AU2017235992B2 - Output current control for usb device - Google Patents

Abstract

Disclosed is an output current control circuit for controlling an output current. The circuit comprises means for varying an output current limit of the output current in accordance with a sensed temperature. Also disclosed is an electrical device containing the output current control circuit, which in some embodiments, may be used to charge an external device such as a USB-enabled device. Also disclosed is a method of controlling an output current. 1/10 100 )10 110 T Figure 1 100 'in 1 105 110 T Figure 2

Description

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OUTPUT CURRENT CONTROL FOR USB DEVICE TECHNICAL FIELD

[0001] The present application relates to wired electrical devices such as charging devices, including Universal Serial Bus (USB) chargers.

PRIORITY

[0002] This application claims priority from Australian Provisional Patent Application No. 2016903958 entitled "Output Current Control For USB Device", filed on 29 September 2016.

[0003] The entire content of this priority application is hereby incorporated by reference.

INCORORATION BY REFERENCE

[0004] The present application refers to the following documents:

- Data Sheet for device TPS2511 entitled USB Dedicated Charging Port Controller and

Current Limiting Power Switch (designated as document SLUSB18-JUNE 2012) by Texas Instruments Incorporated.

[0005] The content of this document is hereby incorporated by reference in its entirety.

BACKGROUND

[0006] The use of mobile devices such as tablets, smart phones, computers and cameras is steadily increasing, as are the requirements for charging these devices. Various options are available for charging these devices, including portable chargers that are able to be plugged into a source of power such as mains or supply power, and charging devices that are built in to a wall power outlet such as a General Power Outlet (GPO) or switch plate. These inbuilt charging devices generally provide a charging current that is able to be applied directly to the external device via a connector.

[0007] Built-in devices will tend to become quite hot during use, due to factors including their enclosed environment which limits their capacity to dissipate heat, and their proximity to other heat generating and radiating electrical components.

[0008] As will be understood by the person skilled in the art, the internal operating temperature of the device increases with output current. The life expectancy of a device decreases with increasing operating temperature. In the case of in-built charging devices, it is undesirable to have to replace charging devices on a regular basis due to more frequent failure.

SUMMARY

[0009] According to a first aspect, there is provided an output current control circuit for use in a wired electrical device, the output current control circuit comprising means for varying an output current limit of an output current in accordance with a sensed temperature.

[0010] According to a second aspect, there is provided an electrical device for providing an output current and for connection to a mains or source power supply, the electrical device comprising the output current control circuit of the first aspect.

[0011] In one embodiment, the electrical device is a USB charger device.

[0012] According to a third aspect, there is provided a power source for providing an output current, the power outlet comprising the electrical device of the second aspect.

[0013] In one embodiment, the power source is provided in a power outlet.

[0014] In one embodiment, the power outlet is a surface mountable power outlet.

[0015] According to a fourth aspect, there is provided a method of controlling an output current, the method comprising sensing a temperature; and varying a limit of the output current in accordance with the sensed temperature.

[0015A] According to a fifth aspect, there is provided an output current control circuit for use in an electrical device that in use provides an output current when connected to a mains or source power supply, the output current control circuit comprising means for varying an output current limit of the output current in accordance with a sensed temperature, representative of a temperature of the electrical device.

[0015B] According to a sixth aspect, there is provided an electrical device for providing an output current and for connection to a mains or source power supply, the electrical device comprising the output current control circuit according to the fifth aspect.

2a

[0015C] According to a seventh aspect, there is provided a power source for providing an output current, the power source comprising the electrical device according to the sixth aspect.

[0015D] According to an eighth aspect, there is provided a method of controlling an output current provided by an electrical device when connected to a mains or source power supply, the method comprising: sensing a temperature representative of a temperature of the electrical device; and varying a limit of the output current in accordance with the sensed temperature.

[0015E] According to a ninth aspect, there is provided an output current control circuit for use in an electrical device that is integrated within a power outlet, or, electrically connected to the power outlet, that in use provides an output current when connected to a mains or source power supply of the power outlet, the output current control circuit comprising means for continuously varying an output current limit of at least a portion of the output current in accordance with a sensed temperature, representative of a temperature of the electrical device.

