SELF-REGULATING, NO LOAD PROTECTION ELECTRONIC BALLAST SYSTEM

Abstract: A self-regulating, no load protected electronic ballast system (10) is provided which includes a power source (12) for actuating at least one gas discharge tube (66) with a regulated current and limited voltage to maintain the gas discharge tubes (66) input and output power at a predetermined value. The self-regulating, no load protected electronic ballast system (10) includes a filter circuit (11) coupled to the power source (12) with an induction circuit (15) coupled to the filter circuit (11). The induction circuit (15) has a tapped primary winding (42) providing an auto-transformer configuration for establishing the magnitude of the regulated current. The induction circuit (15) includes a trigger control winding (48) for generating a control current and further includes a no load protection circuit (99) for generating a voltage across the gas discharge tube (66) responsive to the regulated current and for maintaining the output voltage at a predetermined value when the gas discharge tube (66) is decoupled from the electronic ballast system (10). In this manner, the output voltage of the ballast system (10) is substantially reduced when the gas discharge tube (66) is electrically decoupled from the overall circuit resulting in a higher reliability and extended life of a particular gas discharge tube (66).

Description: This invention relates to electronic ballast systems for fluorescent or gas discharge tubes. Electronic ballast systems for gas discharge or fluorescent tubes are known in the art, for example, patent application EP-A-0210310 discloses a system comprising: a filter circuit connectable to the power source for producing a substantially smooth direct-current output and for suppressing feedback to the power source of harmonic frequencies generated by the system; an oscillator powered by the output of the filter circuit and including an oscillator auto-transformer having a primary winding connected in series with a capacitor across the output of the filter circuit, the primary winding having a tapped portion connected in series with a switching circuit across the output of the filter circuit, and the oscillator auto-transformer having a secondary winding for generating a control current to the switching circuit; regulating means comprising a toroidal transformer having a first winding in series with the tapped portion of the primary winding of the auto-transformer, and a second winding (55) in series with the secondary winding of the auto-transformer for modifying the control current produced by the secondary winding; and means for generating an output voltage across the gas discharge tube in dependence upon an oscillating current through the primary winding of the auto-transformer. In particular, in this system, the output voltage for the gas discharge tube is obtained by placing the tube in series with the oscillator capacitor. However, such a system suffers from the disadvantage that removal of the gas discharge or fluorescent tube from the circuit causes excessive voltage outputs from the circuit, which can have a deleterious effect on the operating life of the system. The present invention is characterised in that the output voltage generating means comprises a separate transformer having a primary winding connected in series with the filter circuit and the oscillator, and a secondary winding connectable across the gas discharge tube. Accordingly, if the gas discharge tube is removed (or otherwise becomes open circuit), the load impedance on the separate transformer becomes very high, and thus the reflected impedance of the primary winding becomes very high, which, due to the primary winding being series connected, reduces the current through the primary winding and oscillator and limits the output voltage to the gas discharge tube. The electronic ballast system of this invention is further described with reference to the accompanying drawings, in which the single Figure is an electrical schematic drawing of the self-regulating, no load protected electronic ballast system in a preferred form. Referring to the Figure, the self-regulating, no load protected electronic ballast system 10 is shown connected to a power source 12 for actuation of at least one gas discharge tube 66. Gas discharge tube 66 may be a standard fluorescent type system having first and second filaments 68 and 70 respectively. Self-regulating, no load protected electronic ballast system 10 is provided to maintain the output and input power of gas discharge tube 66 at a substantially predetermined value which is relatively constant during operation and substantially constant and independent of electrical component tolerances from one electronic ballast system 10 to another. In overall concept, self-regulating, no load protected electronic ballast system 10 is provided for maximization of efficiency of light output from gas discharge tube 66 with respect to a power input from power source 12. Additionally, self-regulating, no load protected electronic ballast system 10 provides for a substantially constant light output in the order of approximately ± 3.0%, regardless of the voltage variation by virtue of the output of gas discharge tube 66 being responsive to a square wave form of pulsating driving current as opposed to a square wave form of pulsating input voltage. As will be seen in following paragraphs, output voltage of electronic ballast system 10 is limited and substantially reduced