GB2515597A - A charging apparatus - Google Patents

Abstract

Apparatus (20, fig. 1) for wirelessly charging a rechargeable storage medium (32) of an electrically powered device (30), the apparatus (20) comprising: a charging inductance 24 for wirelessly supplying energy to the device (30) containing the storage medium to be charged; a tuning circuit for setting a resonant frequency of the charging inductance; and a driver circuit 40 for driving the charging inductance, wherein the driver circuit (40) comprises a neutral point clamp (NPC) circuit. The apparatus may also comprise a modulator which generates amplitude modulated signals. The modulator could also impart a binary phase shift keying (BPSK) modulation to the signal that drives the charging inductance.

Description

A CHARGING APPARATUS

Technical Field

The present application relates to a charging apparatus. In particular, the present application relates to apparatus for charging a rechargeable storage medium of an electrically powered device such as a secure transport container.

Background to the Invention

It is commonplace for electronic products to be powered by one or more primary batteries. For example, secure containers for the secure transport and storage of valuable items such as bank notes are typically powered by primary batteries.

Primary batteries have a finite lifespan and must be replaced after a certain time period.

Tn many applications it is imperative that a constant level of power is provided at all times by the batteries to the product in which they are used. This necessitates replacement of the batteries before they reach the end of their useful working life.

Replacing batteries adds to the lifetime cost of an electronic product, and, where the batteries are replaced before the end of their life, gives rise to significant wastage.

In many products, the batteries are not accessible to the user, and so battery replacement involves either transporting the product to a specialist technician or transporting the technician to the product. In industries such as the cash in transit industry the cost of transporting multiple secure transport containers to a technician, or alternatively transporting technicians to multiple locations to service muhiple secure transport containers individually, is significant.

A partial solution to this problem is to use batteries with high power ratings, to reduce the frequency with which the batteries must be replaced. However, space and weight constraints, as well as dangerous goods regulations and cost, act as limiting factors to the maximum capacity of batteries that can be used.

Many products use in-built rechargeable storage media such as Lithium-Ion batteries.

Such products typically include a charging port or socket by means of winch the product can be connected to a mains electricity supply, usually via an adaptor, to charge the in-built rechargeable batteries. However, for products such as secure transport containers where structural integrity is of high importance, this option may not be viable, as the charging port or socket can present a point of weakness or failure of the product.

Other charging options have been proposed, including installing solar cells or other energy harvesting devices on electronic products. Typically, however, such devices are not able to harvest enough energy to provide sufficient power for reliable charging of the product's batteries.

Wireless charging systems have also been proposed. However, current wireless charging systems often do not represent an efficient or cost-effective way of charging batteries in electronic devices such as secure transport containers, Accordingly, a need exists for an improved means for charging batteries in electronic devices such as secure transport containers.

Summary of Invention

According to a first aspect of the present invention there is provided apparatus for charging a rechargeable storage medium of an electrically powered device, the apparatus comprising: a charging inductance for wirelessly supplying energy to the device containing the storage medium to be charged; a tuning circuit for setting a resonant frequency of the charging inductance; and a driver circuit for driving the charging inductance, wherein the driver circuit comprises a neutral point clamp (NPC) circuit, The charging apparatus provides an effective charging solution, particularly for devices such as secure transport containers where structural integrity considerations preclude the use of charging ports or sockets. The charging apparatus is able efficiently to charge storage media such as batteries of compatible devices without requiring a physical (wired) connection between the charging apparatus and the device containing the storage medium to be charged. Moreover, the particular driver circuit used in the charging apparatus gives rise to highly efficient energy transfer between the charging apparatus and the device containing the storage medium to be charged.

In addition, the charging apparatus provides the ability for efficient bidirectional communication of data between the charging apparatus and the electrically powered device (e.g. an intelligent secure transport container). This obviates the need for a dedicated physical communications link, thereby removing a weakness by maintaining the structural integrity of the device, as well as answering a need for a reliable communications system that can be tailored to specific needs.

The tuning circuit may comprise a series combination of a capacitance and the charging inductance.

The tuning circuit may be configured to set the resonant frequency of the charging inductance to between around 70 kI-Iz and around 130 kl-Iz.

