WO2011057442A1

WO2011057442A1 - A method and system for controlling power supplied to a lamp tube in a lighting system having a non-dimmable ballast circuit

Info

Publication number: WO2011057442A1
Application number: PCT/CN2009/074918
Authority: WO
Inventors: Shu-Hung Henry Chung; Nan Chen
Original assignee: City University Of Hong Kong; E. Energy Double Tree Limited
Priority date: 2009-11-12
Filing date: 2009-11-12
Publication date: 2011-05-19

Abstract

A method and system for controlling active power supplied to a lamp tube in an existing lighting system having an electronic ballast circuit for said lamp tube bydrawingreactive power througha circuit path parallel to said lamp tube. The existing lighting system is modified by addition of a reactive power control circuit in parallel to said lamp tube. The reactive power control circuit is configured to share with said lamp tube current drawn from said non-dimmable ballast circuit whereby the reactive power control circuit draws reactive power and thereby controls active power consumption by the lamp unit. The reactive power control unit thereby enables dimming of the lamp tube with a decrease in consumed power by the lighting system without detriment to the lighting system power factor.

Description

A Method and System for Controlling Power Supplied to a Lamp Tube in a Lighting System Having a Non-dimmable Ballast Circuit

Field of the Invention

The invention relates to a method and system for controlling power supplied to a lamp tube in a lighting system having a non-dimmable ballast circuit and, more particularly, to controlling active power supplied to a lamp tube in an existing lighting system having an electronic ballast circuit for said lamp tube by processing reactive power in a circuit path parallel to said lamp tube.

Background of the Invention

A wide range of different types of lamps and lighting systems are used in various applications. These include incandescent lamps, fluorescent lamps, high- and low-pressure discharge lamps. For both aesthetic and energy saving reasons, various attempts have been made in the prior art to provide such lamps with a dimming control so that the brightness of the lamps can be adjustable.

Dimming function is particularly useful for high intensity discharge (HID) lamps, which are widely used in public lighting systems due to the HID lamps' manifold advantages such as longevity and high luminous efficacy. Unlike incandescent lamps, HID lamps generally require a long warm up time to reach full brightness. After being shut off, they need a cooling down period before they can be restarted again. It is this "re -strike" characteristic that makes dimming a very attractive alternative to simply turning the lights off, because dimming does not require the turning off the lamps and can therefore avoid considerable warm-up time of the lamps after re- ignition. Dimming also has other advantages such as reduction of peak power demand, increase of flexibility for multi-use spaces, safer driving in light traffic conditions and avoidance of light pollution.

Existing dimming methods for existing lighting systems include Triac-based dimmers for incandescent lamps and gaseous discharge lamps compatible with triac dimmers, dimmable electronic ballasts for gaseous lamps, and a range of disparate techniques for dimming lamps driven by magnetic ballasts.

Triac-based dimmers have been popularly used as dimming devices for Edison-type incandescent lamps and some triac -dimmable fluorescent lamps. A typical triac circuit connection comprises a triac and a triggering circuit which controls the phase or delay angle of turning the triac on over a cycle of the mains voltage. By controlling the delay angle, the output root -mean-square voltage, and thus the power to the lamp, can be controlled. This control of ac voltage results in the ability to adjust the brightness of the lamp.

However, the waveshape of the mains input current through the triac dimmer is dependent on the delay angle. When the delay angle is nonzero, the input current will deviate from the sinusoidal shape of the mains voltage. When the delay angle is increased, the conduction time of the triac is diminished. The input current will then consist of high harmonic components and this generates undesirable harmonics into the power system. In addition, as the input power factor is the product of the displacement factor and the distortion factor, the input power factor becomes small when the delay angle is large. It is because the displacement factor is equal to the cosine of the delay angle (if the delay angle is large, the displacement factor will become small) and the distortion factor deteriorates as the current harmonic content increases. The ultimate effect of this low input power factor is the presence of reactive power flow between the ac mains and the lighting system. This reactive power can cause serious defects over the power system. The lower the power factor, the larger the rating of the transformers and the larger the size of the conductors of transmission must be. In other words, the greater the cost of generation and transmission will be. That is the reason why supply undertakings always stress upon consumers to increase the power factor. Electronic ballasts for fluorescent lamps (low-pressure discharge lamps) have been widely used and it has been shown that their use has an overall economic benefit. Operating at high frequency (typically above 20kHz) electronic ballasts can eliminate the flickering effects of the fluorescent lamps and achieve a higher efficacy than mains-frequency (50Hz or 60Hz) operated magnetic ballasts. Therefore fluorescent lamps driven by electronic ballasts consume less energy for the same light output when compared with lamps driven by magnetic ballasts.

