BR112020004214B1 - LIGHTING CONTROL CIRCUIT, LIGHTING INSTALLATION AND METHOD - Google Patents

Abstract

The present invention describes a lighting installation having an LED lamp (19), typically consisting of a series chain of individual LEDs (18), which is supplied by a rectifier (20, 200). A control circuit (23, 23 and C1) is interposed between the rectifier and the AC source that feeds the rectifier. Various circuits are disclosed for filtering, power factor control, multiphase operation, and dimming, for example, by phase switching. In particular, the control carried out by the control circuit occurs on the AC side of the rectifier. Also disclosed are the control circuit itself and a method for converting a High Intensity Discharge (HID) lamp installation into a Light Emitting Diode (LED) installation. The control circuit may take the form of an inductor, an inductor and capacitor in series, a shunt inductor, a leakage reactance transformer, a constant current transformer, an autotransformer, an isolation transformer, or a ferromagnetic transformer. resonant.

Description

Field of invention

[0001] The present invention relates to lighting and, in particular, to external lighting, such as public lighting or stadium lighting. The present invention also relates to internal lighting (such as high ceilings). Until now, this lighting has been provided using High Intensity Discharge (HID) lamps.

Fundamentals of the technique

[0002] HID lamps are essentially AC devices and have a negative resistance characteristic. As a consequence, power is supplied by an AC mains source and a ballast or choke is connected in series with the lamp to prevent damage to the lamp due to excessive currents. A typical HID lamp, such as a 1000W metal halide, has an ignition voltage of 750V peak, an operating voltage of 260V and an operating current of 4A, giving a lamp power of approximately 1000W and an output of 100,000 lumen light.

[0003] In recent years, lamps with one or more light-emitting diodes (LEDs) have replaced incandescence lamps and low-intensity discharge lamps, such as fluorescent lamps. As the power capacity of these LEDs increased, LEDs began to be used in outdoor lighting, such as street lighting and stadium lighting, where considerable powers (e.g. more than 1 kW) are required.

[0004] Clearly, as the word "diode" implies, LEDs are DC (or unidirectional current) devices. Additionally, an LED has a positive resistance characteristic (in which the resistance varies with the current flowing through the LED). Like DC devices, where the LEDs are supplied by the AC mains, there is typically a rectifier of some description supplied by the mains, and the LEDs (along with their drivers or control circuits) are powered by the output of the rectifier. As a consequence, DC control circuits for LED lamps are completely different from AC control circuits for HID lamps.

[0005] An example of a high power LED has a nominal voltage of 6V and consumes a nominal current of 2.1A. However, the LED can have a current of up to 4.8A. An LED lamp fixture may typically include 24 such LEDs connected in series. This LED lamp fixture has an operating voltage of 150V and a rated operating current of 2.1A, giving a lamp power of approximately 315W and a light output of 30,000 lumens. In some cases, corresponding LEDs are connected in parallel; in this case, the total current is shared.

[0006] Another example of a high power LED has 36 LEDs in a string, has a nominal voltage of 225V, a nominal current of 2.1A, a nominal voltage of 475W and a light output of 45,000 lumens.

[0007] Typically, a control circuit in the form of an electronic driver of some type (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. Furthermore, the output of the rectifier may have unacceptable ripple, and therefore a filter including one or more capacitors may be connected between the rectifier and the LED.

[0008] Where some additional control function, such as dimming, is required, this is performed 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.

[0009] A particular problem with the aforementioned prior art is that the drivers or control circuits typically include one or more electrolytic capacitors with high levels of capacitance. This has the consequence of very high initial transient currents at start-up, since the capacitor(s) require large amounts of charge to reach their operating voltage. Prior art research carried out after the conception of the present invention disclosed US Patent No. 9,497,811 (Schijffelen), which is illustrative of this prior art. All control is carried out between the rectifier and the LED chain.

[0010] Furthermore, high voltage semiconductors are very expensive. Consequently, designers are often forced, for economic reasons, to use multi-driver systems incorporating low voltage and more economical semiconductors. Multi-driver systems mean more control wires and more complexity, with a greater chance of circuit failure in service.

