GB2526532A

GB2526532A - Power supply apparatus and method

Info

Publication number: GB2526532A
Application number: GB1407803.4A
Authority: GB
Inventors: Phillip Knowles
Original assignee: Harvard Engineering PLC
Current assignee: Harvard Engineering PLC
Priority date: 2014-05-02
Filing date: 2014-05-02
Publication date: 2015-12-02
Anticipated expiration: 2034-05-02
Also published as: GB2526532B; GB201407803D0

Abstract

Power supply apparatus comprises: a first power supply 1 having a first pair of input terminals 11 for connecting to a power source 1000, and a first pair of output terminals 12 for connecting to a first load 100; a second power supply 2 coupled to the first output terminals, and having a second pair of output terminals 22 for connecting to a second, dynamic load 200. The first power supply is operable to drive a first current I1 through the first load. The second power supply is arranged to draw a substantially fixed second current I2 from the first power supply when the first power supply is operating to drive the first current through the first load, and use the second current to generate an output voltage across the second pair of output terminals to power the second, dynamic load when connected to the second pair of output terminals.

Description

Intellectual Property Office Application No. GB1407803.4 RTTVT Date:25 September 2014 The following term is a registered trade mark and should be read as such wherever it occurs in this document: Z 1GB EE Intellectual Property Office is an operating name of the Patent Office www.ipo.govuk Power Supply Apparatus and Method

Field of the Invention

The present invention relates to power supply apparatus and corresponding methods for supplying power to two separate loads from a power source, and particularly, although not exclusively, where one of the loads requires operation at substantially constant current and the other load is a dynamic load, that is a load whose operating current typically varies with time.

Background to the Invention

There are a number of applications in which power supply apparatus is required to drive a first load at a substantially constant current (and this constant current may, of course, be adjustable) and also where the apparatus is required to power a second, independent load.

If this second load is dynamic, that is if its opelating current varies with time, then this can result in unwanted variations in the current being supplied or driven through the first load.

Light Emitting Diode (LED) drivers, for example, are typically constant current source (CCS) power supplies (PSU). When designing such power supplies there may be the need for an auxiliary rail referenced to the output of the driver. In such circumstance it is cost effective and energy efficient to derive this rail, which may be 3V or 5V or any other voltage, directly from the output of the PSU. In many modes of operation this itself is not a problem. In circumstances where the load on the auxiliary supply is dynamic, step changes in the current being supplied from the output to the auxiliary rail will result in a step change in the output current of the LED driver. Depending on the ratio of this change in current to the average output current, the changing light output that results can be perceived by the human eye as flicker in a lighting application.

Figure 1 shows an overview of a typical LED driver with auxiliary rail The problem described above may be at its worst in isolated single stage power factor corrected (PFC) PSUs, since these typically have a slow control loop response to give good power factor and cannot compensate for the changing load current fast enough. 1.

Dimmable LED drivers are also a very prone to the problem. The output current on such LED drivers could be reduced down to a level much lower than the load demand of the auxiliary power rail. In this scenario step changes could be larger than the output current itself, requiring impractical amounts of filtering.

Prior art attempts at solving these problems include the following.

Multiple power stage LED drivers allow the LED Outputs to regulated separately from the auxiliary supply rails but add size, complexity, cost, and inefficiency to the design.

Separate auxiliary power supplies can be added to the LED PSU design, which adds further cost and complexity to the design (see fig. 2).

Filtering the steps in current by the use of large PSU output capacitors and/or auxiliary supply input capacitors can reduce the amount of change in current and therefore flickering of an LED driver due to step changes in the load. In many applications the LED driver output will require so much capacitance (particularly at low output voltages and low current) that cost and printed circuit board (PCB) size can become extreme (see fig. 3).

Summary of the Invention

It is an aim of certain embodiments of the invention to solve, mitigate or obviate, at least partly, at least one of the problems and/or disadvantages associated with the prior art.

Certain embodiments aim to provide at least one of the advantages described below.