There has been an increasing trend of using dimmable electronic ballasts for discharge lamps such as fluorescent lamps and high intensity discharge (HID) lamps. A dimmable electronic ballast usually has a 4-wired connection arrangement on the input side. Two connections are for the "live" and "neutral" of the ac mains, the other two are for the dimming level control signal, which is typically a dc signal within 1V-10V. A general structure of a typical dimmable electronic ballast comprises an active or a passive power factor correction circuit, a high-frequency dc/ac converter, and a resonant tank circuit. The power factor correction circuit and the dc/ac converter are interconnected through a high voltage dc link. The dc/ac converter is used to drive the lamp through the resonant tank circuit. It is usually switched at a frequency slightly higher than the resonant frequency of the resonant tank circuit. The resonant tank is used to preheat the electrodes, provide a high voltage to ignite the lamp and ballast the lamp current. Dimming function is achieved by controlling the dc link voltage and/or the switching frequency of dc/ac converter. The input power factor can be kept high at any power level. The waveform of the input current is sinusoidal and in phase with the ac mains, but its magnitude varies .

Among various light sources, high-intensity-discharge (HID) lamps exhibit the best combination of the high luminous efficacy and good colour rendition with the high power compact source characteristics. Through appropriate choice of dose, full spectrum (white light) sources with excellent color rendering properties can be produced with good efficacy and compact size. Applications of HID lamps have been used in many applications, such as wide area floodlighting, stage, studio, and entertainment lighting to UV lamps.

Use of high frequency electronic ballast can reduce the size and the weight of the ballast and improve the system efficacy. This feature is especially attractive for low wattage HID lamps because the overall lighting system is expected to be of small size. Moreover, as the operating frequency increases, the re-ignition and extinction peaks disappear, resulting in a longer lamp lifetime. The high-frequency load characteristic of a HID lamp can be approximated as a resistor and the lamp (power) factor approaches unity. There is no flickering effect and the stroboscopic effect in the light output and the light lumen can be improved. However, the operation of high pressure HID lamps with high-frequency current waveforms is offset by the occurrence of standing pressure waves (acoustic resonance). This acoustic resonance can lead to changes in arc position and light color or to unstable arcs. Instability in the arcs could sometimes cause the arcs to extinguish. Fig. 1 shows the equivalent circuit 10 of a typical dimmable ballast 12 and fluorescent lamp 14 in accordance with a known dimming technique. The ballast 12 is modeled by a voltage source V_b 16 connecting in series with an output reactance X_b 18, which is typically inductive. At high-frequency operation, the fluorescent lamp is modeled by a resistance 20. The ballast control 22 will sense the lamp current I_L to control the power supplying to the fluorescent lamp 12.

The ultimate purpose of the above dimming control method is to control the lamp current I_L. Fig. 2 shows the equivalent circuit 30 of the entire lighting system, in which the lamp current

I_L is controlled by a variable current source I_b 32. The magnitude of I_b is determined by the ballast 16. However, many existing lighting systems include lamps having non-adjustable or non- dimmable ballasts, particularly non-adjustable or non-dimmable electronic ballast circuits. The structure of a non-dimmable electronic ballast is similar to a dimmable ballast circuit, except there is no external dimming control. Thus, the output current of the non-dimmable ballast I_b , and hence the lamp current I_L , is regulated at a preset value rather than a variable value as in the dimmable arrangement. Fig. 3 shows the equivalent circuit 40 of the non-dimmable electronic ballast circuit.

A problem is that in many existing lighting systems having electronic or magnetic ballast circuits, the ballast circuits are of the non-adjustable or non-dimmable type therefore providing no means for adjusting lamp current and thus lighting control beyond the preset value inherent in the ballast circuit. The logical solution to such a problem is to replace the existing non-adjustable or non-dimmable electronic or magnetic ballast circuits by suitable adjustable or dimmable ballast circuits. However, this solution involves considerable capital costs in providing the replacement adjustable or dimmable ballast circuits or modules, considerable costs in rewiring the existing lighting systems to replace the existing ballast circuits with replacement ballast circuits and environmental issues in disposing of the removed ballast circuits.

Objects of the Invention

An object of the invention therefore is to provide a method of adapting an existing lighting system having a non-adjustable or non-dimmable ballast circuit to be adjustable or dimmable whilst reducing power consumed without detriment to the power factor of the system.

Another object of the invention is to provide a module or circuit for adapting an existing lighting system having a non-adjustable or non-dimmable ballast circuit to be adjustable or dimmable.

Another object of the invention is to mitigate or obviate to some degree one or more problems associated with known lighting systems having non-adjustable or non-dimmable electronic or magnetic ballast circuits.

One skilled in the art will derive from the following description other objects of the invention. Therefore, the foregoing statements of object are not exhaustive and serve merely to illustrate some of the many objects of the present invention. Summary of the Invention

The invention provides a simpler, less expensive and more efficient solution to the aforementioned problems by providing a means and method of converting a lighting system or network using non-dimmable ballast circuitry (more particularly one using electronic ballast circuitry) to be dimmable without requiring replacement of the existing ballast circuitry.

The invention generally comprises installing at each lamp tube in a lighting system a dimming module so that lamp brightness can be adjusted. The adjustment may be a fixed adjustment or a variable adjustment. The invention is applicable to lighting systems or networks comprising a plurality of ballasted lamp tubes or to a lighting system comprising a single ballasted lamp tube.

The dimming module is connected across a lamp tube, i.e. the module's terminals are connected to respective terminals of the lamp tube lighting circuit at the lamp tube. As such, the function of the dimming module is to divert current being supplied to the lamp tube away from the lamp tube thereby dimming the lamp tube. The module in its simplest form may comprise a capacitor or a circuit operated as a capacitor connected in parallel with the lamp tube. The capacitor or the capacitance of the circuit may be variable.

