BALLAST

Abstract: A ballast for powering a high intensity discharge lamp includes at least one voltage divider for sampling a lamp operating condition and producing a sampled signal based on a scaling factor applied to the sampled lamp operating condition. The ballast also includes a controller for driving an inverter in response to the sampled signal. The scaling factor varies based on the mode of lamp operation and is substantially larger during lamp ignition than during steady state lamp operation.

Description: Ballast The present invention relates to a ballast and a method for poweπng and controlling the operation of a high intensity discharge lamp and, more specifically for poweπng and controlling a high intensity discharge lamp duπng lamp ignition and steady state operation. A conventional high intensity discharge (HID) ballast, shown in Figure 1, receives a power from an AC mains 100. A preconditioner/up-converter 110 includes circuitry for rectification of the AC power, power factor correction and for boosting of the AC peak voltage. A regulated DC voltage (Vbus) is supplied to an inverter 120 (e.g. full-bπdge type). The mverter output is supplied to the seπal combination of a blocking capacitor Cs, inductor L and capacitor C. Inductor L and capacitor C form a resonant circuit. Inverter 120 typically operates at a switching frequency so as to be in an inductive mode (i.e. above the resonant frequency of the resonant circuit). Lamp 130 is connected across capacitor C. A fixed voltage divider circuit 210 includes a plurality of resistors Rl, R2, R3 and R4 and a pair of taps tl and tl ' between resistors Rl and R2 and between resistors R3 and R4, respectively. A circuit 210 including a pair of voltage dividers divides down/sets the differential lamp voltage such that a sample 220 of the latter is at a conventional electronics level suitable for input to an integrated circuit 3. Integrated circuit 3 serves as a controller for controlling operation of lamp 130. An ignitor (not shown) typically can produce ignition voltages of about 3.5 kilovolts (KV). In order to sample the lamp voltage duπng ignition, the lamp voltage is lowered or divided down to a usable level of less than 15 volts through circuit 210. Unfortunately, voltage divider circuit 210 will lower or divide down steady-state voltages, typically of about 200 volts, to less than 1-volt full scale. Voltages of such small magnitude are too small to yield good resolution, that is, are too small in magnitude to accurately reflect lamp operating conditions. It is therefore desirable to provide a new and improved ballast which includes a sampling scheme for accurately reflecting lamp operating conditions duπng ignition and steady-state conditions In accordance with a first aspect of the invention, a ballast for poweπng a high intensity discharge lamp includes an inverter for supplying power to the lamp, at least one voltage divider for sampling a lamp operating condition and producing a sampled signal based on a scaling factor applied to the sampled lamp operating condition and a controller for dπving the inverter in response to the sampled signal The scaling factor vanes based on the mode of lamp operation The sampled signal is always of a sufficient magnitude to yield good resolution Consequently, lamp operating conditions can be accurately monitored The magnitude of the sampled signal is maintained at an acceptable level by varying the scaling factor based on the mode of lamp operation It is a feature of this first aspect of the invention that the mode of lamp operation includes lamp ignition and steady-state lamp operation Typically, the scaling factor applied is substantially larger duπng lamp ignition than duπng steady-state lamp operation It is another feature of this first aspect that the at least one voltage divider includes two stπngs of resistors and further includes a pair of switches Each switch selects which of the resistors from an associated stπng are to be coupled to the controller m response to instructions from the controller In accordance with a second aspect of the invention, a method of operating a high intensity discharge lamp ballast includes the steps of applying a scaling factor to a sampled lamp operating condition in producing a sampled signal, dπving an inverter in response to the sampled signal, and varying the scahng factor based on the mode of lamp operation The step of varying the scaling factor includes substantially reducing the scaling factor applied to the sampled lamp operating condition duπng steady-state lamp operation as compared to lamp ignition operation Accordingly, it is an object of the invention to provide an improved ballast which includes a sampling scheme for accurately reflecting lamp operating conditions during ignition and steady-state conditions Still other objects and advantages of the invention will, in part, be obvious and will, in part, be apparent from the specification The invention accordingly compπses several steps in the relation of one or more such steps with respect to each of the others, and a device embodying features of construction, combination of elements, and arrangements of parts which are adapted to effect such steps, all is exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims The foregoing objects and advantages of the present invention may be more readily understood by one skilled in the art with reference being had to the following detailed description of a preferred embodiment thereof, taken in conjunction with the accompanying drawings wherein like elements are designated by identical reference numerals throughout the several views, and in which: Figure 1 is a schematic diagram of a prior art HID ballast circuit; and Figure 2 is a schematic diagram of an HID ballast circuit in accordance with the invention. The present invention, shown in Figure 2, adds additional control logic circuitry to a voltage divider 210'. The additional circuitry includes additional resistors R5 and R6, a tap t 2 which is positioned between resistors R2 and R5 and a tap t 2 - which is inserted between resistors R4 and R6. Resistors Rl, R2 and R5 form a first voltage divider/string of resistors and resistors R3, R4 and R6 form a second voltage divider/string of resistors. A switch SI associated with the first string selects the voltage at tap t] or at tap t 2 based on instructions generated by a controller 15. These instructions are supplied to switch SI along a line 16. A switch S2 associated with the second string selects the voltage at tap - or at tap t 2 - based on instructions generated by a controller 15. These instructions are supplied to switch S2 along a line 17. The voltage supplied by a line 151 to controller 15 represents a sample of the voltage at the high potential end of lamp 130. The voltage supplied by a line 150 to controller 15 represents a sample of the voltage at the low potential end of lamp 130. The voltage difference between lines 151 and 150 represents the lamp voltage. Controller 15 also receives a signal representing the level of lamp current from a line 140. The switching frequency of inverter 120 is controlled by controller 15 and is based on the input signals from lines 140, 150 and 151. Controller 15 determines the mode of lamp operation (i.e. lamp ignition and steady state operation) based on the level of lamp current. The instructions from controller 15 to switches SI and S2 reflect this determination. Each of the sampled signals supplied by lines 150 and 151 can be varied in magnitude (i.e. scaled) by changing the tap voltage selected by the associated switch. Switch SI selects either the voltage at tap t] or at tap t 2 based on the instructions received from controller 15. Switch S2 selects either the voltage at tap t or at tap t 2 > based on the instructions received from controller 15. The voltages at tap ti and at tap ty will be of a larger magnitude than the voltages at tap t and at tap t 2 >, respectively. More particularly, by selecting tap t 2 rather than tap ti a larger scaling factor will be applied to the voltage being sampled by the first voltage divider formed by resistors Rl, R2 and R5. By selecting tap t 2 ' rather than tap ti- a larger scaling factor will be applied to the voltage being sampled by the second voltage divider formed by resistors R3, R4 and R6. The sampled signals supplied by lines 150 and 151 are always of a sufficient magnitude to yield good resolution. The larger scaling factor (i.e. selection of taps t 2 and t >) is applied during lamp ignition when very high magnitudes of lamp voltage are experienced. The smaller scaling factor (i.e. selection of taps ti and tp) is applied during steady state lamp operation when much smaller magnitudes of lamp voltage are experienced. The voltage levels of the sampled signals supplied by lines 150 and 151 are typically kept at or near full scale during the ignition and during the steady state operation. Consequently, lamp operating conditions can be accurately monitored during either mode of lamp operation by controller 15. In other words, the magnitudes of these sampled signals are maintained at an acceptable level by varying the scaling factors based on the mode of lamp operation. While the invention has been particularly shown and described with respect to an illustrative and preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention that should be limited only by the scope of the appended claims.

