in Figure 3 for an example IPT system 1 . The current drawn by the load 1 1 in amps is shown on the "x" axis and the unit of the "y" axis is volts. The line Vioad 301 shows the voltage being delivered to the load 1 1 , which will typically be a fixed value regardless of what current flows into the load 1 1 . The line Vrect,max 302 shows the maximum voltage that could be achieved at Vrect 206 if primary side regulation were disabled, i.e., if the inductive power transmitter were set to always transmit at maximum power. The line Vrect,optimai 303 shows an example of a voltage for Vrect 206 which balances robust load voltage regulation with good power transfer efficiency. In the case of Vrect,optimai 303, greater voltage drop across the regulator 202 is desired when the output current is low because a sudden load step from a light load to full load will cause more of a volage dip in Vrect 206 than will a sudden load step from a medium load to full load. Further, at a light load the current through regulator 202 is low and so the losses on it are are tolerable even with a larger voltage drop across it.

There is increasing interest in IPT systems 1 in which the inductive power transmitter 2 includes one or an array of transmitter coils beneath a charging surface (commonly referred to as "charging mats"). In certain applications with multiple inductive power receivers 3 placed on a charging mat, secondary side regulation may be used. This enables each inductive power receiver 3 to independently control its own received power, voltage or current. A single inverter 6 in the inductive power transmitter 2 may thereby power multiple inductive power receivers 3, each of which may have a different position, orientation, output voltage, loading, sleeve and/or design from the others. Use of secondary side regulation can decrease the bulk and component count of inductive power transmitter 2 and increase the efficiency of the IPT system 1 . Therefore, a key advantage to using secondary side regulation is that it allows multiple inductive power receivers 3 to share a single inductive power transmitter 2 having a single inverter 6. If the output power of the inductive power transmitter 2 is fixed or constrained, the addition of secondary side regulation means that it is possible for the inductive power receivers 3 to regulate own their own received power.

In cases where multiple inductive power receivers 3 are powered by a single inverter 6, primary side regulation is still useful and can help to improve the efficiency of the IPT system 1 . This is because each inductive power receiver 3 will request only the power it needs. A possible way to resolve the conflict which occurs when different inductive power receivers 3 request different amounts of power from a single inductive power transmitter 2 having a single inverter 6 is for the inductive power transmitter 2 to provide the only the highest of the requested output power levels, and providing this to all of the inductive power receivers 3. However, even with multiple inductive power receivers 3 being powered by a single inverter 6, the highest power requirement of one of these inductive power receivers 3 may be lower than the maximum possible power output of the inverter 6. For example, if both a 1W and a 2W inductive power receiver 3 are placed on a charging mat inductive power transmitter 2 which is capable of powering two 10W inductive power receivers 3, the inductive power transmitter 2 may set its output power to meet the needs of the 2W inductive power receiver 3 and thereby also providing enough power for the 1 W inductive power receiver 3 while still running at a better efficiency than if the inductive power transmitter 2 were outputting at its maximum rated output power. Therefore, the efficiency of the IPT system 1 is be improved through having primary side control and secondary side control in combination.

The use of linear regulation to perform secondary side regulation in a receiver can be efficient so long as the voltage drop on linear regulator 202 is not too great. However, in some IPT systems, including some charging mats, it may be desirable that individual inductive power receivers 3 are able to regulate the received voltage from a high level down to a significantly lower level or step-up their voltage, or change the received power in some other way. For example, inductive power receivers 3 with significantly different power requirements may be present on a single charging mat inductive power transmitter 2 that has only a single inverter 6. Using linear regulation in these cases may not be efficient or even possible. The linear regulator 202 may be either supplemented with or replaced by a switch mode regulator. An example of this is shown in Figure 4.

Figure 4 shows an example embodiment of an inductive power receiver 3 with a switch mode regulator 401 for power flow control. Figure 4 builds on the circuit described in Figure 2. A linear regulator 202 and/or a load disconnect switch may also be used in combination with the switch mode regulator 401 , or may be omitted. Many different regulator types are possible for switch mode regulator 401 , including but not limited to buck, boost, buck-boost, "coupled coil", and a regulator of the types described in US patent application number 62/109,552 filed 29 January 2015, the contents of which are incorporated herein by reference. For example, regulators that create an AC voltage in series with the receiving coil or coils 9 so as to regulate the AC power from the receiving coil or coils 9 may be possible for switch mode regulator 401 . Some switch mode regulators 401 vary their duty cycle in order to change the relationship between the output voltage or current and the input voltage or current and to thereby regulate their output voltage or current. Other regulator types vary their operating frequency, phase or some other parameter in order to regulate their output voltage or current. Any of these may be possible for switch mode regulator 401 .