[0011] Document US2005/0030192, for example, discloses a power supply system for LED aerodrome lighting where the LEDs are supplied with a pulse width modulated DC current that is controlled by a DC control circuit (regulators). switching current 430A and 430B) to vary the intensity of the LEDs. An AC circuit including a ferroresonant transformer 415 is used to supply an AC voltage to a full-wave rectifier 420. The ferroresonant transformer acts as a power supply for the switching current regulators. The 419 ferroresonant capacitor does not carry the current flowing through the rectifier circuit and the 465A and 465B LED light sources. Ferroresonant activity occurs regardless of whether current flows through rectifier 420.

[0012] The prior art circuit monitors the magnitude of current in a current loop 405 and a CPU processor 450 interprets the loop current and uses an algorithm to change the duty cycle of the switching current regulators 430A and 430B to make the intensity of the LEDs equivalent to the intensity of an incandescent lamp connected to the same current loop.

[0013] The ferroresonant transformer arrangement 415-419 is a constant voltage transformer with a conventional "tank" circuit (418-419) and supplies a constant AC voltage to the rectifier 420. The rectifier 420 and the filtering capacitor 425 provide a constant DC voltage for the 430A and 430B switching current regulators. The switching current regulators, not the ferroresonant transformer (415), control the current to the LEDs.

Genesis of the invention

[0014] The genesis of the present invention is a desire to avoid such problems by adopting an alternative control arrangement for LED lamps.

Summary of the invention

[0015] According to a first aspect of the present invention, there is disclosed a control circuit for an LED lamp unit, said circuit comprising AC inputs for connection to an AC mains source, a pair of lamp outputs for connection to said LED lamp unit and a rectification circuit supplied from said AC inputs and providing said lamp outputs, wherein a control circuit is interposed between said AC inputs and said rectification circuit.

[0016] According to a second aspect of the present invention, there is disclosed a lighting installation comprising an LED lamp fixture supplied by a rectifier, said rectifier being supplied by an AC mains source and a control circuit interposed between said rectifier and said AC mains source.

[0017] According to another aspect of the present invention, there is provided a method for converting a High Intensity Discharge (HID) lamp fixture supplied by an AC mains source and including a HID lamp fixture, a ballast and a control mechanism associated with an LED lamp fixture including an LED lamp fixture and provided by said source, said method comprising the steps of: replacing said HID lamp fixture with said LED lamp fixture, replacing the ballast and the control mechanism associated by a rectifier having an input and an output, connecting the output of said rectifier to said LED lamp fixture, and interposing a control circuit between said AC mains source and said input of the rectifier.

[0018] A particular advantage of the above method is that it allows the use of existing cabling and control mechanism boxes that were previously used for HID installation, resulting in a simple and economical conversion process.

[0019] Various forms of control circuit are disclosed, as well as the variants above, where the AC mains source is a three-phase source.

