Patent Description:
HID lamps are essentially AC devices and have a negative resistance characteristic. As a consequence, power is supplied by an AC mains supply and a ballast or choke is connected in series with the lamp so as to prevent damage to the lamp from excessive currents. A typical HID lamp such as a 1000W Metal Halide has an ignition voltage of <NUM> V peak, an operating voltage of 260V and an operating current of 4A giving a lamp power of approximately 1000W and a light output of <NUM>,<NUM> Lumens.

In recent years lamps having one or more light emitting diodes (LEDs) have replaced filament lamps and low intensity discharge lamps such as fluorescent lamps. As the power capability of such LEDs has increased, so LEDs have come to be used in outdoor lighting such as street lighting and stadium lighting where considerable powers (for example in excess of <NUM> kW) are required.

Clearly, as the word "diode" implies, LEDs are DC (or unidirectional current) devices. In addition, a LED has a positive resistance characteristic (in which the resistance varies with the current flowing through the LED). As DC devices, where LEDs are supplied from the AC mains, there is normally a rectifier of some description supplied by the mains and the LEDs (together with their drivers or control circuits) are powered by the output of the rectifier. As a consequence, the DC control circuits for LED lamps are completely different from the AC control circuits for HID lamps.

One example of a high powered LED has a nominal voltage of 6V and draws a nominal current of <NUM>. However, the LED can have a current of up to <NUM>. A LED lamp fixture may typically include <NUM> such LEDs connected in series. Such a LED lamp fixture has an operating voltage of 150V and a nominal operating current of <NUM>. 1A giving a lamp power of approximately 315W and a light output of <NUM>,<NUM> Lumens. In some cases matched LEDs are connected in parallel, in which case the total current is shared.

Another example of a high powered LED has <NUM> LEDs in one string, has a nominal voltage of 225V, a nominal current of <NUM>. 1A, a nominal wattage of 475W and a light output of <NUM>,<NUM> Lumens.

Normally a control circuit in the form of an electronic driver of some kind (either a constant current circuit and/or a constant voltage circuit) is required to maintain the LED current within its rated limits. This control circuit is connected between the rectifier and the LEDs and in its simplest form comprises a single resistor. In addition, the output of the rectifier may have an unacceptable ripple and therefore a filter including one or more capacitors can be connected between the rectifier and the LED.

Where some further control function, such as dimming, is required then this is carried out by a control circuit connected between the rectifier and the LEDs. For example, pulse width modulation (PWM) is often used to control the brightness of LEDs. Thus a control circuit in the form of a PWM modulator is connected between the rectifier and the LEDs.

A particular problem with the above-mentioned prior art is that the drivers or control circuits normally include one or more electrolytic capacitors having high levels of capacitance. This has the consequence of very high initial transient currents at starting since the capacitor(s) require large amounts of charge in order to reach their operating voltage. Prior art novelty searches carried out after the conception of the present invention have disclosed <CIT>) which is illustrative of this prior art. All control is carried out between the rectifier and the string of LEDs.

In addition, high voltage semiconductors are very expensive. Consequently designers are often forced for economic reasons to use multiple driver systems incorporating lower voltage and less expensive semiconductors. Multiple driver systems mean more control wires and more complexity with an increased chance of circuit failure in service.

D1 <CIT> Weaver discloses a power supply system for LED airfield lighting where the LEDs are supplied with a pulse width modulated DC current which is controlled by a DC control circuit (Switching Current Regulators 430A and 430B) to vary the intensity of the LEDs. An AC circuit including a ferro-resonant transformer <NUM> is used to supply an AC voltage to a full wave rectifier <NUM>. The ferro-resonant transformer acts a power supply for the Switching Current Regulators. The ferro-resonant capacitor <NUM> does not carry the current flowing through the rectifying circuit and the LED light sources 465A and 465B. The ferro-resonant activity occurs irrespective of whether current flows through the rectifier <NUM>.

The prior art circuit monitors the magnitude of current in a current loop <NUM> and a CPU processor <NUM> interprets the loop current and uses an algorithm to change the duty cycle of the Switching Current Regulators 430Aand 430B to make the intensity of the LEDs equivalent to the intensity of an incandescent lamp connected to the same current loop.

