Title of Invention

"A BALLAST CIRCUIT FOR A GAS DISCHARGE LAMP"

Abstract

A ballast circuit for a gas discharge lamp comprises a resonant load circuit incorporating a gas discharge lamp and including first and second resonant impedances whose values determine the operating frequency of the resonant load circuit A dc.-to-ac. converter circuit is coupled to the resonant load circuit to induce an ax. current in the resonant load circuit. It comprises first and second switches aerialh/ connected between a bus conductor at a dx. voltage and ground, with a common node through which the bidirectional load current flows. An arrangement allows a user to select a setpoint signal during lamp operation mat determines the amount of light output by the lamp. A feedback arrangement regeneratively controls the first and second switches; it includes a circuit for sensing ax. current in the resonant load circuit and producing an ax. feedback signal in proportion to the ax. current; a circuit for producing a dx-. feedback signal representing an average value of alamp output parameter; *a circuit for generating an error signal between the setpoint signal and the dx. feedback signal; a comparator circuit for comparing the ax. feedback signal with a periodic reference signal, and for producing a comparator output signal mat changes state when a one of the compared signals becomes greater than the other of the compared signals, and vice-versa; and a circuit for generating the periodic reference signal in response to both the comparator output signal and the error signal. A conditioning circuit receptive of the comparator output signal controls the first and second switches.