[0015F] According to a tenth aspect, there is provided an electrical device that is integrated within a power outlet, or, in use, electrically connected to the power outlet, for providing an output current and for connection to a mains or source power supply of the power outlet, the electrical device comprising the output current control circuit of the ninth aspect.

[0015G] According to an eleventh aspect, there is provided a power outlet containing the electrical device of the tenth aspect.

[0015H] According to a twelfth aspect, there is provided a method of controlling an output current provided by an electrical device when connected to a mains or source power supply of a power outlet, the electrical device being either integrated within the power outlet or in use, electrically connected thereto, the method comprising: sensing a temperature representative of a temperature of the electrical device; and continuously varying an output current limit of at least a portion of the output current in accordance with the sensed temperature.

BRIEF DESCRIPTION OF DRAWINGS

[0016] Embodiments of the various aspects described herein will be detailed with reference to the accompanying drawings in which:

[0017] Figure 1 - shows a simplified arrangement of an output current control circuit according to one aspect described herein;

[0018] Figure 2 - shows an embodiment of the output current control circuit of Figure 1;

[0019] Figure 3 - shows a general embodiment of an electrical device comprising the output current control circuit described herein;

[0020] Figure 4 - shows an embodiment of the output current control circuit using an integrated circuit with a programming resistance;

[0021] Figure 5A - shows an embodiment of the programming resistance using two serial resistors and a thermal switch in a first state;

[0022] Figure 5B - shows the arrangement of Figure 5A with the thermal switch in a second state;

[0023] Figure 6A - shows another embodiment of a programming resistance using two resistors in parallel and a thermal switch in a first state;

[0024] Figure 6B - shows the arrangement of Figure 6A with the thermal switch in a second state;

[0025] Figure 7A - shows a plot of sensed temperature vs time for the arrangement of Figures 6A and 6B;

[0026] Figure 7B - shows a plot of output current limit vs time for the arrangement of Figures 6A and 6B;

[0027] Figure 8 - shows another embodiment of the programming resistance using a thermistor;

[0028] Figure 9A - shows a plot of sensed temperature vs time for the arrangement of Figure 8;

[0029] Figure 9B - shows a plot of output current limit vs time for the arrangement of Figure 8;

[0030] Figure 1OA - shows a plot of sensed temperature vs time for the arrangement of Figure 8 with the output current limit changing in discrete steps;

[0031] Figure 1OB - shows a plot of output current limit vs time for the arrangement of Figure 8 with the output current limit changing in discrete steps;

[0032] Figure 11 - shows another embodiment of the programming resistance using a resistor in series with a thermistor in parallel with a thermal switch in a first state;

[0033] Figure 1lB - shows the arrangement of Figure 11A with the thermistor in a second state;

[0034] Figure 12A - shows a plot of sensed temperature vs time for the arrangement of Figures 11A and I1B;

[0035] Figure 12B - shows a plot of output current limit vs time for the arrangement of Figures 11A and IB;

[0036] Figure 13 - shows a circuit schematic of an embodiment of the arrangement of Figure 4;

[0037] Figure 14 - shows a flowchart of a method of controlling an output current;

[0038] Figure 15 - shows an embodiment of an electrical device as a USB charging device;

[0039] Figure 16 - shows an embodiment of the electrical device of Figure 15 connected to a power outlet;

[0040] Figure 17 - shows a front view of the arrangement of Figure 16;

[0041] Figure 18 - shows another embodiment of a USB charger device;

[0042] Figure 19 - shows another embodiment of a USB charger device;

[0043] Figure 20 - shows another embodiment of a USB charger device;

[0044] Figure 21 - shows another embodiment of a USB charger; and

[0045] Figure 22 - shows another embodiment of a USB charger

DESCRIPTION OF EMBODIMENTS

[0046] Figure 1 shows a simplified arrangement of an output current control circuit 100 for providing an output current 1. In one embodiment, the output current control circuit 100 is used in a wired electrical device.