The driver circuit may comprise: a first capacitance and a second capacitance connected in series; a switch sub-circuit comprising a plurality of switches connected in parallel with the first and second capacitances; a first diode and a second diode, the first and second diodes being connected to the first and second capacitances and the switch sub-circuit to provide current paths from the first and second capacitances; and a controfler, wherein the controller is operative to actuate the plurality of switches in a predetermined sequence such that the driver circuit is operative to convert a direct current input into an alternating current at an output of the switch sub-circuit, The switch sub-circuit may comprise four switches.

The switches may comprise N-channel or P-channel MOSFETs.

The apparatus may thrther comprise a modulator for imparting a binary phase shift keying (BPSK) modulation to a signal that drives the charging inductance, The modulator may be configured to reverse a timing sequence of the signal that drives the charging inductance, The apparatus may further comprise a demodulator for demodulating amplitude modulated (AM) signals.

The apparatus may further comprise a modulator for generating amplitude modulated (AM) signals.

According to a further aspect of the invention there is provided a secure transport container for transporting or storing valuable items, the secure transport container including apparatus according to the first aspect, According to a thither aspect of the invention there is provided a charging system comprising: an electrically powered device having a rechargeable storage medium; and an apparatus according to the first aspect, The electrically powered device may comprise a modulator for generating an amplitude modulated signal, The modulator may be configured to add a load to and to remove a load from output nodes of a rectifier of the electrically powered device to generate the amplitude modulated signal.

The electrically powered device may comprise a demodulator for demodulating binary phase shift keying (BPSK) signals.

Brief Description of the Drawings

Embodiments of the invention will now be described, strictly by way of example only, with reference to the accompanying drawings, of which: Figure I is a schematic representation of a charging system including a charging apparatus and a device containing a battery to be charged; Figure 2 is a schematic representation of a charging circuit of the charging apparatus of Figure 1; Figure 3 is a schematic representation of an equivalent circuit illustrating operation of the charging circuit shown in Figure 2 in a first phase of operation; Figure 4 is a schematic representation of an equivalent circuit illustrating operation of the charging circuit shown in Figure 2 in a second phase of operation; Figure 5 is a schematic representation of an equivalent circuit illustrating operation of the charging circuit shown in Figure 2 in a third phase of operation; Figure 6 is a schematic representation of an equivalent circuit illustrating operation of the charging circuit shown in Figure 2 in a fourth phase of operation; Figure 7 is a schematic representation of a charging circuit for use in a device containing a battery to be charged; Figure 8 is a further schematic representation of the charging circuit of Figure 7, including detail of a modulation system; Figure 9 is a schematic representation of waveforms used in a Binary Phase Shift Keying (BPSK) modulation scheme; Figure 10 is a schematic diagram illustrating an exemplary realisation of the circuit shown in Figure 2; and Figure 11 is a schematic representation of waveforms used in Amplitude Modulation (ANI

Description of the Embodiments

Referring first to Figure 1, a system for charging rechargeable storage media such as rechargeable batteries, supercapacitors or the like, is shown generally at 10, and includes a charging apparatus 20 and a device 30 containing a rechargeable storage medium 32 to be charged. For the sake of simplicity, the rechargeable storage medium 32 will be referred to as a rechargeable battery, but it is to be understood that the term "battery" as used herein is intended to encompass any rechargeable medium capable of storing electrical energy.

The charging apparatus includes a driver circuit 22 which drives a charging inductance 24, via a tuning circuit 26. The driver circuit 22 generates, from a constant direct current (DC) input, an alternating current (AC) output, which drives the charging inductance 24.

The alternating current supplied to the charging inductance 24 causes a time-varying magnetic field to be generated around the charging inductance 24.

The device 30 includes an inductance 34 which is connected to a charging circuit 36, the output of which is connected to terminals of the battery 32, such that the battery 32 can be charged by the charging circuit 36, When the device 30 is brought into proximity with die charging apparatus 20, an alternating electrical current is induced iii the inductance 34 of the device 30 by the time-varying magnetic field around the charging inductance 24 of the charging apparatus 20. This alternating current is converted by the charging circuit 36 into a direct current suitable for charging the battery.

The structure and operation of the charging apparatus will now be explained in more detail, with reference to Figures 2 -6 of the drawings.

