GB2498836A - LED power supply circuit - Google Patents

Abstract

A linear power supply circuit 100, operable to receive a rectified AC voltage supply having an instantaneous voltage which varies between a minimum zero-crossing value and peak value as a function of time, comprises an LED arrangement 106 having a minimum drive voltage above which supply current is operable to flow through said LED arrangement, a drive circuit section 108 operable to control the supply current through the LED arrangement, and a load ballast circuit 110 arranged in parallel with said LED arrangement and operable to enable supply current to flow therethrough when the instantaneous voltage across the LED arrangement is less than the minimum drive voltage such that the circuit is operable to provide current control across the whole voltage range of the AC voltage supply. The circuit provides a high power factor supply with low harmonic distortion suitable to power aircraft lighting.

Description

LED Power Supply Circuit Thc prcscnt invcntion rclatcs to a power supply circuit. More preferably, the present invention relates to power supply circuit for LED lighting applications with low supply current distortions.

Increasingly, light emitting diodes (LEDs) arc replacing conventional filament or fluorescent light sources in domestic and commercial lighting applications. LEDs have a low power consumption and a long lifetime. In addition, being solid-state devices, they have an intrinsic resistance to shock and damage. These, and other properties, make LEDs suitable for a variety oflighting applications.

As LED technology improves, LEDs will become more efficient. Therefore, the need to make LED drivers highly efficient to compensate for poor LED efficiency will be reduced. Consequently, the technological demands from the power supply will focus less on power supply efficiency and more on clean power management with reduced exported noise and supply current distortion. The combined effect of thousands of lower power loads (such as in lighting of aircraft cabins, big builds and thousands of homes) may cause a significant problem if exported noise, current harmonics and conducted emissions are not controlled. This may lead to unnecessarily larger power generators and supply cables to reduce current losses in the distribution of electricity.

One such application for clean power management is that of lighting for commercial or private aircraft. In general, commercial aircraft operate on AC power having a frequency which is generally in the range from under 300Hz to over 800Hz.

The requirements for low interference on commercial aircraft demand that onboard electronic devices have extremely low supply current harmonic distortion and conducted emissions, and unity power factor devices. This is to reduce the effect of interference on sensitive aircraft instrumentation. However, known arrangements for powering LEDs from an AC supply suffer from a number of technical problems which prevent such low emission and distortion standards from being reached.

Figure I shows a conventional linear constant-current circuit I 0 for an LED arrangement 12. The linear constant-current circuiL 10 comprises an AC power source 14, a rcctifler 16, a fcedback resistor 18, a current control and detection transistor pair 20 and a current detection resistor 22.

A problem with such a circuit is that, for a considcrable portion of a given drive cycle of the circuit, the LED arrangement will not be illuminated. Figure 2 illustrates the problem. An LED requires a particular threshold forward voltage drop Vf across it before it will illuminate. This is due to the bandgap of the semiconductor material from which the LED is formed. Below Vf the LED will not illuminate and no current will flow through the LED. Above Vfthc current drawn by the LED is approximately exponentially dependent upon the voltage drop across the LED, limited by thc current control and detection transistor pair 20.

As shown in Figure 2, when a rectified AC voltage is used to power an LED (or a series array of LEDs, in which case Vf of the array will be the sum of the individual Vf values of the LEDs comprising the array), the LED arrangement is only lit when the voltage is above Yr. Therefore, for the remainder of a voltage cycle, the LED is unlit and no current flows therethrough, the supply current being turned off Therefore, the current in the circuit will not follow the voltage directly. This leads to generation of unwanted harmonics in the circuit which renders the circuit unsuitable for usc in environments where low harmonies and conducted emissions are required, such as on board aircraft and, increasingly, in domestic and commercial lighting.

Attempts to improve the power efficiency of LED power circuits have been proposed. US-A-201 1/0199003 discloses an arrangement whercby, during a power cycle, LED circuits are gradually added in to the circuit to create a stepped current draw. EP-A- 2 400 819 discloses a power saving LED lighting apparatus which switches LED strings in and out as the rectified mains voltage rises and falls. US-A-7,08 1,722 discloses an arrangement whereby LED strings are progressively switched in/out to improve power efficiency within a power cycle. US-A-201 1/02731 03, US-A-20l2/0001 558 and WO-A- 12/034102 disclose arrangcmcnts whereby strings of LEDs arc switched in and out during a cycle. However, with improvcmcnts in LED tcchnologics thcsc arrangcmcnts to improvc powcr cfficicncy will bccomc lcss important.