[0047] In a broad sense, there is provided an output current control circuit for use in a wired electrical device, the output current control circuit comprising means for varying an output current limit of an output current in accordance with a sensed temperature. Figure 1 shows a general system of an output current control circuit 100, with a means 110 for varying the output current limit of the output current in accordance with a sensed temperature T,. The sensed temperature T, is the value representative of the actual temperature T of the surrounding environment such as within an enclosure of a device incorporating the output current control circuit 100.

[0048] In one embodiment, as shown in Figure 2, the output current control circuit 100 comprises an output current limiter 105 for providing an output current limit, and means 110 for varying the output current limit of the output current limiter 105 in accordance with a sensed temperature T,, representative of the surrounding temperature T.

[0049] In this arrangement, an input current li, is provided to the input of the output current limiter 105. The input current can be provided by any suitable source, such as from a power supply including an AC-to-DC Converter or Buck DC-to-DC converter. The magnitude of the output current Ic is then limited by the output current limiter 105 to provide an output current with a given limit. It will be appreciated that the input current li, is provided by a power source with capacity to supply current up to a maximum of Imax (subject to the current demand from the load). The current limiter 105 then further limits this to im where Inm < Imax.

[0050] According to one aspect described herein, this given limit is able to be dynamically varied over time in accordance with the sensed temperature T,. The ability to vary the output current limit provides a number of advantages over a fixed output current limit. For example, in the case where an output current limit is set at a specific, static limit, this limit needs to be set at a level low enough to ensure that the temperature T related to the device never exceeds desired levels. This means that even when the operating temperature of the device is low, the current that is able to be provided by the output current limiter 105 will more often than not be lower than is desired or necessary. The provision of a variable output current limit allows the output current limiter to provide a more effective current output for a greater proportion of the time of use.

[0051] Figure 3 shows a simplified block diagram of an embodiment of an electrical device 500 utilising the output current control circuit of Figures 1 and 2. In this embodiment, an alternating current (AC) signal is applied to the input of the electrical device, which is connected to an AC-to DC converter 510. In some embodiments, the AC signal is a mains or source power signal, and can differ depending on different countries, as will be understood by the person skilled in the art. For example, in Australia, the mains power is a nominal 240V AC signal with a nominal frequency of Hz.

[0052] The output of the converter 510 is applied to the input of supporting electronics 520, which will dictate the function of the electrical device. In one embodiment, the electrical device is a charging device for charging a piece of equipment connected to the output of the electrical device. In one embodiment, the charging device is a Universal Serial Bus (USB) charging device, and the output of the electrical device 500 is a USB port for receiving a USB connector connected to the device to be charged. In other embodiments, the electrical device 500 simply provides a source of power to a piece of electrical equipment for operation.

[0053] Figure 4 shows a specific embodiment of output current control circuit 100. In this embodiment, current limiter 105 is provided by a current limiting integrated circuit 105a, and in this specific embodiment, an integrated circuit device TPS2511 provided by Texas Instruments Incorporated. Other suitable devices include a type MP5010B device provided by Monolithic Power Systems, Inc. In accordance with an aspect described herein, a means for varying output current limit 110 is provided as shown in Figure 4, and connected to the ILIMSET input of the integrated circuit device 105a. This arrangement provides for the current limit set by the device 105a to vary as directed by the means 110. In this particular embodiment, the means 110 provides a resistance to input ILIMSET, the value of which dictates the limit of the current output by the integrated circuit 105a.

[0054] In this embodiment, means 110 is provided by a programming resistor and can be provided by any suitable arrangement that can vary its resistance in accordance with or as a function of temperature.