Figure 2 is a schematic diagram showing an exemplary equivalent circuit for use in the charging apparatus 20 to drive the charging inductor 24. The circuit, shown generally at 40, is a neutral point clamp circuit, and includes first and second capacitors 42, 44, connected in series between positive and zero volt power supply rails 46, 48. In the exemplary circuit shown in Figure 2, the power supply rails 46, 48 are at 15 volts and 0 volls respectively, but it will be appreciated that any supply voltage that is appropriate to the application for which the circuit 40 is intended could equally be used. Similarly, although the capacitors 42, 44 are each shown in Figure 2 as having a value of 1 000iF, it will be appreciated that other capacitance values may be selected, depending upon the application for which the circuit 40 is intended. It is important, however, that the first and second capacitors 42, 44 are matched, i.e. that the capacitance value of the second capacitor 44 is equal to that of the first capacitor 42.

A node 50 connecting a lower plate of first capacitor 42 to an upper plate of second capacitor 42 acts as a neutral point in the circuit 40. The node 50 is connected to one terminal of the charging inductance 24, whilst the other terminal of the charging inductance 24 is connected to an output of' a tuning capacitance 60, which in turn is connected to an output of a storage inductance 62. It will be appreciated that the capacitance 60 and charging inductance 24 together form a series resonant circuit. The values of the tuning capacitance 60 and charging inductance 24 are selected to tune the resonant frequency of this series resonant circuit to a desired frequency. For example, the values of the capacitance 60 and charging inductance 24 may be selected such that the resonant frequency of tile series resonant circuit is beiween around 70 kHz and around kI-lz, depending upon the application for which the circuit 40 is intended. However, other resonant frequencies may be used if appropriate to the application for which the circuit 40 is intended. The resonant frequency of the series resonant circuit is likely to increase, for example, when a higher charging voltage or a greater distance is required between the charging apparatus 20 and the device 30 containing a rechargeable storage medium 32 to be charged. For instance, a vehicle charging system may typically require a charging voltage of around 48 volts, with a distance of tens of centimetres between the charging apparatus 20 and the rechargeahie storage medium (battery) of the vehide to be di charged, The formula V=L-shows that in order to achieve a larger voltage a higher di frequency is required, as well as a larger value of inductance.

An input of the storage inductance 62 is connected to an output of a switching sub-circuit 70. The switching sub-circuit 70 includes first, second, third and fourth switches 72, 74, 76, 78, connected in series between the positive and zero volt power supply rails 46, 48.

The switches 72, 74, 76, 78 may be, for example, N-or P-channel MOSFETS, or any other switch appropriate to the application for which the circuit 40 is intended. A node SO connecting the second and third switches 74, 76 constitutes the output of the switching sub-circuit 70.

The switching sub-circuit 70 also includes a first diode 82. The first diode 82 connects the node 50 to a node 84 which connects the first and second switches 72, 74. The first diode 82 is forward biased, that is to say its anode is connected to the node 50, whilst its cathode is connected to the node 84.

The switching sub-circuit 70 also includes a second diode 86. The second diode 86 connects the node 50 to a node 88 which connects the third and fourth switches 76, 78.

The second diode 86 is reverse biased, that is to say its anode is connected to the node 88, whilst its cathode is connected to the node 50.

When the circuit 40 is activated, the switches 72, 74, 76, 78 are all open. The first capacitor 42 charges up from the positive power supply rail 46. The switches 72, 74, 76, 78 of the switching sub-circuit are subsequently selectively actuated by a controller 88 in a predetermined four-phase sequence to generate at the output 80 of the switching sub-circuit 70 an alternating current to drive the charging inductance 24, as will now be described with reference to Figures 3 to 6 of the drawings.

In a first phase of operation of the circuit 40, the first and second switches 72, 74 are closed, whilst the third and fourth switches 76, 78 remain open. Figure 3 is a schematic diagram of an equivalent circuit for the circuit 40 in this first phase.

With the first and second switches 72, 74 closed and the third and fourth switches 76, 78 remaining open, the first and second capacitors 42, 44 charge from the positive supply rail 46. Current flows from the first capacitor 42 through the first and second switches 72, 74, storage inductance 62, tuning capacitance 64 and charging inductance 24, returning to its origin at the first capacitor 42. This current forms the rising edge of the positive half-cycle of a sine wave.

