WO2017105256A1 - Inductive power receiver - Google Patents

Abstract

An inductive power receiver comprising an LCL pickup; a power rectifier and regulator including a plurality of control devices configured to connect to the pickup; and a current transformer configured to provide a current signal representative of the incoming current from the pickup to the rectifier and regulator, the current signal having a predetermined phase advance with respect to the incoming current, wherein drive signals for the control devices are determined based on the current signal.

Description

INDUCTIVE POWER RECEIVER

FIELD This invention relates generally to a converter, particularly though not solely, to a converter for 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 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 on a 'charging mat' or for power transfer in an industrial or commercial environment, such as in wind turbines).

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. SUMMARY

The present invention may provide an improved inductive power receiver or which provides the public with a useful choice.

According to an example embodiment there is provided an inductive power receiver comprising: an LCL pickup; a power rectifier and regulator including a plurality of control devices configured to connect to the pickup; and a current transformer configured to provide a current signal representative of the incoming current from the pickup to the rectifier and regulator, the current signal having a predetermined phase advance with respect to the incoming current, wherein drive signals for the control devices are determined based on the current 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 documents in this specification does not constitute an admission that those documents are prior art or form 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, in which:

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

Figure 2 is a block diagram of an example receiver;

Figure 3 is a circuit diagram of an example receiver;

Figure 4 is a circuit diagram of an example current transformer;

Figure 5 is a graph of the currents associated with the current transformer;

Figures 6 and 7 are graphs of the voltage signal from the current transformer;

Figure 8 is a circuit diagram of an example PID circuit;

Figure 9 is a graph of the ramp signals;

Figure 10 is a circuit diagram of an example ramp generator;

Figure 11 is a graph of the error signals;

Figure 12 is a graph of the drive signals; and

Figure 13 is a graph of the output voltage being regulated simultaneously with ZCS with a variable transmitter IPT frequency. DETAILED DESCRIPTION

Various different topologies for inductive power transfer (IPT) are used depending on the application. For example the transmitter and/or the receiver may be resonant or non-resonant. Resonant power transfer has the advantage that the range of power transfer can be increased, compared to non resonant transfer.

Resonant topologies may include series resonant circuits or parallel resonant circuits. Another topology is LCL. Typically a series resonant circuit can be modelled as a voltage source and a parallel resonant circuit can be modelled as a current source. Each option has pros and cons. LCL on the other hand has the desirable input/output characteristics of a parallel tuned topology without the downsides. This is useful as it means that when the receiver is not present, the transmitter does not have to switch high current through the bridge. LCL circuits may allow power transfer at unity power factor compared to a parallel resonant circuit which usually has a significant reactive power requirement.

According to an example embodiment, an LCL topology may be implemented by an LCL based power transmitting coil and an LCL based power receiving coil. When applied to IPT, in particular to a rotating IPT coupling for industrial applications, it may be desirable to operate the transmitter and receiver in a tuned manner. This may reduce the reactive current flowing in the circuit, operating at close to unity power factor, which may increase efficiency, and allow for lower component rating. It may also enable zero voltage switching in relation to the inverter and/or rectifier, which may further increase efficiency and/or reduce component stress. In order to maintain close to unity power factor and/or minimise reactive current flow, it may be necessary to make adjustments to keep the two LCL circuits tuned, despite changes in the circuit. One example of a change is a change in coupling. This may happen due to a change in distance between the transmitter and receiver Retuning can be done in a numbers of ways; the particular methodology can be chosen according to the requirements of the application. One example for retuning is to modify the IPT frequency of the transmitter to the new "resonant" frequency.

An 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. Additional coils may be provided, for example in an LCL configuration.

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.

The inductive power receiver 3 includes a power pick up stage 9 connected to power conditioning circuitry 10 that in turn supplies power to a load 11. The power pick up stage 9 includes inductive power receiving coil or coils. 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. The receiving coil or coils may be connected to capacitors (not shown) either in parallel or series to create a resonant circuit. Additional coils may be provided, for example in an LCL configuration.

In some inductive power receivers, the receiver may include a controller 12 which may control tuning of the receiving coil or coils, operation of the power conditioning circuitry 10 and/or communications.

The term "coil" may include an electrically conductive structure where an electrical current generates a magnetic field. For example inductive "coils" may be electrically conductive wire in three dimensional shapes or two dimensional planar shapes, electrically conductive material fabricated using printed circuit board (PCB) techniques into three dimensional shapes over plural PCB 'layers', and other coil-like shapes. Other configurations may be used depending on the application. The use of the term "coil", in either singular or plural, is not meant to be restrictive in this sense.

Figure 2 shows a receiver 3 according to an example embodiment, with the power rectifier 202 combined with the power regulation circuit 204 as an integrated converter to provide regulation using a synchronous rectifier. This may reduce the component count which may allow for a smaller footprint, reduce the cost of the device, improve efficiency and/or reduce heat generation.

An example receiver 300 circuit is shown in Figure 3. The LCL resonant pickup is formed from LI, C2 and L2. A synchronous rectifier is formed by Ml, M2, M2 and M4, which additionally provides regulation.

The control strategy and switching algorithm are based on measuring the input current into the rectifier for ZCS and measuring the output voltage to regulate it to 24V or another desired voltage. The control methodology is explained in more detail below, in relation to a) the current transformer which is used to measure the current and detect zero crossings, b) the PID controller which provides a control signal relative to the load voltage, and c) combining the signals to generate drive signals for the synchronous rectifier. The control methodology may be implemented in discrete circuitry, completely in software inside a processor, or a combination of both.

