[0001] The technical field of this disclosure is power supplies, particularly, driver circuitry for a gas discharge lamp driver and related methods. The disclosure particularly relates to driving a high intensity discharge lamp (HID), i.e., a high-pressure lamp, such as a high-pressure sodium lamp, a high-pressure mercury lamp, a metal-halide lamp, or the like. The disclosure is specifically explained hereinafter with reference to an exemplary HID lamp, but is not restricted thereto, as it can also be applied more generally to other types of gas discharge lamps.

[0002] Gas discharge lamps generally include two electrodes located in a closed vessel filled with an ionizable gas or vapor. The vessel is typically made of quartz or of a ceramic material, such as polycrystalline alumina (PCA). The electrodes are arranged at a certain distance from each other, and an electric arc is maintained between these electrodes during operation.

[0003] A gas discharge lamp proceeds through several stages when starting: ignition, takeover, run up, and steady state stages. The ignition stage includes the breakdown portion. The takeover stage can be further divided into vapor-arc, glow/glow-to-arc, and arc portions. Typically, the takeover stage lasts a few seconds depending on the type of gas discharge lamp and lamp driver performance. Generally, the shorter the takeover stage, the better for lamp performance and lifetime. The electronic ballast driving the gas discharge lamp must provide power that is suitable for each stage. During the ignition stage, the gas discharge lamp is cold and has high impedance, so the electronic ballast must provide high frequency, high voltage power to start the lamp. When the lamp ignites, the resistance and the gas discharge lamp voltage decrease rapidly in the breakdown portion. During the takeover stage, the gas discharge lamp is still cold but has an unstable lower impedance, with the discharge lamp voltage decreasing slightly in the glow/glow-to-arc portion and more rapidly in the arc portion. During the run up stage, the arc is established, the gas discharge lamp voltage increases, and the gas discharge lamp warms until steady state stage is achieved. The gas discharge lamp voltage remains substantially constant during the steady state stage. Generally, the voltage to the gas discharge lamp decreases during the breakdown and takeover stages and then increases to the steady state voltage during the run up stage. [0004] Electronic ballasts can be used to power gas discharge lamps by providing either high or low frequency AC power to the lamps. Electronic ballasts commonly perform a number of power-related functions including, inter alia, the conversion of power from the primary sources to AC voltages and frequencies corresponding to the requirements of respective lamps, and the limiting and control of the flow of electrical current to the lamps.

[0005] Electronic ballasts often include a first circuit, which converts AC from a mains source to DC power, and a power circuit which receives the DC power and provides open circuit voltage to the lamp. During the takeover stage, the gas discharge lamp tends to extinguish, so the electronic ballast must supply sufficient open circuit voltage together with sufficient power to carry the lamp through the takeover stage to the run up stage. To accomplish this, the DC bus voltage for the DC power must be high enough to maintain the transition between stages. A smooth takeover transition after lamp breakdown is the key factor to achieve long life and solid lighting performance. Unfortunately, maintaining a high DC bus voltage reduces the efficiency of the electronic ballast during steady state operation. In the present generation of electronic ballasts, it is necessary to sacrifice efficiency for reliability and lamp lifetime. Thus, it would be desirable to have a gas discharge lamp driver and method that would address the above disadvantages.

[0006] Generally, in one aspect, the present invention relates to a driver for a gas discharge lamp including output terminals operable to connect the gas discharge lamp; a lamp current generating section operable to generate a low frequency voltage from a DC voltage, the low frequency voltage alone being insufficient to prevent the gas discharge lamp from extinguishing in a takeover stage, the lamp current generating section having output terminals operably connected to the output terminals; a high frequency section comprising a waveform generator operable to generate high frequency voltage; and a coupling device operable to add the high frequency voltage to the low frequency voltage and generate an open circuit voltage at the gas discharge lamp. The high frequency section is operable in an ignition stage to generate ignition pulses in the open circuit voltage; and the high frequency section is further operable in the takeover stage to generate the high frequency voltage sufficient to prevent the gas discharge lamp from extinguishing in the takeover stage. [0007] Another aspect of the present invention generally relates to a driver for a gas discharge lamp including a lamp current generating section having a power circuit operable to generate a low frequency voltage from a DC voltage; a high frequency section operable to generate high frequency voltage; and a coupling device operable to superimpose the high frequency voltage on the low frequency voltage to generate an open circuit voltage. The low frequency voltage alone is insufficient to prevent the gas discharge lamp from extinguishing in the takeover stage and the open circuit voltage is sufficient to prevent the gas discharge lamp from extinguishing in the takeover stage.