Claims: CLAIMS: 1. A ballast for powering a high intensity discharge lamp comprising: an inverter for supplying power to the lamp; at least one voltage divider for sampling a lamp operating condition and producing a sampled signal based on a scaling factor applied to the sampled lamp operating condition; and a controller for driving the inverter in response to the sampled signal; wherein the scaling factor varies based on the mode of lamp operation. 2. The ballast of claim 1, wherein the mode of lamp operation includes lamp ignition and steady-state lamp operation. 3. The ballast of claim 2, wherein the scaling factor applied is substantially smaller during lamp ignition than during steady-state lamp operation. 4. The ballast according to claim 1, 2 or 3 wherein the scaling factor is controlled by the controller. 5. The ballast of claim 1, wherein the at least one voltage divider includes two strings of resistors and further includes a pair of switches, each switch selecting which of the resistors from an associated string are to be coupled to the controller in response to instructions from the controller. 6. The ballast of claim 5, wherein the controller further determines the mode of lamp operation based on the level of lamp current, the instructions from the controller to the switches reflecting this determination. 7. The ballast of claim 3, wherein the at least one voltage divider includes two strings of resistors and further including a pair of switches, each switch selecting which of the resistors from an associated string are to be coupled to the controller in response to instructions from the controller, the controller determining the mode of lamp operation based on the level of lamp current wherein the instructions from the controller to the switches reflect this determination. 8. A method of operating a high intensity discharge lamp ballast, comprising the steps of: applying a scaling factor to a sampled lamp operating condition in producing a sampled signal; driving an inverter (120) in response to the sampled signal; and varying the scaling factor based on the mode of lamp operation. 9. The method of claim 8, wherein the step of varying the scaling factor includes substantially reducing the scaling factor applied to the sampled lamp operating condition during steady-state lamp operation as compared to lamp ignition operation.

Inventors: STEIN MITCHEL J, STEIN, MITCHEL, J.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2000
Main classification: H05B41
IPC codes: H05B41