A coupled coil regulator uses a tertiary coil coupled to receiving coil or coils 9. This tertiary coil may then be shorted, opened, or the impedance across its terminals otherwise modified in order to control the power, voltage and/or current developed by receiving coil or coils 9.

The position of the switch mode regulator 401 within the inductive power receiver 3 will depend on the type of switch mode regulator used, and the position shown in Figure 4 is only an example of a possible placement location. For example, the regulator is on the AC side of rectifier 201 and in series with the coil or coils 9 if a regulator of the type in US patent application number 62/109,552 is used, and magnetically coupled with the coil or coils 9 if a coupled coil regulator is used. The position of the switch mode regulator 401 shown in Figure 4 could correspond to a buck regulator, a boost regulator, or a buck-boost regulator for example. Using the circuit shown in Figure 4, it is possible for each inductive power receiver 3 to efficiently regulate its own output voltage or current, independently of the magnitude of the magnetic field received by the power receiving coil or coils 9. Because efficient secondary side regulation is now available to the IPT system 1 , primary side regulation is no longer always necessary to regulate the output voltage at the load 1 1 . However, using primary side and secondary side regulation in combination may further help to improve the efficiency of the IPT system 1 . Additionally, primary side regulation may be necessary in order to meet wireless power interoperability standards.

With any of the inductive power receivers 3 described herein, the linear regulator 202 can be made to act like a closed switch if the target output voltage of the linear regulator 202 is made to be higher than its input voltage, in which case the linear regulator 202 may just pass its input voltage to its output terminal with minimal voltage drop. This is useful for periods where regulation is already being done by the switch mode regulator 401 . In a preferred embodiment, the output voltage of the switch mode regulator 401 is designed to be slightly lower than the output voltage of the linear regulator 202 so that the linear regulator 202 will act like a closed switch when switch mode regulator 401 is regulating, and as a normal linear regulator when switch mode regulator 401 is not regulating.

By using a combination of the switch mode regulator 401 and the linear regulator 202 in the power receiver 3, secondary side regulation, high average efficiency, rapid regulator response time, tight regulation accuracy and a reliable backscatter communications channel may be achieved within the same IPT system 1 . This makes the combination of linear regulator 202 and switch mode regulator 401 particularly beneficial. Figure 5 is a simplified circuit diagram which shows an inductive power receiver 3 using a buck regulator as switch mode regulator 501 . Receiving coil or coils 9 comprise receiving coil 501 and tuning capacitor 502. Rectifier 201 comprises four of rectifier diodes 503, connected in a full bridge configuration. Intermediate DC capacitor 504 smoothens voltage from rectifier 201 so that switch mode regulator 401 has an approximately DC input voltage. The switch mode regulator 401 is connected in a buck configuration and comprises MOSFET 505, which has a duty cycle D and may be a PMOS FET, freewheeling diode 506, DC inductor 507 and intermediate DC capacitor 508. Linear regulator 202 comprises MOSFET 509, which may be a PMOS FET and which may also be used as a load disconnect switch and output DC capacitor 510. The AC-side modulator 203, the DC-side modulator 204 and the current sensing circuit 205 are not shown in this figure.

A first method, which an inductive power receiver 3 can use to achieve a desired receiver voltage regulation headroom, outlined in Figure 6, can be applied to the inductive power receivers 3 shown in Figure 2 and Figure 5; that is, a linear regulator 202 or a buck regulator for secondary side regulation. In either case and in general, regulator should have both a DC input voltage and a DC output voltage and the input voltage should not be regulated by the regulator itself.

The first method follows these steps:

Initially, in step 601 , a desired value for Vioad 207 and a look-up table (which can be supplemented or replaced by an equation) for determining Vrect,optimai from the current drawn by the load 1 1 are known. The present values of Vrect 206, Vrect,optimai, the current drawn by the load 1 1 and the current voltage regulation headroom may be unknown. In step 602, the controller 12 measures the value of the current drawn by the load 1 1 and the voltage of Vrect 206. These values are stored in memory.

In step 603, the controller 12 uses the stored value of the current drawn by the load 1 1 and the look-up table to find what the value of Vrect,optimai is. This value is stored in memory.

In step 604, the controller checks to see if the stored value of Vrect 206 is below the stored value of Vrect ,optimal.