According to a first aspect of the invention there is provided power supply apparatus comprising: a first power supply having a first pair of input terminals for connecting to a power source, and a first pair of output terminals for connecting to a first load; a second power supply coupled to said first output terminals, and having a second pair of output terminals for connecting to a second load (e.g. a dynamic load); wherein the first power supply is operable, when the first load is connected to the first pair of output terminals, to drive a first current between said first pair of output terminals through the first load; and the second power supply is arranged to draw a substantially fixed second current from said first power supply when said first power supply is operating to drive said first current through the first load, and use said second current to generate an output voltage across said second pair of output terminals to power the second (e.g. dynamic) load when connected to the second pair of output terminals.

It will be appreciated that dynamic load" in the context of the specification means that the second load is one whose operating current varies with time. In other words, the dynamic load is one which, when connected to a power supply providing a constant supply voltage will draw a current (from that power supply) whose magnitude varies with time. For example, the dynamic load may be a circuit or circuit module arranged to provide some function, for example wireless communication, the power requirements for which vary with time (for example according to the variations in the volume of data being transmitted and/or received over time).

Advantageously, as the second power supply is arranged to draw a substantially fixed second current from the first power supply, the magnitude of the second current remains constant overtime, irrespective of the magnitude of the current being drawn by the second load from the second power supply at a particular instant. In other words, although the current drawn by the second load from the second power supply may vary with time, the second current drawn from the first power supply by the second power supply is constant (it does not vary with time). The second current is substantially unaffected by changes in the second load current.

It will be appreciated that one of each pair of input and/or output terminals may be provided by a ground terminal, ground rail, or connection to a ground rail. Thus, in certain embodiments, a pair of terminals may comprise a first, or "high" terminal, and a ground terminal, rail, or connection.

In certain embodiments the output voltage generated by the second power supply may not be fixed; it may be variable. The output voltage may vary with time and/or with power requirements of the second load, but the second current drawn from the first power supply remains substantially fixed (constant).

In certain alternative embodiments, the second power supply is arranged such that the output voltage is substantially fixed. Thus, the second load may be provided with (driven by) a substantially fixed supply voltage, derived from the fixed second current extracted from the first power supply. As power demands of the second load change with time, this will of course result in changes in the magnitude of the current being drawn by the second load from the second power supply, but this does not affect the substantially fixed second current being drawn from the first power supply.

In certain embodiments the second power supply is arranged to drive a third current (e.g. a first portion of said second current) through the second load when connected, and to drive a fourth current (e.g. a second portion of said second current) through a current path between said second pair of output terminals and in parallel with the second load.

The fourth current can be regarded as an unneeded current (i.e. a current not required by the second load at a particular instant in view of its operational conditions) which the second power supply dumps through the parallel current path.

Advantageously, the second power supply may be arranged to maintain the combined magnitude of the third and fourth currents substantially fixed, i.e. with the magnitude of the fourth current being decreased or increased to correspond with increases or decreases respectively in the third current through the second load.

In certain embodiments the second power supply is adapted to control a magnitude of said fourth current, in response to changes in the magnitude of the third current driven through the dynamic load, so as to maintain a substantially constant voltage across said second pair of output terminals.

In certain embodiments the current path includes a resistor.

In certain embodiments the second power supply comprises a transistor, said current path comprises a controllable current path through the transistor, and the second power supply further comprises control means arranged to supply a control signal to the transistor to control a conductivity of said controllable current path.

In certain embodiments the third current is a first portion of the second current, and the fourth current is a second portion of the second current.

In certain embodiments the second power supply comprises current feedback means arranged to provide an indication of the combined magnitude of the third and fourth currents and the second power supply is arranged to use said indication of the combined magnitude to maintain said combined magnitude substantially constant.

In certain embodiments the second power supply comprises voltage feedback means arranged to provide an indication of a voltage across said second pair of output terminals, and the second power supply is arranged to use said indication of a voltage to maintain said voltage substantially constant.