Brief description of the drawings

[0020] The preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a prior art circuit showing an electronic driver for an LED lamp, Figure 2 is a circuit diagram for operating an LED lamp utilizing a single inductor control circuit of a first embodiment and an optional filter capacitor and optional power factor correction capacitor, Figure 3 is a modification of the circuit diagram of Figure 2 to improve its current control, Figure 4 is a circuit diagram of a third embodiment incorporating a high leakage reactance transformer, Figure 5 is a circuit diagram of a fourth embodiment incorporating an isolated constant current transformer, Figure 6 is a circuit diagram of a fifth embodiment being a combination of the circuits of figures 2 and 3, Figure 7 is a three-phase circuit diagram of a sixth embodiment, Figure 8 is a modification of the circuit diagram of Figure 7, A Figure 9 is a circuit diagram of a seventh embodiment incorporating a dimming function, Figure 10 is a combination of the circuits of Figures 7 and 9, Figure 11 is a circuit diagram of a ninth embodiment being a three-phase version of the circuit of Figure 3, Figure 12 is a circuit diagram of a tenth modality being a three-phase version of the circuit of Figure 4, Figure 13 is a circuit diagram of another three-phase modality, Figure 14 is a circuit diagram of an eleventh Figure 15 is a circuit diagram of a twelfth embodiment being a three-phase modification of the circuit of Figure 14. Figure 16 is a circuit diagram of a further embodiment being a control circuit. single-phase double rectification, Figure 17 is an additional single-phase circuit with saturable shunt inductor, Figure 18 is a circuit diagram of another embodiment being a three-phase modification of the circuit of Figure 17, Figures 19 and 20 are single-phase embodiments incorporating a autotransformer, Figures 19A and 20A illustrate variations that can be made to the circuits of Figures 19 and 20, and done more generally, Figures 21 to 24 are single-phase circuits incorporating an isolated high leakage transformer with dual output windings, Figure 25 is a single-phase circuit incorporating a voltage doubler, Figure 26 is a single-phase constant current transformer circuit with a single secondary winding, and Figure 27 is another single-phase constant current transformer circuit with two secondary windings.

Detailed Description

[0021] As seen in Figure 1, a prior art LED lamp circuit takes the form of a transformer 11 supplied by an AC mains source and a full-wave bridge rectifier 20 that provides the required DC voltage. The rectifier 20 includes four regular diodes 21. The LED lamp 19 takes the form of a string of light-emitting diodes 18.

[0022] To control the lamp current with fluctuations in the mains voltage, a control circuit is provided in the form of resistors R1 and R2, a transistor Q1 and a controlled voltage. Controlled voltage where dimming is not required can take the form of a reverse biased Zener diode. The controlled voltage is equal to the base-emitter voltage of transistor Q1 and the voltage across resistor R1. Since the base-emitter voltage does not vary significantly with the collector-emitter current flowing through the transistor, this means that the voltage across resistor R1 is substantially constant. This, in turn, makes the current through the lamp 19 substantially constant.

[0023] Where lamp 19 is to be dimmed, the controlled voltage itself is adjustable by an additional dimmer adjustment circuit that allows the controlled voltage to be adjusted. As a consequence, the lamp 19 and its associated circuits, which are positioned at the top of a pole or tower, are connected to the rest of the circuit by four wires.

[0024] Returning now to figure 2, the lamp 19 is as before consisting of a sequence of light-emitting diodes connected in series 18. The lamp 19 is connected directly to a full-wave bridge rectifier (FWB) 20 composed of four 21 regular diodes connected in the traditional way for a full-wave rectification. The rectifier 20 is supplied by an AC mains source with an active terminal A and a neutral terminal N. An iron core inductor 23 is interposed between the mains source and the rectifier 20, preferably in the active conductor, such as illustrated. Inductor 23 operates as a control circuit. The current supplied by the bridge rectifier 20 to the LED lamp 19 is maintained within the upper and lower limits of LED current conduction by the impedance of the inductor 23.

[0025] Furthermore, inductor 23 provides a phase change in the mains current, so that the mains current is continuous and essentially sinusoidal. Two capacitors Ci and C27 are drawn in figure 2 in dashed lines to indicate that they are optional and can be provided if desired. The function of capacitor C27 is to smooth the ripple voltage provided by the full-wave bridge rectifier 20. The function of capacitor Ci is to improve the power factor of the mains current.

[0026] It will be seen that only two wires are needed to power the LED lamp 19. As a consequence, the lamp 19 can be located at the top of a pole or tower (not illustrated) and the operating circuit in the form of the inductor 23 and the Rectifier 20 may be located at the base of the pole or tower. This allows for easy retrofitting to replace existing HID lighting installations.

[0027] Returning now to figure 3, in this embodiment, the full-wave bridge rectifier 20 and the LED lamp 19 and the inductor 23 are as before, however, a capacitor C1 is added in series with the inductor 23 to convert the low power factor lagging circuit of Figure 2 (without capacitor Ci) into a low main power factor circuit. The inclusion of capacitor C1 also reduces the variation in current supplied to the LEDs 19 due to variations in mains voltage. Further reductions in current variation due to mains voltage variations can be achieved by introducing a degree of nonlinearity in inductor 23.