Full Text

LAMP POWER SUPPLY CIRCUIT WITH FEEDBACK CIRCUIT FOR DYNAMICALLY ADJUSTING LAMP CURRENT CROSS-REFERENCE TO RELATED APPLICATION This application is related to commonly assigned application Serial No. (attorney docket no. LD 10606). entitled "Lamp Power Supply Circuit with Electronic Feedback Circuit for Switch Control,- by Louis R. Nerone, the present inventor. The foregoing application was concurrently filed with, and is co-pending with, the instant application. FIELD OF THE INVENTION The present invention relates to a power supply, or ballast, circuit for a gas discharge lamp. Mote particularly, it relates to a power supply circuit with plural power switches controlled by a feedback circuit in sucb manner as to allow a user -to dynamically adjust die level of light output by die lamp. BACKGROUND OF THE INVENTION A gas discharge lamp, such as a fluorescent lamp, typically utilizes a power supply, or ballast, circuit to convert an a.c. tine voltage to a high frequency a.c, voltage which is impressed across a resonant load circuit containing the gas discharge lamp. The resonant load circuit includes a resonant inductor and a resonant capacitor for determining the frequency of resonance of current in the resonant load circuit. The power supply circuit includes a series half-bridge d.c.-to-a.c. converter having a pair of power switches that alternately connect one end of the resonant load circuit to a d.c. bus voltage and then to a ground, thereby impressing the mentioned a.c. voltage across the resonant load circuit. A prior ballast circuit of the foregoing type is disclosed in co-pending U.S. patent 5.341,068, issued August 23, 1994, entitled 'Electronic Ballast Arrangement for a Compact Fluorescent Lamp," by Louis R. Nerone. the present inventor, and assigned to the present assignee. The disclosed ballast circuit utilizes a feedback circuit for controlling the mentioned pair of power switches of the d.c.-to-a.c. convener. The feedback circuit operates in response to a feedback signal representing an ax. current in the resonant load circuit. The use of die foregoing circuit for controlling the power switches beneficially results in regenerative, or so-called self-resonant, control of the power switches. Accordingly, the ballast circuit of the foregoing patent application avoids the expense and bulk of extra circuitry for switch control. The feedback circuit of the foregoing patent application, however, utilizes a magnetic-core transformer with multiple windings. The inclusion of such transformer adds considerable bulk to ballast size, which is an especially acute issue with so-called compact fluorescent lamps having a standard screw-type base for insertion into a conventional incandescent lamp socket. A feedback circuit avoiding the use of a magnetic transformer is disclosed in the above cross-referenced application Serial No. (attorney docket no. LD 10606). The feedback circuit is of the self-resonant type, and is comprised of components which can be implemented in solid-state form, and the majority of which can be incorporated into an integrated circuit. Different levels of lamp current can be accommodated by the circuitry disclosed in the foregoing application Serial No. (attorney docket no. LD 10606). However, mis typically requires the selection of values for the components (e.g. resistors) of the feedback circuit which, for size considerations, are preferably discrete components rather man adjustable. For applications where control of lamp current is required while the lamp is operating, such as for dimmable lamps, it would be desirable to provide, in a lamp ballast, a feedback circuit for facilitating simple control of lamp current during lamp operation. It would also be desirable to enable more accurate operation of a lamp at a user-selected setpoint. OBJECTS AND SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a ballast circuit for a gas discharge lamp that includes, for controlling a pair of power switches, a self-resonant feedback circuit which allows a user to adjust the output of the lamp during lamp operation, A further object of the invention is to provide, for a ballast circuit of the foregoing type, a user-selectable setpoint of an output parameter of the lamp at which the lamp will more accurately operate. Another object of the invention is to provide, for a ballast circuit of the foregoing type, a self-resonant feedback comprised of components which can be implemented in solid-state form, and the majority of which can be incorporated into an integrated circuit. In accordance with one form of the invention, there is provided a ballast circuit for a gas discharge lamp, comprising a resonant load circuit incorporating a gas discharge lamp and including first and second resonant impedances whose values determine the operating frequency of the resonant load circuit A d.c.