The ferro-resonant transformer arrangement <NUM>-<NUM> is a constant voltage transformer with a conventional "tank" circuit (<NUM>-<NUM>) and supplies a constant AC voltage to the rectifier <NUM>. The rectifier <NUM> and filtering capacitor <NUM> supply a constant DC voltage to the Switching Current Regulators 430A and 430B. The Switching Current Regulators, and not the ferro-resonant transformer (<NUM>), control the current to the LEDs.

The Genesis of the present invention is a desire to avoid such problems by adopting an alternative control arrangement for LED lamps.

In accordance with a first aspect of the present invention there is disclosed a control circuit for supplying a substantially constant DC current to an LED lamp unit which constitutes a load, said control circuit comprising AC inputs for connection to an AC supply which is subject to variations including supply voltage fluctuations and transients, a pair of lamp outputs for connection to said LED lamp unit, a full wave rectifying circuit supplying a DC voltage and said substantially constant DC current to said pair of lamp outputs, and an AC control circuit, wherein said AC control circuit is connected between said AC inputs (A, N) and said rectifying circuit to reduce variations in the magnitude of an AC current supplied to said rectifying circuit from said AC control circuit to thereby reduce corresponding variations in said substantially constant DC current, said AC control circuit including a capacitor in series with an inductive winding of a magnetic component having a magnetically permeable core that at least part of which in operation is at least partially saturated by ferro-resonance, wherein said capacitor, said rectifying circuit and said LED lamp unit are electrically connectable in series to conduct a load current therethrough which is controlled by said ferro-resonance, and said ferro-resonance is created by said load current.

In accordance with a second aspect of the present invention there is disclosed a lighting installation comprising the above control circuit and an LED lamp fixture supplied with said substantially constant DC current via said LED lamp unit.

According to another aspect of the present invention there is provided a method of converting a High Intensity Discharge (HID) lamp installation supplied by an AC mains supply subject to variations including supply voltage variations and transients, and including an HID lamp fitting, a ballast and associated control gear, to an LED lamp installation including a LED lamp fitting supplied with a substantially constant DC current, said method comprising the steps of:.

A particular advantage of the above method is that it enables the use of existing cabling and control gear housings that were previously used for the HID installation resulting in a simple and cost effective conversion process.

Various forms of control circuit are disclosed as are variants of the above where the AC mains supply is a three-phase supply.

The circuits illustrated in <FIG>, <FIG>, <FIG>, <FIG>, <FIG>, <FIG>, <FIG>, <FIG>, and <FIG> do not form part of the invention but represent background art that is useful for understanding the invention.

As seen in <FIG>, a prior art LED lamp circuit takes the form of a transformer <NUM> supplied by an AC mains supply, and a full wave bridge rectifier <NUM> which supplies the necessary DC voltage. The rectifier <NUM> includes four regular diodes <NUM>. The LED lamp <NUM> takes the form of a string of light emitting diodes <NUM>.

In order to control the lamp current with fluctuations in mains voltage, a control circuit in the form of resistors R1 and R2, a transistor Q1 and a controlled voltage, is provided. The controlled voltage where no dimming is required can take the form of a reverse biased Zener diode. The controlled voltage is equal to the base-emitter voltage of the transistor Q1 and the voltage across the resistor R1. Since the base emitter voltage does not vary in any significant fashion with the collector-emitter current flowing through the transistor, this means that the voltage across the resistor R1 is substantially constant. This in turn renders the current through the lamp <NUM> substantially constant.

Where the lamp <NUM> is to be dimmed, the controlled voltage itself is adjustable by a further dimmer setting circuit which enables the controlled voltage to be adjusted. As a consequence, the lamp <NUM> and its associated circuitry which are positioned at the top of a pole or tower, are connected to the remainder of the circuit by four wires.