If the result is "YES" then in step 605, the controller sends a message to the inductive power transmitter 2 to increase the amount of available power. The message may include information about how much of an increase is needed. If the increase is sufficiently small, a message may not be necessary. Return to step 602.

If the result is "NO" then in step 606, the controller checks to see if the stored value of Vrect 206 is above the stored value of Vrect,optimai.

If the result is "YES" then in step 607 the controller sends a message to the inductive power transmitter 2 to decrease the amount of available power. The message may include information about how much of an decrease is needed. If the decrease is sufficiently small, a message may not be necessary. Return to step 602.

If the result is "NO" then Vrect 206 must be equal to Vrect.optimai. Return to step 602.

Figure 7 is a simplified circuit diagram which shows an embodiment of the invention using a boost regulator as switch mode regulator 401 . The switch mode regulator 401 is connected in a boost configuration and comprises MOSFET 701 , which has a duty cycle D and may be an NMOS FET, freewheeling diode 506, DC inductor 507 and intermediate DC capacitor 508. The AC-side modulator 203, the DC-side modulator 204 and the current sensing circuit 205 are not shown in this figure but may be required in a practical implementation. Rather than an intermediate DC capacitor 504 to ensure a DC voltage at the output of the rectifier 201 , this circuit uses the DC inductor 507 to ensure approximately DC current flow through the rectifier 201 and a DC value for Vrect 206 is not readily available. Vrect 206 may have a large AC voltage component. The voltage across the intermediate DC capacitor 508 also cannot be used to estimate a meaningful value for Vrect 206 because it is already regulated by the switch mode regulator 401 . Therefore, the first method for setting a desired receiver voltage regulation headroom, shown in Figure 6, cannot be directly applied to the boost regulator type of inductive power receiver 3.

Figure 8 is a simplified circuit diagram which shows an embodiment of the invention using a coupled coil regulator as switch mode regulator 401 . In this embodiment, the position of the switch mode regulator 401 is different from that shown in Figure 4. The coupled coil switch mode regulator 401 has a tertiary coil 801 which is well coupled to receiving coil 501 . By selectively allowing and blocking current flow through the tertiary coil 801 using MOSFET 802 and MOSFET 803, the amount of magnetic flux which can enter receiving coil 501 can be controlled, thereby controlling the current flow through the rectifier 201 and the voltage across intermediate DC capacitor 504. In one embodiment, the tertiary coil 801 and the receiving coil 501 may both have an inductance of 10.5μΗ and a coupling coefficient between them of 0.9 to 0.95. Both the tertiary coil 801 and the receiving coil 501 may have a coupling coefficient to the transmitting coil or coils 7 of 0.58. The AC-side modulator 203, the DC-side modulator 204 and the current sensing circuit 205 are not shown in this figure.

MOSFET 802 and MOSFET 803 may be driven with a duty cycle D and they may also be driven with a phase angle φ in relation to the zero crossing of the current in the tertiary coil 801 or with a phase angle φ in relation to some other current or voltage. The duty cycle D and the phase angle φ of the signals driving MOSFET 802 and MOSFET 803 may independently or in combination determine the output voltage and/or current that is delivered to the load 1 1 . The control effort of the switch mode regulator 401 can be estimated from a duty cycle D and/or a phase angle φ. The particular way that MOSFET 802 and MOSFET 803 are driven on a cycle-by-cycle basis does not limit the application of the present invention to the coupled coil inductive power receiver 3 circuit shown in Figure 8.

In a first possible driving scheme for use with the coupled coil regulator that is shown in Figure 8 a pair of square waves drive MOSFET 802 and MOSFET 803. The negative edge of each of these square wave are phase synchronized with the positive and negative zero crossings of the current in the tertiary coil 801 . In this way, the phases φ of the driving signals are fixed at 0° and 180° under this driving scheme and so the duty cycle of the driving signal is used to regulate output voltage to the load 1 1 . Lowering the duty cycle D will result in a greater output voltage being delivered to the load 1 1 . The output voltage of the coupled coil regulator is at a maximum when the duty cycle D is at 0% and is at a minimum when the duty cycle D is at 100%.

In a second possible driving scheme for use with the coupled coil regulator that is shown in Figure 8, MOSFET 802 and MOSFET 803 are driven by a pair of 50% duty cycle square waves that are 180° apart from each other. The rising edges of these square waves have phase delays φ with respect to the positive and negative zero crossings respectively of the current in tertiary coil 801 . Because the duty cycle D of both of these square waves is fixed at 50% under this driving scheme, the phase delay φ is used to regulate the output voltage to the load 1 1 . Within the operating range of phase delay φ, increasing the phase delay φ will increase Vioad 207. The output voltage of the coupled coil regulator is at a maximum when the phase angle φ is at 180° and is at a minimum when the phase angle φ is at O°.