-to-a.c. convener circuit is coupled to the resonant load circuit to induce an ax. current in the resonant load circuit. It comprises first and second switches serially connected between a bus conductor at a d.c. voltage and ground, with a common node through which the bidirectional load current flows. An arrangement allows a user to select a setpoint signal during lamp operation that determines the amount of light output by the lamp. A feedback arrangement regcoeian'vely controls the first and second switches; it includes a circuit for ng a.c. current in the resonant load circuit and producing an a.c. feedback signal in proportion to the ax. current; a circuit for producing a d.c. feedback signal represeniting an average value of a lamp output parameter; a circuit for generating an error signal between the setpoint signal and the d.c. feedback signal; a comparator circuit for comparing the a,c. feedback signal with a periodic reference signal, and for producing a comparator output signal mat changes state when a one of the compared signals becomes greater than the other of the compared signals, and vice-versa; and a circuit for generating the periodic reference signal in response to both the comparator output signal and the error signal, A conditioning circuit receptive of the comparator output signal controls the first and second switches. The invention provides for A ballast circuit for a gas discharge lamp, comprising: (a) a resonant load circuit incorporating a gas discharge lamp and including first and second resonant impedances whose values determine the operating frequency of said resonant load circuit; (b) a d.c.-to-a.c. converter circuit coupled to said resonant load circuit so as to induce an a.c. current in said resonant load circuit, and comprising first and second switches serially connected between a bus conductor at a d.c. voltage and ground, and having a common node through which said bidirectional load current flows; Cnoev-oS c u . &eleoi«5o cf cv . . (c) an arrangement tor allowing a user tn irmurt i setpoint signal during lamp operation that determines the amount of light output by the lamp; and (d) a feedback arrangement for regeneratively controlling said first and second switches; said arrangement including a circuit for sensing a.c. current in said resonant load circuit and producing an a.c. feedback signal in proportion to said a.c. current; a circuit for producing a d.c. feedback signal representing an average value of a lamp output parameter, and a circuit for generating an error signal between said setpoint signal and said d.c. feedback signal; said feedback arrangement further including: (i) a comparator circuit for comparing said a.c. feedback signal with a periodic reference signal, and for producing a comparator output signal that changes state when a first one of the compared signals becomes greater than the second of the compared signals, and that further changes state when the second of the compared signals then becomes greater than the first of the compared signals; (ii) a circuit for generating said periodic reference signal in response to both said comparator output signal and said error signal; and (iii) a conditioning circuit receptive of said comparator output signal for controlling said first and second switches. BRIEF DESCRIPTION OF THE DRAWINGS In the following detailed description, reference will be made to the attached drawings in which like reference numerals refer to like, or corresponding elements, throughout the following figures, and in which: Fig, 1 is a schematic representation of a ballast cnicuit for a gas discharge lamp which exclusively uses electronic components in a feedback circuit for implementing self-resonant control of a pair of power switches, and which allows a user to dynamically select a level of light output Fig. 2 shows various voltage waveforms together with state diagrams for the output V. of comparator 31 and of switch 39 of Fig. 1. Fig. 3 is a schematic representation of an exemplary implementation of phase splitter, dead time A level shift circuit 50 of Fig. 1. Fig. 4 is a schematic representation of an exemplary implementation of dead time circuit 60 of Fig. 3. Fig. S is a schematic representation of an exemplary implementation of a low pass filter circuit that approximates integrator 37 of Fig. 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in Fig. 1, an electronic ballast arrangement for a compact fluorescent lamp, referenced generally as 10, is effective for converting a standard power line current into a high frequency pulsed signal for driving a compact fluorescent lamp 12. The ballast converter circuit 10 receives a d.c. input signal at terminals a-a', which input signal has been derived from an ax. input power line and has been rectified and, optionally, corrected for power factor by a rectification and power factor correction circuit (not shown). The d.c. input signal received by ballast circuit 10 is coupled across a pair of serially connected power MOSFETs designated Q, and Qj, which can be provided by International Rectifier of El Segundo, California under the product designation HEXFET MOSFET (HEXFET is a registered trademark of International Rectifier). The power MOSFETs Q, and O2 are effective upon receiving a control signal at their respective gate, or control, terminals thereto (g, and g2) for alternately switching between an on and an off condition at a high frequency. Connected across nodes b-b' is a series resonant lamp circuit which consists of the resonant circuit 14. having the inductor and capacitor elements LR and CR selected in size using conventional design means to achieve