[0055] In one embodiment, in a simple form, the means for varying output current limit is a thermistor. In one embodiment, the thermistor is provided by a Positive Temperature Coefficient Thermistor (PTC Thermistor) sold under the brand name Posistor provided by Murata Manufacturing Co. Ltd.. In another embodiment, the thermistor is provided by a Negative Temperature Coefficient Thermistor (NTC Thermistor) such as the NC types provided by Murata Manufacturing Co. Ltd. These devices could be used alone or in an arrangement of two or more, in series and/or parallel with one or more fixed resistors to provide a change in the value of the programming resistance which corresponds to the desired change in value of current limit as required by the chosen programmable current limit switch. Such an arrangement could provide a continuously variable current limit as described further below.

[0056] In other embodiments, as shown in Figures 5A to 8, the means for varying the output current limit 110 is provided by arrangements using two or more resistors in series or in parallel, with one or more thermally-operated switches such that when the thermally-operated switch actuates in response to a change in temperature, the resistance presented by the means for varying output current limit changes, thereby changing the output current limit.

[0057] Figures 5A and 5B show one embodiment in which the means for varying the output current limit 110 is provided by a first resistance RI and a second resistance R2 in series. A thermal switch Slis connected across one of the resistances (e.g. R2 as shown in Figure 5A). As will be understood by the person skilled in the art, when the thermal switch Si is closed, the resistance provided by this arrangement will be substantially the value of first resistance RI since any current will effectively bypass second resistance R2 to go through the substantially closed circuit path through thermal switch S1. This particular resistance value will result in an output current limit according to the set relationship between the value of the programming resistor and the output current limit.

[0058] As the device operates at that output current limit, the temperature of the device, and as sensed by thermal switch Si, will increase over time. When the temperature sensed by thermal switch S Ireaches a set value, switch Si will open as shown in Figure 5B, causing an open circuit across second resistance R2, forcing current to pass through R2 and RI. This will result in a change to the resistance provided by this arrangement, to the effect of RI plus R2. This will result in a change (reduction) to the output current limit, again, according to the relationship between the value of the programming resistor ILIM and the output current limit. This relationship will depend upon the device used and is obtainable from the relevant device data sheet. For example, for the TPS2511 device referred to above with reference to Figure 4, this relationship can be determined using the equations and graphs set out on page 17 of the data sheet for the TPS2511 device, entitled USB Dedicated Charging Port Controller and Current Limiting Power Switch (designated as document SLUSB18-JUNE 2012) previously incorporated by reference and as understood by the person skilled in the art. In one example of the arrangement of Figures 6A and 6B in which a single USB port is provided, RI =44.2k and R2= 95.3k. This provides current limits of 1.7A and 1.16A in the two different switching states. In another example, and in which two USB ports are provided, R1=R2=44.2k, providing current limits of 2.32A and 1.16A in the two different switching states.

[0059] Other set up and programming parameters may also be determined from the remainder of the content of this document, incorporated by reference.

[0060] With the reduced output current limit, the temperature of the device as it operates and thus that sensed by thermal switch S1, will reduce again over time. When the temperature sensed by thermal switch S I reduces to below the set value thermal switch S Icloses again as shown in Figure A, and the resistance of the arrangement returns to the resistance of R1, again resulting in an increase in the output current limit.

[0061] It will be understood that the thermal switch can be used in other modes, such as one that is normally open and triggers closed when the temperature reaches the set value. The programming of the chip will be commensurately modified for this operation.

[0062] In another embodiment, as shown in Figures 6A and 6B, the programming resistance is provided by first resistance RI and second resistance R2 in parallel, with a thermal switch Si in series with one of the resistances, for example R2.

[0063] When the temperature sensed by thermal switch S I is below the set value, the resistance provided by the arrangement will be substantially equal to the equivalent resistance of RI in parallel with R2, that is (R1.R2)/(R1 + R2).

[0064] As the temperature increases over time, the temperature sensed by the thermal switch Si reaches the set point and causes Si to open. This causes an open circuit in the path of R2, causing the current to flow only through RI as shown in Figure 6B, thus providing the effective resistance of RI alone. This change in resistance causes the output current limit to change (reduce) which in turn results in a reduction of temperature over time, until the set point is reached, and thermal switch S I closes again as in Figure 6A, bringing the resistance of R2 back into the circuit, and so on. It will be appreciated, that in other embodiments, using different programmable devices, the output current limit can change to increase, as the effective programming resistance changes.