[0008] Yet another aspect of the present invention generally relates to a method of driving a gas discharge lamp including igniting the gas discharge lamp; determining when the gas discharge lamp enters a takeover stage; generating a low frequency voltage from a DC voltage; generating high frequency voltage; and superimposing the high frequency voltage on the low frequency voltage to generate an open circuit voltage at the gas discharge lamp when the gas discharge lamp enters the takeover stage. The low frequency voltage alone is insufficient to prevent the gas discharge lamp from extinguishing in the takeover stage and the open circuit voltage is sufficient to prevent the gas discharge lamp from extinguishing in the takeover stage.

[0009] The foregoing and other features and advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention, rather than limiting the scope of the invention being defined by the appended claims and equivalents thereof.

[0010] FIG. 1 is a block diagram schematically illustrating an embodiment of a gas discharge lamp driver according to the present invention;

[0011] FIG. 2 is a graph of a voltage versus time simulation of open circuit voltage in the takeover assist mode for a gas discharge lamp driver according to the present invention;

[0012] FIG. 3 is a flowchart for a method of driving a gas discharge lamp driver according to the present invention.

[0013] FIG. 1 is a block diagram schematically illustrating an embodiment of a driver 1 for driving a HID lamp 2. The driver 1 includes a lamp current generating section 10, a high frequency section 20, and a coupling transformer 40 coupling the output of the high frequency section 20 to the output of the lamp current generating section 10. The lamp current generating section 10 includes a first circuit 11 and a power circuit 12. Although it is possible to power the driver 1 from a DC power source and omit the first circuit 11, the driver 1 is typically powered from a mains socket providing alternating current, such as 230 VAC @ 50 Hz in Europe or 120 VAC @ 60 Hz in the United States. The first circuit 11 performs the functions of power factor correction, rectification, and conversion from mains AC to DC voltage. The power circuit 12 converts the DC voltage from the first circuit 11 into a commutating lamp current I _L at a commutation frequency. In one embodiment, the commutation frequency can be of the order of about 100 Hz. Those skilled in the art will appreciate that different designs are possible for the first circuit 11 and power circuit 12 of the lamp current generating section 10, and for the lamp current generating section 10 itself, as desired for a particular purpose. In this embodiment, the half-bridge commutation forward (HBCF) design is described.

[0014] Still referring to FIG. 1, in one embodiment, the driver 1 has driver output terminals 3, 4 for connecting the lamp 2. The driver output terminals 3, 4 are operably connected to the power circuit output terminals 13, 14, in series with a secondary winding 42 of the coupling transformer 40. A first capacitor 43 is coupled in parallel with the secondary transformer winding 42. The secondary inductance of the transformer 40 in parallel with the first capacitor

43 form a parallel resonant circuit having a resonance frequency f _RES defined by f^s ^"1 = 2πV LC , in which L indicates the secondary inductance of the transformer 40, and C indicates the capacitance of the first capacitor 43. In one example, the resonance frequency f _RES is in the range from 60 kHz to 200 kHz, such as on the order of about 120 kHz. Those skilled in the art will appreciate that the resonance frequency f _RES can differ from device to device due to tolerances, and that the resonance frequency f _RE s can be selected as desired for a particular purpose.

[0015] The high frequency section 20 includes a waveform generator 21 having a half- bridge topology, including two power rails 22, 23, a first branch with two controllable switches 24, 25 coupled in series between the two power rails 22, 23, a second branch with two capacitors 26, 27 coupled in series between the two power rails 22, 23, and a diagonal branch coupled between a first node A between said two controllable switches 24, 25 and a second node B between said two capacitors 26, 27. The diagonal branch includes a series arrangement of the primary winding 41 of the coupling transformer 40, an inductor 28, and a capacitor 29. The waveform generator 21 further includes a switch controller 30, which can be a microcontroller or the like, for driving the two controllable switches 24, 25, which can be FETs or the like. In the example of the HBCF topology, the power circuit 12 also includes two controllable switches and a switch controller, which can be integrated with the switch controller 30 or can be separate.