However, such arrangements would bc unsuitable for a commercial aircraft. Thc switching in!out of LEDs within a circuit gcncratcs additional harmonics and current distortion in the form of current spikes which renders this arrangement unsuitable for commcrcial aircraft, without additional flltcring. Howcvcr, additional flltcring adds additional complexity to a powcr supply circuit and introduccs additional current control problcms associated with inductivc clcmcnts.

Further, none of these arrangements is operable to control the supply current when the instantaneous supply voltage is so low that the lowest-voltage LED chain cannot be lit. Furthermore, the above disadvantages aside, switching of LEDs would be impractical for the long strip lighting used in aircraft, where such brightness variations would be readily apparent.

Therefore, a technical problem exists in the art that known solutions to improve power efficiency of LED arrangements are impractical for many lighting purposes and are unable to reduce emissions and harmonic distortion to the required level for applications such as commercial aircraft or cvcn domestic/commercial lighting. In ordcr to meet fhture emission requirements, thc controlling supply harmonics and emissions of conductcd noise will bc of significant importance. Thc present invcntion addresses, in one aspect, this issue.

According to a first aspect of the present invention, there is provided a linear power supply circuit operable to receive a rectified AC voltage supply having an instantaneous voltage which vanes between a minimum zero-crossing value and peak S value as a lunction of time, the linear power supply circuit comprising; an LED arrangement having a minimum drive voltage above which supply current is operable to flow through said LED arrangement; a drive circuit section operable to control the supply current through the LED arrangement; and a load ballast circuit arranged in parallel with said LED arrangement and operable to enable supply current to flow therethrough when the instantaneous voltage across the LED arrangement is less than the minimum drive voltage such that the circuit is operable to provide current control across an entire power cycle of the AC voltage supply and such that the current is controlled across the whole instantaneous voltage range of the AC voltage supply between the zero crossing point to the maximum instantaneous voltage.

By providing such an arrangement, an additional load ballast circuit introduced to a linear circuit design is operable to drain the supply current when the instantaneous mains AC voltage is insufficient to drive the load. This forces the remaining supply current to follow the supply voltage curve, creating a simple unity power factor power supply with extremely low supply current distortion. The present invention thus provides a power supply which is operable at frequencies ranging from DC to over 800 Hz, and from low to high voltages. Whilst LEDs are a preferred application, this need not be so and other load devices may be driven.

In one embodiment, the drive circuit section comprises an operational amplifier operable to control the supply current and the load ballast circuit comprises switching means to enable current to flow through the load ballast circuit when the instantaneous voltage across the LED arrangement is less than the minimum drive voltage of said LED arrangement.

In one embodiment, the switching means are switched in dependence upon the output of the operational amplifier.

Tn one embodiment, the switching means comprises a diode.

In one embodiment, the load ballast circuit comprises at least one field effect transistor.

In one embodiment, the drive circuit section comprises a transistor pair.

In one embodiment, the LED arrangement comprises a plurality of LEDs arranged in series connection.

In one embodiment, the circuit further comprises a rectifier connectable to an AC power source and operable to generate the rectified AC voltage signal therefrom.

In one embodiment, the AC power source comprises an aircraft power supply.

Tn one embodiment, the AC power source has a frequency in the range of 300- 800 Hz. In one embodiment, the AC power source has a RMS voltage of II 5V.

According to a second aspect of the present invention, there is provided an aircraft lighting apparatus comprising the circuit the first aspect.

In one embodiment, there is provided a power supply network comprising an AC power source connected to a multiplicity of power supply circuits according to the first aspect.