the desired resonant operating frequency of the lamp 12; capacitor C2 which blocks d.c. current from reaching lamp 12; and the lamp 12, which may comprise a compact fluorescent lamp. Node b* is also shown as ground 15. Self-resonant feedback control of power MOSFET switches Q, and Q2, in Fig. 1 is implemented in Fig. 1 with electronic components, the majority of which can be conveniently incorporated into an integrated circuit for controlling operation of ballast arrangement 10. In Fig. 1, phase control for controlling the alternate switching of i * MOSFET switches Q, and Q2 is provided by a phase control-circuit 30. A comparator 31 in circuit 30 receives at its inverting input a feedback voltage VFI that is proportional to current in lamp 12, Shown in dashed lines leading from lamp 12 are alternative conductors 42 and 43. Either one of such conductors is used, the other being omitted. Conductor 42 is preferably used for a relatively low d.c. input vohage to circuit 10, e.g. 160 volts, whereas conductor 43 is preferably used for a relatively high dc. input voltage, e.g. 300 volts. In any event, the current directed through a resistor 44 and the resulting vohage across such resistor have the same phase as the current in lamp 12. Feedback voltage VFt is derived from the voltage across resistor 44, via a d.c. blocking capacitor 45. Resistor 46 provides a d-c. path to ground 38 from the inverting input of comparator 31. Feedback voltage VFt has an a_c. waveform that typically approximates * sinusoidal waveform, as shown in Fig. 1. Voltage Vr, is approximately symmetrical about zero-voltage axis, or analog ground 38, and. if necessary, may be provided with a d.c. offset before being input into comparator 31. (Such an offset may typically be required for comparator 31 to properly operate within its input common mode range if Incorporated in an integrated circuit having only a single-polarity voltage supply.) At its non-inverting input comparator 31 receives a voltage V., which is preferably a triangular waveform as shown in Fig. 1. Voltage VT, in turn, is produced in response to a setpoint voltage Vs and a feedback voltage VF. Setpoint voltage Vs is typically proportional to a user-selected value for the amount of light output desired, and could be provided from a rheostat-type control, for instance. Feedback voltage V, represents an average value of a lamp output parameter, such as lamp current or lamp lumens. It could, for instance, comprise the same voltage as a.c. feedback signal VF, that is applied to the inverting input of comparator 31. A circuit 32 produces a dc. feedback voltage Vn as an average value of feedback voltage Vp. By "average" value is meant herein a d.c. avenge value such as an r.m.s, value, or a time-averaged value (e.g. during half a cycle of lamp current). Thus, circuit 32 could comprise an ax. to r.m.s. converter circuit, or an integrator circuit, both of which are known in the art. A summing amplifier 33 receives setpoint voltage V, on its inverting input, and d.c. feedback voltage Vn on its non-inverting input. The error signal produced by summing ampliGer 33 is then scaled by a gam factor of G by an amplifier 34, and fed through a p-n, Schottky or other rectifying diode 35. At this point, the voltage on cathode 36 of diode 35 will have a non-negative value, owing to the diode. Such voltage may be as shown in the voltage (V) versus time (T) graph shown in Fig. 1 for a value that is constant for some time. The voltage on cathode 36 is received by an non-inverting integrator 37 as a rectangular waveform, as shown in Fig. l, which is referenced to an analog ground 38. Analog ground 38 will usually be different from previously mentioned ground 15 shown beneath transistor Q, if me supply voltage (not shown) for phase control circuit 30 is unipolar, if the supply voltage is bipolar, analog ground 38 and ground IS can be the same. Integrator 37 may be approximated by a low pass filter of the resistor-capacitor type, by way of example, as described below (Fig. 5). Through the switching action of a single-pole, double throw switch 39, integrator 37 is alternately connected directly to p-n diode cathode 36 (via conductor 40) in one switch condition, or connected to cathode 36 through a standard unity-gain inverter 41 in another switch condition. Switch 39 may comprise, for instance, an analog switch sold under product designation CD40S3 by Harris Semiconductor, of Melbourne, Florida; or, an analog switch sold under product designation MCI4053B by Motorola, of Phoenix, Arizona. Switch 39 is responsive to output voltage V0 of comparator 31 such that, when the comparator output changes state, the switch 36 alternates in its connection to p-n diode cathode 36. This switching action is illustrated in Fig. 2. Fig. 2 shows voltage waveforms for comparator input voltages VFt and Vr on the same voltage scale, and shows a d.c. offset voltage, which is assumed the same for both voltages VFI and V.. Fig. 2 also shows a high/low state diagram for comparator output voltage V0, and an up/down state diagram for switch 39 as will be further explained below; these state diagrams have arbitrary • vertical scales relative to comparator input voltages VF1 and V.. In Fig. 2, comparator output