Turning now to <FIG>, the lamp <NUM> is as before consisting of a sequence of series connected light emitting diodes <NUM>. The lamp <NUM> is directly connected to a full wave bridge (FWB) rectifier <NUM> composed of four regular diodes <NUM> connected in the traditional manner for a full wave rectification. The rectifier <NUM> is supplied by an AC mains supply having an active terminal A and a neutral terminal N. An iron cored inductor <NUM> is interposed between the mains supply and the rectifier <NUM>, preferably in the active lead as illustrated. The inductor <NUM> operates as a control circuit. The current supplied by the bridge rectifier <NUM> to the LED lamp <NUM> is maintained within the upper and lower limits of LED current conduction by the impedance of the inductor <NUM>.

In addition, the inductor <NUM> provides a phase shift in the mains current so that the mains current is continuous and essentially sinusoidal in shape. Two capacitors Ci and C27 are drawn in <FIG> in broken lines in order to indicate that they are optional and can be provided if desired. The function of capacitor C27 is to smooth out the ripple voltage provided by the Full Wave Bridge rectifier <NUM>. The function of the capacitor Ci is to improve the power factor of the mains current.

It will be seen that, there are only two wires required to supply the LED lamp <NUM>. As a consequence, the lamp <NUM> can be located at the top of a tower or pole (not illustrated) and the operating circuitry in the form of the inductor <NUM> and rectifier <NUM> can be located at the base of the tower or pole. This enables an easy retrofit in order to replace existing HID lighting installations.

Turning now to <FIG>, in this embodiment the full wave bridge rectifier <NUM> and LED lamp <NUM> and inductor <NUM> are as before, however, a capacitor C1 is added in series with the inductor <NUM> to convert the lagging low power factor circuit of <FIG> (without capacitor Ci) into a leading low power factor circuit. The inclusion of the capacitor C1 also reduces the variation in the current supplied to the LEDs <NUM> due to mains voltage variations. Further reductions in current variation due to mains voltage variations can be achieved by introducing a degree of non-linearity into the inductor <NUM>.

Additionally if desired an optional shunt inductor <NUM> (illustrated by broken lines in <FIG>) can be connected across the mains terminals to improve the power factor. A further optional addition is the connection of a filter <NUM> across the output of the rectifier <NUM>. The provision of the filter <NUM> attenuates the ripple in the current through the LEDs <NUM>.

As illustrated in <FIG>, an isolated leakage reactance transformer T1 incorporating magnetic shunts, may be connected across the mains and to substitute for the inductor <NUM>. The bridge rectifier <NUM> and LED lamp <NUM> are as before. The optional filter <NUM> is also as before. An optional capacitor C2 can be connected across the mains terminals to improve the power factor. The high leakage reactance of the transformer T1 provides essentially the same phase shift in the output current as provided by the inductor <NUM> of <FIG>. As a result, there is a continuous and essentially sinusoidal current flow in both the primary and secondary windings of the isolated leakage reactance transformer T1.

An embodiment based on <FIG> is illustrated in <FIG>. An isolated leakage reactance transformer T2 of <FIG> has a capacitor C1 connected in series with its secondary winding. The magnetic circuit associated with the secondary winding permits partial saturation. Preferably, this is achieved by modifying the magnetic circuit. This non-linear nature of the secondary winding inductance in conjunction with the capacitor C1, results in a relatively constant current through the LEDs <NUM> irrespective of variations in the mains supply voltage.

The inclusion of an optional capacitor C3 (illustrated in broken lines in <FIG>) in series with the capacitor C1 and the secondary winding of the transformer T2, increases the capacitive reactance in the output circuit of the transformer T2. This therefore reduces the current through the LEDs <NUM> and dims the light output. Closing the optional switch S1 restores the full light output.

An alternative technique to achieve dimming is to substitute parallel capacitors for the capacitor C1. Full light output is achieved when both capacitors are in circuit but a dimmed light output is achieved when one of the two parallel capacitors is switched out of the circuit. More than one dimming level of light output can be achieved by using various combinations of series and/or parallel switched capacitors.

In <FIG>, the circuits of <FIG> and <FIG> are combined to produce a high power factor circuit which can operate two lamps <NUM> in a lead-lag configuration. This is very beneficial for the mains supply network.