For the coupled coil inductive power receiver 3 shown in Figure 8, Vrect 206 is controlled by the switch mode regulator 401 . Therefore, the value of Vrect 206 cannot be used to find how far the input voltage to the switch mode regulator 401 can fall before the voltage Vioad 207 falls out of regulation. Therefore, the first method for setting a desired receiver voltage or current regulation headroom, shown in Figure 6, cannot be directly applied to the coupled coil regulator type of inductive power receiver 3.

Inductive power receivers 3 with switch mode regulators 401 that regulate on the AC side of the rectifier 201 , or those which do not have a DC voltage at Vrect 206 or a DC current through the rectifier 201 cannot use the first method for setting the desired receiver voltage or current regulation headroom. However, it is still important that the output voltage or current which appears across the load 1 1 remains constant in the case of load steps, load dumps or other system parameter changes which are too fast for the primary side control loop to counteract. It is also important for efficiency reasons that the voltage received across the receiving coil or coils 9 not be too high. Therefore a second method is required in order to request the optimal amount of power from the inductive power transmitter 2 in the case of certain types of inductive power receivers 3. The second method uses different parameters on the inductive power receiver 3 to infer whether the inductive power transmitter 2 is transmitting the right amount of power. For example, depending on the switch mode regulator 401 type one or more of a phase angle, a duty cycle, a frequency and a time delay may be used to estimate whether more or less power is required from the inductive power transmitter 2. A second method, which an inductive power receiver 3 can use for setting the desired output power from the inductive power transmitter 2, is outlined in Figure 9 and can be applied to the inductive power receivers 3 shown in Figure 5, Figure 7 and Figure 8; that is, a buck regulator, a boost regulator and a coupled coil regulator. It can also be applied to a regulator as described in US patent application number 62/109,552. In general, this method can be applied to switch mode regulator 401 circuits in inductive power receivers 3 wherein at least one of a duty cycle D, a phase angle φ, a time delay t and a frequency f correspond in some way to how close the switch mode regulator 401 is to letting its output voltage fall out of regulation. Other circuit parameters can also be used where appropriate, depending on the type of switch mode regulator 401 that is used. These parameters can be described as regulator control signals and are generated by the controller 12.

As an example of a circuit to which the second method can be applied, in a given regulator circuit a lower frequency f of switch mode regulator 401 operation may correspond to a lower output voltage and a higher frequency f might correspond to a higher output voltage (normally the lower the freq the higher the voltage, the higher the freq, the lower the voltage. We tune receivers at the low end of the freq range eg: tuned at 100kHz for a transmitter within the 1 10-205kHz frequency range compliance with Wireless Power Consortium Qi standard 1 .2.0). If the maximum operating frequency fmax of the switch mode regulator 401 is 1 MHz, then the closer that the operating frequency f gets to fmax, the closer the switch mode regulator is to falling out of regulation. In another example, for a coupled coil switch mode regulator 401 knowing the duty cycle D and the phase angle φ of the switch mode regulator 401 may be sufficient to estimate how close it is to allowing its output voltage to fall out of regulation. Depending on the type of inductive power receiver 3 regulator circuit used, many different regulator control signals may be used together or individually to help estimate how close the switch mode regulator 401 is to falling out of regulation. For simplicity, this method assumes that knowing the duty cycle and the phase of the regulator circuit is sufficient, but can be adapted to work with the other regulator control signals.

The second method assumes that in principle a function exists for mapping a pair of duty cycle D and phase angle φ values which are produced by a controller 12 for switch mode regulator 401 into an ordered list or a continuum representing how close the switch mode regulator 401 is to falling out of regulation for a given load 1 1 current. In other words, it is assumed, for a given load current and a first pair of duty cycle and phase values and a second pair of duty cycle and phase values that it will be known which of the pairs of values corresponds to the switch mode regulator being closer to falling out of regulation.

The second method follows these steps:

Initially, in step 901 , a desired value for Vioad 207, a look-up table and/or an equation for determining the optimal pair of phase angle φ and duty cycle D values from the current drawn by the load 1 1 is known, and that a function for mapping a pair of phase angle φ and duty cycle D values to an ordered list of receiver regulation headrooms is known. Present values for cpoptimai and Doptimai and the current drawn by the load 1 1 may be unknown.