when gas discharge tube 66 is electrically removed from the circuit. Of importance, electronic ballast system 10 provides initially for a regulation control which eliminates the need for adjusting or pre-selecting transistors of specific gain, in order to provide a relatively constant light output substantially independent of manufacturing tolerances associated with the manufacture of contained electronic components. The output voltage is reduced below its normal operating voltage when the gas discharge tube 66 is electrically removed from the circuit, either by its failure or by its physical removal from the system. Additionally, self-regulating, no load protected electronic ballast system 10 provides for a frequency control mechanism using the inductive characteristics of inverter transformer 40 and the tank circuit formed by the secondary winding 160 of transformer 100 and capacitor 130 allowing for frequency stabilization and having the advantage of permitting electronic ballast system 10 to operate in a normal manner without bothersome visual flickering. Particularly, operation of gas discharge tube 66 is maintained at a minimum level due to the higher efficiency obtained by self-regulating, no load protected electronic ballast system 10. Of importance to the reliability of system 10 is the minimization of electrireliability of system 10 is the minimization of electrical components coupled with the simplicity of the circuitry associated with ballast system 10. This concatenation of elements has the effect of increasing reliability of self-regulating, no load protected electronic ballast system 10 while simultaneously maximizing the operating lifetime of gas discharge tube 66. Referring now to the Figure of electronic ballast system 10, having a power source 12 for actuating gas discharge tube 66, such includes filter circuit 11 coupled to power source 12 for establishing a substantially direct current voltage signal and suppressing harmonic frequencies generated by electronic ballast system 10. In still further overall concept, self-regulating, no load protected electronic ballast system 10 includes induction circuitry 15 which is electrically coupled to filter circuit 11 for establishing the magnitude of a pulsating driving current established by switching network 13. As will be detailed in following paragraphs, induction circuitry 15 includes no load protection circuit 99 for generating a voltage across gas discharge tube 66 responsive to the pulsating driving current, and for maintaining the output voltage at a predetermined value when gas discharge tube 66 is not electrically connected to ballast system 10. Induction circuit 15 is coupled to regulation control circuit 17 for maintaining a gain value of switching network 13 to a predetermined level. Thus, filter circuit 11 is connected to power source 12 for (1) maintaining a substantially smooth direct current voltage signal; and, (2) suppressing harmonic frequencies generated by electronic ballast system 10. Induction circuitry 15 also includes base drive winding 48 for generating a switching signal. Switching network 13 generates a pulsating driving current responsive to the switching signals generated in base driving winding 48. As will be further described, no load protection circuit 99 is coupled to filter circuit 11 and includes a tuned high voltage output secondary winding 160 for generating across gas discharge tube 66 responsive to the regulated driving current. Referring further to the Figure, there is shown power source 12 to provide an electrical power input for self-regulating, no load protected electronic ballast system 10. In the embodiment shown in the Figure, power source 12 may be an AC power source of standard voltage such as 120, 240, 270 volts, or any acceptable standardized power voltage generated at approximately 50.0 or 60.0 Hz. In broad general concept, power source 12 may be a DC power electrical source applied internal or external to self-regulating, no load protected electronic ballast system 10 in a manner well-known in the art by removal of some elemental circuitry and filtering elements. For the purposes of illustration, power source 12 will be in the following paragraphs designated as a 210-240 volt, 50.0 Hz., AC power source, and will be used in the embodiment to be described. Power for system 10 is supplied by power source 12 to switch 14 which may be a standard switch element such as a single pole, single throw switch mechanism. Power is applied from switch element 14 to choke 32 and harmonic filter capacitor 28. Harmonic filter capacitor 28 is coupled in parallel relationship with power source 12 and is designed to shunt high frequency components which would be fed back from electronic ballast system 10 to the power source 12. Choke element 32 is coupled in series relation with power source 12 and rectification circuit 16, which is used for providing full-wave rectification of the power source AC voltage. Rectification circuit 16 may be a full-wave bridge circuit well-known and standard in the art. In the embodiment illustrated, full wave bridge circuit 16 is formed of diode elements 18, 20, 22, and 24 for providing the necessary rectification of AC voltage from power source 12. Diode elements 18, 20, 22 and 24 may be one of a number of standard diode elements, and in one form of self-regulating, no load protected electronic ballast system 10, diode elements 18, 20, 22 and 24 have a standarized designation of 1N4005. Rectification by full wave bridge