In certain embodiments the second power supply comprises a constant current source circuit and a constant voltage shunt circuit, the constant current source circuit being arranged to draw said substantially fixed second current from the first power supply and provide a substantially fixed current to the constant voltage shunt circuit, the constant voltage shunt circuit being arranged to supply a substantially constant voltage across said second pair of output terminals to drive the dynamic load.

In certain embodiments the first power supply comprises current feedback means arranged to provide an indication of the magnitude of the first current driven through the first load, and the first power supply is arranged to use said indication of the first current's magnitude to maintain the magnitude of the first current substantially constant.

In certain embodiments the first power supply comprises current feedback means arranged to provide an indication of the combined magnitude of the first current and the second current, and the first power supply is arranged to use said indication of the first and second currents' combined magnitude to maintain the combined magnitude of the first and second currents substantially constant.

In certain embodiments the apparatus further comprises said first load connected to said first pair of output terminals.

In certain embodiments said first load comprises at least one LED arranged to be powered by at least a portion of the first current.

In certain embodiments the apparatus further comprises said second load connected to said second pair of output terminals.

In certain embodiments said second load comprises a wireless communication module or circuit.

In certain embodiments the first power supply is an LED driver arranged to drive a first load comprising at least one LED at constant current (which may be adjustable/dirnmable).

S

Another aspect provides a power supply adapted for use as the second power supply of apparatus in accordance with the first aspect. The power supply may also be described as an adaptor, for example an adaptor for connection to an LED driver so as to provide an auxiliary power supply to an auxiliary circuit or module, such as a wireless communication module. Thus, the power supply may comprise means for coupling the power supply to a first power supply (e.g. an LED driver) arranged to drive a first load, and an output terminal for connecting to a second load (e.g. a dynamic load), the power supply being arranged to draw a substantially fixed current from said first power supply when said first power supply is operating to drive current through the first load, and use said fixed current to generate an output voltage on said output terminal to power the second load when connected to the output terminal.

Another aspect of the invention provides a method of driving a first load and a second load (for example wherein the second load is a dynamic load), the method comprising: operating a first power supply, connected to a power source, to drive a first current through the first load; operating a second power supply, coupled to the first power supply, to draw a substantially fixed second current from said first power supply when said first power supply is operating to drive said first current through the first load, and use said second current to generate an output voltage to power the second load.

In certain embodiments the output voltage is substantially fixed.

In certain embodiments operating the second power supply comprises driving a third current through the second load and driving a fourth current through a current path in parallel with the second load.

In certain embodiments operating the second power supply comprises controlling a magnitude of said fourth current in response to changes in the magnitude of the third current driven through the dynamic load so as to maintain said output voltage substantially constant.

In certain embodiments operating the second power supply comprises using current feedback means to provide an indication of the combined magnitude of the third and fourth currents, and using said indication of the combined magnitude to maintain said combined magnitude substantially constant.

In certain embodiments operating the second power supply comprises using voltage feedback means to provide an indication of said output voltage, and using said indication of output voltage to maintain said output voltage substantially constant.

In certain embodiments the second power supply comprises a constant current source circuit and a constant voltage shunt circuit, and operating the second power supply comprises operating the constant current source circuit to draw said substantially fixed second current from the first power supply and provide a substantially fixed current to the constant voltage shunt circuit, and operating the constant voltage shunt circuit to supply said output voltage to drive the dynamic load.

In certain embodiments operating the first power supply comprises using current feedback means to provide an indication of the magnitude of the first current driven through the first load, and using said indication of the first current's magnitude to maintain the magnitude of the first current substantially constant.

In certain embodiments operating the first power supply comprises using current feedback means to provide an indication of the combined magnitude of the first current and the second current, and using said indication of the first and second currents' combined magnitude to maintain the combined magnitude of the first and second currents substantially constant.

