ADAPTIVE FLUORESCENT LAMP DRIVER CIRCUIT

The present invention relates generally to a circuit for adapting a fluorescent lamp to a power source. The invention has particular use in adapting compact fluorescent lamps to a variety of magnetic and electronic transformers originally intended for use with extra low-voltage halogen reflector lamps, and it will be convenient to describe the invention in relation to that exemplary application. It is to be understood though, that the invention is not limited to use in that exemplary application only. Almost every new house built in the last ten to fifteen years has been built with extra low voltage (ELV) tungsten halogen downlights installed. These downlights are most frequently known as "MR-16" halogen lights, referring to the MR-16 standard format for halogen reflector lamps used by a variety of manufacturers. A typical installation will use significant numbers of such downlights (it is not uncommon for 60 or more units to be fitted throughout the house). Originally, large inefficient iron-core, magnetic transformers were used to provide power for the halogen lamps. Each downlight consists of a fixture, a halogen lamp and a 240 volt AC to 11.5 volt AC 50 Hz transformer. Over the years, efficient electronic transformers have been used instead of magnetic transformers. A wide variety of electronic transformers exist having different output waveforms, such as saw tooth, square, triangular, sinusoidal and mixtures thereof, and different operating frequencies.

Recently, compact fluorescent lamps (CFLs) have been used as an energy saving replacement for mains (240Vac & 120Vac) powered halogen downlights. There is currently no fluorescent lamp replacement for extra low voltage halogen lamps that operate reliably from a wide variety electronic transformers types as well as magnetic transformers As electronic transformers provide an output signal having a wide variety of waveforms and frequencies, the operation of CFLs when connected to an electronic transformer will vary. One reason for that variable performance is because the

ELV CFLs use internal oscillating circuits that cannot sustain correct and

satisfactory operation nor are specifically designed for use with high frequency output signals of electronic transformers.

It would therefore be desirable to provide an adaptive fluorescent lamp driver circuit to adapt a fluorescent lamp to a power source that ameliorates or overcomes one or more disadvantages, or at least provides an alternative, to existing lamp circuitry.

With this in mind, the invention provides an adaptive fluorescent lamp driver circuit for adapting a fluorescent lamp to a power source, the fluorescent lamp including: a vapour filled tube and two filaments projecting within the tube, the adaptive fluorescent lamp driver circuit including: a primary driver circuit for generating an AC voltage between the two filaments, the AC voltage having a magnitude sufficient to cause arcing through the vapour within the fluorescent lamp and a frequency sufficiently high to avoid lamp flicker; a voltage conditioner for converting frequency components in a first frequency range of the power source output signal into an internal DC power supply for the primary driver circuit; and a secondary driver circuit for supplying frequency components in a second frequency range of the power source output signal through each filament, wherein the second frequency range is higher than the first frequency range.

In one or more embodiments of the invention, the primary driver circuit includes: a step-up transformer including a primary input winding and an output winding; and a push-pull stage driving current through the step-up transformer primary input winding at a frequency in the second frequency range, the step- up transformer output winding being connected in series with the two filaments.

In this case, the primary driver circuit may further include a current limiting capacitor connected in series with the step-up transformer output winding and the two filaments.

The push-pul! stage may include two base-to-base connected transistors, each transistor having a collector connected to a different end of the step-up transformer primary input winding.

The push-pull stage may further include a step-up transformer secondary input winding, wound in the opposite sense to the step-up transformer primary input winding, interconnecting the bases of the transistors. The bases of the transistors may be biased by a separate resistor connected to a positive supply rail of the voltage regulator.

The push-pull stage may further include series-emitter resistors connecting the emitters of the transistors to a negative supply rail of the voltage regulator. The primary driver circuit may further include a choke for removing AC signal components from the current driven through the step-up transformer primary input winding.