According to a third aspect ofthe present invention, there is provided a method of controlling the supply current in a linear power supply circuit, the method comprising: receiving a rectified AC voltage supply having an instantancous voltage which varies betwecn a minimum zero-crossing value and peak value as a ffinction of time; driving an LED arrangement with said rectiFied AC voltage supply, the LED arrangement having a minimum drive voltage above which supply current is operable to flow through said LED arrangement; and controlling the supply current by: utilising a drive circuit section operable to control the supply current when the AC voltage drop across the LED arrangement is equal to or greater than the minimum drive voltage; and switching to a load ballast circuit arranged in parallel with said LED arrangement and operable to enable supply current to flow therethrough when the instantaneous voltage across the LED arrangement is less than the minimum drive voltage such that the circuit is operable to provide current control across an entire power cycle of the AC voltage supply and such that the current is controlled across the whole instantaneous voltage range of the AC voltage supply between the zero crossing point to the maximum instantaneous voltage.

In one embodiment, the drive circuit section comprises an operational amplifier operable to control the supply current and the load ballast circuit comprises switching means to enable current to flow through the load ballast circuit when the instantaneous voltage across the LED arrangement is less than the minimum drive voltage of said LED arrangement.

Tn one embodiment, the switching means are switched in dependence upon the output of the operational amplifier.

In one embodiment, the switching means comprises a diode.

In one embodiment, the load ballast circuit comprises at least one field effect transistor.

In one embodiment, the drive circuit section comprises a transistor pair.

Tn one embodiment, the LED arrangement comprises a plurality of LEDs an-angcd in scncs connection.

In one cmbodimcnt, the AC power sourcc comprises an aircraft powcr supply.

In one cmbodimcnt, the AC powcr sourcc has a frequency in thc rangc of 300- 800 Hz. In one cmbodimcnt, thc AC powcr sourcc has a RMS voltage of 11 5V.

Embodimcnts ofthc present invention will flOW bc described in dctail with reference to the accompanying drawings, in which: Figurc 1 is a schematic diagram of a known linear powcr circuit; Figure 2 is a schematic diagram of a rectified AC voltage signal illustrating the conduction cycle of a driven LED; Figure 3 shows a circuit according to an embodiment of the present invention; Figure 4 shows a schematic diagram similar to Figure 2 but illustrating the two regimes of operation of the circuit of Figure 3; FigureS shows an example of the total input power to the circuit of Figure 3 across a half cycle; Figurc 6a) shows the current loss across an LED arrangcment across the half cycle shown in Figure 5 and for thc samc input powcr; Figurc 6b) shows the currcnt loss across a load ballast circuit across thc half cyclc shown in Figure 5 and for the same input power; Figure 7a) shows the power output from the LED arrangement across a half cycle and for the same input power; and

S

Figure 7b) shows the data of Figures 5, 6a), 6b) and 7a) superimposed.

The present invention relates to a low power linear circuit operable to force the supply current to follow the supply voltage wave shape and frequency. In one embodiment, this is implemented utilising a circuit which does not require expensive filtering and thus does not require the use of capacitors or wound components in the supply line. This circuit full fills the requirements specifically for but not limited to aircraft applications where in many cases the supply frequency can vary from as low as 300Hz and up to over 8 00Hz whilst requiring very low supply current harmonic distortion and very low Conducted Emissions. The circuit is operable down to DC power and thus can cover a range of frequency and voltage requirements.

Figure 3 shows an embodiment of the present invention. A linear power supply circuit 100 is provided. The linear power supply circuit 100 is operable to receive power from an AC supply 102. Tn this embodiment, the AC supply comprises an aircraft supply of 1 15V 400Hz AC.

A rectifier 104 is provided to rectifr the received AC supply 102. A first resistor R1 is located between the positive voltage rail (shown by +V on Figure 3) and Ground (OV). Resistor Ri is operable to provide a minimum load to the circuit during the time when the remainder of the circuit 100 is inactive at very low voltages (in this embodiment, below about 1.5V).

Resistor R2 and Zener diode Z1 are also provided in series between the positive voltage rail and ground in parallel with capacitor C1 to provide a fixed micro-power supply for the Op Amp A1 and the bias for the gate of FETs T1 and T2. In other words, capacitor CI is used to hold up the I OV supply for the micro-power Op Amp A1 and to S provide the bias For the gate oF the FETs T1 and T2.