[0016] The driver 1 includes a lamp voltage sensor 50 operably connected to the power circuit output terminals 13, 14 to receive the lamp voltage as input and provide the switch controller 30 with a sensor output signal indicating the sensed lamp voltage. The switch controller 30 is operable to compare the sensed lamp voltage to a voltage threshold V _TH to determine an operating mode, such as the takeover assist mode or the arc straightening mode. The switch controller 30 compares this received sensor signal with one or more threshold signals V _TH and can change the operating mode as desired when the sensed lamp voltage passes the threshold signal V _TH - In one embodiment, passing the transition threshold signal triggers the switch controller 30 to operate in the takeover assist mode. In another embodiment, passing the arc straightening threshold signal triggers the switch controller 30 to operate in the arc straightening mode. Those skilled in the art will appreciate that the threshold signal can be an electrical signal or can be a numerical value stored in memory.

[0017] The lamp 2 proceeds through ignition, takeover, run up, and steady state stages when starting. The high frequency section 20 can perform a number of functions. In the ignition stage, the high frequency section 20 can provide the desired ignition voltage. In the takeover stage, the high frequency section 20 can provide takeover power in a takeover assist mode. In the steady state stage, the high frequency section 20 can be switched off so that the first circuit 11 provides a DC voltage and the power circuit 12 converts this voltage to an alternating current to drive the lamp, or the high frequency section 20 can generate an arc straightening high frequency current component in an arc straightening mode. The lamp 2 operates as follows:

[0018] In the ignition stage when the lamp 2 is off, the power circuit 12 of the lamp current generating section 10 generates an alternating low frequency voltage with a square wave shape having a relatively low frequency of the order of 100 Hz to 200 Hz, for example, although the frequency can be higher as desired for a particular application. For the exemplary HBCF topology, the voltage maximum amplitude of the low frequency voltage can be half of the DC bus voltage from the first circuit 11. For electronic ballasts without a takeover assist mode, the DC bus voltage is typically designed to be higher than 510V to provide a low frequency voltage of more than 250V. This is sufficient to allow a smooth transition from an ignition phase to an arc phase, but is insufficient to ignite an arc. When the driver 1 includes the takeover assist mode, the low frequency voltage from the lamp current generating section 10 can be lower than the voltage without the takeover assist mode. In one example, the low frequency voltage is less than about 230V. The low frequency voltage is supplemented by the added high frequency voltage, so the total voltage is more than the 250V normally required to prevent the lamp 2 from extinguishing during the takeover stage.

[0019] The high frequency section 20 is operable in an ignition stage to generate ignition pulses in the open circuit voltage. During the ignition stage, the high frequency section 20 generates high voltage pulses on the order of about 3.5 kV, for example. To this end, the switch controller 30 alternately opens and closes the two controllable switches 24, 25 in counter phase, such that an alternating current is generated in the primary winding 41 of the transformer 40, causing an alternating voltage to be generated across the secondary winding 42. In one example, the switch controller 30 initially uses a relatively high frequency, of the order of about 200 kHz, and slowly lowers this frequency to a frequency of the order of about 80 kHz. In doing so, the switching frequency approaches the frequency f _RE s. When this frequency f _RES is reached, said parallel circuit 42, 43 resonates, resulting in high resonance voltage pulses causing ignition. The function of the capacitor 29 in series with the primary winding 41 of the transformer 40 is to block DC voltages and currents. The function of the inductor 28 in series with the primary winding 41 of the transformer 40 is primarily to limit the current.

[0020] The lamp 2 passes through the breakdown portion of the ignition stage as the lamp 2 ignites and the lamp voltage decreases. The lamp 2 then enters the takeover stage, which includes vapor-arc, glow/glow-to-arc, and arc portions. In the takeover stage, lamp voltage is unstable. Most of the time, the lamp voltage is very low with high voltage spikes from entry of the lamp 2 into the various portions of the takeover stage. After a few seconds, the lamp 2 passes through takeover stage and enters the run up stage and lamp voltage increases to the steady state stage. After the takeover stage, the switch controller 30 can turn off the two controllable switches 24, 25, so that no current flows in the primary winding 41 of the transformer 40. This is defined herein as the OFF mode of the high frequency section 20. In this OFF mode, only the lamp current generating section 10 is active to provide lamp current.