The choke may be connected between the voitage conditioner and a central tap on the step-up transformer primary winding. In one or more embodiments of the invention, the voltage conditioner includes a rectifier bridge and a filtering capacitor. Further filtering may also be used utilising additional discrete components such as capacitors, inductors, resistors and diodes or a combination thereof. The voltage conditioner may also contain components such as Zenor Diodes and Voltage Regulators In one or more embodiments of the invention, the secondary driver circuit includes a secondary transformer including an input winding and two output windings; and a filtering capacitor connected in series with the secondary transformer input winding, wherein each secondary transformer output winding is connected in series with a separate filament.

The secondary transformer input winding may form an impedance matching inductor acting to substantially match the impedance of the power source.

The first frequency range may include mains frequency. In specific embodiments of the invention, the mains frequency may be either 50 Hz or 60 Hz, however it will be appreciated in other embodiments of the invention a different mains frequency may be used.

The second frequency range may be from 100 Hz to 200 kHz, and preferably in the range of 20 kHz to 60 kHz. In one or more embodiments, the adaptive fluorescent lamp driver circuit may include an impedance circuit connected across the output of the power source acting to increase current drawn from the power source.

Any discussion of documents, devices, acts or knowledge in the specification is included to explain the context of the invention. It should not be taken as an admission that any of the material formed part of the prior art base or the common general knowledge in the relevant art on or before the priority date of the claims attached hereto.

The following description refers in more detail to the various features of the adaptive fluorescent lamp driver circuit of the present invention. To facilitate and understanding of the invention, reference is made in the description to the accompanying drawings where the adaptive fluorescent iamp driver circuit is illustrated in a preferred embodiment. It is to be understood that the adaptive fluorescent iamp driver circuit of the present invention is not limited to the preferred embodiment as illustrated in the drawings.

In the drawings:

Figure 1 is a schematic diagram of a fluorescent lamp including an integrated adaptive fluorescent lamp driver circuit; and

Figure 2 is a circuit diagram of such an adaptive fluorescent lamp driver circuit.

Figure 1 depicts a CFL assembly 1. The CFL assembly 1 may typically include a Sight emitting portion 2, a base portion 3 and a connector portion 4. The light emitting portion 2 may typically include an ELV MR-16 type CFL, designed for operation at 12 Volt AC and 5 W - 50 W, in the form of a spiral or U-Tube, finger tube, CFL tube housed in an aluminium or plastic reflector. The connector portion 4 includes two pins 5 and 6 for connecting the CFL assembly to a power source, whilst the base portion 3 includes notably an adaptive fluorescent lamp driver circuit for adapting the light emitting portion 2 to the power source. Although this figure shows a CFL, it will be understood that the invention is also suitable for use in adapting fluorescent lamps of varying sizes to various power sources. It may be also understood that the assembly maybe in the form of a remote and externa! ballast driving a remote and external lamp.

Figure 2 shows an exemplary embodiment of such an adaptive fluorescent lamp driver circuit 10 for adapting a fluorescent lamp 12 to a power source 14. The compact fluorescent lamp is filled with a gas, such as low pressure mercury vapour and xenon. The inner surface of the lamp 12 is coated with a fluorescent coating made of varying blends of metallic and rare- earth phosphor salts. Two filaments 16 and 18 project within the lamp 12. When the lamp is turned on, the electric power supplied heats up the filaments enough for them to emit electrons. These electrons collide with ionised atoms in the lamp surrounding the filament and form a plasma which enables current to flow through the interior of the lamp between the filaments, lonisation of the gas in the lamp causes the emission of light in the ultraviolet region of the spectrum. The ultraviolet light is absorbed by the lamp's fluorescent coating, which re-radiates the energy at lower frequencies to emit visible light.

The adaptive fluorescent lamp driver circuit 10 includes a primary driver circuit 20 for generating an AC voltage between the two filaments 16 and 18. The AC voltage generated by the primary driver circuit 20 has a magnitude sufficient to cause arcing through the vapour within the lamp 12 and a frequency sufficiently high to avoid lamp flicker. The adaptive fluorescent lamp driver circuit 10 also includes a voltage conditioner 22 for converting frequency components in a first frequency range of the output signal of the power source

14 into an internal DC power supply for the lamp drive circuit 20. Finally, the adaptive fluorescent lamp driver circuit 10 includes a secondary driver circuit 24 for supplying frequency components in a second frequency range of the output signal of the power source 14 through each filament 16 and 18. The second frequency range is higher than the first frequency range. In a typical embodiment of the invention, the first frequency range includes mains frequency, such as 50 Hz or 60 Hz, whereas the second higher frequency range is adapted to efficient operation of the lamp 12. For example, the second frequency range may be from 100Hz to 200 kHz and preferably from 20 kHz to 60 kHz.

