Series-resonant electronic ballast circuit

Abstract

Subject invention relates to an inverter-type electronic fluorescent lamp ballast wherein a series-resonant LC circuit connected across the inverter's output is used for matching the inverter's operating characteristics to those of the fluorescent lamp--the fluorescent lamp being connected in parallel with the tank-capacitor of this LC circuit. In particular, the invention relates to the use of a Varistor coupled in parallel with this tank-capacitor, thereby limiting the voltage developed thereacross to a magnitude suitable for proper lamp starting. Moreover, by providing for means whereby the inverter shuts itself off in case current flows through this Varistor for more than about one second or so (which is the maximum length of time that it should normally take for a fluorescent lamp to start), inverter as well as Varistor overload protection is obtained. Without such overload protection the inverter and/or the Varistor would self-destruct in case the fluorescent lamp failed to start or if it were removed from the circuit. Also, to mitigate the possibility of electric shock hazard for persons servicing fluorescent lighting fixtures using ballasts based on subject invention--as might occur if someone were to touch a terminal at one end of a fluorescent lamp while inserting the other end into its socket--the inverter will shut itself off immediately in case there is flow of current directly from one of the ballast terminals and to earth ground.

I claim: 
     
       1. A ballast for a gas discharge lamp, said lamp having a pair of electrodes and requiring thereacross in order to initiate lamp operation an initial relatively high-magnitude lamp starting voltage for a relatively brief period of time, said ballast being adapted to be powered from an ordinary electric utility power line ande comprising: rectifier-means connected in circuit with said power line by way of a pair of power-line-wires and operative to provide a DC voltage;   inverter-means connected with said DC voltage and operative to provide a high-frequency-voltage at an AC output, said high-frequency-voltage having a fundamental frequency that is substantially higher than that of the voltage normally present on said power line;   tuned-circuit-means connected in circuit with said AC output, said tuned-circuit-means being substantially resonant at the fundamental frequency of said high-frequency-voltage and having a pair of output terminals with a tuned-circuit-voltage thereacross, the presence of said tuned-circuit-voltage being a response to the presence of said high-frequency-voltage;   lamp-socket-means connected in circuit with said pair of output terminals and operable to permit disconnectable connection of said gas discharge lamp therewith;   disable-means connected in circuit with said inverter-means and operable, upon the receipt of a disable-signal, to disable the operation of said inverter-means, thereby to remove said tuned-circuit-voltage from said pair of output terminals;   voltage-sensing-means connected in circuit with said pair of output terminals and operable to provide said disable-signal to said disable-means whenever the magnitude of said tuned-circuit-voltage has remained at or above a pre-determined level for a time longer than a certain first length of time, said pre-determined level being of a magnitude at least equal to that of said relatively high-magnitude lamp starting voltage and said certain first length of time being at least equal in length to that of said relatively brief period of time; and   current-sensing-means operative to sense the flow of high-frequency-current through said pair of power-line-wires and to provide said disable-signal to said disable-means whenever said high-frequency-current has been flowing for a time longer than a certain second length of time, said high-frequency-current having a fundamental frequency that is substantially the same as that of said high-frequency-voltage.   
     
     
       2. The ballast of claim 1 wherein the length of said relatively brief period of time is on the order of one second. 
     
     
       3. The ballast of claim 2 wherein said certain second length of time is substantially shorter than about one second. 
     
     
       4. The ballast of claim 1 wherein said tuned-circuit-means comprises a series-resonant LC circuit. 
     
     
       5. A ballast for a fluorescent lamp, said fluorescent lamp having a pair of thermionic cathodes, each such cathode having a lamp input terminal, said lamp requiring in order to get started the presence of a relatively high-magnitude starting voltage across said pair of cathodes for a relatively brief period of time, said ballast being adapted to be powered from an ordinary electric utility power line and comprising: rectifier-means connected in circuit with said power line by way of a pair of power-line-wires and operative to provide a DC voltage;   inverter-means connected with said DC voltage and operative to provide a high-frequency-voltage across a pair of output terminals, said high-frequency-voltage having a fundamental frequency that is substantially higher that that of the voltage normally present on said power line;   lamp-connect-means connected in circuit with said pair of output terminals and operable to permit disconnectable connection with each of said lamp input terminals;   disable-means connected in circuit with said inverter-means and operable, upon the receipt of a disable-signal, to disable the operation of said inverter-means, thereby to remove said high-frequency-voltage, from said pair of output terminals;   voltage-sensing-means connected in circuit with said pair of output terminals and operable to provide said disable-signal to said disable-means whenever the magnitude of said high-frequency-voltage has remained equal to or larger than that of said relatively high-magnitude starting voltage for a length of time that is at least equal to that of said relatively brief period of time; and   current-sensing-means operative to sense the flow of high-frequency-current through said pair of power-line-wires and to provide said disable-signal to said disable-means whenever said high-frequency-current has been flowing for longer than a certain short length of time, said high-frequency-current being of a frequency substantially the same as that of said high-frequency-voltage.   
     