In order to achieve low ripple current through the LEDs <NUM> without the need for a filter <NUM> or filter capacitor C27 as shown in <FIG> and <FIG>, three-phase mains power inputs can be used. <FIG> illustrates a simple three-phase modification of the circuit of <FIG>. A conventional three-phase full wave bridge rectifier <NUM> utilising six regular diodes <NUM> replaces the rectifier <NUM> of <FIG>. An inductor <NUM> is provided for each of the <NUM> phases P1-P3.

<FIG> illustrates a modification of the circuit diagram of <FIG> in that an input shunt capacitor C2 is added for each phase. This improves the power factor of the circuit of <FIG>. The shunt capacitors C2 can be connected in Wye configuration to an optional Neutral terminal N as shown which results in a <NUM> wire supply. Alternatively, in a <NUM> wire supply arrangement, the shunt capacitors can be connected in Wye configuration to a floating star point. Alternatively, a Delta connection between the phases can be used for these power factor correcting capacitors C2, thereby again resulting in a <NUM> wire supply.

<FIG> illustrates a further embodiment in which the three-phase circuit of <FIG> is modified by the provision of a switch S1 which enables a single phase to be turned off. This has the consequence of dropping the current supplied via the three-phase rectifier <NUM> to the LED lamp <NUM>. The reduced current through the lamp <NUM> is, however, still within the specified current range of the lamp <NUM> but results in the lamp being dimmed. Such dimming is particularly useful in introducing lighting at sporting fixtures during the onset of evening. In addition to a saving in power, the eyes of the spectators and players are allowed to adjust to the artificial lighting environment. Alternatively, such dimming is an acceptable lower lighting level for training activities, as opposed to match competition. The identical dimming function is also available by switching power off to one of the phases of the circuits of <FIG> and <FIG>. In the particular embodiment of <FIG>, a series capacitor C1 is provided for each phase.

In a manner analogous to <FIG>, the embodiment of <FIG> is a combination of the circuits of <FIG> and <FIG> which permits two LED lamps <NUM> to be operated in a high power factor lead-lag arrangement. The switch S1 enables one of the phases to be dropped to dim both lamps <NUM>.

Similarly, the embodiment of <FIG> represents a three-phase version of the circuit of <FIG> utilising shunt inductors <NUM>, one for each phase. The switch S1 can be open to provide a first level of dimming of the LEDs <NUM>. If desired, an optional second switch S2 (illustrated in broken lines in <FIG>) can be provided in another phase. If both switches S1 and S2 are opened, a second lower dimming level can be achieved (provided the available voltage is adequate to provide the LEDs forward voltage drop and drive sufficient current through the LEDs).

Similarly, the embodiment of <FIG> represents a three-phase version of the circuit of <FIG> utilising three isolated leakage reactance transformers T1, again one for each phase. Again the switch S1 enables a dimming function to be achieved. The three primary windings are connected in star or Wye configuration and preferably the star point is floating so as to form a <NUM> wire supply. Alternatively, the star point can be connected to a mains neutral terminal N so as to form a <NUM> wire supply. As for <FIG>, power factor correction capacitors C2 can be connected to the power supply terminals in either Delta or star configurations.

Still further, the embodiment of <FIG> represents a three-phase version of the circuit of <FIG> utilising three isolated constant current transformers T2, again one for each phase. A series capacitor C1 is provided for each phase.

Turning now to <FIG>, in a single phase circuit a ferro-regulating transformer T3 has a capacitor C3 connected across its secondary winding to form a tank circuit. The secondary winding is tapped to provide the appropriate input voltage to the full wave bridge rectifier <NUM>. The lamp <NUM> is as before. If desired, an optional filtering capacitor C27 can be connected across the output of the rectifier <NUM>.

<FIG> illustrates a three-phase version of the circuit of <FIG> utilising three ferro-regulating transformers T3. The three primary windings are connected in star or Wye configuration and preferably the star point is floating so as to form a <NUM> wire supply. Alternatively, the star point can be connected to a mains neutral terminal N so as to form a <NUM> wire supply.