circuit 16 provides a pulsating DC voltage signal passing on output line 26 which is applied to shunt capacitor 34 of filter network 11. Filter network 11 filters the voltages input and output from rectification circuit 16, and is electrically connected to bridge circuit 16 by input line 19 and output line 26. Rectification bridge circuit 16 is coupled to return line 64 which is the return path for the DC supply for the opposing ends of bridge circuit 16, providing DC power input to shunt capacitor 34 of filter network 11. The smoothing filter portion of filter network 11 includes choke element 32 and shunt capacitor 34. Choke element 32 is coupled on a first end to power source 12 and on a second end to rectification circuit 16 through input line 19. As is seen, shunt capacitor 34 is coupled in parallel relation with the output of rectification circuit 16 through output line 26. Shunt capacitor 34 is connected on a first end to rectification circuit 16 output line 26 and coupled on opposing end to DC return line 64. In combination, shunt capacitor 34 and choke element 32 function to average out the 100 Hz pulsating DC voltage supplied by full wave bridge circuit 16. Additionally, this combination substantially maintains the current draw at an average value without creating a power factor which is either unacceptably leading, or unacceptably lagging. Deleterious lead or lag may be found wherein a large capacitance is used, or a large inductance, as the sole filtering means for smoothing a pulsating DC voltage. For purposes of illustration, in the event choke element 32 were not incorporated within self-regulating, no load protected electronic ballast system 10, shunt capacitor 34 would draw an increase current commonly referred to as a surge current on each cycle as capacitor 34 began to charge. By incorporation of choke element 32, the inductance stores energy during each half-cycle to supply current for initial charging of shunt capacitor 34 which provides a substantially smooth, average current as seen by power source 12. In the embodiment herein provided, choke element 32 may be an inductor approximately 2.0 Henries, with a resistance of less than 40.0 Ohms, and shunt capacitor 34 is a commonly available 100.0 microfarad, 250.0 volt electrolytic capacitor. Filter network 11 includes harmonic filter capacitor 28 which in combination with choke element 32 substantially reduces harmonic frequencies generated by induction circuit 15. The tuning of harmonic filter capacitor 28 in combination with choke element 32 has been designed to provide significant reduction in amplitude of at least the first five harmonic frequencies coupled from the DC supply of ballast system 10. As is typical in filters of this nature, harmonic frequencies which are multiples of these first five harmonic frequencies are also reduced. In the embodiment herein provided, harmonic filter capacitor 28 is approximately a 1.5 microfarad, 400.0 volt Mylar type capacitor, which is coupled in parallel relation with power source 12 and bridge circuit 16 through choke element 32. No load protection circuitry 99 is coupled between filter network 11 and induction circuit 15. No load protection circuit 99 includes transformer 100, with primary winding 125 and tuned high voltage output secondary winding 160 as well as a pair of filament excitation windings 140 and 150. Tuned, high voltage output secondary winding 160 is coupled in parallel relation with tuning capacitor 130 to form a tank circuit for generating the output voltage to gas discharge tube 66. Filament excitation windings 150 and 140 are coupled to gas discharge tube 66 filaments 70 and 68, respectively. In the embodiment herein provided, no load protection circuit 99 is comprised of transformer 100 with the core material being a ferrite composition having the designation of Ferroxcube 2616 with primary winding 125 composed of 29 turns, and tuned high voltage output secondary winding having 50 turns and each of filament excitation windings 140 and 150 being composed of a single turn each. Transformer 100 has a linear magnetic core and does not saturate. Tuning capacitor 130 is a ten nanofarad capacitor. Self-regulation control circuit 17 is coupled between no load protection circuit 99 and inverter network 15. Self-regulation control circuit 17 includes first capacitor 54, toroid transformer 56, and shunt resistor 51. Shunt resistor 51 is coupled in parallel relation with first winding 55 of toroid transformer 56. First winding 55 of toroid transformer 56 besides being parallel coupled to shunt resistor 51 is coupled on a first end to return line 64 and on a second end to base coupling capacitor 54. Base coupling capacitor 54 is coupled on one end to first winding 55 of toroid transformer 56 and on the opposing end to base drive winding 48 of induction circuit 15. Although not important to the inventive concept as herein described, shunt resistor 51 may have a value of approximately 200 Ohms. Toroid transformer 56 may have a ferrite core material composition which is a Ferroxcube designated 3B7-266T125, or 3B7-266CT125, with first winding 55 having 12 turns of number 28 wire, and a second winding 57 of a single turn formed by DC power input or filter output line 36 passing through the axis of the toroid core. Base coupling capacitor 54 may be 0.22 microfarad, 100 volt Mylar type capacitor. The series combination of first winding 55 of toroid transformer 56, and base coupling capacitor 54 provides for return paths for