[0028] Alternatively, or additionally, if desired, an optional shunt inductor 25 (illustrated by dashed lines in figure 3) can be connected across the mains terminals to improve the power factor. A further optional addition is the connection of a filter 27 across the output of the rectifier 20. The provision of the filter 27 attenuates the ripple in current through the LEDs 19.

[0029] In the embodiment of figure 4, an isolated leakage reactance transformer T1 incorporating magnetic taps is connected across the electrical network and therefore replaces the inductor 23. The bridge rectifier 20 and the LED lamp 19 are as before. Optional filter 27 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 transformer T1 provides essentially the same phase shift in the output current provided by inductor 23 of figure 2. As a result, there is a direct and essentially sinusoidal current flow in the primary and secondary windings of the leakage reactance transformer. isolated T1.

[0030] A variation of the circuit of figure 4 is illustrated in figure 5. An isolated leakage reactance transformer T2 of figure 5 has a capacitor C1 connected in series with its secondary winding. The magnetic circuit associated with the secondary winding allows partial saturation. Preferably, this is achieved by modifying the magnetic circuit. This non-linear nature of the inductance of the secondary winding in conjunction with the capacitor C1, results in a relatively constant current through the LEDs 19, regardless of variations in the mains source voltage.

[0031] The inclusion of an optional capacitor C3 (illustrated in dashed lines in figure 5) in series with the capacitor C1 and the secondary winding of transformer T2 increases the capacitive reactance in the output circuit of transformer T2. This therefore reduces the current through the LEDs 19 and decreases the light output. Closing optional switch S1 restores full light output.

[0032] An alternative technique to obtain dimming is to replace parallel capacitors with capacitor C1. Full light output is achieved when both capacitors are in circuit, but weak light output is achieved when one of the two parallel capacitors is disconnected from the circuit. It is possible to achieve more than one level of dimming of the light output using various combinations of series and/or parallel switched capacitors.

[0033] In figure 6, the circuits of figures 2 and 3 are combined to produce a high power factor circuit that can operate two lamps 19 in a lead-lag configuration. This is very beneficial for the power grid supply network.

[0034] In order to obtain low ripple current through the LEDs 19 without the need for a filter 27 or filter capacitor C27, as shown in figure 2 and figure 3, power inputs from the three-phase electrical network can be used. Figure 7 illustrates a simple three-phase embodiment of the circuit of Figure 2. A conventional three-phase full-wave bridge rectifier 200 using six regular diodes 21 replaces the rectifier 20 of Figure 2. An inductor 23 is provided for each of the 3 phases P1 to P3.

[0035] Figure 8 illustrates a modification of the circuit diagram of Figure 7, in which an input bypass capacitor C2 is added for each phase. This improves the power factor of the circuit in figure 7. Shunt capacitors C2 can be connected in the Wye configuration to an optional N neutral terminal as shown, resulting in a 4-wire supply. Alternatively, in a 3-wire source 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 C2 power factor correction capacitors, again resulting in a 3-wire supply.

[0036] Figure 9 illustrates an additional embodiment in which the three-phase circuit of figure 7 is modified by the provision of a switch S1 that allows a single phase to be turned off. This results in a drop in the current supplied by the three-phase rectifier 200 to the LED lamp 19. The reduced current through the lamp 19 is still within the specified current range of the lamp 19, but results in dimming of the lamp. This dimming is particularly useful in introducing lighting into sports fixtures during the early evening. In addition to saving on power, the eyes of spectators and players can adjust to the artificial lighting environment. Alternatively, this dimming is a lower lighting level acceptable for training activities, as opposed to competition. The identical dimming function is also available by turning off one of the phases of the circuits of figures 7 and 8. In the particular embodiment of figure 9, a series capacitor C1 is provided for each phase.