Resistors R3 and R4 are also provided in series between the positive voltage rail and ground and provide the rcfcrcncc signal for the Op Amp A1. In other words, resistors R3 and R4 attenuate the rectified rail voltage to dcrivc the signal V1 for the Op Amp A1.

The circuit 100 further comprises a load section 106, a constant current driver section 108 and a load ballast circuit section 110. In this embodiment, the load section 106 comprises a plurality of LEDs arranged in series (or a string). However, other loads may be used; for example, a single LED or alternative constant current devices.

The constant current driver section 108 comprises an operational amplifier A1, and two Th-ansistors T and T2. In this embodiment, T comprises an n-channel MOSFET, and 12 comprises a bipolarjunction flnsistor. Resistors R5 and R6 are provided to ensure an equal load is provided to the gate of transistor T2.

The load ballast circuit section 110 is arranged in parallel with the load section 106. In this embodiment, the load ballast circuit section 110 comprises a diode DI, and two transistors T2 and T4. In this embodiment, T2 comprises an n-channel MOSFET, and T4 comprises a bipolarjunction transistor (BJT). Resistors R7 and R8 are provided to ensure an equal load is pmvided to the gate of transistor 12.

The non-inverting operational amplifier A1 and the respective gates of thc FETs T and T3 arc powered by a rcfcrcncc voltage V1 provided from the differential of the voltage drop across resistors R3 and R4 (the amplifier voltage being taken across R) Resistor R2, Zcncr diode Z1 and capacitor Ci ensure that the supply rcccivcd is a stable DC power supply (which is 1 OV in this embodiment). Utilizing micro-power control circuits Operational amplificr A1 acts to maintain thc voltagc V2 across resistor R9 equal o voltage Vi. Therefore, in a case where V2 < VI, A1 will generate a greater voltage at Lhe gate oftransistor 12, increasing thc collcctor-cmittcr current of 12 flowing from thc positive rail through the load section 106. Concomitantly, when V., »= V1, no bias is applied to the gate of transistor T2 and so no current flows therethrough.

ID Since the gate ofT1 is powered by a constant DC source, the MOSFET is always open and acts as a current-limiting device in the circuit, controlled by the action of T2 on the source ofT1. Transistor Ti is also a voltage level shifter to operate over an instantaneous voltage up to I 000V.

In alternative embodiments, fiirthcr controlling elcrnents may bc added; for examplc, a modified voltage rcfcrcncc V1 to stabilisc the current in R9against variations in supply RMS voltage.

The circuit 100 is operable to provide a supply current within the circuit that follows exactly the supply voltage within the circuit. Thcrcforc, whilst conventional circuits provide an LED current that follows the supply current during the conduction time of the LEDs (i.e. during the shaded portion shown in Figure 2), the present invention enables the supply current to follow the supply voltage throughout a power cycle, resulting in a unity power factor circuit with low noise, low emissions and low harmonics. ftrther, because there is no requirement to switch LEDs in and out of the circuit, no current spikes (which generate harmonics) will result.

The operation of the circuit 100 according to an embodiment of the present invention will now be described with reference to Figures 3 and 4.

Similarly to Figure 2, Figure 4 shows a graph of instantaneous voltage across the load section 106 in the circuit 100 as a function of time. Each cycle is subdivided into two regions -region A and region B. Region A is when the instantaneous voltage is equal to or greater than the forward voltage Vfofthe load section 106 (in this case, a series array of LEDs, iii which case the forward voltageYf is the sum of the Forward voltages oF each of the LEDs in the string). In other words, region A covers thc voltage range between Vf and the maximum (or peak) voltage Vrnax. In region A, the LEDs emit light.

Region B is when the instantaneous voltage is insufficient to drive the load section 106. This is when the instantaneous voltage across the load section 106 is in the range from the zcro crossing minimum value up to but not including thc fonvard voltage Vf. Consequently, no current flows through the LEDs forming the load section. No light is emitted by the LEDs in this region.

Consider first region A. When the instantaneous voltage across the load section 106 is equal to, or greater than, the forward voltage Vf of the load section 106, current flows through the load section 106, through transistor T1 (which is in an "on" state by virtue of the gate ofT1 being biased at voltage Vi) which acts as a current limiting device to the collector of BJT T2. The base of BJT T2 is biased by the output of operational amplifier A1 and controls the current flow through T2 in order to maintain V2 = V1.