the base drive signal of switching network 13 subsequent to self-regulating, no load protected electronic ballast system 10 going into an oscillation phase. Self-regulating, no load protected electronic ballast system 10 further includes switching network 13 which is feedback coupled to induction circuitry 15 for establishment of a pulsated current. As will be seen in following paragraphs, switching network 13 includes a regulation mechanism for maintaining the power output of gas discharge tube 66 at a predetermined and subsequently constant value. Switching network 13 includes transistor 72 connected in feedback relation to bias or trigger control winding 48 of inverter transformer 40. This coupling allows switching of a current signal responsive to a b s signal produced. Referring to bias control winding 48 of inverter transformer 40, current entering the first end of bias control winding 48 passes through winding 48 to base element 78 of transistor 72. Transistor 72 includes respectively base element 78, collector element 74, and emitter element 76. It is to be understood that self-regulating, no load protected electronic ballast system 10 is designed to provide a consistency in visual light output, as well as power input to gas discharge tube 66 by maintaining the current of collector element 74 substantially constant independent of the current gain of a particular transistor 72 used in electronic ballast system 10. It has been determined that the light output should not fluctuate more than ± 3.0% while the current gain of transistor 72 used in system 10 may vary in the extreme between 10 and 60. It is to be further understood that although system 10 as shown in the illustrated embodiment operates a single gas discharge tube 66, the principle as herein described is general in concept and may be used in dual systems, since in such cases, transistor current gains would not necessarily have to be matched by pairs. Initially, a positive voltage provided to base element 78 by resistor 53 assures a small but sufficiently initiating current flow through base element 78 for initiation of conduction through transistor 72. A value of 1.0 megohms has been used successfully for resistor 53. When transistor 72 goes into a conduction or "on" state, current from power source 12 flows through choke 32, bridge circuit 16 and primary winding 125 of transformer 100 to DC output line 36. DC output line 36 is coupled to primary winding 42 of inverter transformer 40 and passes through the axis of the core of toroidal transformer 56. Such current passes through first section 46 of primary winding 42 to tap line 25 which is coupled to collector element 74 of switching transistor 72. Current flows through transistor 72 from collector 74 to emitter 76 and then from emitter element 76 through return line 64. The increase in collector current established by switching transistor 72 induces a voltage in bias control winding 48 which is coupled to base element 78 of transistor 72. Base current flows from base element 78 to emitter element 76 in transistor 72 and from emitter element 78 to return line 64. In completion of the circuit, the current flows through first winding 55 of toroid transformer 56, and base coupling capacitor 54. The series combination of elements as aforementioned, creates a pulse type base drive for switching transistor 72 from an "on" state to an "off" state after a predetermined period of time. The pulse which drives switching transistor 72 controls the duration of the "on" time during the frequency of operation of self-regulating, no load protected electronic ballast system 10. At the terminating point of this pulse, transistor 72 goes to an "off" state and the pulse differentiation through capacitor 54 supplies a negative signal to base element 78 which is limited in value magnitude by diode 38. The energy stored in inverter transformer 40 is discharged from primary winding 42 to capacitor 62 and therethrough to return line 64. Concurrently, as the collector current passes through the primary winding 125 of transformer 100, a voltage is induced in each of transformer 100 secondary windings. The voltage induced in filament excitation windings 140 and 150 causes a current flow through filaments 68 and 70 respectively of gas discharge tube 66. The voltage induced in tuned high voltage output secondary windings 160 is coupled across tuning capacitor 130 and gas discharge tube 66. The voltage induced at this point in the cycle is sufficient to maintain the discharge within gas discharge tube 66 but not sufficient to initiate the discharge itself. Reiterating, transistor 72 is initially switched to an "on" state by the small current flowing from output line 36, through resistor 53, base drive winding 48 to base 78 of transistor 72. This provides a current flow from output line 36 through first section 46 of primary winding 42, line 25, collector 74, emitter 76 and then to return line 64. Due to the aforementioned, the magnetic field in the ferrite magnetic core of transformer 40 increases from substantially zero to a predetermined value of magnetic induction, B, on the hysteresis cycle in a manner such that the circuit operates in a linear region of the characteristic. The change in the magnetic field induces a voltage in the windings surrounding the magnetic core of transformer 40 proportional to the number of turns of a particular winding. The windings are coupled in a manner to enhance the positive voltage applied to the base 78 which increases the