The module is applicable to all types of electric discharge lamp tubes and, in particular, high intensity discharge (HID) lamp tubes including, but not limited to, fluorescent lamps, HID lamps HPS lamps, and mercury arc lamps.

The module is preferably arranged to be added in a piggyback manner to existing lamp tubes whereby it is unnecessary to change the choke or existing ballast circuitry for the lamp tube.

Instead of dimming a lamp in a lighting system using a dimmable ballast, the invention generally provides a method of adapting a lighting system to dim a lamp having an existing non- dimmable electronic ballast circuit.

The proposed dimming method is based on connecting a reactive power controller (RPC) across the lamp, so that the actual current flowing through the lamp can be varied. As the RPC only handles reactive power, it neither dissipates nor generates electric energy. Its function is to share the ballast current with the lamp current.

In a first main aspect of the invention, there is provided a method of controlling power supplied to a lamp tube in a lighting system having a non-dimmable ballast circuit, comprising connecting across said lamp tube a reactive power control circuit configured to share with said lamp tube current drawn from said non-dimmable ballast circuit whereby the reactive power control circuit processes reactive power. Preferably, the reactive power control circuit consumes a preset share of the current drawn from the non-dimmable ballast circuit to provide a preset amount of dimming of the lamp tube. Alternatively, the reactive power control circuit consumes a variable share of the current drawn from the non-dimmable ballast circuit to provide variable dimming of the lamp tube. The lighting system may be an existing lighting system and the non-dimmable ballast circuit may comprise an existing circuit of the lighting system and the lighting system is modified using the reactive power control circuit to adapt an existing non-dimmable lighting system to a dimmable lighting system. Preferably, the non-dimmable ballast circuit is an electronic ballast circuit for an electric discharge lamp tube.

The reactive power control circuit may comprise a passive device connected across the lamp tube.

The reactive power control circuit may comprise a frequency independent current source which diverts therethrough a portion of the lamp current being drawn from the electronic ballast circuit. In a second main aspect of the invention, there is provided a module for adjusting active power supplied to a lamp tube in a lighting system having a non-dimmable ballast circuit, comprising a reactive power control circuit coupled across said lamp tube, said reactive power control circuit being configured to share with said lamp tube current drawn from said non- dimmable ballast circuit whereby the reactive power control circuit draws reactive power.

Brief Description of the Drawings

The foregoing and further features of the present invention will be apparent from the following description of preferred embodiments which are provided by way of example only in connection with the accompanying figures, of which:

Fig. 1 is an equivalent circuit of a typical dimmable electronic ballast;

Fig. 2 is an equivalent circuit of a lamp system including a typical dimmable electronic ballast; Fig. 3 is an equivalent circuit of a lamp system with a non-dimmable ballast; Fig. 4 is a an equivalent circuit diagram illustrating a lamp system with a non-dimmable ballast including a reactive power control circuit according to the invention;

Fig.5 is a graph showing the relationship between and lamp power for the circuit of fig. 4;

Fig. 6(a) is a circuit diagram of a passive RPC in the form of a capacitor;

Fig. 6(b) is a circuit diagram of a passive RPC in the form of an inductor;

Fig. 7 is a circuit diagram of an active circuit RPC;

Fig. 8 is a system diagram of typical modern electronic ballast circuit;

Fig. 9 is a circuit schematic of a voltage-fed half-bridge series-resonant parallel-loaded inverter typically used for the second stage of the typical electronic ballast circuit of fig. 9;

Fig.10 illustrates waveforms associated with the inverter of fig. 9;

Fig. 11 shows an equivalent circuit model in which the inverter of fig. 9 is modeled by a low-frequency sinusoidal voltage source;

Fig. 12 shows an equivalent circuit of an entire lighting system, in which the non- dimmable ballast is modeled as a current source i_inv supplying current to the lamp;

Fig. 13 illustrates the connection of the dimming module according to the invention to the entire lamp system of fig. 12;

Fig. 14 shows a phasor diagram of the system operating at undimmed power and dimmed power;

Fig, 15 is a circuit schematic of the dimming module according to the invention;

Fig. 16 shows waveforms associated with the dimming module of fig. 15;

Fig. 17 shows the voltage and current phasors of the dimming module of fig. 15;

Fig. 18 shows the relationships between the lamp power p and v_dc at different switching frequencies for the dimming module of fig. 15;

Fig. 19 shows the relationship between \ v₀ | and | ν_ζ'| for the dimming module of fig. 15;

Fig. 20 is a schematic of an experimental circuit setup to verify the dimming module according to the invention;

Fig. 21 is a circuit schematic of the experimental setup of fig. 20;

Fig. 22 is a flowchart of the software program used in testing the dimming module according to the invention using the experimental setup of figure 20;

Fig. 23(a) shows the waveforms of the input voltage and current of the ballast at the rated power without connecting the dimming module of the invention in the experimental setup of figure 20;

Fig. 23(b) shows the corresponding waveforms to fig. 23(a) when the lamp power of Lamp 1 is 8.1W; Fig. 24(a) shows the voltage and current waveforms of the two lamps at the rated power without connecting the dimming module of the invention in the experimental setup of figure 20;

Fig. 24(b) shows the corresponding waveforms to fig. 24(a) when the lamp power of Lamp 1 is 8.1W;

Fig. 25(a) shows the filament voltages of the two lamps at the rated power without connecting the dimming module of the invention in the experimental setup of figure 20;

Fig. 25(b) shows the corresponding waveforms to fig. 25(a) when the lamp power of Lamp 1 is 8.1W;

Fig. 26 shows the startup transient of the two lamps with the dimming module according to the invention connected; and

Fig. 27 is a perspective view of a lighting system including the dimming module of the invention.