     
       6. The ballast of claim 5 wherein the length of said relatively brief period of time is on the order of one second. 
     
     
       7. The ballast of claim 6 wherein said certain short length of time is on the order of 25 milli-second. 
     
     
       8. A ballast for a gas discharge lamp, said ballast adapted to be powered from a voltage source and comprising: voltage-conversion-means connected in circuit with said voltage source and operable to provide a current-limited AC voltage across a pair of output terminals;   connect-means connected with said pair of output terminals and operative to permit connection thereto of a gas discharge lamp, said gas discharge lamp requiring a starting voltage of relatively high magnitude for a relatively brief period of time in order to initiate proper lamp operation;   disable-means operative, upon the receipt of a disable-signal, to remove said AC voltage from said output terminals;   voltage-sensing-means connected in circuit with said pair of output terminals and operative to provide said disable-signal to said disable-means whenever the magnitude of said AC voltage has remained equal to or larger than that of said starting voltage for longer than a relatively brief period of time; and   current-sensing-means operative to sense the flow of ground-fault-current between said pair of output terminals and said voltage source, and to provide said disable-signal to said disable-means after said ground-fault-current has been flowing for longer than a very short period of time.   
     
     
       9. The ballast of claim 8 wherein said gas discharge lamp is a fluorescent lamp. 
     
     
       10. The ballast of claim 8 wherein said voltage source is an ordinary electric utility power line. 
     
     
       11. The ballast of claim 8 wherein said very short period of time is one the order of 25 milli-second. 
     
     
       12. The ballast of claim 9 wherein said relatively brief period of time is on the order of one second. 
     
     
       13. The ballast of claim 8 wherein said voltage-conversion-means comprises an inverter means. .Iadd. 
     
     
       14.  A ballast for a gas discharge lamp, said lamp having a pair of electrodes and requiring thereacross, in order to initiate lamp operation, an initial relatively high-magnitude lamp starting voltage for a relatively brief period of time, said ballast being adapted to be powered from an ordinary electric utility power line and comprising: rectifier-means connected in circuit with said power line by way of a pair of power-line-wires and operative to provide a DC voltage;   inverter-means connected with said DC voltage and operative to provide a high-frequency-voltage having a fundamental frequency that is substantially higher than that of the voltage normally present on said power line;   tuned-circuit-means connected in circuit with said AC output, said tuned-circuit-means being substantially resonant at the fundamental frequency of said high-frequency-voltage and having a pair of output terminals with a tuned-circuit-voltage thereacross, the presence of said tuned-circuit-voltage being a response to the presence of said high-frequency-voltage;   lamp-socket-means connected in circuit with said pair of output terminals and operable to permit disconnectable connection of said gas discharge lamp therewith;   disable-means connected in circuit with said inverter means and operable, upon the receipt of a disable-signal, to disable the operation of said inverter-means, thereby to remove said tuned-circuit-voltage from said pair of output terminals;   voltage-clamping-means connected in circuit with said pair of output terminals and operative, by drawing current therefrom, to limit the magnitude of said tuned-circuit-voltage present thereacross to a level that is about equal to or somewhat larger than said relatively high-magnitude lamp starting voltage; and   sensing-means connected in circuit with said voltage-clamping-means and operable to provide said disable-signal to said disable-means whenever said voltage-clamping-means has drawn current from the output terminals for longer than a brief period of time;   whereby, if a gas discharge lamp is not connected with said lamp-socket-means and thereby operative to constitute a load for said tuned-circuit-means, the inverter means is disabled within a brief period of time..Iaddend. .Iadd.15. The ballast of claim 14 wherein said voltage-clamping-means comprises non-linear resistive means..Iaddend.   
     
     
        .Iadd.16.  An arrangement comprising: power supply means conditionally operable to supply a magnitude-limited current from a pair of output terminals adapted to connect with a load, the power supply means having disable means operative, on the receipt of a disable signal, to prevent said magnitude-limited current from being supplied;   voltage clamping means connected in circuit with the output terminals and operative, by way of drawing current therefrom, to prevent the magnitude of any voltage present thereat from substantially exceeding a predetermined level; and   sensing means connected in circuit with the voltage clamping means and operative to provide said disable signal whenever the voltage clamping means has been drawing current for longer than a certain period of time..Iaddend. .Iadd.17. The arrangement of claim 16 wherein the voltage clamping means comprises non-linear resistance means..Iaddend. .Iadd.18. The arrangement of claim 16 wherein the voltage clamping means draws substantially no current when the magnitude of any voltage present at the output terminals is below said predetermined level..Iaddend. .Iadd.19. The arrangement of claim 16 wherein the power supply means: (i) comprises frequency converter means connected in circuit between an ordinary electric utility power line and the output terminals, and (ii) is operative to cause the magnitude-limited current supplied from those output terminals to be of a frequency substantially higher than that of the voltage on the power line..Iaddend. .Iadd.20. The arrangement of claim 19 wherein the frequency converter means comprises tuned LC circuit means connected in circuit with the output terminals and being substantially resonant at the frequency of the magnitude-limited current..Iaddend. .Iadd.21. The arrangement of claim 16 wherein a load is indeed connected and wherein this load is a gas discharge lamp..Iaddend. .Iadd.22. The arrangement of claim 21 wherein, whenever the load is indeed connected and drawing power, the magnitude of the voltage present at the output terminals is prevented from reaching said predetermined level. .Iaddend.   
     