current flow in the loop comprising collector 74, emitter 76 and all elements in series coupling therewith, until the current reaches a maximum value determined by the impedance of first section 46 and the voltage level on line 36. When the current in the path of collector 74 and emitter 76 ceases to increase, the magnetic induction having reached the predetermined value B abruptly collapses and induces a voltage of opposite polarity in base drive winding 48, which terminates the flow through collector 74 and emitter 76. At this time, the induced voltages are large and classically are equal to the inductance sum multiplied by the differential of the current flowing with respect to time. The inductance sum is the sum of the inductances of transformer primary winding 125, first section 46, primary winding second section 44 and twice the mutual inductance between sections 46 and 44 which results in a frequency of discharge equal to one over 2 times the square root of the sum of the inductances multiplied by the capacitance of capacitor 62. The discharge current assumes a sinusoidal shape dependent on the inverse of the frequency of discharge which is the "off" time and is the time taken to discharge the energy stored during the "on" time, namely one-half the inductance sum multiplied by the current squared. Once again, an abrupt fall in the magnetic induction is provided with the direction of the current opposite in direction to that portion of the sine wave. The substantially instantaneous voltages and currents are large enough to induce the starting voltages either positive or negative in winding 160. The magnitude of the voltage across winding 160 of transformer 100 is equal to the mutual inductance between primary winding 125 and winding 160 multiplied by the current flow through the winding 125 and 2 times the discharge frequency. It is to be understood that the electromotive forces across windings 125 and 160 of transformer 100 are opposite in phase and current flowing in windings 140, 150 as well as 160 are such that it opposes changes in magnetic flux in primary winding 125. This has the effect of reducing the impedance of primary winding 125 and allows a larger current to flow in the loop inducing such. Thus when a current flows in any secondary winding, the current in primary winding 125 increases. When discharge tube 66 is removed from the circuit, there is no current flow in windings 140, 150 or 160 and the impedance of primary winding 125 increases which substantially reduces the current flow in the circuit loop including primary winding 125, first section 46 and transistor 72. Switching network 13 further includes switching diode 38 which is coupled in parallel relation to the base emitter junction of transistor 72 and as is seen, the polarity of switching diode 38 is provided to prevent negative voltage from damaging transistor 72. Switching diode 38 may be a commercially available 1N4005 type and is connected such that its polarity is opposite to that formed by the base-emitter junction of transistor 72. Primary winding 42 of inverter transformer 40 is a tapped winding which is connected in an autotransformer configuration such that the voltage induced in primary winding second section 44 is coupled in series relation and adds to the voltage across primary winding first section 46. The total voltage across primary winding 42 is coupled to capacitor 62 which is connected in series relation with primary winding 42. Obviously, as seen in the Figure, capacitor 62 is coupled on a first end to primary winding 42 of inverter transformer 40 and is further coupled on a second end to return line 64. For purposes of the embodiment herein described, capacitor 62 may be a 3.5 nanofarad, 1.0 kilovolt Mylar capacitor, for discharge during transistor 72 "off" cycle of energy stored in inverter transformer 40 during transistor 72 "on" cycle. Primary winding 125 of no load protection transformer 100, as previously described, is one element in the series combination of elements through which the transistors 72 collector current flows. Thus, the magnitude of the current in each of those elements is identical and the maximum value for that current is equal to the DC supply voltage on output line 36 divided by the sum of the series impedances in the current path. The impedance of primary winding 125 of transformer 100 is a function of the impedance of the winding itself and the impedance which is reflected from the secondary windings. When the tuned high voltage output secondary winding 160 is loaded, the resulting impedance of primary winding 125 is at a minimum value. The filament excitation windings 140 and 150 have a negligible effect on the reflected impedance, since each is composed of only one turn. With the impedance of primary winding 125 at its minimum value, the collector current is therefore at its maximum, assuming the DC supply voltage on line 36 is at its designed value. When the gas discharge tube 66 is no longer connected to electronic ballast system 10, the resulting reflected impedance of tuned high voltage secondary winding 160 in the primary winding 125 becomes very high. The now high series impedance of primary winding 125 causes the collector current to be proportionally reduced. Thus, the energy stored in transformer 100 is likewise proportionately reduced and the resulting voltage across the output terminals of winding 160 will be reduced as well. Primary winding 125 of no load protection transformer 100 therefore functions as