voltage V0 is shown initially as being in a low state. During this time, switch 39 is connected to conductor 40, which is shown above the switch in Fig. 1 and, during this time, is thus designated as being "UP" in Fig. 2. When non-inverting voltage V«. then rises above feedback voltage VF, at time t,, comparator output voltage V0 rises to its high state as shown. Switch 39 is men connected to unity-gain inverter 41, which shown below the switch in Fig. 1 and, during this time, is thus designated as "DOWN" in Fig. 2. When non-inverting input voltage V. then falls below feedback voltage VF1 at time t,, comparator output voltage V0 switches back to its low state as shown. Non-inverting comparator input voltage V, will approximate a triangular waveform where integrator 37 is of the resistive-capacitor type. It is preferred that the upward and downward slopes of voltage V. be fairly linear. This insures at time t, in Fig. 3, for instance, that input voltage V, distinctly surpasses the magnitude of input voltage VPI so that comparator 31 will reliably change its output state to a low value; if. in contrast, voltage V. rounded off to a more level slope at time t,, the desired change of output state of the comparator would be less certain to occur. (The invention would work, however, even if voltage V. is comprised of a predominantly positive-going portion and a predominantly negative-going portion, so long as the following comparator operation occurs.) In accordance with operation of comparator 31 (Fig. 1), comparator output V0 changes from a low level to a high level when input non-inverting input V, exceeds feedback voltage input VPI, shown at time t, in Fig. 2, and changes from a high level to a low level when feedback voltage VFI exceeds input V., shown at time t, in Fig. 2. The output of comparator 31 may be alternatively described as changing levels whenever the absolute value of the a.c. portion of non-inverting voltage V. becomes higher than the absolute value of the a.c! portion of feedback voltage VFI, where the following condition is satisfied: Both the waveform for voltage V, and the waveform for feedback voltage VFJ have excursions above an axis of strength that are substantially mirror-images of their excursions below such reference axis, although shifted in time with respect to each other. Comparator output voltage V0 will typically have a phase advance of, e.g., 20a, to offset the phase delay of, e.g. 20", occurring in ballast arrangement 10 (Fig. 1) between power switches Q, and Q: and current-sensing resistor 44. More generally, however, the phase advance of comparator output voltage VO could additionally include 360°, or multiples of 360°, phase advance. Comparator output V. contains the necessary phase information to control operation of power MOSFET switches Q, and Q2, as will be further described below. Beneficially, using the phase control circuit 30 of Fig. 1, the power level of lamp 12 can be adjusted by changing the upward and downward slopes of non-inverting voltage V,. By making the slopes both steeper, the comparator output will change earlier, causing the frequency of switching of switches Q, and Q, to increase, as is apparent from Fig. 2, which decreases the power level of the lamp. This can be done by a user decreasing setpoint voltage Vs. On the other hand, to increase the power of the lamp, a user would increase setpoint voltage V5, causing the frequency of switching of switches Q, and Q, to decrease, which increases the power level of the lamp. Referring again to Fig. 1, comparator output V0 is received by phase splitter, dead time &. level shift circuit SO, described below in connection with Fig. 3. Circuit 50 provides appropriate signals on lines 50A and 50B that are respectively received by conventional buffers 70 and 72. These buffers are used to drive the gates of MOSFET switches Q, and Q2 with a low impede Fig. 3 shows an exemplary implementation of phase splitter, dead time & level shift circuit 50 of Fig. !. As Fig. 3 shows, comparator output voltage V0 is applied to a dead time circuit 60, one implementation of which is shown in Fig. 4. In Fig. 4, a circuit comprising resistor 63 and capacitor 65 receives comparator output voltage VO, and provides a delayed input, shown as voltage V65, to a logic NOT gate 67. Gate 67 is of the type having hysteresis, as indicated by the hysteresis notation in the symbol for gate 67, whereby its input threshold voltage is a function of the state of its output voltage. Gate 67 produces an output voltage V60 which transitions in the opposite manner from comparator output V0, but only after respective delay (or dead-time) intervals 61. A typical delay interval 61 for a lamp operating at a frequency of 50 kilo-hertz is one microsecond. Referring back to Fig. 3, dead-time circuit output voltage V60 is then input into a logic AND gate 51. The other input to gate 51 is the output of logic NOT gate 52, which inverts comparator output Vo. The output of AND gate 51, voltage V3I, is shown in Fig. 3. The output V5, is then level-shifted by a conventional level shift circuit 54. to provide an appropriate signal on conductor 50A to drive the gate of upper MOSFET Q, in Fig. 2 after passing through buffer 70 (Fig. 2). For driving tfae gate