In step 902, the controller 12 measures the value of the current drawn by the load 1 1 , the value of the duty cycle D and of the value of phase angle φ of the signal(s) driving the switch(es) within switch mode regulator 401 . These measured values are stored in memory.

In step 903, the controller 12 uses the stored value of the current drawn by the load 1 1 and the look-up table or equation to find what the optimal values phase and duty cycle are, i.e., cpoptimai and Doptimai. These values are stored in memory.

In step 904, the controller 1 2 compares the present values of phase angle φ and duty cycle D with the optimal values cpoptimai and Doptimai to see if the switch mode regulator 401 is closer to falling out of regulation than is optimal.

If the result is "YES" then in step 905, the controller sends a message to the inductive power transmitter 2 to increase the amount of available power. The message may include information about how much of an increase is needed. Return to step 902.

If the result is "NO" then in step 906, the controller 1 2 compares the present values of phase angle φ and duty cycle D with the optimal values cpoptimai and Doptimai to see if the switch mode regulator 401 is further from falling out of regulation than is optimal.

If the result is "YES" then in step 907 the controller sends a message to the inductive power transmitter 2 to decrease the amount of available power. The message may include information about how much of a decrease is needed. Return to step 902.

If the result is "NO" then the available power must be at or near the optimal level for the inductive power receiver 3. Return to step 902.

Both of the first method and the second method for setting the desired transmitter output power are applicable to the inductive power receiver 3 which uses a buck regulator as shown in Figure 5. There may be complexity or performance advantages associated with using one method over the other. For example, fewer analogue voltage sensors are required with the second method as compared to the first method. This may mean that the second method is less complex to implement or that it remains functional under greater levels of electromagnetic interference. The regulator control signal(s) such as the duty cycle, phase angle, time delay and frequency of the switch mode regulator 401 can be filtered prior to use by the controller 12, or can be filtered using the controller 12 itself. For example, a low-pass filter may be used to establish an average over a number of cycles of the phase angle of the switch mode regulator 401 .

In the case of both the first method and the second method, even if a message from the inductive power receiver 3 to the inductive power transmitter 2 is unnecessary for power flow control purposes, a message may be sent anyway if the time passed since the previous message was sent exceeds a certain value, for example one second. This may be require for compliance with certain inductive power transfer standards.

For both of the methods for setting the desired output power from a inductive power transmitter 2, the look-up table or equation for finding the optimal regulator control signal(s) (such as cpoptimai and Doptimai) can be updated during use of the inductive power receiver 3. For example, if the controller 12 detects the output voltage Vioad 207 at the load 1 1 is occasionally falling out of regulation during use, the controller 12 can update the desired regulator control signal(s) in its look-up table or equation to request more power from the inductive power transmitter 2 and make future drop-outs less likely. The same learning technique can also be applied to excess temperature rises, where the inductive power receiver 3 can learn to lower the power it requests from the inductive power transmitter 2 in order to improve the efficiency of the IPT system 1 . Learned behaviour can persist for an arbitrary period, such as for the life of the inductive power receiver 3 device or for just a single charging session or for even more briefly. In particular, learning that is used for a particular session only and then discarded may be useful because it relates to a particular inductive power transmitter 2 and a particular charging location and coupling coefficient. Instead of or in addition to a look-up table, an equation may be used to help determine the optimum regulator control signals from a given load 1 1 current. In the same way, an equation may also be used to help determine which of two sets of regulator control signals corresponds to the switch mode regulator 401 being closer to falling out of regulation.

The particular look-up table that is used for finding the optimal regulator control signal(s) can be dependent on the ambient or internal temperature of a device within IPT system 1 , on the position or coupling coefficient between devices within IPT system 1 , on the type or serial number of inductive power transmitter 2 and on the type of inductive power receiver 3. When using a look-up table interpolations may be made between adjacent entries if the exact value is not available.

The magnitude of the "more power" or "less power" request in order to achieve the optimal output power from the inductive power transmitter 2 may be pre-programmed into or learned by the controller 12. In this way, the right size of "more power" or "less power" request can be sent by the inductive power receiver 3 and the operating point can be reached more quickly. For example an "error percentage" for transmission to the inductive power transmitter 2 can be calculated based on the difference between the present and desired values of the regulator control signal(s). "More power" may be expressed in terms of a positive error percentage and "less power" may be expressed in terms of a negative error percentage. The magnitude of the error percentage is then dependent on how big the error is.