a variable impedance which is inversely proportional to the load seen on the secondary winding 160 of transformer 100. Since the impedance of primary winding 125 is substantially all inductive, there is very little dissipation in the form of heat in primary winding 125 to effect the efficiency or component life of electronic ballast system 10. Referring still to the Figure of self-regulating, no load protected electronic ballast system 10, it is seen that when this system is in oscillation and transistor 72 is in an "on" state, the collector current which is the system driving current flows from power source 12 through filter network 11 including rectification circuit 16, through primary winding 125 of transformer 100 via line 26 to line 36. DC power output line 36 passes through the axis of toroidal transformer 56 and is coupled to one end of primary winding 42 of inverter transformer 40. The driving current flows through first section 46 of primary winding 42 to tap line 25 where it is coupled to collector element 74 of switching transistor 72. Transistor 72 being in an "on" state allows current to pass from collector element 74 to emitter element 76 and back to the return of the power supply through line 64. This current being of an increasing nature, induces a voltage in bias control winding 48 which is wound in a direction to produce a voltage on base element 78 of transistor 72 which is positive with respect to emitter element 76 and greater than approximately 0.7 volts which is required to keep transistor 72 in an "on" state, thus, the voltage produced in bias control winding 48 reinforces the "on" condition of transistor 72. The collector current increases in a substantially linear manner until a maximum value is reached. The maximum value is a function of the power supply voltage and the impedance of the collector circuit. Thus, when the collector current flows through first section 46 of primary winding 42, during the transistor "on" state, a magnetic flux is generated within the core of inverter transformer 40 which induces the voltage in secondary winding 48 reinforcing the "on" condition and provides the base drive current. The same driving current also flowing through primary winding 125 of transformer 100 generates a magnetic flux within the core of transformer 100 which induces the voltages in all of the secondary windings of no load protection transformer 100. The induces voltages in secondary windings 140 and 150 provide the heater current for respective filaments 68 and 70 of gas discharge tube 66. The voltage induced in tuned high voltage output secondary winding 160 is coupled to tuning capacitor 130 and gas discharge tube 66 for generation of visible light output from gas discharge tube 66. The induced voltage in second section 44 of primary winding 42 of inverter transfomer 40 being monopolar only charges capacitor 62 during this portion of the cycle. When transistor 72 is turned to an "off" state, the collector current which was flowing through first section 46 of inverter transfomer primary winding 42 and primary winding 125 of no load protection transformer 100 terminates abruptly. The rapid change in collector current induces voltages again in second section 44 of inverter transformer primary winding 42 and secondary winding 48 as well as secondary windings 150, 140 and 160 of transformer 100. As is known from classical theory, the polarity of the voltages induced by the rapid collapse of the collector current is such that transformer 40 and transformer 100 attempt to maintain the direction of the original current in the respective primary windings 46 and 125. Due to the direction of current flow in windings 46 and 48 as indicated by nomenclature dots 77, the voltage induced in bias control winding 48 of inverter transformer 40 is of the opposite polarity as previously described when the collector current was flowing. Thus, a negative signal on base element 78 with respect to emitter 76 is generated and transistor 72 is switched to an "off" condition. It is of importance to maintain a relatively uniform gain of transistors used in self-regulating, no load protected electronic ballast system 10 such that light output may be relatively consistent from one particular unit to another within a range of approximately ± 3.0%. However, due to normal manufacturing techniques known in the industry, the gain of transistors 72 may vary between 10.0 - 50.0 or greater. Thus, a self-regulation control is a requirement as an advantage over having to manually adjust gain control elements or in the alternative, to preselect devices within a small tolerance in order to obtain an output of light which is relatively constant from one electronic ballast system 10 to another. Self-regulating, no load protected electronic ballast system 10 makes use of the concept of a variable inductance in the form of toroidal core 27 wound with 12 turns in which the base current passes. Line 36 passes through the axis the toroidal core 27 which carries the collector current of transistor 72. The direction in which current flows through the two windings is such that the respective magnetic fields are additive within toroidal core 27 of toroidal transformer 56. Therefore, the inductance which is seen in first winding 55 of toroidal transformer 56 is a function of both the base current and the collector current multiplied by the respective turn ratios and the permeability of magnetic core 27. In actual practice, the inductance variations of second winding 