of the lower MOSFET Q, in Fig. 2, a gate-driving voltage V58 is produced by the circuit of Fig. 3. To accomplish this, a logic NOT gate 56 first inverts dead-time circuit output V60 and applies the resulting voltage as one input to logic AND gate 58. The other input to AND gate 58 is comparator output voltage V0. Due to the inclusion of NOT gate 56, the gate-driving output voltage V98 of AND gate 58 appears as shown in the figure, with its phase shifted 180* from the phase of gate-drive signal V,,. This realizes the phase-splitting function of circuit 50. As further shown in Fig. 3, the high states of gate-drive signal V58 are separated from the high states of gate-drive signal V51, on both leading and trailing sides, by dead-time intervals 61. This assures high speed operation of MOSFET switches Q, and Q2, since so-called soft switching techniques (e.g., zero-voltage switching) can be employed. The various functions of phase splitter, dead time & le-el shift circuit 50 of Figs. 1 and 3, as well as the function of buffers 70 and 72 in Fig. 1, can be implemented in an obvious manner by those of ordinary skill in the art. For instance, an IR2155 self-oscillating power MOSFET/IGBT gate driver from International Rectifier of El Segundo, California, could be utilized with the connections illustrated for "bootstrap operation" in its Provisional Data Sheet 6.029, dated January 13,1994. With the foregoing gate driver, the so-called RT input can be left open, and the present comparator output Vo can be applied to tile so-called CT input of the gate driver. However, the ability to set the dead time of present dead-time circuit 60 of Figs. 3 and 4 is not present with the use of the foregoing gate driver. Fig. S shows a low pass filter circuit that approximates integrator 37 of Fig. 1. As shown in the figure, a resistor 80 is connected between the output of analog switch 39 and the non-inverting input of comparator 31. A capacitor 81 is connected between the non-inverting input of the comparator and analog ground 38. The values of resistor 80 and capacitor 81 are preferably chosen to result in non-inverting input voltage V. (Fig. 2) being approximately linear, for the reasons mentioned above. Exemplary circuit values for a circuit for 20-watt fluorescent lamp 12 with an operating current of SO kilo-hertz, with a d.c. input voltage of 300 volts, are as follows: In Fig. 1, resonant inductor LR, 7SO micro henries; resonant capacitor CR. 6.8 nanofarads; d.c. blocking capacitor C2, 0.47 microfarads; resistor 44, 1.5 ohms; d.c. blocking capacitor 45, 0.5 microfarads; in Fig. 4, resistor 63 and capacitor 65 selected to provide a delay interval 61 of one microsecond; and in Fig. 5, resistor 80, 6.2 k ohms; and capacitor 81, 3.3 nanofarads. The foregoing describes a ballast circuit for a gas discharge lamp mat includes, for controlling a pair of power switches, a self-resonant feedback circuit whose components can be implemented in solid-state form, as opposed to the use of a magnetic transformer. A user-controllable input is provided to adjust the level of light produced by the lamp. Moreover, the majority of the components of the self-resonant feedback circuit can beneficially be incorporated into an integrated circuit.As will be realized from the foregoing description, the invention provides a ballast circuit for a gas discharge lamp that includes, for controlling, a pair of power switches, a self-resonant feedback circuit which allows a user to adjust the output of the lamp during lamp operation. Additionally, the ballast circuit enables a user to select a setpoint for an output parameter of the lamp at which the lamp will more accurately operate. The ballast circuit, moreover, includes a self-resonant feedback comprised of components which can be implemented in solid-state form, and the majority of which can be incorporated into an integrated circuit. While the invention has been described with respect to specific embodiments by way of illustration, many modifications and changes will occur to those skilled in the art. For instance, either electrodeless or electroded gas discharge lamps may be used with the invention. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true scope and spirit of the invention. We claim: 1. A ballast circuit for a gas discharge lamp, comprising: (a) a resonant load circuit incorporating a gas discharge lamp and including first and second resonant impedances whose values determine the operating frequency of said resonant load ckcuit; (b) a d.c.