The second phase now commences, with the first switch 72 being opened, the second switch 74 remaining closed and the third and fourth switches 76, 78 remaining open. An equivalent circuit for this second phase is shown schematically in Figure 4, In this phase, current is able to "freewheel" around the circuit, from the first capacitor 42, through the first diode 82, storage inductance 62, tuning capacitance 60 and charging inductance 24, and back to the first capacitor 42. No additiona' energy is being supplied to the circuit 40 in this phase second phase, because the node 50 is a neutral point, and so as the freewheeling current experiences impedance as it flows around the circuit 40, it declines. Thus, the current flowing in the circuit 40 in this second phase forms the falling edge of the positive half-cycle of the sine wave.

The third phase of the sequence now commences, with the both the first and second switches 72, 74 being opened and the third and fourth switches 76, 78 being closed. An equivalent circuit for this third phase is shown schematically in Figure 5.

With the third and fourth switches 76, 78 closed and the first and second switches 72, 74 remaining open, the first and second capacitors 42, 44 charge from the positive supply rail 46. Current flows from the second capacitor 44 in the opposite direction to the direction of current flow in the first and second phases, i.e. the current flow is negative.

Current flows from the second capacitor 44 through the charging inductance 24, tuning capacitor 60, storage inductance 62, third and fourth switches 76, 78, returning to its origin at the second capacitor 44, This current forms the falling edge of the negative half-cycle of the sine wave.

The fourth and final phase now commences, with the fourth switch 78 being opened, the third switch 76 remaining closed and the First and second switches 72, 74 remaining open. An equivalent circuit for this fourth phase is shown schematically in Figure 6.

In this phase, current is able to "free-wheel" around the circuit, again in the opposite direction to the direction of current flow in the first and second phases, i.e. the current flow is negative. Thus, current flows from the second capacitor 44, through the charging inductance 24, tuning capacitance 60, storage inductance 62, third switch 76 and second diode 86, returning to the second capacitor 42. No additional energy is being supplied to the circuit 40 in this fourth phase, because the node 50 is a neutral point, and so as the freewheeling current experiences impedance as it flows around the circuit 40, it declines.

Thus, the current flowing in the circuit 40 in this fourth phase forms the falling edge of the negative half-cycle of the sine wave.

At the end of the fourth phase of the sequence, the sequence restarts, such that the output of node 80 of the switching sub-circuit 70, which drives the charging inductance 24 (via the storage inductance 62 and tuning capacitance 60) is a sine wave of a frequency determined by the switching frequency of the switches 72, 74, 76, 78.

The values of the tuning capacitance 60 and charging inductance 24 are s&ected such that the charging inductance 24 is resonant at the frequency of the alternating current output by the switching sub-circuit 70. This ensures that the amount of energy converted into the magnetic fleW generated by the charging inductance 24 as a result of the alternating current driving it can be maximised, which in turn maximises the amount of energy that can be supplied to the battery 32 to be charged.

Figure 7 is a schematic diagram of a charging circuit 36 for use in the device 30 to harvest the energy generated by the charging apparatus 20 to charge the battery 32, The charging circuit includes a tuning capacitor 102, a first terminal of which connected in series with a first terminal of the inductance 34 of the device 30. The tuning capacitor I 02 and the inductance 34 form a series resonant circuit whose resonant frequency is deterniined by the value of the tuning capacitor 102 and the inductance 34. For optimum transfer of energy from the charging apparatus 20 to the device 30, the values of the tuning capacitor 102 and the inductance 34 should be selected so that the resonant frequency of the series resonant circuit formed of the tuning capacitor 102 and the inductance 34 is substantially equal to the resonant frequency of the series resonant circuit formed from the capacitance 64 and charging inductance 24 of the charging apparatus 20. For example, the values of the tuning capacitor 102 and inductance 36 may be selected to produce a resonant frequency of between around 70 kHz and around 130 kllz.