[0037] Analogous to figure 6, figure 10 is a combination of the circuits of figures 7 and 9, which allows two LED lamps 19 to be operated in a high power factor lead-lag arrangement. Switch S1 allows one of the phases to be dropped to dim both lamps 19.

[0038] Likewise, figure 11 represents a three-phase version of the circuit of figure 3 using shunt inductors 25, one for each phase. Switch S1 can be opened to provide a first level of dimming of the LEDs 19. If desired, a second optional switch S2 (illustrated in dashed lines in figure 11) can be provided in another phase. If both switches S1 and S2 are open, a second, lower dimming level can be achieved (provided that the available voltage is adequate to provide forward voltage drop across the LEDs and drive sufficient current through the LEDs).

[0039] Likewise, figure 12 represents a three-phase version of the circuit of figure 4 using three isolated leakage reactance transformers T1, again one for each phase. Again, switch S1 allows a dimming function to be achieved. The three primary windings are connected in star or Wye configuration and preferably the star point is floating to form a 3-wire source. Alternatively, the starpoint can be connected to a neutral terminal N of the mains to form a 4-wire source. As in figure 8, power factor correction capacitors C2 can be connected to the mains source terminals in Delta or star configurations.

[0040] Furthermore, figure 13 represents a three-phase version of the circuit of figure 5 using three isolated constant current transformers T2, again one for each phase. A series capacitor C1 is provided for each phase.

[0041] Returning now to figure 14, in a single-phase circuit, a ferroregulator transformer T3 has a capacitor C3 connected through its secondary winding to form a tank circuit. The secondary winding is threaded to provide the appropriate input voltage to the full-wave bridge rectifier 20. Lamp 19 is as before. If desired, an optional filtering capacitor C27 can be connected across the rectifier output 20.

[0042] Figure 15 illustrates a three-phase version of the circuit in figure 14 using three T3 ferro-regulator transformers. The three primary windings are connected in star or Wye configuration and preferably the star point is floating to form a 3-wire source. Alternatively, the starpoint can be connected to a neutral terminal N of the mains to form a 4-wire source.

[0043] Figure 16 illustrates an additional single-phase embodiment that is similar to Figure 6. Six diodes 21 constitute a double full-wave bridge rectifier 20 that is fed by an inductor 23, on the one hand, and a series-connected inductor 23 and capacitor C1 on the other hand. As a consequence, the main and lagging currents are supplied to the LEDs 19 simultaneously. The resulting current from the LED has very little ripple (approximately 5%) without any filter capacitors. Furthermore, the circuit has a very high power factor (almost unity) and low total harmonic distortion of the mains current.

[0044] Returning now to figure 17, in this single-phase circuit inductor L1 there is a relatively linear inductor, while inductor L2 has at least partial saturation of its core, so that the voltage in inductor L2 remains relatively constant. As a consequence, there is a substantially constant current flowing through capacitor C1 and therefore a constant current flowing through the LEDs 19 despite fluctuations in the mains supply voltage.

[0045] Figure 18 illustrates a three-phase version of figure 17. The common connection point of the L2 inductors can be a floating star point with a 3-wire mains source or can be connected to a neutral terminal N of a source 4-wire electrical network.

[0046] Figures 19 and 20 each show an arrangement with a T5 autotransformer supplying the rectifier 20 from a threaded winding. An inductor 23 is connected in series to autotransformer T5. Threading can be selected to allow a correspondence between the mains voltage and the X-Y reflected load voltage (with the voltage at the LEDs 19 being reflected in the mains supply circuit). In figure 19, the reflected voltage from the LED is increased. While in figure 20 the reflected voltage from the LED decreases. In both cases, power factor correction can be added by connecting a capacitor across the source terminals.

[0047] Figures 19A and 20A illustrate variations that can be made to the T5 autotransformer to dim the LEDs 19. In Figure 19A, a switch S5 is connected between some of the main turns of the T5 autotransformer winding. With switch S5 in position 2, a lower voltage is applied to inductor 23 and thus LEDs 19 are dimmed. With switch S5 in position 1, the mains voltage is applied through a small number of turns of the primary winding and thus the voltage applied to the inductor 23 and the rectifier 20 is increased and therefore the LEDs 19 are not dimmed .