Description of Preferred Embodiments

The invention relates to a method and system for controlling power supplied to a lamp tube in a lighting system having a non-dimmable ballast circuit and. More particularly, the invention relates to controlling active power supplied to a lamp tube in an existing lighting system having an electronic ballast circuit for said lamp tube by processing reactive power in a circuit path parallel to said lamp tube.

Generally, there is provided a method of controlling power supplied to a lamp tube in a lighting system having a non-dimmable ballast circuit, comprising connecting across said lamp tube a reactive power control circuit configured to share with said lamp tube current drawn from said non-dimmable ballast circuit whereby the reactive power control circuit draws reactive power.

Furthermore, there is provided a module for adjusting active power supplied to a lamp tube in a lighting system having a non-dimmable ballast circuit, comprising a reactive power control circuit coupled across said lamp tube, said reactive power control circuit being configured to share with said lamp tube current drawn from said non-dimmable ballast circuit whereby the reactive power control circuit draws reactive power.

The invention provides a simpler, less expensive and more efficient solution to the aforementioned problems by providing a means and method of converting a lighting system or network using non-dimmable ballast circuitry (more particularly one using electronic ballast circuitry) to be dimmable without requiring replacement of the existing ballast circuitry. The invention generally comprises installing at each lamp tube in a lighting system a dimming module so that lamp brightness can be adjusted. The adjustment may be a fixed adjustment or a variable adjustment. The invention is applicable to lighting systems or networks comprising a plurality of ballasted lamp tubes or to a lighting system comprising a single ballasted lamp tube.

The dimming method of the invention is based on connecting a reactive power controller

(RPC) across the lamp, so that the actual current flowing through the lamp can be varied. As the RPC only handles reactive power, it neither dissipates nor generates electric energy. Its function is to share the ballast current with the lamp current. Since I_B is almost constant, the lamp current

I_L decreases as the RPC current is increased. The dimming method of the invention is illustrated by Fig. 4 which comprises a RPC 52 connected in parallel with the lamp (modeled by RL) 54 in a light system 50 having a non-dimmable electronic ballast It can be shown that:

where P is the lamp power, R_L(P) is the lamp resistance at the lamp power P, V_L(P) = a P + b is the lamp voltage at the lamp power P. The values of a and b are dependent on the lamp characteristics, for example, the values of a and b are -0.98VW^"1 and 147V, respectively, for T8 36W fluorescent lamp.

Fig. 5 comprises a graph illustrating the required in order to reduce the lamp power of a T8 36W lamp from 36W to 4W.

I_ft_pc is maintained at 90 or 270 degrees out of phase with the lamp voltage V_L across the said apparatus, and wherein the magnitude of is used for varying the lamp current I_L . The

RPC 52 can be implemented by a passive or active circuit. The passive RPC can be realized by a variable capacitor 60, as illustrated in Fig. 6(a), or a variable inductor 70, as illustrated in Fig. 6(b). In practice, the former one is preferred as the required value of the inductor 70 is much larger than that of the capacitor 60 for a given value of . can be varied by adjusting the equivalent value of the capacitor or inductor. The limitation of this method is that 1) only discrete value of can be obtained, and 2) the value of is dependent on the operating frequency of the ballast. Thus, this method is only applicable for the case that the operating frequency of the ballast is known. If the capacitor value or inductor value is not chosen properly, the lamp arc cannot be maintained and the lamp will extinguish.

Fig. 7 shows an example of an active RPC 80. The RPC structure is based on an ac/dc converter 82. The switches S_A 84 and S_B 86 of the converter 82 are operated in anti -phase at the same frequency of the lamp voltage. The duty cycle of S_A and S_B is both 0.5. The inductor L 88 and capacitor C_B 90 form a resonant path so that a near sinusoidal voltage across C_B . The value of is adjusted by controlling the phase difference between V_L and, S_A and S_B . This will be better understood from the following more detailed discussion of an active RPC.

More specifically, the system diagram of a modern electronic ballast 100 is shown in Fig. 8. It consists of two power processing stages. The first stage 101 is an active or a passive power factor correction circuit 102 and the second stage 103 is a high-frequency inverter 104 having an output resonant tank circuit 106. The two stages are interconnected by a high- voltage dc link 108. The inverter 104 generates a square voltage waveform at the input of the resonant tank 106. The resonant tank 106 is used to preheat the lamp filaments 1 10, maintain the filament temperature, generate a sufficiently high voltage to ignite the lamp 1 12, facilitate soft-switching of the inverter 104, and give a near sinusoidal lamp current.