At the transition between regions A and B, when the instantaneous voltage drops below Vt; no current will flow through the load section 106, and through T1 and T2. This will cause V2 to drop. In other words, as the LED current falls the ability of the BJT T2 to supply the current in resistor R9 to maintain V2 Vi is diminished. Consequently, the output of Operational Amplifier A1 incrcascs to increase the current flow to the base of BJT T2. In a conventional arrangement, this will make no difference since the current flowing from the positive rail through the load section 106 is small or zero.

However, by provision of the load ballast section 110, the circuit 100 of the present invention enables current to flow to resistor R9 to increase V2 seamlessly as follows.

As Lhe volLage output oloperational amplifier A1 increases, the diode D1 will be biascd on. This will enable current to flow to thc base of BJT T4, which is switched on.

When BiT T4 switches on, current flows from thc positive rail, through current limiting MOSFET T and the collector-emitter region of T4, to resistor R9. In other words, in region B, transistors 13 and T4 arc operable to maintain the current in R9 when the supply voltage is too low to power the load section 106 (i.e. the LEDs). The operational amplifier Al therefore continues to control the current via T4 and T3 through the remainder of region B until the supply voltage rises again during the next half cycle.

When the instantaneous voltage is equal to the forward voltage Vf of the load section 106 (i.e. at the start of region A of the next cycle), then current will flow through Ti andT2, resulting in an increase in V2. Concomitantly, the output of the operational amplifier A1 will be reduced, switching off diode D1. In other words, in region A, the combination ofTi and T2 start to supply current to resistor R9 again. The process then reverts to the original condition of driving through the LEDs enabling the LED current to rise and fall in line with the instantaneous supply voltage again.

As set out above, the change over between the regimes of region A and region B is achieved by the use of Diode D1 between the controlling element for the load section 106 (i.e. T1 andT2) and the controlling elements (i.e. T3 andT4) of the load ballast section 110. As an additional benefit, the use of the MOSFETs Ii and T2 enables the voltage range of the current controlling Bipolar Transistors T2 and T4 to be increased.

In summary, the additional circuit (the load ballast section 110) provided in parallel with the LEDs is operable to take up the supply current waveform when the supply voltage is insufficient to power the LEDs. The load ballast section 110 enables the circuit 100 to control the current in the circuit right down to the zero crossing minimum voltage. This reduces or even eliminates harmonic distortion and noise, and power efficiency is concomitantly increased. The provision of the arrangement of the present invention enables this transition to occur inherently and elegantly as the LED current falls S oFT. This, thereFore, creates a near seamless transiLion bek een the two elenienLs and will happen even as the LED forward voltage drifts.

In other words, because the transition is controlled by the output from the operational amplifier A1 and the switch on voltage of the diode Di, the transition is independent of V for the load section 106. Consequently, the same circuit can bc utiliscd with different load section 106 components; for example, one or more LEDs; LEDs of different colour; or alternative components.

Figures 5 to 7 illustrate cxamplcs of the operational parameters of the circuit 100.

FigureS shows a typical input power into the circuit 100 across a half cycle (i.e. from a given zcro-crossing point to the next zero-crossing point). In this example, the average total input power is 9.15 Watts.

Figures 6a) and 6b) show the power loss (in Watts) across the half cycle shown in FigureS. Figure 6a) shows the power loss (in Watts) due to the LED arrangcment. There is no power loss when the instantaneous voltage is below Vç (i.e. in region B shown in Figure 4). However, in region A (shown in Figure 4), the LED arrangement is switched on and current can flow. In this region, the magnitude of the power loss is, of course, a ftnction of the magnitude of the instantaneous power shown in FigureS.

Figure 6b) shows the power loss due to the load ballast section 110. Given the nature of the switching as described above, current only flows through the load ballast section 110 when the instantaneous voltage is below Vf(i.c. in region B shown in Figure 4).

Figure 6b) clearly shows the power loss in the load ballast section 110 being right down to thc zero crossing point, indicating that the load ballast section ItO is dissipating current in the circuit 100 throughout the entire portion of a half cycle in which the instantaneous voltage is below Vf and the LED arrangement is not conducting current.