[0048] Figure 20A illustrates a similar arrangement, but with switch S5 connected to the secondary side of autotransformer T5. With switch S5 in position 1, a maximum voltage is applied to inductor 23 and rectifier 20 and therefore LEDs 19 are not dimmed. However, when switch S5 is in position 2, a lower voltage is applied to inductor 23 and rectifier 20 and therefore the LEDs are dimmed.

[0049] It will be apparent to those skilled in the art that the above-mentioned switching at the primary location or secondary side to obtain dimming is applicable to transformers other than autotransformers and is therefore applicable, for example, to the transformer arrangements illustrated in figures 5, 12 to 15 and 21 to 25.

[0050] In addition, Figures 19A and 20A also illustrate switched dimming techniques applicable to impedances located in the electrical network source for rectifiers 20, 200. In Figure 19A, inductor 23 has a thread in the winding. With switch S8 in position 1, there is a lower impedance and the LEDs 19 are not dimmed. With switch S8 in position 2, inductor 23 has a higher impedance and LEDs 19 are dimmed.

[0051] A similar arrangement is illustrated in figure 20A. In a first arrangement, an inductor L8 is connected in series with the rectifier 20. A switch S7 connected in series with another inductor L9 can be operated to connect the inductor L9 in parallel with the inductor L8, thus increasing the current to the rectifier 20. Thus, with switch S7 open, LEDs 19 are dimmed and with switch S7 closed, LEDs 19 are not dimmed.

[0052] Alternatively or additionally, switch S8 can be provided as in figure 19A to change the effective number of turns of inductor 23. With switch S8 in position 1, the impedance of inductor 23 is reduced, the current to rectifier 20 is at maximum and the LEDs 19 are not dimmed. However, with switch S8 in position 2, the impedance of inductor 23 is maximum and therefore the current in rectifier 20 is reduced and the LEDs are dimmed. It will be apparent to those skilled in the art that, in some circuit arrangements, capacitor switching can be used as an alternative form of impedance changing. Other forms of switchable series and/or parallel interconnections for changing the current level will also be apparent to those skilled in the art.

[0053] Figures 21 to 24 each illustrate a single-phase circuit with a T10 high leakage isolated transformer with double output windings. These are used to supply a pair of LED lamps 19 at an appropriate and acceptable voltage, especially in cases where a 2-wire LED voltage circuit would require the use of unacceptably high voltages. Dimming can be achieved by including additional parallel or series capacitors in the dual output windings, which are switched in or out of the circuit. Power factor correction can be achieved with an optional capacitor connected between the active and neutral terminals.

[0054] In Figures 21 and 23, the two full-wave bridge rectifiers 20 are completely isolated from each other, thus creating a 4-wire supply for the LED lamp modules 19. Alternatively, as illustrated in Figures 22 and 24 , the two full-wave bridge rectifiers 20 may have a common connection, thus creating a 3-wire supply for the LED lamp modules 19.

[0055] In the circuits of figures 23 and 24, the double output windings of the high leakage isolated transformer T10 each have a capacitor C10 connected in series. This arrangement provides essentially the same benefits as those associated with Figure 5, in that the current through the LED lamp modules 19 will be relatively constant regardless of changes in mains voltage. However, as for the circuits in Figures 21 and 22, there is the additional benefit of a reduced voltage applied to the LED lamp modules 19.

[0056] Figure 25 illustrates a single-phase circuit similar to figure 2, except that the rectifier 20 is replaced by a transformer T4 and a voltage doubler unit formed by a pair of regular diodes 21 and a pair of capacitors C4. An advantage of the circuit of Figure 25 is that the voltage across the LED lamp 19 is greatly increased and can therefore accommodate an increased number of series-connected LED diodes 18. Thus, the lamp 19 of Figure 25 can have an increased power. in relation to the lamps 19 of the other circuits.