[0021] In one embodiment, the driver 1 employs a takeover assist mode during the takeover stage to assure that the lamp 2 does not extinguish. The power circuit 12 of the lamp current generating section 10 is operable to generate a low frequency voltage from a DC voltage, but the low frequency voltage alone is insufficient to prevent the gas discharge lamp (2) from extinguishing in the takeover stage. For the exemplary HBCF topology, the low frequency voltage can be a square wave with a maximum amplitude less than or equal to one half of the DC bus voltage provided to the power circuit 12 of the lamp current generating section 10. With the takeover assist mode, the low frequency voltage from the lamp current generating section 10 can be lower than the voltage without a takeover assist mode. In one example, low frequency voltage can be 230V, so the DC voltage can be lowered to 460V. The switch controller 30 operates during in a takeover assist mode that provides a smooth transition in the takeover stage, i.e., between the break down stage and the run up stage. In one embodiment, the takeover assist mode can extend into the run up stage. The takeover assist mode allows a much lower DC bus voltage from the first circuit 11 of the lamp current generating section 10 while providing the required open circuit voltage to the lamp 2 from the power circuit 12 for the takeover stage. A high frequency voltage is superimposed on the low frequency voltage from the lamp current generating section 10 to increase the voltage of the low frequency wave. The high frequency section 20 is operable in the takeover stage to generate the high frequency voltage sufficient to prevent the gas discharge lamp 2 from extinguishing in the takeover stage.

[0022] The driver 1 enters the takeover assist mode when the lamp voltage sensor 50 detects the lamp voltage decrease of the breakdown stage, which indicates that the lamp 2 has ignited. In one embodiment, the driver 1 enters the takeover assist mode when the lamp voltage is less than a predetermined takeover assist entrance voltage threshold. The switch controller 30 changes the operation from the ignition phase to superimpose high frequency voltage on the square wave shaped alternating low frequency voltage provided by the lamp current generating section 10. The superimposed higher frequency voltage increases the voltage of the low frequency alternating voltage. In one embodiment, the higher frequency voltage is 100 kHz to 200 kHz or higher. The switch controller 30 alternately opens and closes the two controllable switches 24, 25 in counter phase, such that an alternating current is generated in the primary winding 41 of the transformer 40, causing the superimposed higher frequency voltage. The driver 1 can exit the takeover assist mode when the lamp voltage sensor 50 detects the lamp voltage increase of the run up stage and passes a takeover assist exit voltage threshold, which indicates that the lamp 2 has passed through the takeover stage. In another embodiment, the switch controller (30) is operable to change operating mode after a predetermined time. For example, the driver 1 can exit the takeover assist mode a predetermined time, such as 0.8 seconds, after the gas discharge lamp 2 enters the takeover assist mode. Either the takeover assist exit voltage threshold or the predetermined time for takeover assist operation can be selected so that the takeover assist mode continues into the run up stage as desired, i.e., the higher frequency voltage is superimposed on the voltage of the alternating low frequency voltage into the run up stage.

[0023] FIG. 2 is a graph of a voltage versus time simulation of open circuit voltage in the takeover assist mode for a gas discharge lamp driver according to some embodiments of the present invention. In this example, the open circuit voltage 100 includes a low frequency voltage 102 and an assist high frequency voltage 104. In the takeover assist mode, the lamp current generating section generates alternating low frequency voltage with the low frequency wave of 100 Hz at a voltage of 230V. The low frequency voltage of 230V is one half the DC bus voltage of 460V from the first circuit of the lamp current generating section. The low frequency voltage can be a square wave. The assist high frequency voltage of about 250V peak-to-peak at 100 kHz is superimposed on the low frequency voltage from the lamp current generating section. This results in an open circuit voltage to the lamp of 350V peak and 250V RMS. Thus, a voltage of 250V RMS with 350V peak is sufficient to prevent the lamp from extinguishing in the takeover stage is provided with the lower DC bus voltage of 460V. During steady state operation, the driver with the lower DC bus voltage of 460V uses 5% to 10% less power than a driver without the takeover assist mode, which would require a DC bus voltage higher than 510V to prevent the lamp from extinguishing in the takeover stage.

[0024] FIG. 3 is a flowchart for a method of driving a gas discharge lamp driver according to various embodiments of the present invention. The method 200 includes igniting the gas discharge lamp 202; determining when the gas discharge lamp enters a takeover stage 204; generating a low frequency voltage from a DC voltage 206; generating high frequency voltage 208; and superimposing the high frequency voltage on the low frequency voltage to generate an open circuit voltage at the gas discharge lamp when the gas discharge lamp enters the takeover stage 210. The low frequency voltage alone is insufficient to prevent the gas discharge lamp from extinguishing in the takeover stage and the open circuit voltage is sufficient to prevent the gas discharge lamp from extinguishing in the takeover stage. The method 200 can further include ceasing the generating high frequency voltage, such as ceasing the generating high frequency voltage when the gas discharge lamp exits the takeover stage, ceasing the generating high frequency voltage during the run up stage, ceasing the generating high frequency voltage when voltage to the gas discharge lamp passes a takeover assist exit voltage threshold, and/or ceasing the generating high frequency voltage a predetermined time after the gas discharge lamp enters the takeover assist mode. The operating stage can be determined from the gas discharge lamp voltage and/or history. The method 200 can also include generating a ripple current component for the gas discharge lamp to provide an arc straightening mode during the steady state stage.