[0065] An example of a thermal switch suitable for this purpose is a bimetallic thermal switch such as the H20 device provided by Dongguan Heng Hao Electric Co., Ltd in China.

[0066] In other embodiments, a proprietary resistor programmable thermal switch such as the type TMP708 by Texas Instruments Incorporated may be used to switch a second resistor in series or in parallel.

[0067] In yet further embodiments, use may be made of a discrete component "snap action": electronic switch using a thermistor as a temperature dependent element in a potential divider which causes a simple electronic switch to operate and switch a parallel resistor arrangement (for example as shown in Figures 6A and 6B). In one embodiment, this particular arrangement uses a PTC thermistor in the lower portion of the potential divider coupled to an inverting switch. Other similar arrangements are possible using appropriate combinations of NTC thermistors, the upper portion of the potential divider and a non-inverting switch as will be understood by the person skilled in the art.

[0068] Figures 7A, 7B provide a general illustration of the process described above with reference to Figures 5A and 5B. Figure 7A is a plot of sensed temperature Ts versus time t, being the temperature sensed by the thermal switch Si in Figures 5A and 5B as described previously, and being representative of the temperature of the device, surrounding thermal switch Si. The dashed line represents the sensed temperature threshold Tire at which the thermal switch Si will trigger and open as previously described. Figure 7B indicates the output current limit at any given time.

[0069] In Figure 7A, the sensed temperature is shown at point A, being relatively low and stable while no current is being drawn. When the device is started to be used and current is being drawn at point B, the temperature as shown in Figure 7A begins to rise. During the period B-C, the sensed temperature is under the sensed threshold temperature, and the output current limit is at a first, high limit as shown in Figure 7B. It will be noted that in this example, the output current is shown to reach this output current limit relatively quickly, but it will be appreciated that in some applications, the output current limit may not be reached at this point.

[0070] As the temperature and sensed temperature continues to rise during the period B-C, it will eventually reach the sensed temperature threshold shown at point C in Figure 7A. At this point, the thermal switch Si will open, resulting in a change to the resistance of the programming resistor, which in turn will cause the output current limit to drop to a lower limit, as shown in Figure 7B.

[0071] During the period C-D in Figure 7A, the temperature will continue to increase slightly, but will reach a maximum and begin to fall at point D, due to the lower output current being drawn. As the temperature and sensed temperature falls back to the sensed temperature threshold, thermal switch Si will close again changing the value of the resistance of the programming resistor, again causing the output current limiter 105 to reinstate the higher current limit as shown in Figure 7A at point E. If the required output current being drawn is greater than the lower output current limit, the output current will rise to its required value or up to the maximum of the output current limit.

[0072] As the output current increases, the temperature and sensed temperature will again increase, until the sensed temperature again reaches the sensed temperature threshold and the sequence previously described will repeat.

[0073] In another embodiment, the programming resistance is simply provided by a thermistor 115, whose resistance continuously changes with temperature, as shown in Figure 8. In this arrangement, the output current limiter 105 can be programmed to vary the output current limit progressively as the value of the programming resistance changes. This can provide a continuously changing output current limit instead of the discrete or stepped varying current limit previously described, which can provide further efficiencies in the operation of the device.

[0074] Figures 9A and 9B illustrate how this arrangement would work. Figure 9A is a plot of sensed temperature Ts versus time t, being the temperature sensed by the thermistor in Figure 8 as described above, and being representative of the temperature of the device, surrounding the thermistor. Figure 9B indicates the output current limit at any given time.

[0075] It will be appreciated that in this embodiment, there need not be a sensed temperature threshold, since the output current limit is varied in a continuous manner as sensed temperature varies.