Fig. 9 shows the circuit schematic of a voltage-fed half-bridge series-resonant parallel- loaded inverter 120 typically used for the second stage 103. Fig. 10 shows the key waveforms in the inverter 120. The duty cycles of the switches S_l 121 and S₂ 123 are both slightly less than

0.5. The capacitor C 122 is used to provide a stable dc voltage of V I 2 at node 'Υ'. C_sl 124 and C_s2 126 create zero-voltage- switching (ZVS) conditions for S and S₂ and the switching frequency of S_l 121 and S₂ 123 is slightly higher than the natural frequency of the resonant tank

106 so as to ensure ZVS of S_l and S₂ . A near-square ac voltage of amplitude V I 2 appears across nodes 'X' and 'Υ'. Since the resonant tank 106 is designed to operate at a high quality factor, the fundamental component of the square wave is the dominant frequency component in the circuit. The inverter output current i_im lags by an angle φ.

At the preheating stage, the lamp is non-conducting and its equivalent resistance is very high. Thus, the quality factor is very high. The switching frequency of S_l 121 and S₂ 123 is much higher than the natural frequency for a fixed duration to preheat the filaments. At the ignition stage, it is decreased towards resonance to generate a high voltage across the lamp 1 12. At the steady state, it is further decreased to the frequency at which the lamp power is at the rated value. Two auxiliary windings are added on the resonant inductor L_r 128 for separately heating the filaments to the appropriate temperature.

Many high-frequency models for the lamp 1 12 have been proposed as illustrated by Fig. 1 1. The lamp is modeled by a resistor r(p) , where p is the lamp power. The filament resistance is r_f . The rms lamp voltage v_L(p) is assumed to be equal to the voltage across r(p) . Based on [2], v_L(p) , r(p) and the lamp current i_L(p) are equal to

v_L (p) = p + b (1) r{ p) = a² p + 2 a b + b² p ^l (2) i_L(p) =——Γ (3) a + b p

where a and b are constants related to the physical dimensions of the lamp 1 12. For example, the values of a and b of the Philips TL5 28W/865 lamp are found to be -4.57 VW^"' and 297 V, respectively. Experimental results show that the above mathematical model is valid for the lamp power varying from 10% to 100% of the full power.

Fig. 1 1 shows the equivalent circuit model in which the inverter 104 is modeled by a low- frequency sinusoidal voltage source v_iHV' . For the sake of simplicity, the filament resistance r_f is ignored in the following analysis. It can be shown that i_inv is inv (4)

^Ζ iίnηv Ρ)

V ( Ό )

where Z_inv(p) = j oo Z_r H L±r± , ω = 2 π f , and /is the switching frequency.

l + j (o C_r r_L(p)

Thus, based on (4), the magnitude of i_inv , rated | i_inv | , is equal to

where P_r is the rated power of the lamp.

The reactance of C_r 130 is larger than the lamp resistance. In practice, the lamp current i_L is regulated indirectly by regulating i_im , in order to control the power supplying to the fluorescent lamp 1 12. There are three possible methods of regulating the value of i_im

1 ) Control of the magnitude of the voltage V - This method requires using a power factor correction circuit having variable dc output voltage.

2) Control of the switching frequency of S_l and S₂ - The reactances of L_r and C_r vary with the switching frequency. Thus, the magnitude of the lamp current, and thus the lamp power, can be regulated.

3) Control of the effective value of L_r and C_r - The output impedance can be varied by changing the effective values of L_r and C_r . In one example, L_r is implemented by using a sloped gap magnetic core. In another example, the resonant capacitor is realized by using a fixed capacitor in series with a switched-capacitor module. The duty time of the switched capacitor module determines the effective capacitance of the resonant capacitor, so that the lamp current can be varied. However, this method will make the lamp current unsymmetrical and is harmful to the lamp. Fig. 12 shows the equivalent circuit 140 of the entire system, in which the ballast is modeled as a current source i_im supplying to the lamp.

For an existing lamp circuit, the value of i_jnv is relatively constant in the electronic ballast. To dim the lamp, a dimming module 150 according to the invention is connected to each lamp in the manner shown in Fig. 13. It is based on using a frequency- independent current source i_m to divert the lamp current i_L . Fig. 13 illustrates the basic concept and the connection of the dimming module 150 to the entire lamp system. If the filament resistance is neglected in the calculations, it can be shown that

m ^linv ^lL Cr (6) where i_Cr is the current flowing through the resonant capacitor C_r , i_L + i_t Cr and z_x(p)

i_m is controlled to be leading v_L by 90°, so the current source does not handle active power, but reactive power.

Fig. 14 shows the phasor diagram of the system operating at the undimmed power P_l and dimmed power P₂. Since i_m is in phase with the resonant capacitor current i_Cr , the current i_c through the filament can be approximated by

\ *C I = I io I + 1 I ⁽⁷⁾ Based on (1) and (3),

v P₂) > v PA (8)

According to (8), the resonant capacitor current i_Cr increases as v_L increases at the dimmed condition. Together with i_m , the filament voltage and thus its power will increase as the lamp power decreases. The filament power p_f can be approximated by

where i_Cr = ω C_r v_L . Such phenomenon provides a favorable condition for dimming the lamp because the filament power will increase with an increase in i_m and i_Cr at the dimmed conditions.