Brief Description of the Drawings

Embodiments of the invention will now be described with reference to the accompanying drawings, of which: Figs. 1-3 illustrate power supply apparatus in accordance with the prior art; Figs 4, and 7-10 illustrate power supply apparatus in accordance with the present invention; and Figs. 5 and 6 illustrate power supplies embodying the invention.

Detailed Description of Embodiments of the Invention Referring now to Fig. 9, this shows power supply apparatus embodying the invention. The apparatus comprises a first power supply 1 having a first pair of input terminals 11 for connecting to a power source 1000, and a first pair of output terminals 12 for connecting to a first load 100. The apparatus also comprises a second power supply 2 having a second pair of input terminals 21 connected to the first output terminals 12, and having a second pair of output terminals 22 for connecting to a second load 200, which may be a dynamic load. The first power supply 1 is arranged to generate a substantially constant (i.e. fixed) current 1T A first portion of that total current is driven through the first load 100, that first portion being first current Ii. A second portion of that total current is drawn from the first power supply by the second power supply 2. That second portion is current 12. The second power supply 2 is arranged such that the magnitude of this second current 12 is fixed, i.e. it remains constant with time irrespective of any changes in current being driven through the second load 200 by the second power supply 2. As this second current 12 is fixed and the total current 1T is fixed, the first current driven through the first load Ii is also substantially fixed. Advantageously, changes with time in the current being drawn by the second load 200 do not affect the substantially fixed current Ii being driven through the first load 100, and in applications where the first load comprises one or a plurality of LED5 this has the advantage of avoiding flicker. It will be appreciated that l = Ii + 12.

The second power supply 2 draws the fixed second current 12 from the first power supply and uses that fixed current to generate an output voltage across its output terminals 22, and that output voltage drives current through the second load 200. In certain embodiments the second power supply is arranged such that this output voltage is substantially fixed, so that the second load 200 is driven with a substantially constant supply voltage. Thus, as its power requirements vary with time, the current it will draw from the second power supply varies. However, the second power supply provides the advantage that such changes in output current do not affect the fixed current 12 being drawn from the first power supply.

In certain alternative embodiments the output voltage from the second power supply may not be fixed, but variable. For example, rather than the second power supply comprising a constant current source driving (i.e. followed by) a constant voltage source (as in the examples shown in figs. 4 to 8), in certain embodiments the second power supply comprises a first constant current source, arranged to draw the substantially fixed second current from the first power supply, and a second constant current source (which may be dynamic and/or switchable) driven by the first constant current source. This second constant current source in certain embodiments is a switchable current source arranged to run (drive) a second load in the form of an indicator LED, which may be required to blink or flash. Thus, the power requirements of the second load are dynamic, not constant. For example, when the LED is on, it may require a constant current drive, but in the intervening off' times, it requires no current from the second constant current source. In such examples, the second constant current source may also comprise a voltage limit/shunt regulator, but the off load shunt voltage may be significantly higher than the on load voltage when the second load is being driven, thus, the output voltage of the second power supply may vary with time (i.e. may not be fixed).

Moving onto Fig. 10, this shows an alternative power supply apparatus embodying the invention. The arrangement is very similar to that shown in Fig. 9, but here the actual current being driven between the pair of output terminals 12 is the total current l, shared between the second power supply and the first load 100.

Referring now to Fig. 4, this illustrates power supply apparatus embodying the invention which enables a first load to be driven with substantially constant current and a second load to be driven with a substantially constant output voltage. When such an embodiment is used to drive a first load comprising one or more LED5 and a dynamic load it effectively represents apparatus in which power supply rails are derived from a constant current LED driver power supply output which eliminates flicker due to step changes in the second load in an energy efficient manner.

Still referring to Fig. 4, the best way to eliminate the transients in the current being taken from the output of the PSU 1 is by removing the transients in the current of the auxiliary rail in the first instance. Forcing the load on the PSU output to be constant current overcomes the problems of transient currents and flickering. This is achieved in the present embodiment by separating the auxiliary supply into two stages. The first stage is a constant current supply 23 which is set to the maximum current demand of the load circuit. The second stage is a constant voltage shunt regulator 24. By adding these two stages together we create an auxiliary supply rail which takes a constant current without transients from the output of the LED PSU 1 and then generates the fixed Voltage auxiliary rail that is used to power the second load 200.