A second terminal of the inductance 34 is connected to a first input node 112 of a bridge rectifier sub-circuit 110. A second temiinal of the tuning capacitor 102 is connected to a second input node 114 of the bridge rectifier sub-circuit 110. As is conventional, the bridge rectifier sub-circuit 110 include four diodes 116, Its, 120, t22 connected in a bridge configuration such that when an alternating current is applied between the first and second input nodes 112, 114 of the bridge rectifier sub-circuit, an output signal present at first and second output nodes 124, 126 of the bridge rectifier sub-circuit is a rectified version of the input alternating current, As is conventional, a smoothing capacitor 130 is connected in parallel with the bridge rectifier sub-circuit 110, between the first and second output nodes 124, 126, to smooth the rectified signal output by the rectifier sub-circuit 110. As can be seen from Figure 7, in the illustrated example, a communications system 140 is also connected to the output of the rectifier sub-circuit 110. The construction and operation of the communications system 140 will be discussed in more detail below, The smoothing capacitor 130 is connected to a DC-DC converter 150, which is operative to convert the DC voltage output by the smoothing capacitor 130 to an alternative DC voltage that is suitable for charging the battery 32. An output of the DC-DC converter is connected to an input of a battery controller 160, which is configured to supply appropriate charging current to the battery 32, to charge the battery 32.

Turning now to Figure 8, it will be noted that in this example the communications system includes a resistance 142 and a switch 144, which may be, for example, a transistor or a MOSFET, connected in parallel with the smoothing capacitor 130, A modulation unit 146 controls the operation of the switch 144, to generate an amplitude modulated signal at the charging apparatus 20, by means of which the device 30 is able to communicate with the charging apparatus 20. The switch 144 can be switched at a predetermined frequency and duty cycle, which adds a predetermined load to first and second output nodes 1 24, 1 26 of the bridge rectifier sub-circuit, thereby establishing amplitude modulation (AM) communication with the charging apparatus. An exemplary AM signal is illustrated in Figure II, from which it can be seen that an AM signal can be achieved by opening and closing the switch 144 to add a load 170 to and to remove a load 172 from a carrier signal.

In order to receive AM signals transmitted by the device 30, the charging apparatus includes a demodulator for demodulating AM signals. in one embodiment, the demodulator is provided as part of the controller 88. The demodulator is configured to demodulate AN'l signals received from the device 30 to extract data from the received AN'l signals. The controller 88 may then take appropriate action, according to the extracted data.

The charging apparatus 20 is also able to communicate with the device 30, using a binary phase shift keying (BPSK) modulation scheme. The BPSK scheme is implemented in software running on the controller 88 of the circuit 40.

An exemplary BPSK signal is illustrated in Figure 9, from which it can be seen that a BPSK signal can be achieved by reversing the timing sequence at zero crossing points 160, 162 of the AC signal driving the charging inductance 24 of the circuit 40. This change in the timing sequence imparts a 180 degree phase shift to the signal, which can be used to represent a data symbol to be transmitted, and this phase shift can be detected by a demodulator at the receiving device 30 which is able to map the phase shift to the data symbol it represents, thereby allowing the data transmitted by the charging apparatus to be received and decoded.

Although the charging apparatus 20 has been described above as implementing a BPSK scheme for data communication with the device 30, and the device 30 is described as implementing an AM scheme for data communication with the charging apparatus 20, it will be appreciated that the charging apparatus may, additionally or alternatively, be provided with an AM modulator for transmitting AM signals to the device 30, to enable bidirectional AM communications between the charging apparatus and the device 30.

Similarly, the device 30 may be provided with a BPSK modulator (implemented either in hardware or software) to enable bidirectional BPSK communications between the device and the charging apparatus 20.

It will be appreciated that the circuit 40 of Figure 2 can be realised in a variety of ways.

For example, Figure 10 is a schematic representation of one implementation of the circuit of Figure 2 which uses a widely available integrated circuit supplied by International Rectifier under the part number IRS21 86(4). However, those skilled in the relevant art will appreciate that the circuit 40 could equally be realised using discrete components, or using one or more general purpose integrated circuits, or as one or more custom integrated circuits, either alone or in combination with one or more discrete components.

The charging apparatus described above provides an effective means for charging batteries in battery operated devices such as secure transport containers where structural integrity is a key requirement and precludes the use of charging ports or sockets for mains powered chargers. The circuit 40 described above is particularly effective, The use of the neutral point clamp circuit described above gives rise to especially efficient wireless charging, particularly where the charging inductors of both the charging apparatus and the charge receiving device are tuned to the same resonant frequency.

Although the charging apparatus 20 has been described above in the exemplary context of a rechargeable battery operated secure transport container, it will be appreciated that the principles described above are equally applicable to wireless charging of other battery powered devices of all sizes, including, for example, mobile telephone, laptop and tablet computers and electric vehicles,