Patent Publication Number: US-6710554-B2

Title: Dimmer arrangement for gas discharge lamp with inductive ballast

Description:
This application is based upon Provisional Patent Application No. 60/359,721 filed Feb. 27, 2002. 
    
    
     FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to arrangements for dimming gas discharge lamps with inductive ballasts. 
     The predominant trend in gas discharge (“fluorescent”) lighting is generally towards the use of electronic ballasts, which provide numerous advantages including the capability of effective dimming of the light. Nevertheless, due to the much higher cost of electronic ballasts for fluorescent lighting, inductive (or “magnetic”) ballasts are still widely used. 
     There is therefore a need for an effective and energy efficient arrangement for dimming a fluorescent light without requiring modification of the lamp or inductive ballast. 
     SUMMARY OF THE INVENTION 
     The present invention is an arrangement for dimming gas discharge lamps with inductive ballasts. 
     According to the teachings of the present invention there is provided, a dimmer arrangement for a gas discharge lamp assembly, the gas discharge lamp assembly including a gas discharge lamp and an inductive ballast, the dimmer arrangement being for connection to a domestic AC power supply, the dimmer arrangement comprising: (a) a first controllable switch deployed in series with the lamp assembly so as to selectively close a circuit from the power supply through the lamp assembly; (b) a second controllable switch deployed in parallel with the gas discharge lamp assembly so as to selectively close a circuit including the gas discharge lamp and the inductive ballast; (c) a controller operatively interconnected with the first and second controllable switches and configured to cyclically: (i) close the first controllable switch and open the second controllable switch, and (ii) close the second controllable switch and open the first controllable switch; and (d) a non-linear saturating inductance connected serially with the first and second controllable switches. 
     According to a further feature of the present invention, the controller closes and opens the switches at a frequency of at least 1 KHz. 
     According to a further feature of the present invention, the controller closes and opens the switches at a frequency of no more than 100 KHz. 
     According to a further feature of the present invention, the controller has an input indicative of a required degree of dimming, the controller being responsive to the input to vary a proportion of each cycle for which the first controllable switch is closed. 
     According to a further feature of the present invention, the controller is operative to close and open the first and second controllable switches such that a time of overlap during which both the first and second controllable switches are closed lies within a range of 1 nanosecond to 1 microsecond. 
     According to a further feature of the present invention, the non-linear saturating inductance has a soft magnetic core of cross-sectional area no greater than one square centimeter. 
     According to a further feature of the present invention, the non-linear saturating inductance includes a coil having no more than 10 turns. 
     According to a further feature of the present invention, the controller is an analogue controller. 
     According to a further feature of the present invention, the controller includes a digital processor. 
     According to a further feature of the present invention, the first and second controllable switches are implemented as transistors. 
     According to a further feature of the present invention, the first and second controllable switches are implemented as photo-transistors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
     FIG. 1 is a schematic circuit diagram showing a dimming arrangement, constructed and operative according to the teachings of the present invention, for discharge lamps with inductive ballast; 
     FIG. 2 is a set of plots (a)-(d) illustrating schematically time variation of a supply voltage, an actuating voltage of first and second switches of the circuit of FIG. 1, and a resulting actuation voltage applied across the gas discharge lamp; 
     FIG. 3 is a schematic circuit diagram of a first alternative implementation of a dimming arrangement, constructed and operative according to the teachings of the present invention, for discharge lamps with inductive ballast; and 
     FIG. 4 is a schematic circuit diagram of a second alternative implementation of a dimming arrangement, constructed and operative according to the teachings of the present invention, for discharge lamps with inductive ballast. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is a dimming arrangement for discharge lamps with inductive ballast. 
     The principles and operation of dimming arrangements according to the present invention may be better understood with reference to the drawings and the accompanying description. 
     Referring now to the drawings, FIGS. 1 and 2 illustrate a first preferred embodiment of a dimming arrangement, generally designated  10 , constructed and operative according to the teachings of the present invention, for discharge lamps with inductive ballast. 
     Generally speaking, dimmer arrangement  10  is interposed between a domestic AC power supply  12  and one or more gas discharge lamp assembly L 1 , L 2  arranged in parallel between output terminals  18  and  20 . Each lamp assembly includes a gas discharge lamp  14  and an inductive ballast  16 . Dimmer arrangement  10  includes a first controllable switch SW 1  deployed in series with the lamp assembly, i.e., between one of the power supply terminals and the corresponding one of terminals  18  and  20 , so as to selectively close a circuit from the power supply through the lamp assembly. A second controllable switch SW 2  is deployed in parallel with the gas discharge lamp assembly, i.e., between terminals  18  and  20 , so as to selectively close a circuit including the gas discharge lamp and the inductive ballast. (Parenthetically, it will be noted that the terms “parallel” and “series” as used herein are defined relative to the power supply. Clearly, in the context of the circuit formed by closing SW 2  across the lamp assembly, the switch is actually in series with the lamp assembly.) 
     A controller  22  is operatively interconnected with controllable switches SW 1  and SW 2 . Controller  22  is configured to cyclically: (1) close switch SW 1  and open switch SW 2  so that the lamp assembly is in closed circuit with the power supply; and (2) close switch SW 2  and open switch SW 1  so that the circuit with the power supply is broken and a current induced by decaying fields in the inductive ballast flows through the lamp. 