This demonstraLes how curreni control is implemented across an enlire power cycle utilising the present invention. In this example, the average power loss across the load ballast section 110 is 1.61 Watts.

Figure 7a) shows the power output from the LED arrangement, In this example, the total power from the LED is 5.81 Watts. Figurc 7b) shows a combined powcr graph.

Figure 7b) thus includes the data from Figures 5, 6a), 6b) and 7a). From this data, the overall power efficiency of the arrangement can be determined and this calculated to be approximately 64%.

However, this value is dependent upon the efficiency of the LED arrangement and this, with development of more efficient LED technology, will rise. Further, by optimising the peak magnitude of the instantaneous Voltage and Vç of the LEDs, the period within a given half cycle for which the LED arrangement is unlit can be shortened.

In summary, power efficiency can be improved in numerous ways known in the art.

However, the more crucial issue addrcsscd by the present invention is to provide a circuit which is operable to significantly reduce or eliminate harmonic distortions due to a lack of current control across the entire power cycle.

Variations of the above embodiments will be apparent to the skilled person. The precise configuration of hardware components may differ and still fall within the scope of the present invention. The circuit of the above-described embodiment is suitable for a solid-state LED cabin lighting power supply. This however is not the limitation of the circuit as the applications can bc more general in offering low power outputs not limited to aircraft or LED lighting.

The circuit can, for example, be used for the low power applications in commercial and domestic lighting and othcr applications where high quality and low supply current distortion is required and for providing high quality low power output for many requirements. This becomes increasingly important as LEDs become more efficient and the required power levels are reduced.

Further, any number of LEDs may be utilised, provided they are included as a single series-connected string. Any colour or configuration of LED may be used. A series string of LEDs as described may comprise a plurality ofseries strings provided that the subsequent strings are matched to the first string in terms of forward voltage and Vf with temperature. The group of series strings then thnetional electronically as a single string.

Further, a diode need not be utilised and other switching arrangements may be used.

The present invention provides a power supply having negligible or very low power supply current distortion. The present invention, thus, addresses a significant problem associated with supplying power to thousands of low power supply loads. Tn conventional arrangements, each low power supply load will export supply current distortion and the effect of this on the supply infrastructure (which may supply many thousands of individual loads) is significant and material. The combined effect of thousands of uncontrolled low power lights/power supplies connected to a power generator will cause extra costs in cabling and generators to handle the current distortion.

This problem is addressed, in embodiments, by means of a load ballast circuit arranged in parallel with the LED load device. This provides control of current in the power supply circuit from the zero crossing voltage up to the maximum instantaneous voltage. In contrast, kno\vn arrangements are unable to control the current across the entire voltage range and would lead to significant power supply distortions.

Embodiments ofthe present invention have been described with particular reference to the examples illustrated. While specific examples are shown in the drawings and are herein described in detail, it should be understood, however, that the drawings and detailed description are not intended to limit the invention to the particular form disclosed. Tt will be appreciated that variations and modifications may be made to the examples described within the scope of the present invention.

Claims (1)