The inductive power receiver 3 may have a maximum rated current which can be drawn by load 1 1 . During development and manufacture of the inductive power receiver 3 a look-up table can be generated for a given IPT system 1 configuration by using an electronic load in place of the load 1 1 , with the electronic load configured to step from a particular initial load current to the maximum rated load current. The values which for time delay toptimai, phase angle cpoptimai, frequency foptimai and duty cycle Doptimai which provide sufficient immunity to Vioad 207 sagging can be determined for the particular initial load 1 1 current. For example for a given load 1 1 current, the receiver voltage regulation headroom can be lowered until the Vioad 207 starts to fall out of regulation during a load step to rated load 1 1 current. Alternately, the receiver voltage regulation headroom can be increased until heating becomes a problem or until the power loss of the inductive power receiver 3 becomes unacceptably high. By repeating this process across a range of initial load 1 1 currents, the look-up table can be populated.

The current flowing through one location within the inductive power receiver 3 may be computed by the controller 12 given a combination of the current flowing through another place within the circuit in combination with other variables such as duty cycle D, Vrect 206, Vioad 207 or other voltages and currents. It is not always necessary to have a current sensor at a location in order to determine the current flowing through that location. Instead of the current drawn by the load 1 1 , the receiver voltage regulation headroom may be defined as a function of the output current of the rectifier 201 or by some other variable. It may be possible to derive or estimate the value of the load current 1 1 or the output current of the rectifier 201 from other circuit parameters.

In some implementations of inductive power receiver 3, a circuit can perform both the function of regulator and the function of modulator for the backscatter communications channel. In this way, a regulator such as the switch mode regulator 401 or the linear regulator 202 may be used to modulate the backscatter communication channel in place of the AC-side modulator 203 and/or the DC-side modulator 204. Therefore, the regulator circuitry may partially or completely be shared with modulator circuitry. For example in addition to its regulation functions, the switch mode regulator 401 may be used as a modulator, varying its duty cycle to modulate the power drawn from receiving coil or coils 9 in order to send a message on the backscatter communications channel. In a preferred embodiment, a regulator sending messages on the backscatter communications channel is digitally controlled.

It is understood that either or both of the inductive power transmitter 2 and the inductive power receiver 3 may be configured as an IPT transceiver, such that power may be sent and also received by a single IPT transceiver.

While the present invention has been illustrated by the description of the embodiments thereof, and while the embodiments have been described in detail, it is not the intention of the Applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departure from the spirit or scope of the Applicant's general inventive concept.

Claims

Claims:

1 . An inductive power receiver comprising: a power receiving coil; a switch mode regulator configured to regulate the power from the power receiving coil to a load depending on a control signal; and a controller configured to determine the control signal and to communicate a power request to an inductive power transmitter based on the control signal.

2. The receiver in claim 1 wherein the control signal is a control effort parameter of the switch mode regulator.

3. The receiver in claim 1 wherein the control signal is a head room parameter of the switch mode regulator.

4. The receiver in claim 1 wherein the control signal relates to a time delay.

5. The receiver in claim 1 wherein the control signal relates to a phase angle.

6. The receiver in claim 1 wherein the control signal relates to a duty cycle.

7. The receiver in claim 1 wherein the control signal relates to a frequency.

8. The receiver of any of claims 1 to 7 wherein the switch mode regulator is a coupled coil regulator.

9. The receiver of any of claims 1 to 7 wherein the switch mode regulator creates an AC voltage in series with the receiving coil so as to regulate the AC power from the receiving coil.

10. The receiver of any of claims 1 to 7 wherein the switch mode regulator is a boost regulator.

1 1 . The receiver of any of claims 1 to 7 wherein the switch mode regulator is a buck regulator.

12. The receiver of any of claims 1 to 7 wherein the switch mode regulator is a buck-boost regulator.

13. An inductive power transfer system comprising: an inductive power transmitter configured to generate a magnetic field; and an inductive power receiver including a power receiving coil coupled to the magnetic field and a switch mode regulator configured to regulate the power from the power receiving coil to a load depending on a control signal; wherein the inductive power transmitter is configured to change the magnetic field in response to a change in the control signal.

14. The system in claim 13 wherein the control signal relates to a time delay.

15. The system in claim 13 wherein the control signal relates to a phase angle.

16. The system in claim 13 wherein the control signal relates to a duty cycle.

17. The system in claim 13 wherein the control signal relates to a frequency.