In a scenario where the load on the auxiliary rail (i.e. how much current is being supplied to the second load) is at maximum, the shunt regulator has to dump little or no current in order to regulate its output at the required voltage. When the load demand on the auxiliary falls to a lower level the shunt regulator dumps the excess current supplied from the constant current supply.

Whilst it may seem wasteful or inefficient to "waste" current and therefore power in this manner, in some applications this is the most energy efficient method.

In an example application the load demand may be only lOmA average with 2OmA peaks.

For a 3.3 Volt output, the peak power would be 66mW and the average supply power would be 33mW. Using the present invention, 66mW (assuming worst case efficiency of 50%), would be permanently dumped as heat energy, but examine the alternative.

Adding a second power conversion stage for the LED driver output will lower the overall efficiency of the driver. Even if the 2nd convertor had a best case efficiency of 95% then a 33W LED driver would lose an extra 1.6W compared to using the present invention.

A further benefit of certain embodiments of the invention is that we now have a way of connecting to existing LED drivers to power external circuits.

Still referring to Fig. 4, it will be appreciated that the input terminals 11 of the first power supply can be connected to an AC or DC power source in different embodiments of the invention. The first power supply 1 can then include an AC/DC or DC/DC converter as appropriate. In this example the first load is an LED load comprising one or more LED5, each arranged to carry at least a portion of the first current Ii. The second power supply 2 is shown connected to the upper or "high" output terminal 121. Input terminals of the second power supply are not explicitly shown in the figure, but it will be appreciated that the second power supply is coupled to the "loW' output terminal 122 via ground. It will also be appreciated that the terms "input terminals" and "output terminals" used throughout the specification may also be regarded as "input rails" and "output rails". The first power supply also comprises a capacitor connected in parallel between the output terminals 121, 122.

The second power supply comprises a capacitor arranged between the constant current source 23 and the constant voltage shunt regulator 24. The second power supply also comprises a further capacitor connected across its output terminals, i.e. across its output supply rail 221 (which can be described as an auxiliary power rail) and the ground rail 222.

The second load 200 is shown connected to the auxiliary power rail 221 and to ground.

Referring now to Fig. 7, this shows another embodiment of the invention. This is an example of connection of the second power supply circuit 2 by-passing the current LED power supply unit current sense. Only current taken by the LED load is sensed by the LED PSU (i.e. the first power supply). In this arrangement, if the LED output was lOOmA and the Vcc circuit constant current source was 5OmA, the LED PSU main power convertor would be supplying l5OmA.

Referring now to Fig. 8, this shows an alternative embodiment of the invention. This is an example of connection of the second power supply circuit 2 directly to the output of an LED driver 1. In this scenario the fixed output current IT of the LED driver will be shared between the LED load and the external circuit.

Thus, with reference to Fig. 7 and 8, embodiments of the invention may be utilised in a wireless communication adaptor product. The adaptor can be supplied directly from the output of the first power supply (e.g. LED driver) before (see Fig. 7) or after (see Fig. 8) the LED PSUs current feedback circuit (taking power after the current feedback will result in reduction of current to the LED). While the wireless adaptors circuitry requires a different load depending on whether or not it is communicating wirelessly, adding the constant current/constant voltage output load PSU results in an unchanging load being applied to the LED output.

Thus, the circuit of Fig. 7 includes current feedback means, comprising a sense resistor and amplifier arranged to amplify a voltage developed to cross the sense resistor, which provides the first power supply (in this example, in particular, the convertor pad of the circuit) with an indication of the current actually flowing in the first load 100 (i.e. an indication of Ii). In such embodiments, the first power supply may be adapted to use this feedback mechanism so as to accurately maintain the output current (Ii) fixed, at the desired value.