<FIG> illustrates a further single phase embodiment which is a similar to <FIG>. Six diodes <NUM> constitute a double full wave bridge rectifier <NUM> which is fed via an inductor <NUM> on the one hand and a series connected inductor <NUM> and capacitor C1 on the other hand. As a consequence, both leading and lagging currents are supplied to the LEDs <NUM> simultaneously. The resultant LED current has very little ripple (approximately <NUM>%) without any filter capacitor. In addition, the circuit has a very high power factor (almost unity) and low total harmonic distortion of the mains current.

Turning now to the embodiment of <FIG>, in this single phase circuit inductor L1 is a relatively linear inductor whereas inductor L2 has at least partial saturation of its core, so that the voltage across inductor L2 remains relatively constant. As a consequence, there is a substantially constant current flowing through capacitor C1 and hence a constant current flowing through the LEDs <NUM> notwithstanding fluctuations in the mains supply voltage.

The embodiment of <FIG> illustrates a three-phase version of <FIG>. The common connection point of inductors L2 can be either a floating star point with a <NUM> wire mains supply or can be connected to a neutral terminal N of a <NUM> wire mains supply.

<FIG> and <FIG> each show an arrangement with an autotransformer T5 supplying the rectifier <NUM> from a tapped winding. An inductor <NUM> is connected in series with the autotransformer T5. The tapping can be selected to enable a match between the supply voltage and the reflected load voltage X-Y (being the voltage across the LEDs <NUM> reflected into the mains supply circuit). In <FIG> the reflected LED voltage is increased. Whereas in <FIG> the reflected LED voltage is decreased. In both cases power factor correction can be added by connecting a capacitor across the supply terminals.

<FIG> and <FIG> illustrate variations able to be made to the autotransformer T5 in order to dim the LEDs <NUM>. In <FIG> a switch S5 is connected between some of the primary winding turns of the autotransformer T5. With the switch S5 in position <NUM>, a lower voltage is applied to the inductor <NUM> and thus the LEDs <NUM> are dimmed. With the switch S5 in position <NUM>, the mains voltage is applied across a small number of turns of the primary winding and thus the voltage applied to the inductor <NUM> and rectifier <NUM> is increased and thus the LEDs <NUM> are not dimmed.

<FIG> illustrates a similar arrangement but with the switch S5 connected to the secondary side of the autotransformer T5. With the switch S5 in position <NUM>, a maximum voltage is applied to the inductor <NUM> and rectifier <NUM> and so the LEDs <NUM> are not dimmed. However, when the switch S5 is in position <NUM>, a smaller voltage is applied to the inductor <NUM> and rectifier <NUM> and so the LEDs are dimmed.

It will be apparent to those skilled in the art that the above-mentioned switching on either of the primary site or the secondary side to achieve dimming is applicable to transformers other than autotransformers and thus is applicable to, for example, the transformer arrangements illustrated in <FIG>, <FIG> and <FIG>.

Furthermore, <FIG> and <FIG> also illustrate switched dimming techniques applicable to impedances located in the mains supply to the rectifiers <NUM>, <NUM>. In <FIG>, inductor <NUM> has a tapping in the winding. With switch S8 in position <NUM>, there is a lower impedance and the LEDs <NUM> are not dimmed. With switch S8 in position <NUM>, the inductor <NUM> has a higher impedance and the LEDs <NUM> are dimmed.

A similar arrangement is illustrated in <FIG>. In a first arrangement an inductor L8 is connected in series with the rectifier <NUM>. A switch S7 connected in series with a further inductor L9 can be operated to connect the inductor L9 in parallel with the inductor L8, thereby increasing the current to the rectifier <NUM>. Thus with the switch S7 open the LEDs <NUM> are dimmed and with the switch S7 closed, the LEDs <NUM> are un-dimmed.

Alternatively or additionally, the switch S8 can be provided as in <FIG> to change the effective number of turns of the inductor <NUM>. With the switch S8 in position <NUM>, the impedance of the inductor <NUM> is reduced, the current to the rectifier <NUM> is at a maximum and the LEDs <NUM> are un-dimmed. However, with the switch S8 in position <NUM>, the impedance of the inductor <NUM> is at a maximum, and so the current to the rectifier <NUM> is reduced and the LEDs are dimmed. It will be apparent to those skilled in the art that in some circuit arrangements switching of capacitors can be used as an alternative form of impedance change. Other forms of switchable series and/or parallel interconnections to alter the level of current will also be apparent to those skilled in the art.