The circuit schematic of the dimming module 150 is shown in Fig. 15. Fig. 16 and Fig. 17 comprise the key waveforms and phasor diagrams of the dimming module of Fig. 15. The dimming module 150 has a high-frequency transformer Tr 152 with the turns ratio of n : 1. It is connected to an inverter 154 formed by the switches S_A 156 and S_B 158 through a resonant tank circuit formed by C_A 160, C_B 162, and the inductor L 164. The dc side of the inverter 154 is connected to a dc capacitor C_dc 166. The gate signals to S_A 156 and S_B 158 are synchronized with the lamp voltage with a phase difference of cp. The value of φ determines the power flow from the ballast to C_dc 166.

Let Z_A =— , Z_B =— , and Z_c = j ω L . It can be shown that

j (o C_A j ω C_B

z z z z

where Z_AB =— ^A— , Z_BC =— ^a— ^ , v_L is the lamp voltage reflected to the secondary side of

A ⁺ Z_B Z_B + Z_c

the transformer, and v₀ is the inverter voltage on the ac side.

Thus, the input current i_m ' of the dimming module is equal to

J (K_{I VL} ' - K₂ V₀ ) (12) where K, = — ω C, and Κ_Ί = ω C _Λ

¹ \ - ω² L (C_A + C_B) ^{A 2} l - ω² L (C_A + C_B) ^A

By taking

v o = | v o \ β^]ψ

(13)

I v₀ I cos φ + j I v₀ I sin φ

The active power p_m and reactive power q_m transferring from the ballast to the dimming module 150 are calculated by substituting (13) into (12). Thus,

= ^K2 Ί | sin cp

where Re[»] and Im[»] are the real and imaginary parts of the function, respectively, and i_L '* is the conjugate of i_L ' .

The direction of energy flow is dependent on cp. If φ > 0°, p_m > 0 and energy is transferred from the ballast to the dimming module. v_dc will increase. If φ < 0°, p_m < 0 and energy is transferred from the dimming module 150 to the ballast 170. v_dc will decrease. If φ = 0°, p_m = 0 and no energy is transferred between the ballast 170 and dimming module 150 v_dc remains unchanged. This is also the steady-state operating condition of the dimming module 150, where v₀ is in phase with v_L Based on (12) and (15), the steady-state module current i_{m s} ' and reactive power q_{m s} handled by the dimming module 150 are

y (^ '| - ^₂ « | vj) (16) = -^ Ί^{2 +} ^ Ί |ν₀ | (17)

By substituting (4) and (16) into (6) and using the phenomenon of (5),

n 2 ' nv I" - I ^{υ aC} _r r_L(p) I" - - ^Kl ^VL (P)

(18) n K₂

where υ = ,

By considering the fundamental frequency component, the rms value of v₀ can be expressed as

\ _v \ = ^ (19) π

where v_dc is the voltage across the dc link capacitor C_dc 166.

Thus, based on (18) and (19), the lamp power can be varied by adjusting the magnitude of v_dc . Fig. 18 shows the relationships between the lamp power p and v_dc . The parameters used are given in Table I. Based on (14), the value of v_dc is regulated by adjusting the angle cp.

Fig. 17 shows the voltage and current phasors of the dimming module. Consider the voltage and current associated with the inductor L. It can be shown that

where Θ is equal to either 0° or 180°.

S_A and S_B are soft-switched if the current i₀ leads v₀ . Thus, if Θ = 180°, i₀ leads v₀ .

Thus, the dimming module is in soft-switching. If Θ = 0°, S_A and S_B will be soft-switched when

| vJ > | v_CT | (21) Therefore, based on (21), the condition for ensuring soft-switching is

, , , ^ZA ^{// Z}B M , , ^Zc " ^ZB

Vo > ^" ₀ + ^Q V_£ '

Z_c + Z_A // Z_B Z_A + Z_c // Z_B (22) I v„ \>K l v "

where £„

c + ^T c B

By using (18), the relationships between | v₀ \ and \ v_L' \ are shown in Fig. 19. The boundary condition of (22) is used to determine the lamp voltage at which the dimming module 150 starts soft-switching operation.

At the preheating stage, the lamp is non-conducting and its equivalent resistance is very high. Thus, the quality factor is very high. The switching frequency of S_l and S₂ is much higher than the natural frequency for a fixed duration to preheat the filaments. At the ignition stage, it is decreased towards resonance to generate a high voltage across the lamp. At the steady state, it is further decreased to the frequency at which the lamp power is at the rated value. Two auxiliary windings are added on the resonant inductor L_r for separately heating the filaments to the appropriate temperature.

The values of C_A 160, C_B 162 and L 164 are designed by the following procedure. The resonant frequency of the output filter formed by L and C_B is chosen to be the same as the operating frequency ω of the ballast 170. Thus,

By substituting (23) into (12), it can be shown that

The maximum value of the dimming module current \ i_m \ , \ i_{m max} | , is chosen to be a fraction of the lamp current. | n v₀ | is chosen to be a value slightly higher than the lamp voltage at the dimmed condition. By using (23) and (24), it can be shown that

\ n v.