[0057] The above describes only some embodiments of the present invention and modifications, obvious to those skilled in the art of LED lighting circuits, can be made to them without departing from the scope of the present invention. In particular, it will be appreciated that 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.

[0058] Additionally, some variations can be categorized as follows. A filter 27 may be located between the rectifier 20, 200 and the LED module(s) 19. This filter may take the form of a bypass capacitor C27 or a series inductor or a combination of series inductors and a bypass capacitor.

[0059] Furthermore, the control circuit may take the form of various transformers, including leakage reactance transformers, ferroresonant transformers and constant current transformers. This can be accomplished by an autotransformer or a conventional isolation transformer. The control circuit can also take the form of a constant current transformer, as illustrated in figures 26 and 27.

[0060] In figure 26, the constant current transformer Tr is drawn with its primary magnetic circuit illustrated with the laminations parallel to the paper plane and its leakage magnetic circuit drawn with the laminations perpendicular to the paper plane. Thus, a primary winding WP is supplied with power from the AC mains and a secondary winding WS provides an output voltage to the load circuit(s). The leakage magnetic circuit leads provide substantial leakage inductance, thereby magnetically decoupling the secondary winding output from variations in mains voltage. The magnetic circuit associated with the WS secondary winding is arranged so that at least a substantial portion thereof enters into magnetic saturation during normal operation. Preferably, this is achieved by modifying the magnetic circuit.

[0061] The non-linear nature of the inductance of the secondary winding WS in conjunction with the reactance of the resonant capacitor CR, results in at least a portion of the magnetic circuit of the secondary winding being maintained in a magnetically saturated state due to ferro-resonance.

[0062] As the magnetic core associated with the WS secondary winding is saturated, changes in the network voltage practically do not affect the output of the WS secondary winding, 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 mainly governed by the size of the resonant capacitor CR.

[0063] Preferably, as indicated in figure 26, the load consists of 4 regular diodes forming a full-wave bridge rectifier with the LED module 19 forming the load. If desired, an optional filtering capacitor C27 can be connected in parallel with the LEDs 19.

[0064] In addition to controlling the charging current so that it is substantially constant, there are other benefits to this circuit. One of these benefits is the very high operating power factor, very close to unity. Another benefit is very low total harmonic distortion in the network circuit, typically less than 10% distortion. An additional benefit is that the secondary circuit has a very high immunity to mains transient voltages, and this immunity is applicable to transverse mode transient voltages and common mode transient voltages.

[0065] If further improvement in line regulation is required, a small bucking winding (not illustrated, but conventional) can be wound on the primary winding WP and connected in series to the secondary winding WS, but in an inductive direction opposite.

[0066] As indicated by the broken lines in Fig. 26, several load circuits, each with its own sequence of LEDs 19, can be operated in parallel with the constant current transformer Tr. The extension to 3 phases is as indicated in figures 12 to 15.

[0067] Returning now to figure 27, it is possible to build a constant current transformer Tr with a single primary winding WP and two completely independent secondary windings WS1 and WS2. The single primary winding WP is located between a pair of magnetic taps 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 providing a number of parallel load circuits in the manner of figure 26.

[0068] In general, the AC mains source can be single-phase or polyphase (normally three-phase). Where a transformer is used for three phase arrangement, the primary windings of the transformer can be connected in either Wye configuration or 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 source phases or between each phase and a star point or neutral connection.

[0069] Power factor correction can also be implemented by duplicating the overall circuit and operating two sets of LED modules 19 in a lead-lag configuration.

[0070] Dimming is possible by turning off one of the duplicate circuits in a lead-lag configuration or turning off one or more of the phases of a polyphase circuit. Dimming is also possible by switching the impedance(s) in and out of the rectifier source circuit 20, 200. Likewise, dimming is also possible by alternating the in and out turns of the transformer arrays that apply the rectifier 20, 200.