<FIG> each illustrate a single phase circuit with an isolated high leakage transformer T10 having dual output windings. These are used to supply a pair of LED lamps <NUM> at an appropriate and acceptable voltage, especially in cases where a <NUM> wire LED voltage circuit would necessitate the use of unacceptably high voltages. Dimming can be achieved by the inclusion of additional series or parallel capacitors in the dual output windings which are switched in or out of circuit. Power factor correction can be achieved with an optional capacitor connected between the active and neutral terminals.

In <FIG>and <FIG> the two full wave bridge rectifiers <NUM> are completely isolated from each other thus creating a <NUM> wire feed to the LED lamp modules <NUM>. Alternatively, as illustrated in <FIG> and <FIG>, the two full wave bridge rectifiers <NUM> can have a common connection, thereby creating a <NUM> wire feed to the LED lamp modules <NUM>.

In the embodiments of <FIG>, the dual output windings of the isolated high leakage transformer T10 each have a capacitor C10 connected in series. This arrangement provides essentially the same benefits as those associated with <FIG> in that the current through the LED lamp modules <NUM> will be relatively constant irrespective of changes in the supply voltage. However, as for the circuits of <FIG>, there is the additional benefit of a reduced voltage applied to the individual LED lamp modules <NUM>.

<FIG> illustrates a single phase circuit similar to <FIG> save that the rectifier <NUM> is replaced by a transformer T4 and voltage doubler unit formed by a pair of regular diodes <NUM> and a pair of capacitors C4. An advantage of the circuit of <FIG> is that the voltage across the LED lamp <NUM> is much increased and thus can accommodate an increased number of series connected LED diodes <NUM>. Thus the lamp <NUM> of <FIG> can have an increased wattage relative to the lamps <NUM> of the other circuits.

The foregoing describes only some embodiments of the present invention and modifications, obvious to those skilled in LED lighting circuitry, can be made thereto without departing from the scope of the present invention. In particular, it will be appreciated that the current control is achieved in the AC portion of the circuit upstream of the rectifier supplying the LEDs. This represents a significant departure from the prior art.

Furthermore, some variations can be categorised as follows. A filter <NUM> can be located between the rectifier <NUM>, <NUM> and the LED module(s) <NUM>. This filter can take the form of either a shunt capacitor C27, or a series inductor, or a combination of series inductor(s) and a shunt capacitor.

In addition, the magnetic component of the control circuit can take the form of various transformers including leakage reactance transformers, ferro-resonant transformers and constant current transformers. These can be realised by either an autotransformer or by a conventional isolation transformer. The control circuit can also take the form of a constant current transformer as illustrated in <FIG> and <FIG>.

In the embodiment of <FIG> the constant current transformer Tr is drawn with its primary magnetic circuit illustrated with the laminations parallel to the plane of the paper and its leakage magnetic circuit drawn with the laminations perpendicular to the plane of the paper. Thus a primary winding WP is supplied with AC mains power and a secondary winding WS provides an output voltage to the load circuit(s). The shunts of the leakage magnetic circuit provides substantial leakage inductance, thereby magnetically de-coupling the output of the secondary winding from variations in the mains voltage. The magnetic circuit associated with the secondary winding WS is arranged so that at least a substantial portion thereof goes into magnetic saturation during normal operation. Preferably, this is achieved by modifying the magnetic circuit.

The non-linear nature of the inductance of the secondary winding WS in conjunction with the reactance of the resonant capacitor CR, result in at least a portion of the secondary winding magnetic circuit being maintained in a magnetically saturated state due to ferro-resonance.

Since the magnetic core associated with the secondary winding WS is saturated, so changes in the mains voltage have virtually no effect on the output of the secondary winding WS, with the result that its voltage remains constant. This has the consequence that the current through the load also remains constant and its magnitude is primarily governed by the size of the resonant capacitor CR.