Referring again to Fig. 8, this type of circuit includes feedback means (again comprising a sense resistor and amplifier) arranged to provide an indication of the combined magnitude of the first current being driven through the first load 100 and the second current 12 being drawn from the first power supply by the second power supply. In such embodiments the first power supply may be arranged to use the feedback signal to maintain this total current substantially fixed.

Referring now to Fig. 5, this shows a circuit (a power supply) which embodies the invention and which may be utilised as the second power supply in certain embodiments of the invention. It will be appreciated that the constant current source 23 in this example is a Buck convertor with both voltage and current feedback, the current feedback being provided by sense resistor R3, and the voltage feedback being provided to IC1 by the voltage divider arrangement of resistors R7 and R15. The constant voltage shunt circuit 24 provides a substantially fixed output voltage (Vcc output). When the second load is connected between the VCC output and ground, current will, in general, flow through the load. Bipolar transistor TR2 is controllable to dived a portion of the total current provided from the constant current source 23 to the constant voltage shunt 24 through a dump resistor R22. Thus, the controllable conduction path through the transistor (between the collector and emitter of TR2) and the resistor R22 form a current path in parallel to the connected second load.

Opamp IC2B is arranged to control the transistor TR2 to vary the fourth" current flowing in the parallel current path, as the "third" current flowing through the second load varies with time, so as to keep the total current flowing the parallel current path and second load substantially constant. Furthermore, the output voltage is therefore maintained at a fixed value.

Referring to Fig. 5 in more detail, Figure 5 is an implementation of an embodiment the invention in a Flyback LED PSU application. Here the constant current source part of the power supply is a typical buck convertor with both voltage and current feedback. The constant voltage shunt regulator is created using a single operational amplifier (opamp) and transistor. In this chosen configuration using an integrated buck convertor microchip, it is necessary to set the buck convertors output voltage regulation set point to a higher voltage than the shunt regulator output voltage (the Vcc rail), doing so forces the buck convertor into current limit mode giving us a constant current source.

In practice it would be possible to omit voltage feedback on the buck convertor entirely, but having it allows for safer operation. Should the shunt regulator fail open circuit, the buck convertor would limit the output voltage to a lower level, which could prevent further damage to other devices connect to the Vcc rail.

It is not important that the current source is a buck convertor. Any suitable step down switch supply such as inverting buck or sepic convertor could be used, but these would increase complexity and cost of the design.

A linear current source such as a LM317 or similar could also be used but this reduces system efficiency of the design significantly.

Similarly, the Voltage shunt could be replaced with an integrated pad such as a TL431 shunt reference or higher current rated shunt regulator. If accuracy wasn't a crucial requirement the shunt could be replaced with a Zener diode.

One problem to be addressed is excess power consumption during off mode operation. The off mode is where the LED PSU lowers its output voltage and turns any output switches off to disconnect the LED load. In this off mode" more power saving can be made from the LED PSU by making the output voltage as low as possible to reduce the energy consumption (caused by quiescent currents and power stage losses).

The way the circuit of fig. 5 works, by supplying the peak current required by the auxiliary circuits, gives us an unnecessary waste of power in the off mode. In the off mode condition there is no need for the circuit to work to eliminate flicker since the LED5 are off, so to save power we can supply the average current required instead. This is achieved by switching the voltage shunt off, and switching the buck convertors voltage feedback set point to 3V. Now the Buck convertor will regulate the Vcc rail at 3V, moving to a constant voltage mode of operation and since the voltage shunt is turned off only the average current required by Vcc will be sourced.

Figure 6 shows a 2 incarnation of Figure 5, in this a 2nd operation amplifier is used to sense and detect the LED PSU output voltage. When the voltage on the LED output falls below a set threshold (set by R13, 14, R19, R12 & R18) the opamp IC2A switched of the Constant voltage shunt regulator by pulling the Non-inverting input of IC2B low via D4. At the same time the Gate of TR3 is pulled low switching TR3 off, this sets the Buck convertors output voltage to a lower level.