[0076] In accordance with this embodiment, the output current limit is continuously varied as shown in Figure 9B to counter the rising temperate and to keep it at a suitable level. As the output current increases, it will increase until it hits the output current limit. At each point in time, the output current will either follow the output current limit or be less than the output current limit. It will also be appreciated that in other embodiments, the output current limiter 105 may be programmed to continuously track the sensed temperature from the programming resistance provided by the thermistor, but to change the output current limit in discrete steps of longer periods.

[0077] Such an embodiment is illustrated in Figures 1OA and 1OB which are equivalent to Figures 9A and 9B with the exception that the output current limit as shown in Figure 1OB is set as a plurality of discrete periods.

[0078] In a further embodiment, a combination of continuous and discrete output current limits can be set by the programming resistance provided by a combination of resistors, thermal switches and/or thermistors. Figures 11A and 1lB show an embodiment of the programming resistance provided by a first resistance RI in series with a thermistor 115. Connected in parallel with thermistor 115 is a thermal switch S. In one embodiment of this arrangement, thermal switch Si may begin in the open state, allowing current to flow through both the thermistor 115 and RI as shown in Figure 11A. When the temperature reaches a threshold, thermal switch Si is triggered to close, and effectively removes thermistor 115 from the programming resistance circuit, as shown in Figure 1IB.

[0079] Figures 12A and 12B show plots of the temperature/sensed temperature (Figure 12A), current limit (Figure 12B) resulting from the arrangement using the programming resistance of Figures 11A and I1B.

[0080] As the output current begins to be drawn, the temperature and thus the sensed temperature as sensed by thermistor 115 begins to increase as seen in region A-B in Figure 12A. At the same time, as the sensed temperature increases and therefore the resistance of thermistor 115 changes, the output current limit begins to decrease. When the sensed temperature reaches the temperature threshold as shown at point B in Figure 12A, thermal switch S triggers and opens, removing thermistor 115 from the programming resistance circuit. This sudden change in resistance causes a step change in the output current limit as shown in Figure 12B.

[0081] During period B-C, the sensed temperature will continue to rise for a time until it reaches a maximum at point C, and then begins to fall due to the reduced output current being drawn. When the sensed temperature returns to the temperature threshold at point D, this triggers thermal switch Si to open again, returning the resistance of thermistor 115 back into the programming resistance circuit and causing the output current limit to track in a continuous fashion again, as the resistance value of thermistor 115 changes as the temperature changes. This cycle continues as previously described and shown in Figures 12A and 12B to maintain the temperature at a stable level while maximising the available output current for a greater period of time.

[0082] Figure 13 shows a circuit schematic of one example of an embodiment of the arrangement of Figure 4.

[0083] In other embodiments, the output current limit may be controlled in an analogue manner. In such arrangements, the means for varying output current limit may be provided by a means for sensing current, a means for comparing that current to a threshold (for example with the use of a comparator) and a means for limiting that current if the sensed current reaches the threshold. An adjustable current limit can be obtained through the use of an adjustable threshold. The adjustable threshold may be controlled in accordance with a sensed temperature, to ultimately provide an adjustable current limit, the magnitude of which is determined by sensed temperature.

[0084] In accordance with another aspect described herein, there is provided a method of controlling an output current. As shown in Figure 14, at step 400, a temperature is sensed from an appropriate region, such as within an enclosure of the device generating the output current, and, in step 410, a limit of the output current is varied in accordance with the sensed temperature.

[0085] In one embodiment, the method comprises reducing the output current limit in accordance with an increase in the sensed temperature from the previously-sensed temperature.

[0086] In one embodiment, the method comprises increasing the output current limit in accordance with a reduction in the sensed temperature from the previously-sensed temperature.

[0087] In another aspect described herein, there is provided an electrical device for providing an output current and for connection to a power outlet, the electrical device comprising the output current control circuit described herein in any one of its various embodiments. In some embodiments of this aspect, the electrical device is used to provide electrical power to an external device to allow it to function. In this regard, the external device is simply connected to the output of the electrical device via a suitable connector.