57 of toroidal transformer 56 may be neglected since second winding 57 is formed of only a single turn and winding 57 inductance is relatively low as well as coupled in series with the inductance of first section 46 of primary winding 42. The inductance of second winding 57 has been found not to be significant when compared with the inductance of first section 46 of primary winding 42 which is substantially larger in absolute value. In order to insure oscillation within self-regulating, no load protected electronic ballast system 10 of switching transistor 72, bias control winding 48 is specifically designed to supply sufficient voltage to turn "on" transistor 72 of the lowest gain which may be expected to be obtained from a manufacturer of these systems. In this manner, it is assured that transistor 72 will go to an "on" state and reach saturation and thus, the base to emitter voltage will be at least 0.7 volts required to switch transistor 72 to the saturation state. Regardless of the gain of transistor 72 used in self-regulating, no load protected electronic ballast system 10, the collector voltage and collector circuit impedance is substantially the same and thus, the substantially same collector current will flow whether a transistor with a gain of 10.0 or 50.0 is being utilized. Therefore, since the base current is a function of the collector current divided by the gain of transistor 72, it is seen that the base current must change if a transistor 72 of different gain value is to be used and function properly in self-regulating, no load protected electronic ballast system 10. Where the base current changes, then an electronic element in the base circuit must change its impedance value which is the function of self-regulating circuit 17 and primary first winding 55 of toroidal transformer 56. In order to achieve self-regulation, the design of toroidal transformer 56 is such that the maximum permeability of core 27 is reached with a transistor whose gain is at a maximum expected value. Likewise, the inductance of first winding 55 of toroidal transformer 56 will therefore be at a maximum and hence a minimum current will flow through the base circuit for transistor 72. The impedance of a winding having a magnetic core is related to the number of turns of the winding and current flowing therethrough, as well as inversely to the length of the magnetic path in the core. The point of operation may be adjusted by either changing the size of the toroid or by inserting parallel resistor 51 in parallel relation with toroid first winding 55 for adjustment of the corresponding exciting field. A value of approximately 200 Ohms has been successfully used for parallel resistor 51. Thus, with first winding 55 of toroidal transformer 56 being at a maximum value of inductance, its impedance is significantly larger than the impedance of base coupling capacitor 54 such that it is the controlling factor in limiting the current to base element 78 of transistor 72. With transistor 72 having a maximum gain value, little current is needed, and for example, if the gain or beta of transistor 72 is 50.0, then it is seen that the base current is 1/50th of the collector current. However, the voltage induced in base drive winding 48 has been designed to turn "on" a transistor of lower gain and therefore, there is excess energy to be dissipated in the base circuit of transistor 72. The excess energy is stored in first winding 55 of toroidal transformer 56. This impedance of first winding 55 is primarily inductive as opposed to resistive, and there is little dissipation in the form of heat, and thus, there is provided an efficient means of dissipating the excess energy which is liberated when transistor 72 is in an "off" state. In opposition, when a transistor of low gain is used in self-regulating, no load protected electronic ballast system 10, the base current obviously must increase and the permeability of core 27 of toroidal transformer 56 shifts in a downward direction to a lower value than would be measured for a high gain transistor and the inductance is less than was seen with a high gain transistor. Thus, the series impedance is reduced which allows a greater base current to flow and compensates for the low gain transistor 72 being used in system 10. Further, with tuned no load protection transformer 100 being series coupled to induction circuit 15 for preventing the generation of voltages above a predetermined value when gas discharge tube 66 is electrically removed from the circuit. The primary winding 125 of tuned no load protection transformer 100 forms a variable inductance which is inversely proportional to the magnitude of the power delivered to gas discharge tube 66. The variable impedance seen in primary winding 125 is a function of the impedance reflected from the tuned output winding 160. That impedance being a function of the number of turns in winding 160, the parallel capacitance of tuning capacitor 130, the magnetic circuit length in the core of transformer 100, and the current in secondary winding 160 which is the load current of gas discharge tube 166. Thus, a change in load current as seen when the gas discharge tube is no longer in the circuit, effects the impedance of winding 160 and is reflected into the impedance of the primary winding 125. By creating a variable inductance whose impedance is inversely proportional to the load current, a limited voltage can be generated by control of the collector current responsive to the load conditions.