Still referring to Fig. 6, it will be appreciated that the positive input potential divider (arranged to provide the positive input to opamp IC2A is set to allow up to 60V on output without the need for protection components. The negative input is set at 217mV. When the LED output voltage drops below 5V the Buck convertor's output is set at 3V and the current sink circuit is turned off. With regard to opamp IC2B, this opamp is used to regulate a 3V rail. Bipolar transistor TR2 is used as a current sink. In run mode operation the Buck convertor is set to output a constant current of 4OmA (fractionally more than the maximum required current of the second load, which in certain embodiments is a wireless communication module, such as a Zigbee module, and its associated circuits). Transistor TR2 sinks excess current to maintain the rail at 3V. In other words, transistor 1R2 is controlled to maintain the output voltage at 3V to the second load. Resistor R18 limits the maximum amount of current that the transistor can take to just over the maximum provided by the Buck, and therefore reduce the gain of the circuit. Like the circuit of Fig. 5, the circuit of Fig. 6 incorporates a Buck convertor with both voltage and current feedback.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, or characteristics described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

It will be also be appreciated that, throughout the description and claims of this specification, language in the general form of "X for Y" (where Y is some action, activity or step and X is some means for carrying out that action, activity or step) encompasses means X adapted or arranged specifically, but not exclusively, to do Y.