Because the transmitter frequency may change dynamically, (according to coupling etc) it may be necessary to implement the switching control mentioned above on a cycle by cycle basis. It may also be desirable to have cycle by cycle control for responding to fast load changes (which may happen faster than frequency change). This means that each cycle with each zero crossing in the incoming current (Ι¾), one of the MOSFETs is switched on, then a small time later (dead time eg: 20-50ns) and the opposite MOSFET is switched on. This dead time avoids shorting the output.

Due to circuit delays, it will take some time between when the zero crossing of the current is recorded and when the switch off of the first MOSFET can occur. This delay in practical terms means that the switching instant is not precisely at the instant of the zero crossing.

It order to provide more precise switching, it may be desirable to advance the phase of lf¾, to account for circuit delay. Practically this means that if the circuit delay between when the zero crossing is detected and when the MOSFETs is switch is 300ns, rather than switching 300ns after the zero-crossing, the phase of li can be advanced by 300ns so that the MOSFET switches exactly on the zero crossing on the current. Switching directly on the zero crossing of the current may provide better performance as it allows for better control of low loads, slightly higher maximum power transfer and reduced switching loss in the synchronous rectifier. It may also mean cheaper components with more delay can be utilised.

Figure 4 shows an example of a current transformer (CT) circuit 400 which may be used to achieve the desired phase advance. The LCL pickup is represented by a current source for simulation purposes. The CT primary 402 is connected in parallel with an alternative current path 404, which has a lower impedance magnitude Z but a higher impedance angle Θ than the CT primary current path 402. Compared with the impedance of the primary coil, this will produce a leading current 502 of much lower magnitude in the primary, compared to the main incoming current 504 as shown in Figure 5. The current 506 through the alternative current path 404 is delayed and slightly lower in magnitude. The desired impedance of the alternative current path 404 may be provided by a short section of PCB track, effectively shorting the CT primary or other circuit components depending on the requirements of the application.

The desired phase advance can then be fine tuned by adjusting capacitors (P_Delay_Cap and N_Delay_Cap). This gives a mechanism to account for different circuit delays and switch exactly on the zero crossing of a waveform. For example as shown in Figure 6 a 5nF Capacitor gives too great a delay so the input voltage 602 to the comparator is delayed compared to the current 504, and in Figure 7 with a 2nF Capacitor the incoming voltage 602 is advanced compared to the current 504.

Figure 8 shows an example PID circuit for providing a control signal based on the output voltage. The output voltage is passed through a circuit which provides a control signal that decreases when the output voltage decreases. The input filter 802 and OpAmp 804 form a hardware PID control so the speed, precision, damping etc of the response to a change in output voltage can be adjusted according to the requirements of the application.

The advanced voltage 900 from the CT secondary is then converted to two complementary square wave 902 voltages (Comp_P Comp_N) to detect the zero crossing by comparing the voltage at either end of the CT secondary as shown in Figure 9. Two ramp 904 signals are started by the failing edges in Comp_P Comp_N respectively and stopped with the rising edges. The ramp generator circuits 1002 are shown in Figure 10. The gradient of the ramp is chosen for an approximate expected transmitter IPT frequency range. For example the IPT frequency may normally be llOKHz to 215kHZ for charging of consumer electronics, but other frequencies may be used depending on the application.

The ramps 904 are then compared to the PID control signal 1102 to provide an error signal 1104 as shown in Figure 11. The two error signals V_errorp Vei N are combined with the Comp_P Comp_N respectively to provide the drive signal 1202 for each of the lower FETs as shown in Figure 12. As a lower PID control signal will result in a higher duty cycle in the error signal, this corresponds to a higher regulation effort i.e. the receiver input is shorted for a higher portion of the cycle. The net result is show in Figure 13, with the error signal 1104 duty cycle increasing as the output voltage 1302 increases and stabilises at the desired level, while simultaneously achieving ZCS with a variable transmitter IPT frequency.

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: an LCL pickup; a power rectifier and regulator including a plurality of control devices configured to connect to the pickup; and a current transformer configured to provide a current signal representative of the incoming current from the pickup to the rectifier and regulator, the current signal having a predetermined phase advance with respect to the incoming current, wherein drive signals for the control devices are determined based on the current signal.

2. The inductive power receiver in claim 1 wherein the drive signals for the control devices are also based on a synchronous rectification strategy.

3. The inductive power receiver in claim 2 wherein a primary winding of the current transformer is connected between the pickup and the rectifier and regulator, the receiver further comprising an alternative current path in parallel with the primary winding, the current signal being determined from a secondary winding of the current transformer.

4. The inductive power receiver in claim 3 wherein the receiver is configured to determine zero crossings in the current signal and the drive signals for the control devices are based on the zero crossings.

5. The inductive power receiver in claim 3 wherein the receiver is configured to determine an error signal based on the output voltage, a selected predetermined output voltage, and the drive signals for the control devices are based on the error signal.

6. The inductive power receiver in claim 1 wherein the synchronous rectifier is a bridge rectifier having four control devices, and two of the control devices are driven according to the error signal, and the other two control devices are driven according to the synchronous rectification strategy.

7. The inductive power receiver in claim 3 wherein the alternative current path is substantially lower in resistance than the primary winding.

8. The inductive power receiver in claim 7 wherein the alternative current path is a PCB track.