     The operation of the dimmer arrangement is represented in FIG. 2 where (a) is the supply voltage, (b) the actuation signal to switch SW 1 , and (c) the actuation signal to switch SW 2 . As shown in (d), during the periods that SW 1  is closed, the voltage seen by the lamp  14  climbs according to the instantaneous supply voltage as damped by the inductive ballast  16 . During the intermediate periods when SW 2  is closed and SW 1  open, the ballast  16  provides a smoothing effect, continuing to drive the lamp as the magnetic fields in the coil decay. The result is a waveform which approximates to a normal sinusoidal actuation voltage, but has an amplitude which can be controlled by varying the relative widths of the control pulses for the switches, i.e., pulse width modulation (“PWM”). 
     In order to prevent sparking and other effects which would reduce the lifetime of the components, it is important that switch SW 1  is not opened without simultaneously closing switch SW 2  to allow controlled discharge of the ballast. The switches are therefore switched as near as possible to simultaneously so that in general one, and only one, of the switches is closed at any time. 
     In practice, it is impossible to achieve absolute precision in the simultaneous switching of the switches. As a result, and in order to avoid leaving an open circuit, there is necessarily some degree of overlap during which one switch has closed and the other has not yet opened. Preferably, controller  22  is operative to close and open the first and second controllable switches such that a time of overlap during which both switches are closed lies within a range of 1 nanosecond to 1 microsecond, and typically in the range of 10-100 nanoseconds. Nevertheless, without special precautions, this simultaneous closing would effectively short across the power supply. 
     One possible solution to avoid causing momentary shorting of a circuit across the power supply would be to include an additional impedance (e.g., a resistor) in the circuit. This, however, would generally cause large losses and lead to inefficiency of the entire circuit. 
     To address this problem, it is a particular feature of most preferred implementations of the present invention that dimmer arrangement  10  includes a non-linear saturating inductance  24  connected serially with the first and second controllable switches. A non-linear inductance of this type is sufficient to damp any voltage spikes which would otherwise be caused by the momentary overlap of the two switches being closed. At the same time, the use of a non-linear inductance ensures that inductance  24  rapidly reaches saturation so that it has minimum impact on the overall efficiency of the circuit when switch SW 2  is open. 
     As mentioned earlier, the momentary overlap of the switches during changeover is preferably in the range of 10-100 nanoseconds. For such values, inductance  24  is preferably implemented as a coil with a closed ferrite core of cross-sectional area no more than about 2 cm 2 , and more preferably no more than about 1 cm 2 , and with no more than about 10, and typically about 6, turns in the coil. It will be clear to one ordinarily skilled in the art that such a coil results in very small and typically negligible losses in performance of the circuit as a whole. 
     In order to avoid generating high frequency electronic noise, and in order to minimize energy losses, the output frequency of the controller is preferably less than 100 KHz, and more preferably in the 1-10 KHz range. An output frequency of about 2 KHz is believed to be optimal. To avoid releasing high frequency harmonics back to the power supply network, a low-pass filter (“LPF”)  26  is preferably interposed between dimmer arrangement  10  and power supply  12 . The operating frequency of controller  22  is preferably not significantly lower than 1 KHz, since selective low-pass filtering on the power supply connections becomes difficult and costly to implement at such low frequencies. 
     It will be appreciated that the present invention can be implemented using various different types of components. In a first set of prepared implementations, the controllable switches are implemented as electrically controllable switches. One preferred example is FET transistors. Alternatively, optically controllable (e.g., photo-transistors) or any other type of controllable switch can be used. 
     Similarly, controller  22  may be implemented as an analogue controller or may include a digital processor. In either case, a suitable input is provided to control receive a signal (analogue or digital) indicative of a required degree of dimming. The controller  22  is responsive to the input to vary a proportion of each cycle for which first controllable switch SW 1  is closed, thereby controlling the overall dimming effect. The dimming may be either continuous or may vary in a predefined number of discrete steps. The input may be generated by one or more manually operated switch associated with the dimmer arrangement, may be received via a remote control system, or may be otherwise supplied via another device. 
     Turning now briefly to FIGS. 3 and 4, these show two alternative implementations of a dimmer arrangement for a discharge lamp with inductive ballast, each of which is believed to be patentable in its own right. In both cases, dimming is achieved by adding a dimming inductance in series with the lamp assembly so that a proportion of the voltage is dropped across the dimming inductance. Adjustment (up to full illumination) is then achieved by varying a capacitance connected in parallel with the lamp assembly, thereby varying a degree of resonant compensation for the dimming inductance. When full resonant compensation is achieved, there is effectively no voltage drop across the dimming inductance and full lamp output power is achieved. 
     In the case of FIG. 3, the adjustable compensation is achieved by selectively closing switches to select one or more of a series of discrete capacitors. In the case of FIG. 4, a fixed capacitor is located in parallel with a “variable inductance” implemented by locating a thyristor pair in series with an inductor. Adjustment of the thyristor thresholds varies the extent of cancellation of the capacitor, and hence the degree of compensation provided for the dimming inductance. 
     It will be appreciated that the above descriptions are intended only to serve as examples, and that many other embodiments are possible within the spirit and the scope of the present invention.