Preferably as indicated in <FIG>, the load consists of <NUM> regular diodes forming a full wave bridge rectifier with the LEDs module <NUM> forming the load. If desired, an optional filtering capacitor C27 can be connected in parallel with the LEDs <NUM>.

In addition to controlling the load current so as to be substantially constant, there are other benefits to this circuit. One such benefit is the very high operating power factor, which is very close to unity. Another benefit is the very low total harmonic distortion in the mains circuit, typically being less than <NUM>% distortion. A further benefit is that the secondary circuit has a very high immunity to transient voltages in the mains supply, and this immunity is applicable to both transverse mode transient voltages and common mode transient voltages.

If a further improvement in line regulation is required, then a small bucking winding (not illustrated but conventional) can be wound over the primary winding WP and connected in series with the secondary winding WS but in an inductively opposing sense.

As indicated by broken lines in <FIG>, multiple load circuits each with its own string of LEDs <NUM> can be operated in parallel with the one constant current transformer Tr. The extension to <NUM> phases is as indicated in <FIG>.

Turning now to the embodiment of <FIG>, it is possible to build a constant current transformer Tr having a single primary winding WP and two totally independent secondary windings WS <NUM> and WS2. The single primary winding WP is located in between a pair of magnetic shunts and results in substantial saturation of the magnetic circuit of each of the secondary windings WS1 and WS2. Each of the secondary windings is capable of supplying a number of parallel load circuits in the manner of <FIG>.

In general, the AC mains supply can be either single phase or poly phase (normally three-phases). Where a transformer is used for three-phase arrangement, the primary windings of the transformer can be connected either in Wye configuration or in Delta configuration. A power factor correction circuit can be used to improve the power factor of the overall circuit. A typical power factor correction circuit is a shunt capacitor connected between the phases of the supply or between each phase and a star point or neutral connection.

Power factor correction can also be implemented by duplicating the overall circuit and operating two sets of LED modules <NUM> in a lead-lag configuration.

Dimming is possible by switching off one of the duplicate circuits in a lead-lag configuration, or switching off one or more of the phases of a poly phase circuit. Dimming is also possible by switching impedance(s) into and out of the supply circuit to the rectifier <NUM>, <NUM>. Similarly, dimming is also possible by switching turns into and out of transformer arrangements applying the rectifier <NUM>, <NUM>.

For a poly phase circuit, such as a three-phase circuit, a <NUM> wire supply or <NUM> wire supply is possible. The <NUM> wire supply can be delta connected, or have a floating star point. The <NUM> wire supply can have the star point connected to a neutral terminal.

The foregoing describes only some embodiments of the present invention and modifications, obvious to those skilled in the electronic arts, can be made thereto without departing from the scope of the present invention.

Claim 1:
A control circuit for supplying a substantially constant DC current to an LED lamp unit (<NUM>) which constitutes a load, said control circuit comprising AC inputs (A, N) for connection to an AC supply which is subject to variations including supply voltage fluctuations and transients, a pair of lamp outputs (+, -) for connection to said LED lamp unit, a full wave rectifying circuit (<NUM>, <NUM>) configured for supplying a DC voltage and said substantially constant DC current to said pair of lamp outputs, and an AC control circuit, characterised in that said AC control circuit is connected between said AC inputs (A, N) and said rectifying circuit (<NUM>, <NUM>) to reduce variations in the magnitude of an AC current supplied to said rectifying circuit from said AC control circuit to thereby reduce corresponding variations in said substantially constant DC current, said AC control circuit including a capacitor (C1, C10, Cr) in series with an inductive winding (L2, Ws) of a magnetic component (L2, T2, T10, Tr) having a magnetically permeable core that at least part of which in operation is at least partially saturated by ferro-resonance, wherein said capacitor (C1, C10, Cr), said rectifying circuit (<NUM>, <NUM>), and said LED lamp unit (<NUM>) are electrically connectable in series to conduct a load current therethrough which is controlled by said ferro-resonance, and said ferro-resonance is created by said load current.