1. <claim-text>CLAIMSA linear power supply circuit operable to receive a rectified AC voltage supply having an instantaneous voltage which varies between a minimum zero-crossing value and peak value as a Function ohmic, the linear power supply circuit comprising: an LED arrangcment having a minimum drive voltage above which supply current is operable to flow through said LED arrangement; a drive circuit scction operable to control the supply current through the LED arrangement; and a load ballast circuit arranged in parallel with said LED arrangement and operable to enable supply current to flow therethrough when the instantaneous voltage across the LED arrangement is less than the minimum drive voltage such that the circuit is operable to provide current control across an entire power cycle of the AC voltage supply and such that the current is controlled across the whole instantaneous voltage range of the AC voltage supply from the zero crossing point to the maximum instantaneous voltage.</claim-text> <claim-text>2. A power supply circuit according to claim 1, wherein the drive circuit section comprises an operational amplifier operable to control the supply current and the load ballast circuit comprises switching means to enable current to flow through the load ballast circuit when the instantaneous voltage across the LED arrangement is less than the minimum drive voltage of said LED arrangement.</claim-text> <claim-text>3. A power supply circuit according to claim 2, wherein the switching means are switched in dependence upon the output of the operational amplifier.</claim-text> <claim-text>4. A power supply circuit according to claim 2 or 3, wherein the switching means comprises a diode.</claim-text> <claim-text>5. A power supply circuit according to claim 2, 3 or 4, wherein the load ballast circuit comprises at least one field effect transistor.</claim-text> <claim-text>6. A power supply circuit according to any one of the preceding claims, wherein the drive circuit section comprises a transistor pair.</claim-text> <claim-text>7. A power supply circuit according to any one oF the preceding claims, wherein the LED arrangement comprises a plurality of LEDs arranged in series connection.</claim-text> <claim-text>8. A power supply circuit according to any one of the preceding claims, ifirther comprising a rectifier connectable to an AC power source and operable to generate a the rectified AC voltage signal therefrom.</claim-text> <claim-text>9. A power supply circuit according claim 8, wherein the AC power source comprises an aircraft power supply.</claim-text> <claim-text>10. A power supply circuit according to claim 9, wherein the AC power source has a frequency in the range of 300-800 Hz.</claim-text> <claim-text>11. A power supply circuit according to claim 10, wherein the AC power source has a RMS voltage of II 5V.</claim-text> <claim-text>12. Aircraft lighting apparatus comprising the circuit of any one ofthe preceding claims.</claim-text> <claim-text>13. A power supply network comprising an AC power source connected to a multiplicity of power supply circuits as claimed in any one of claims ito 11.</claim-text> <claim-text>14. A method of controlling the supply current in a linear power supply circuit, the method comprising: receiving a rectified AC voltage supply having an instantaneous voltage which varies between a minimum zero-crossing value and peak value as a ftinction of time; driving an LED arrangement with said rectified AC voltage supply, the LED arrangement having a minimum drive voltage above which supply current is operable to flow through said LED arrangement; and controlling the supply current by: S utilising a drive circuit section operable to control the supply current when the AC voltage drop across the LED arrangement is equal to or greater than the minimum drive voltage; and switching to a load ballast circuit arranged in parallel with said LED arrangement and operable to enable supply current to flow therethrough when the instantaneous voltage across the LED arrangement is less than the minimum drive voltage such that the circuit is operable to provide current control across an entire power cycle of the AC voltage supply and such that the current is controlled across the whole instantaneous voltage range of thc AC voltage supply from the zero crossing point to the maximum instantaneous voltage.</claim-text> <claim-text>IS. A method according to claim 14, wherein the drive circuit section comprises an operational amplifier operable to control the supply current and the load ballast circuit comprises switching means to enable current to flow through the load ballast circuit when the instantaneous voltage across the LED arrangement is less than the minimum drive voltage of said LED arrangement.</claim-text> <claim-text>16. A method according to claim IS, wherein the switching means are switched in dependence upon the output of the operational amplifier.</claim-text> <claim-text>17. A method according to claim 15 or 16, wherein the switching means comprises a diode.</claim-text> <claim-text>18. A method according to claim 15, 16 or 17, wherein the load ballast circuitcomprises at least one field effect transistor.</claim-text> <claim-text>19. A method according to any one of claims 14 to 18, wherein the drive circuit section comprises a transistor pair.</claim-text> <claim-text>20. A method according to any one of claims 13 to 18, wherein the LED arrangement S comprises a plurality oCLEDs arranged in series connection.</claim-text> <claim-text>21. A method according to any one of claims 14 to 20, wherein the AC power source comprises an aircraft power supply.</claim-text> <claim-text>22. A method according to claim 21, wherein the AC power source has a frequency in the range of300 -800 Hz.</claim-text> <claim-text>23. A method according to claim 22, wherein the AC power source has a RMS voltage of 115 V. 24. A power supply circuit substantially as heitinbefore described and/or illustrated with reference to the accompanying drawings.25. Aircraft lighting apparatus substantially as hereinbefore described and/or illustrated with reference to the accompanying drawings.26. A method of controlling current in a power supply circuit substantially as hereinbefore described andior illustrated with reference to the accompanying drawings.</claim-text>