[0088] In another embodiment, the electrical device is a charging device for charging a battery of an external device, and may include associated functional circuitry to allow it to perform the charging function once the external device is connected to the electrical device via a suitable connector.

[0089] In one embodiment, the electrical device is integrated with a power source. In one embodiment, the power source is provided in a power outlet for, in one embodiment, mounting to a wall or other surface. In other embodiments, the electrical device is able to be separately and in some cases, removably connected to the power outlet.

[0090] Figure 15 shows one such example of a charging mechanism in the form of a Universal Serial Bus (USB) charger, providing the power source, which in one embodiment is able to be connected to a power outlet or a switch plate.

[0091] Figure 16 shows the electrical device 500 of Figure 15 connected to the rear of a power outlet 600.

[0092] Figure 17 shows a front view of the power outlet 600 of Figure 16. In one aspect described herein, there is provided a power outlet 600 for providing an output current. In this aspect, the power outlet comprises an electrical device 500 as previously described.

[0093] In one embodiment, the power outlet is a surface mountable power outlet.

[0094] Figures 18, 19, 20, 21 and 22 show different embodiments of possible electrical devices 500, in these embodiments being USB chargers, incorporating the output current control circuit described herein.

[0095] Throughout the specification and the claims that follow, unless the context requires otherwise, the words "comprise" and "include" and variations such as "comprising" and "including" will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers.

[0096] The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement of any form of suggestion that such prior art forms part of the common general knowledge.

[0097] It will be appreciated by those skilled in the art that the various aspects described herein are not restricted in its use to the particular application described. Neither is the present disclosure restricted with regard to the particular elements and/or features described or depicted herein. It will be appreciated that the various embodiments disclosed are capable of numerous rearrangements, modifications and substitutions without departing from the scope as set forth and defined by the following claims.

Claims (13)

1. An output current control circuit for use in an electrical device that that is integrated within a power outlet, or, electrically connected to the power outlet, that in use provides an output current when connected to a mains or source power supply of the power outlet, the output current control circuit comprising means for continuously varying an output current limit of at least a portion of the output current in accordance with a sensed temperature, representative of a temperature of the electrical device.

2. An output current control circuit as claimed in claim 1 comprising: an output current limiter for providing the output current limit; and means for continuously varying the output current limit of the output current limiter in accordance with the sensed temperature.

3. An output current control circuit as claimed in any one of claims 1 or 2 wherein the means for continuously varying the output current limit comprises means for continuously reducing the output current limit in accordance with an increase in the sensed temperature.

4. An output current control circuit as claimed in any one of claims 1 to 3 wherein the means for continuously varying the output current limit comprises means for continuously increasing the output current limit in accordance with a reduction in the sensed temperature.

5. An output current control circuit as claimed in any one claims 1 to 4 wherein the means for continuously varying the output current limit comprises a current limiting integrated circuit whose current limit is controllable by a programming resistance.

6. An output current control circuit as claimed in claim 5 wherein the programming resistance is provided by a thermistor.

7. An electrical device that is integrated within a power outlet, or, in use, electrically connected to the power outlet, for providing an output current and for connection to a mains or source power supply of the power outlet, the electrical device comprising the output current control circuit of any one of claims 1 to 6.

8. An electrical device as claimed in claim 7 wherein the electrical device is a Universal Serial Bus (USB) charger.

9. A power outlet containing the electrical device as claimed in any one of claims 7 or 8.

10. A power outlet as claimed in claim 9 wherein the power outlet is a surface mountable power outlet.

11. A method of controlling an output current provided by an electrical device when connected to a mains or source power supply of a power outlet, the electrical device being either integrated within the power outlet or in use, electrically connected thereto, the method comprising: sensing a temperature representative of a temperature of the electrical device; and continuously varying an output current limit of at least a portion of the output current in accordance with the sensed temperature.

12. A method as claimed in claim 11 further comprising continuously reducing the output current limit in accordance with an increase in the sensed temperature.

13. A method as claimed in any one of claims 11 or 12 further comprising continuously increasing the output current limit in accordance with a reduction in the sensed temperature.