The voltage conditioner 22 includes a fuse 26, rectifier bridge 28 and filtering capacitor 30. Current from the power source 14, which may typically be from 11.5Vac Electronic or Magnetic Transformer enters the adaptive fluorescent lamp driver circuit 10 via the fuse 26, and is then rectified by the bridge rectifier 28 to provide a positive supply rail 32 and a negative supply rail 34. The filtering capacitor 30 is connected between the positive supply rail 32 and the negative supply rail 34, thereby forming a rudimentary internal DC power supply for the lamp drive circuit 20.

The primary driver circuit 20 includes a step-up transformer 36 including a primary input winding split into two portions, here referenced 38 and 40, and an output winding 42. A choke 44 in the form of an inductor is connected between the positive supply rail 32 and a central tap 46 separating the two portions 38 and 40 of the step-up transformer primary winding. The choke acts to remove AC signal components from current supplied to the step-up transformer 36 and to prevent electrical noise from appearing back at the power source.

The primary driver circuit 20 also includes a push-pull stage 48 driving current through the step-up transformer primary input winding 38, 40. The push-pull stage 48 includes two base-to-base connected transistors 50 and 52. Each of the transistors 50 and 52 has a collector connected respectively to different ends 54 and 56 of the step-up transformer primary input winding 38, 40. A capacitor 58 is connected across the step-up transformer primary input winding 38 and 40. A secondary input winding 60 of the step-up transformer

36, wound in the opposite sense to the step-up transformer primary input winding 38, 40, interconnects the bases of the transistors 50 and 52, The bases of the transistors 50 and 52 are each biased by separate resistors 62 and 64 connected to the positive supply rail 32 of the voltage conditioner 22. Series emitter resistors 66 and 68 connect the emitters of the transistors 50 and 52 to the negative supply rail 34 of the voltage conditioner 22.

In operation, each of the transistors 50 and 52 is switched on only when its complement is switched off. When the transistor 50 is switched on, current is drawn through the first portion 38 of the step-up transformer primary input winding and through the transistor 50. Alternately, when the transistor 52 is switched on, current is drawn through the second portion 40 of the step-up transformer primary input winding and through the transistor 52, resulting in current flowing through the step-up transformer primary input winding in an opposite direction. The series emitter resistors 66 and 68 ensure that the current drawn through each of the transistors is substantially matched.

Accordingly, as the transistors 50 and 52 alternately switch on and off, current is caused to flow through the step-up transformer primary input winding in alternating directions at a frequency determined by the value of the capacitor 58, the switching speed of the transistors 50 and 52, the inductance of the step-up transformer 36, the operating characteristics of the lamp and the value of the series lamp capacitor 70. The final operating frequency of the push-pull stage of the lamp drive circuit 20 is ideally set to fall within a desired operating frequency range suited to the lamp 12. This frequency range may be between 100 Hz to 200 kHz, and preferably within the range 20 kHz to 60 kHz.

The alternating current created in the step-up transformer primary input winding 38, 40 thereby creates an AC current in the step-up transformer output winding 42, The output of the step-up transformer 36 is supplied to both lamp filaments 16 and 18 via a series capacitor 70. The capacitor 70 allows the output voltage from the step-up transformer 36 to be applied directly to both filaments of the lamp before the start of the lamp discharge, when the impedance of the iamp 12 is high and there is no apparent glow. After the discharge starts, the capacitor 70 and the internal impedance of the step-up

transformer output winding 42 limits the final operating current of the lamp.