L (25) ω I i m max and C_B = ' ^m ^ ' (26) ω | n v

The value of C_A is designed by considering the maximum voltage stress across it. At the maximum dimmed condition, the voltage v_CA across C_A is the maximum. Thus, the value of C_A is designed such that v_CA is less than the voltage v_CA ^ . Thus,

1 ,

max ^CA max

ωΟ_Α

(27) m max

The soft-switching operation can be ensured if equation (22) is satisfied.

To verify the RPC of the invention, a test setup as shown in Fig. 20 was implemented. The electronic ballast 200 used is a "Hoye HPP-228" non-dimmable electronic ballast 200. The ballast is used to drive two 28W T5 fluorescent tubes 202, 204. To study the feasibility of the dimming module, "Lamp 1 " 202 has the dimming module 206 of the invention connected while "Lamp 2" 204 does not have. Fig. 21 shows the circuit schematic of the experimental prototype. Based on the foregoing, Table I give the values of the components used in the prototype. A separate winding is added for supplying power to the controller, gate driver and electronic components. The controller is a microcontroller "STC12C5410AD". It firstly senses the lamp voltage v_L and then adjust the angle φ to generate appropriate gate signals to S_A and S_B . Then, the magnitude the dc link voltage v_dc can be varied. The ultimate function is to regulate v_L ' at the set value v_{L ref} ' . As stated in (1), v_L is a linear function of the lamp power. By regulating v_L, the lamp power can be varied. For the sake of safety concern, v_dc is also sensed continuously. If its value is higher than a maximum allowable voltage v_{dc mRX} or is zero, it means that the dimming module does not perform the proper power transfer function. The dimming module will generate an alarm signal and the dimming module will stop the function. Fig. 22 shows the flowchart 300 of the software program used in the test set up. The controller will also detect the filament voltages of the two ends so that the dimming module will be connected to the pins that will make the filament power increase.

Table II shows the input and output characteristics of the ballast, lamp characteristics and dimming module characteristics. The input and output characteristics of the ballast include the input current ( I_ac ), power factor (PF), total harmonic distortion of the input current (THD) and input power ( P_in ). The lamp characteristics include the lamp voltage ( V_lamp ), lamp current ( I_lamp ), lamp power ( P_lamp ), crest factor (CR), and filament voltage ( V_f ) of the two lamps. The dimming module characteristics include the input current of the module ( I_M), power consumption (P_M ) and dc link voltage ( V_dc ).

Without using the dimming module, the ballast input current, input power, power factor, input current total harmonic distortion are 258mA, 59.9W, 0.995, and 8.87%, respectively. The lamp voltages of the two lamps are both 158V. The lamp currents of the two lamps are 170mA and 173mA, respectively. The power consumption of each lamp of is about 27W and the filament voltage is 3.92V. After connecting the dimming module to Lamp 1 , the ballast input power can be reduced from 59.9W to 45.7W. The input current is reduced from 258mA to 196mA. The power factor and input current total harmonic distortion remain at the high level. The lamp power of Lamp 1 can be varied from 27W down to 8.1W while the filament voltage of Lamp 1 is increased from 3.92V to 6.93V. Even though Lamp 1 is dimmed, the lamp power and filament voltage of Lamp 2 remain unchanged. The crest factors of the two lamps are almost unchanged when Lamp 1 is dimmed. They are all less than 1.4. The input current of the dimming module is increased from OA to 149mA over the dimming process. This results in an increase in the power consumption because the conduction loss is increased. The maximum power consumption of the module is 2.6W.

The dimming module current is increased from 0 to 149mA when Lamp 1 is dimmed from 26.9W to 8.1W. The power consumption of the module is increased from 1.8W to 2.6W. The dc link voltage of the dimming module is increased from 0V to 21 IV. The total loss in the entire system is increased from 5.6W (undimmed condition) to 7.1W. The total system efficiency is reduced from 90.6% to 78.7%.

Fig. 23(a) shows the waveforms of the input voltage and current of the ballast at the rated power without connecting the dimming module. Fig. 23(b) shows the corresponding waveforms when the lamp power of Lamp 1 is 8.1W. The input currents in both cases are in phase with the input voltage. Fig. 24(a) shows the voltage and current waveforms of the two lamps at the rated power without connecting the dimming module. Fig. 24(b) shows the corresponding waveforms when the lamp power of Lamp 1 is 8.1W. Fig. 25(a) shows the filament voltages of the two lamps at the rated power without connecting the dimming module. Fig. 25(b) shows the corresponding waveforms when the lamp power of Lamp 1 is 8.1W. The filament voltage of Lamp 1 is increased at the dimmed condition while the one of Lamp 2 remains unchanged. Fig. 26 shows the startup transient of the two lamps. Both of them has the startup time of 1.5s.

The experimental results confirm the theoretical predictions. The proposed dimming module can provide a simple solution to reduce the power consumption and brightness of the entire lamp system without replacing the ballast and luminaries.