[0025] In another embodiment, the high frequency section 20 is capable of operating in an arc straightening mode during the steady state operation phase, to generate a ripple current component for the lamp. The high-frequency section (20) is capable of operating both in an ignition mode, in which the high-frequency section (20) generates ignition pulses at an ignition frequency which is higher than the commutation frequency; and in an arc-straightening mode, in which the high-frequency section (20) generates a current ripple at a ripple frequency which is higher than the commutation frequency and differs from the ignition frequency, the ripple having an amplitude which is lower than the amplitude of the ignition pulses.

[0026] To this end, the switch controller 30 alternately opens and closes the two controllable switches 24, 25 in counter phase, at a higher frequency, typically of the order of about 20-40 kHz, such that an alternating current is generated in the primary winding 41 of the transformer 40, causing an alternating current to be generated in the secondary winding 42 and constituting the current ripple. This current ripple has an amplitude which is lower than the amplitude of the ignition pulses, for instance, an amplitude of the order of about 500 mA, which is primarily determined by the inductance of the inductor 28 in series with the primary winding 41 of the transformer 40, and by the "resistance" of the burning lamp. The frequency is maintained constant, and the actual value of this fixed frequency can be set in the controller software. In an experimental setup, a value of 33.5 kHz proved to be satisfactory. In any case, the frequency is higher than the normal operating frequency of the power circuit 12, and differs from the frequency of the ignition pulses. The frequency is preferably lower than f _RES .

[0027] The switch controller 30 decides on the transition from the OFF mode to the arc- straightening mode on the basis of the lamp voltage. To this end, the driver 1 includes a receiving the lamp voltage as input and providing the switch controller 30 with a sensor output signal indicating the sensed lamp voltage. The switch controller 30 compares this received sensor signal with an arc straightening threshold signal V _TH and starts the arc straightening mode as soon as the sensor signal exceeds the arc straightening threshold signal V _TH . It will be evident to a person skilled in the art that, dependent on the lamp type, a suitable threshold for the lamp voltage is about 80 V, which corresponds to the nominal voltage during the steady state. The arc straightening results in a slight decrease of the lamp voltage, so that the temperature also decreases.

[0028] While the invention has been illustrated and described in detail, it will be evident to a person skilled in the art that it is not limited to the disclosed embodiments. Several variations and modifications are possible within the protective scope of the invention as defined in the appending claims. For instance, in FIG. 1, the two power rails 22, 23 are connected to the output of the first circuit 11 of the lamp current generating section 10. Alternatively, the two power rails 22, 23 may be connected to a different source of substantially constant voltage. In another example, the low frequency voltage can vary from a pure square wave as desired for a particular application. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention from a study of the drawing, the disclosure, and the appended claims. In the claims, use of the verb "comprise" and its conjugations does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference sign placed between parentheses shall not be construed as limiting the claim.

[0029] The present invention has been explained with reference to a block diagram, which illustrates functional blocks of the device according to the invention. It is to be understood that one or more of these functional blocks may be implemented in hardware, in which the function of such a functional block is performed by individual hardware components. However, it is also possible to implement one or more of these functional blocks in software, so that the function of such a functional block is performed by one or more program lines of a computer program or a programmable device such as a microprocessor, a microcontroller, a digital signal processor, etc.

[0030] While several inventive embodiments have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the function and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the inventive embodiments described herein. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the inventive teachings is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific inventive embodiments described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, inventive embodiments may be practiced otherwise than as specifically described and claimed. Inventive embodiments of the present disclosure are directed to each individual feature, system, article, material, kit, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the inventive scope of the present disclosure.

[0031] It should also be understood that, unless clearly indicated to the contrary, in any methods claimed herein that include more than one step or act, the order of the steps or acts of the method is not necessarily limited to the order in which the steps or acts of the method are recited. In the claims, as well as in the specification above, all transitional phrases such as "comprising," "including," "carrying," "having," "containing," "involving," "holding," "composed of," and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases "consisting of" and "consisting essentially of" shall be closed or semi-closed transitional phrases, respectively.