The characteristics of the capacitor 70 and the step-up transformer 42 are selected to ensure that the AC voltage between the two filaments 16 and 18 has a magnitude sufficient to cause arcing through the vapour within the lamp 12, and that the frequency of that AC voltage is sufficiently high to avoid lamp flicker. When a fluorescent lamp is viewed directly, lamp flicker can be observed at frequencies less than 70 Hz, whilst lamp flicker can be observed when the lamp is viewed peripherally at frequencies less than 100 Hz.

The secondary driver circuit 24 includes a secondary transformer 72 including an input winding 74 and two output windings 76 and 78. The secondary driver circuit 24 also includes a filtering capacitor 80 connected in series in a circuit branch with the secondary transformer input winding 74, that circuit branch being connected to the raw output signal from the power source

14. Each secondary transformer output winding 76 and 78 is connected in series with a separate filament 16 and 18. The secondary driver circuit 24 supplies higher frequency components of the output signal from the power source 14 to each of the filaments 16 and 18 in order to heat those filaments and thereby minimise the power and maximise the brightness of the lamp 12.

This also enables the voltage supplied by the step-up transformer 36 to be able to be minimised.

The secondary transformer input winding 74 and capacitor 80 effectively create a high pass filter so that two groups of frequency components of the output signal from the power source 14 can be directed through different parts of the adaptive fluorescent lamp driver circuit 10, thereby optimising the efficiency with which the output signal from the power source 14 is used irrespective of whether that output signal is derived from a magnetic transformer or electronic transformer. The secondary transformer input winding 74 effectively forms an inductor and, together with capacitor C1 , substantially matches the impedance of the power source 14, thus maximising the current through the filaments 16 and 18, This will effectively raise and stabilise the power output from the power source 14, when the power source is an electronic transformer, reducing stress on the components of th,e power source and minimising ill effects on lamp performance. The value of the

inductance of the booster transformer input winding 74 will typically vary between 0.1 to 50 μ H.

The capacitor 80 connected in series with the booster transformer input winding 74 filters the high and low frequencies that are presented to it by the aforementioned ELV transformers. The lower frequencies (typically 50 Hz or 60 Hz) are blocked and used by the voltage conditioner 22 to provide power to the primary driver circuit 20. Higher frequencies, typically in the range of 10 kHz to 200 kHz, pass through the filter unimpeded and provide stabilising power to the lamp. Some types of electronic transformers include transistors which do not operate in an active mode when connected to the above-described adaptive fluorescent lamp driver circuit. In order to ensure operation of these transistors in active mode, the adaptive fluorescent lamp driver circuit further includes an impedance circuit 82 connected across the output of the power source 14 which acts to increase current drawn from the power source. In the embodiment shown in Figure 2, the impedance circuit is realised as a simply resistive shunt 84, but in other embodiments different impedance elements or combinations of impedance elements may be used.

The above-described adaptive fluorescent lamp driver circuit enables compact fluorescent lamps to be driven by a wide variety of ELV electronic transformers and magnetic transformers regardless of the operating frequency or voltage waveform output by those transformers. Accordingly, the adaptive fluorescent lamp driver circuit enables compact fluorescent lamps to be provided as a direct replacement for tungsten halogen lamps irrespective of transformer type provided with the tungsten halogen lamp.

Whilst the embodiment of the adaptive fluorescent lamp driver circuit is described here above as forming an integral part of a fluorescent lamp assembly, in other embodiments adaptive fluorescent lamp driver circuit may be manufactured and sold separately from the lamp. In this way, the adaptive fluorescent lamp driver circuit can be used to adapt existing CFL M R- 16 or other fluorescent lamps to a variety of power supplied.

Moreover, the primary and secondary drive circuits, voitage conditioner and impedance circuit need not ait be included in the adaptive fluorescent lamp driver circuit. One or more of these elements may be manufactured as modules for option use with the adaptive fluorescent lamp driver circuit. For example, embodiments of the adaptive fluorescent lamp driver circuit may be manufactured with the primary and secondary drive circuits and voltage conditioner only, and a separate impedance circuit manufactured for use with the adaptive fluorescent lamp driver circuit when required.

Finally, it is to be understood that various modifications and/or additions may be made to the adaptive fluorescent lamp driver circuit without departing from the spirit or ambit of the present invention as defined in the claims appended hereto.