     
        .Iadd.23.  The arrangement of claim 16 wherein the duration of the certain period of time is not longer than about one second..Iaddend. .Iadd.24. The combination comprising: a power supply conditionally operable to supply current from a pair of output terminals, the power supply having disable means operative, on receipt of a disable input, to stop the supply of current from the output terminals;   voltage limiting means connected with the output terminals and operative, by drawing current therefrom, to prevent any voltage present thereacross from substantially exceeding a predetermined level;   sensing means connected in circuit with the voltage limiting means and with the disable means and operative to provide said disable input in case the voltage limiting means has drawn current from the output terminals for longer than a certain period of time; and   load means operative to connect with the output terminals and, when indeed so connected and when in proper operating condition, functional to prevent the magnitude of any voltage present across the output terminals from reaching the predetermined level for any length of time longer than said certain period of time..Iaddend. .Iadd.25. The combination of claim 24 wherein the load means comprises a gas discharge lamp..Iaddend. .Iadd.26. The combination of claim 24 wherein said power supply comprises:   rectifier means connected with an ordinary electric utility power line and operative to provide a DC voltage at a pair of DC terminals;   inverter means connected with the DC terminals and operative to provide a squarewave voltage at a pair of squarewave terminals;   a series-combination of a capacitor and an inductor connected across the squarewave terminals, the series-combination being series-resonant at or near the frequency of the squarewave voltage; and   connect means operative to connect the output terminals in parallel-circuit   
     
     
        with the capacitor..Iaddend. .Iadd.27.  An arrangement comprising: rectifier means connected with an ordinary electric utility power line and operative to provide a DC voltage at a set of DC terminals;   inverter means connected with the DC terminals, the inverter means having disable means receptive of a disable input and being operative, except after having received the disable input, to provide a substantially squarewave voltage at a set of squarewave terminals;   LC circuit having a capacitor interconnected with an inductor and being connected with the squarewave terminals, the LC circuit means being series-resonant at or near the frequency of the squarewave voltage;   output terminals connected in parallel-circuit with the capacitor and operative to provide output current therefrom;   gas discharge lamp means operative to be connected with the output terminals and operative, when so connected, to receive at least part of the output current therefrom;   voltage-limiting means connected with the output terminals and operative, by way of drawing at least part of the output current therefrom, to prevent the magnitude of the voltage present thereacross from substantially exceeding a predetermined level; and   sensing means connected in circuit between the output terminals and the disable means, the sensing means being operative to provide the disable input in case the voltage-limiting means has drawn the output current for longer than a certain length of time..Iaddend. .Iadd.28. The arrangement of claim 27 wherein the gas discharge lamp is characterized by having a starting voltage of magnitude about equal to said predetermined level and an operating voltage of magnitude substantially smaller than said predetermined level, the starting voltage prevailing during a starting mode of relatively brief duration, during which starting mode the lamp ignites and enters into a substantially continuous operating mode, the operating voltage prevailing during the operating mode;   thereby to prevent the voltage-limiting means from drawing any significant part of the output current during the operating mode..Iaddend. .Iadd.29. The arrangement of claim 28 wherein the certain length of time is on the   
     
     
        order of one second or less..Iaddend. .Iadd.30.  An arrangement comprising: source means conditionally operable to provide an AC voltage at a set of AC terminals, the source means having disable means operative to disable its operation on receipt of a disable input, thereby to remove the AC voltage from the AC terminals;   a series-combination of a capacitor and an inductor, the series-combination: (i) being series-resonant at or about the frequency of the AC voltage, (ii) being connected with the AC terminals, (iii) having a pair of output terminals effectively connected in parallel-circuit with the capacitor, and (iv) being operative to provide an AC output current from these output terminals;   voltage-limiting means connected with the output terminals and operative, by way of absorbing at least part of the AC output current, to limit the magnitude of any voltage present thereacross to a predetermined level;   load means operative to be connected with the output terminals and to absorb at least part of the AC output current; and   sensor means connected in circuit between the voltage-limiting means and the disable means and operative to provide the disable input in case the voltage-limiting means were to absorb a substantial part of the AC output current for longer than a certain brief period of time..Iaddend. .Iadd.31. The arrangement of claim 30 wherein the load means, as long as indeed connected, is operative to absorb substantially all of the AC output current, thereby preventing the voltage-limiting means from having to absorb a substantial part of the AC output current..Iaddend. .Iadd.32. The arrangement of claim 31 wherein the load means comprises a gas discharge lamp..Iaddend. .Iadd.33. The arrangement of claim 31 combined with ground fault detection means connected in circuit with the power line and operative, in response to any flow of unbalanced AC current from the output terminals, to provide the disable input..Iaddend. .Iadd.34. The arrangement of claim 31 wherein: (i) the voltage-limiting means comprises non-linear resistive means, and (ii) the certain brief period of time is of a length not exceeding about one second, thereby preventing an excessive amount of energy from being dissipated in the non-linear resistive means.