[0071] For a polyphase circuit, such as a three-phase circuit, a 3-wire or 4-wire source is possible. The 3-wire source can be delta connected or have a floating star point. The 4-wire source can have the star point connected to a neutral terminal.

[0072] The foregoing describes only some embodiments of the present invention and modifications, obvious to those skilled in the art of electronics, can be made to it without departing from the scope of the present invention.

[0073] The term "comprising" (and its grammatical variations), as used here, is used in the inclusive sense of "including" or "having" and not in the exclusive sense of "consisting only of".

Claims (15)

1. Control circuit for supplying a constant DC current to an LED lamp unit constituting a load, said circuit comprising AC inputs (A, N) for connection to an AC source that is subject to variations including fluctuations and transients. source voltage, a pair of lamp outputs for connection to said LED lamp unit (19), and a full-wave bridge rectification circuit (20, 200) that supplies a DC voltage and said DC constant current to said lamp outputs, and an AC control circuit connected between said AC inputs (A, N) and said rectification circuit (20, 200) to reduce variations in the magnitude of an AC current supplied to said rectification circuit from the said AC control circuit to thereby reduce corresponding variations in said DC constant current, characterized in that said AC control circuit includes a capacitor (C1, C10, Cr) and an inductive winding (L2, Ws) of a magnetic component (L2, T2, T10, Tr) having a magnetically permeable core which at least part of which in operation is at least partially saturated by ferroresonance, wherein said capacitor (C1, C10, Cr), said rectification circuit (20, 200) and said LED lamp unit (19) are electrically connected to conduct a load current which is controlled by said ferroresonance, and said ferroresonance depends on said load current. 2. Control circuit according to claim 1, characterized in that said capacitor (C1, C10, Cr) comprises both a resonant reactance for said ferroresonance and a current limiting impedance for said DC constant current. 3. Control circuit, according to claim 1 or 2, characterized in that a filter circuit (27) is connected to the output of said rectification circuit. 4. Control circuit according to claim 3, characterized in that said filter circuit comprises a bypass capacitor (C27). 5. Control circuit according to any one of claims 1 to 4, characterized in that said magnetic component (L2, T2, T10, Tr) comprises an inductor, an autotransformer or an isolation transformer. 6. Control circuit according to any one of claims 1 to 5, characterized in that dimming of the LED is achieved by switching (S5, S8) the inward or outward turns of a winding of said magnetic component (T5). 7. Control circuit according to any one of claims 1 to 6, characterized in that said AC source comprises a three-phase source (P1, P2, P3). 8. Control circuit, according to claim 7, characterized by including at least one dimming switch (S1) that deactivates a corresponding phase (P3) of said three-phase source. 9. Control circuit according to any one of claims 1 to 8, characterized by including at least one dimming switch (S1) that switches one or more impedances into or out of said AC source. 10. Control circuit, according to claim 9, characterized in that said impedances are selected from the class consisting of inductors (23) and capacitors (C3). 11. Control circuit, according to claim 9 or 10, characterized in that said switched impedances are in series, or in parallel, with said series capacitor (C1, C10, Cr). 12. Control circuit, according to claims 10 or 11, characterized by including a power factor correction circuit (C2). 13. Control circuit according to any one of the preceding claims, characterized in that the rectification circuit is a full wave bridge (FWB) rectification circuit. 14. Lighting installation characterized by comprising an LED lamp fixture (19) supplied with a constant DC current by a control circuit, as defined in any one of claims 1 to 13. 15. Method of converting a high intensity discharge (HID) lamp fixture supplied by an AC source (A, N) subject to variations including source voltage fluctuations and transients, and including a HID lamp fixture, a ballast and a control mechanism associated with an LED lamp fixture as defined in claim 14, said method comprising the steps of: replacing said HID lamp fixture with said LED lamp fixture (19), replacing said ballast and the associated control mechanism by a full-wave bridge rectification circuit (20, 200) having an input and an output, connecting the output of said rectification circuit to said LED lamp fixture, and connecting an AC control circuit between said AC source and said rectification circuit.