-to-a.c. converter ckcuit coupled to said resonant load ckcuit so as to induce an a.c. current in said resonant load ckcuit, and comprising first and second switches serially connected between a bus conductor at a d.c. voltage and ground, and having a common node through which said bidirectional load current flows; , . cn&*JT>Z> , „ . gg) action of o- . . (c) an aifangpment for jlnnin^ n n~rf M .Miiiu a setpomt signal during lamp operation that determines the amount of light output by the lamp; and (d) a feedback arrangement for regeneratively controlling said fkst and second switches; said arrangement including a ckcuit for sensing a.c. current in said resonant load ckcuit and producing an a.c. feedback signal in proportion to said a.c. current; a ckcuit for producing a d.c. feedback signal representing an average value of a lamp output parameter, and a ckcuit for generating an error signal between said setpoint signal and said d.c. feedback signal; said feedback arrangement further including: (i) a comparator ckcuit for comparing said a.c. feedback signal with a periodic reference signal, and for producing a comparator output signal that changes state when a fkst one of the compared signals becomes greater than the second of the compared signals, and that further changes state when the second of the compared signals then becomes greater than the fkst of the compared signals; (ii) a ckcuit for generating said periodic reference signal in response to both said comparator output signal and said error signal; and (iii) a conditioning ckcuit receptive of said comparator output signal for controlling said fkst and second switches. 2. The ballast ckcuit as claimed in claim 1, wherein said circuit for generating a periodic reference signal comprises: (a) a ckcuit responsive to said error signal for producing a bi-level output signal having one level when an output of said comparator ckcuit is at a fkst level, and having another level when said output of said comparator ckcuit is at a second level; and (b) a conversion ckcuit for producing an upgoing portion of said periodic reference signal when said bi-level output signal is at one level and for producing a Saif/Essenese/ downgoing portion of said periodic reference signal when said bi-level output signal is at another level. 3. The ballast ckcuit as claimed in claim 2, wherein said conversion ckcuit comprises: (a) a resistor receiving said bi-level output signal on one end and producing said periodic reference signal on another end; and (b) a capacitor connected between said another end of said resistor and a ground potential. 4. The ballast ckcuit as claimed in claim 1 wherein said ckcuit for generating a periodic reference signal is effective for producing a substantially triangular reference waveform whose excursions above a reference axis of strength are substantially mirror-images of excursions of the foregoing signal below said reference axis, although shifted in time with respect to each other. 5. The ballast ckcuit as claimed in claim 4, wherein: (a) said ckcuit for producing a feedback signal is effective for producing a feedback waveform whose excursions above a reference axis of strength are substantially mkror-images of excursions of said signal below the foregoing reference axis, although shifted in time with respect to each other; and (b) the mentioned reference axes of strength of said periodic reference signal and of said feedback signal are approximately equal to each other. 6. A ballast ckcuit for a gas discharge lamp as claimed in any preceding claim, comprising: (a) a resonant load ckcuit incorporating a gas discharge lamp and including first and second resonant impedances whose values determine the operating frequency of said resonant load ckcuit; (b) a d.c.-to-a.c. converter ckcuit coupled to said resonant load ckcuit so as to induce an a.c. current in said resonant load ckcuit, and comprising first and second switches serially connected between a bus conductor at a d.c. voltage and ground, and having a common node through which said bidkectional load current flows; (c) an arrangement for allowing a user to select a setpoint signal during lamp operation that determines the amount of light output by the lamp; and (d) a feedback arrangement for regenetatively controlling said fkst and second switches; said arrangement including a ckcuit for sensing a.c. current in said resonant load ckcuit and producing an a.c. feedback signal in proportion to said a.c. current; a Saif/Essenese/ circuit for producing a d.c. feedback signal representing an average value of lamp current; and a circuit for generating an error signal between said setpoint signal and said d.c. feedback signal; said feedback arrangement further including: (i) a comparator circuit for comparing said a.c. feedback signal with a periodic reference signal, and for producing a comparator output signal that changes state when a first one of the compared signals becomes greater than the second of the compared signals, and that further changes state when the second of the compared signals then becomes greater than the first of the compared signals; (ii) a circuit for generating said periodic reference signal in response to both said comparator output signal and said error signal; and (iii) a conditioning circuit receptive of said comparator output signal for controlling said first and second switches. 7. The ballast circuit for a gas discharge lamp substantially as herein described with reference to and as illustrated by the accompanying drawings.

Documents:

1109-del-1997-abstract.pdf

1109-del-1997-claims.pdf

1109-del-1997-correspondence-others.pdf

1109-del-1997-correspondence-po.pdf

1109-del-1997-description (complete).pdf

1109-del-1997-drawings.pdf

1109-del-1997-form-1.pdf

1109-del-1997-form-19.pdf

1109-del-1997-form-2.pdf

1109-del-1997-form-3.pdf

1109-del-1997-form-4.pdf

1109-del-1997-form-6.pdf

1109-del-1997-gpa.pdf

1109-del-1997-petitio-138.pdf

1109-del-1997-petition-others.pdf