Claims

CLAIMS: 1. Power supply apparatus comprising: a first power supply having a first pair of input terminals for connecting to a power source, and a first pair of output terminals for connecting to a first load; a second power supply coupled to said first output terminals, and having a second pair of output terminals for connecting to a second, dynamic load; wherein the first power supply is operable, when the first load is connected to the first pair of output terminals, to drive a first current between said first pair of output terminals through the first load; and the second power supply is arranged to draw a substantially fixed second current from said first power supply when said first power supply is operating to drive said first current through the first load, and use said second current to generate an output voltage across said second pair of output terminals to power the second, dynamic load when connected to the second pair of output terminals.
2. Apparatus in accordance with claim 1, wherein the output voltage is substantially fixed.
3. Apparatus in accordance with any preceding claim, wherein the second power supply is arranged to drive a third current through the second load when connected, and to drive a fourth current through a current path between said second pair of output terminals and in parallel with the second load.
4. Apparatus in accordance with claim 3, wherein the second power supply is adapted to control a magnitude of said fourth current in response to changes in the magnitude of the third current driven through the dynamic load so as to maintain a substantially constant voltage across said second pair of output terminals.
5. Apparatus in accordance with claim 3 or claim 4, wherein said current path includes a resistor.
6. Apparatus in accordance with any one of claims 3 to 5, wherein the second power supply comprises a transistor, said current path comprises a controllable current path through the transistor, and the second power supply further comprises control means is arranged to supply a control signal to the transistor to control a conductivity of said controllable current path.
7. Apparatus in accordance with any one of claims 3 to 6, wherein the third current is a first portion of the second current, and the fourth current is a second portion of the second current.
8. Apparatus in accordance with any one of claims 3 to 7, wherein the second power supply comprises current feedback means arranged to provide an indication of the combined magnitude of the third and fourth currents and the second power supply is arranged to use said indication of the combined magnitude to maintain said combined magnitude substantially constant.
9. Apparatus in accordance with any preceding claim, wherein the second power supply comprises voltage feedback means arranged to provide an indication of a voltage across said second pair of output terminals, and the second power supply is arranged to use said indication of a voltage to maintain said voltage substantially constant.
10. Apparatus in accordance with any preceding claim, wherein the second power supply comprises a constant current source circuit and a constant voltage shunt circuit, the constant current source circuit being arranged to draw said substantially fixed second current from the first power supply and provide a substantially fixed current to the constant voltage shunt circuit, the constant voltage shunt circuit being arranged to supply a substantially constant voltage across said second pair of output terminals to drive the dynamic load.
11. Apparatus in accordance with any preceding claim wherein the first power supply comprises current feedback means arranged to provide an indication of the magnitude of the first current driven through the first load, and the first power supply is arranged to use said indication of the first current's magnitude to maintain the magnitude of the first current substantially constant.
12. Apparatus in accordance with any preceding claim wherein the first power supply comprises current feedback means arranged to provide an indication of the combined magnitude of the first current and the second current, and the first power supply is arranged to use said indication of the first and second currents' combined magnitude to maintain the combined magnitude of the first and second currents substantially constant.
13. Apparatus in accordance with any preceding claim, further comprising said first load connected to said first pair of output terminals.
14. Apparatus in accordance with claim 13, wherein said first load comprises at least one LED arranged to be powered by at least a portion of the first current.
15. Apparatus in accordance with any preceding claim, further comprising said second load connected to said second pair of output terminals.
16. Apparatus in accordance with claim 15, wherein said second load comprises a wireless communication module or circuit.
17. A method of driving a first load and a second load, wherein the second load is a dynamic load, the method comprising: operating a first power supply, connected to a power source, to drive a first current through the first load; operating a second power supply, coupled to the first power supply, to draw a substantially fixed second current from said first power supply when said first power supply is operating to drive said first current through the first load, and use said second current to generate an output voltage to power the second, dynamic load.
18. A method in accordance with claim 17, wherein the output voltage is substantially fixed.
19. A method in accordance with any one of claims 17 or 18, wherein operating the second power supply comprises driving a third current through the second load and driving a fourth current through a current path in parallel with the second load.
20. A method in accordance with claim 19, wherein operating the second power supply comprises controlling a magnitude of said fourth current in response to changes in the magnitude of the third current driven through the dynamic load so as to maintain said output voltage substantially constant.
21. A method in accordance with any one of claims 19 and 20, wherein the third current is a first portion of the second current, and the fourth current is a second portion of the second current.
22. A method in accordance with any one of claims 19 to 21, wherein operating the second power supply comprises using current feedback means to provide an indication of the combined magnitude of the third and fourth currents, and using said indication of the combined magnitude to maintain said combined magnitude substantially constant.
23. A method in accordance with any one of claims 17 to 22, wherein operating the second power supply comprises using voltage feedback means to provide an indication of said output voltage, and using said indication of output voltage to maintain said output voltage substantially constant.
24. A method in accordance with any one of claims 17 to 23, wherein the second power supply comprises a constant current source circuit and a constant voltage shunt circuit, and operating the second power supply comprises operating the constant current source circuit to draw said substantially fixed second current from the first power supply and provide a substantially fixed current to the constant voltage shunt circuit, and operating the constant voltage shunt circuit to supply said output voltage to drive the dynamic load.
25. A method in accordance with any one of claims 17 to 24, wherein operating the first power supply comprises using current feedback means to provide an indication of the magnitude of the first current driven through the first load, and using said indication of the first current's magnitude to maintain the magnitude of the first current substantially constant.
26. A method in accordance with any one of claims 17 to 25, wherein operating the first power supply comprises using current feedback means to provide an indication of the combined magnitude of the first current and the second current, and using said indication of the first and second currents' combined magnitude to maintain the combined magnitude of the first and second currents substantially constant.
27. A power supply comprising: means for coupling the power supply to a first power supply arranged to drive a first load; and an output terminal for connecting to a second load, the power supply being arranged to draw a substantially fixed current from said first power supply when said first power supply is operating to drive current through the first load, and use said fixed current to generate an output voltage on said output terminal to power the second load when connected to the output terminal.
28. A power supply in accordance with claim 27, wherein said output voltage is variable, dependent on a power demand of the second load.
29. A power supply in accordance with claim 27, arranged such that the output voltage is substantially fixed.
30. A power supply, power supply apparatus, or a method of driving a first load and a second load, substantially as hereinbefore described with reference to figures 4 to 10 of the accompanying drawings.