In fig. 27, a lighting system 400 in accordance with the invention comprises a lamp holder 402 with lamp powering circuitry contained therein (not shown) including a non- dimmable ballast circuit for the lamp tube and a lamp tube 404 inserted in a holding part of the lamp holder. Included is a dimming module 406 according to the invention which connects to the lamp holder 402 and connects with the lamp holder circuitry in the manners hereinbefore described. It will be understood that fig. 27 is merely illustrating the constituent parts of a modified existing lighting system according to the invention and does not represent an actual physical form of the modified system. Where the dimming module of the invention has a variable capacity, this might be implemented by any suitable known control method of manually operated adjuster switch means, remote control means, wireless control means, by way of example but without limitation. Whilst the diming module of the present invention is particularly described for use in modifying existing lighting systems having electronic ballasts, it will be understood that such a dimming module could be adapted for use with existing lighting systems having magnetic ballasts. In such a case, because the operating frequency of typical magnetic ballasts is low (compared to electronic ballasts), some changes to the operating parameters of the dimming module would be required for low frequency operation at 50 to 60Hz.

The dimming module of the invention is particularly advantageous in that it enables an existing non-dimmable lighting system to be modified with minimal changes to the existing lighting system.

The dimming module of the present invention could also be used in new lighting systems employing salvaged or re-conditioned non-dimmable ballasts where said salvaged or re- conditioned ballasts are obtained from re -cycling of ballasts of decommissioned lighting systems.

In general, the invention provides a method and system for controlling active power supplied to a lamp tube in an existing lighting system having an electronic ballast circuit for said lamp tube by drawing reactive power through a circuit path parallel to said lamp tube. The existing lighting system is modified by addition of a reactive power control circuit in parallel to said lamp tube. The reactive power control circuit is configured to share with said lamp tube current drawn from said non-dimmable ballast circuit whereby the reactive power control circuit draws reactive power and thereby controls active power consumption by the lamp unit. The reactive power control unit thereby enables dimming of the lamp tube with a decrease in consumed power by the lighting system without detriment to the lighting system power factor.

While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only exemplary embodiments have been shown and described and do not limit the scope of the invention in any manner. It can be appreciated that any of the features described herein may be used with any embodiment. The illustrative embodiments are not exclusive of each other or of other embodiments not recited herein. Accordingly, the invention also provides embodiments that comprise combinations of one or more of the illustrative embodiments described above. Modifications and variations of the invention as herein set forth can be made without departing from the spirit and scope thereof, and, therefore, only such limitations should be imposed as are indicated by the appended claims.

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country. Table I Values of components in prototype

Table II Input and output characteristics of the ballast

5

Claims

1. A method of controlling power supplied to a lamp tube in a lighting system having a non- dimmable ballast circuit, comprising:

connecting across said lamp tube a reactive power control circuit configured to share with said lamp tube current drawn from said non-dimmable ballast circuit whereby the reactive power control circuit draws reactive power.

2. The method of claim 1, where the reactive power control circuit consumes a preset share of the current drawn from the non-dimmable ballast circuit to provide a preset amount of dimming of the lamp tube.

3. The method of claim 1, where the reactive power control circuit consumes a variable share of the current drawn from the non-dimmable ballast circuit to provide variable dimming of the lamp tube.

4. The method of claim 1 , wherein the lighting system is an existing lighting system and the non-dimmable ballast circuit comprises an existing circuit of the lighting system and the lighting system is modified using the reactive power control circuit to adapt an existing non-dimmable lighting system to a dimmable lighting system.

5. The method of claim 1, wherein the non-dimmable ballast circuit is an electronic ballast circuit for an electric discharge lamp tube.

6. The method of claim 5, wherein the reactive power control circuit comprises a passive device connected across the lamp tube.

7. The method of claim 5, wherein the reactive power control circuit comprises a frequency independent current source which diverts therethrough a portion of the lamp current being drawn from the electronic ballast circuit.

8. A module for adjusting active power supplied to a lamp tube in a lighting system having a non-dimmable ballast circuit, comprising: a reactive power control circuit coupled across said lamp tube, said reactive power control circuit being configured to share with said lamp tube current drawn from said non- dimmable ballast circuit whereby the reactive power control circuit draws reactive power.

9. The module of claim 8, where the reactive power control circuit is configured to consume a preset share of the current drawn from the non-dimmable ballast circuit to provide a preset amount of dimming of the lamp tube.

10. The module of claim 8, where the reactive power control circuit is configured to consume a variable share of the current drawn from the non-dimmable ballast circuit to provide variable dimming of the lamp tube.

11. The module of claim 8, wherein the lighting system comprises an existing lighting system and the non-dimmable ballast circuit comprises an existing circuit of the lighting system and the reactive power control circuit is coupled across said lamp tube in order to adapt an existing non- dimmable lighting system to a dimmable lighting system.

12. The module of claim 8, wherein the non-dimmable ballast circuit comprises an electronic ballast circuit for an electric discharge lamp tube.

13. The method of claim 12, wherein the reactive power control circuit comprises a passive device connected across the lamp tube.

14. The module of claim 12, wherein the reactive power control circuit comprises a frequency independent current source which diverts therethrough a portion of the lamp current being drawn from the electronic ballast circuit.