Claims: An electronic ballast system for actuating one or more gas discharge tubes from a power source, comprising: a filter circuit (11) connectable to the power source (12) for producing a substantially smooth direct-current output and for suppressing feedback to the power source of harmonic frequencies generated by the system; an oscillator (13,15,62) powered by the output of the filter circuit and including an oscillator auto-transformer (15) having a primary winding (42) connected in series with a capacitor (62) across the output of the filter circuit, the primary winding having a tapped portion (46) connected in series with a switching circuit (13) across the output of the filter circuit, and the oscillator auto-transformer (15) having a secondary winding (48) for generating a control current to the switching circuit; regulating means comprising a toroidal transformer (56) having a first winding (57) in series with the tapped portion (46) of the primary winding (42) of the auto-transformer (15), and a second winding (55) in series with the secondary winding (48) of the auto-transformer for modifying the control current produced by the secondary winding; and means for generating an output voltage across the gas discharge tube in dependence upon an oscillating current through the primary winding (42) of the auto-transformer (15); characterised in that: the output voltage generating means comprises a separate transformer (100) having a primary winding (125) connected in series with the filter circuit (11) and the oscillator, and a secondary winding (160) connectable across the gas discharge tube (66). A system as claimed in claim 1, characterised in that the separate transformer (100) has a multiplicity of such secondary windings (140,150,160) connectable to the gas discharge tube (66). A system according to claim 2, characterised in that the multiplicity of secondary windings of said separate transformer (100) includes a tuned high voltage output secondary winding (160) and a pair of filament excitation windings (140, 150), each of the filament excitation windings being coupled to respective filaments (68, 70) of the gas discharge tube (66). A system according to claim 3, characterised in that the tuned high voltage secondary winding (160) is coupled in parallel relation to both a tuning capacitor (130) and to the gas discharge tube (66), the tuned high voltage secondary winding (160) generating a reduced voltage when the gas discharge tube is decoupled from the system. A system according to claim 4, characterised in that the separate transformer (100) includes a ferrite core. A system according to any one of claims 1 to 5, characterised in that the switching circuit (13) includes a transistor (72) for cycling the current through the auto-transformer, the transistor (72) including a base element (78), a collector element (74), and an emitter element (76) coupled to the power source (12). A system according to claim 6, characterised in that the first winding (57) of the toroidal transformer (56) in the regulation means is connected to the tapped primary winding (42) of the auto-transformer (15) and to the emitter element (76) of the transistor (72) in series relation each with respect to the other. A system according to claim 6, characterised in that the regulation means comprising the toroidal transformer (56) is coupled to the auto-transformer (15) and to the transistor (72) of the switching circuit (13) in series relation. A system according to claim 8, characterised in that the toroidal transformer (56) provides a predetermined variable inductance for regulating a power output to the gas discharge tube (66). A system according to any one of claims 1 to 9, characterised in that the regulation means includes a base coupling capacitor (54) connected on opposing ends thereof to the second winding (55) of the toroidal transformer (56) and to the secondary winding (48) of the auto-transformer (15) for substantially blocking a direct current component.

Inventors: HANLET, JACQUES MARIE
Applicants: INTENT PATENTS A.G.
Application year: 1988
Main classification: H05B41
IPC codes: H05B41, H02M7