Abstract:
A self-oscillating inverter-type fluorescent lamp ballast has two modes of operation: (a) a first mode in which the inversion frequency is about 70 kHz and is resonant with a first tuned circuit by which power is supplied to the cathodes of the fluorescent lamp; and (b) a second mode in which the inversion frequency is about 30 kHz and is resonant with a second tuned circuit by which main lamp power is supplied. When the ballast is initially powered-up, it starts operation in its first mode, thereby providing cathode heating power without yet providing main lamp power. About one second later, after the cathodes have reached full incandescence, the inverter automatically changes into its second mode, thereby providing main lamp power while at the same time removing cathode heating power. If for some reason the lamp were not to ignite within about 10 milli-seconds, the inverter reverts back into its first mode; thereafter cycling between its two modes until the lamp does ignite.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electronic ballasts for gas discharge lamps, particularly to ballasts wherein the load is powered by way of a series-excited parallel-loaded resonant L-C circuit. 
     2. Description of Prior Art 
     There are two predominant types of electronic ballasts for gas discharge lamps: (a) a first type may be referred-to as the parallel-resonant type and involves the use of a current-excited (i.e., parallel-excited) parallel-loaded resonant L-C circuit; and (b) a secnd type that may be referred-to as the series-resonant type and involves the use of a voltage-excited (i.e., series-excited) parallel-loaded resonant L-C circuit. 
     An example of the parallel-resonant type of electronic ballasts is described in U.S. Pat. No. 4,277,726 to Burke. An example of the series-resonant type of electronic ballasts is described in U.S. Pat. No. 4,538,095 to Nilssen. 
     Of these two types of electronic ballasts, the parallel-resonant type is conducive to yielding a stable easy-to-control self-oscillating inverter-type ballast; whereas the series-resonant type, although potentially simpler and more efficient, is harder to control in that it has a natural tendency to self-destruct in case the lamp load be removed. 
     To mitigate this tendency to self-destruct under no-load conditions, various protection circuits have been developed, such as for instance described in U.S. Pat. No. 4,638,562 to Nilssen. 
     GENERAL PURPOSE OF PRESENT INVENTION 
     The general purpose of the present invention is that of providing a method for cost-effectively controlling the operation of a series-resonant electronic inverter-type ballast for fluorescent lamps. 
     SUMMARY OF THE INVENTION 
     1. Objects of the Invention 
     An object of the present invention is the provision of a cost-effective control arrangement for attaining proper operation of an electronic ballast wherein the lamp load is powered by way of a series-excited predominantly parallel-loaded resonant L-C circuit. 
     This as well as other objects, features and advantages of the present invention will become apparent from the following description and claims. 
     2. Brief Description 
     A self-oscillating inverter-type fluorescent lamp ballast has two modes of operation: (a) a first mode in which the inversion frequency is about 70 kHz and is resonant with a first tuned L-C circuit by which power is supplied to the cathodes of the fluorescent lamp; and (b) a second mode in which the inversion frequency is about 30 kHz and is resonant with a second tuned L-C circuit by which main lamp power is supplied. 
     When the ballast is initially powered-up, it starts operation in its first mode, thereby providing cathode heating power without yet providing main lamp power. About one second later, after the cathodes have reached full incandescence, the inverter automatically changes into its second mode, thereby providing main lamp power while at the same time removing cathode heating power. If for some reason the lamp were not to ignite within about 10 milli-seconds, the inverter reverts back into its first mode; thereafter cycling (with a period of about one second) between its two modes until the lamp does ignite. 
     Thus, the first tuned L-C circuit is resonant at 70 kHz; and, due to inherent frequency-selectivity characteristics, this first tuned circuit provides cathode heating power only when being excited at or near 70 kHz. Likewise, the second tuned L-C circuit provides main lamp starting voltage and operating power only when being excited at or near 30 kHz. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawing diagrammatically illustrates the circuit arrangement of the invention in its preferred embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     1. Details of Construction 
     The drawing schematically illustrates the preferred embodiment of the invention in the form of a half-bridge inverter-type two-mode electronic ballast for a fluorescent lamp. 
     In the drawing, 277 Volt/60 Hz power line voltage from an ordinary electric utility power line PL is provided to the AC power input terminals of a rectifier and filter means RFM, the DC output from which is applied between a B+ bus and a B- bus. 
     A filter capacitor FCa is connected between the B+ bus and a junction J1; a filter capacitor FCb is connected between junction J1 and the B- bus. A tank capacitor TC is connected between junction J1 and a junction J2. An auxiliary inductor AI is connected between junction J2 and a junction J3; and a main tank inductor TI is connected between junction J3 and a junction J4. 
     Junction J4 is connected with a junction J5 by way of series-connected primary windings SCTap and SCTbp of saturable current transformers SCTa and SCTb, respectively. 
     A first main inverter transistor Qa is connected with its collector to the B+ bus and with its emitter to junction J5; a second main inverter transistor Qb is connected with its collector to junction J5 and with its emitter to the B- bus. 
     Secondary winding SCTas of saturable current transformer SCTa is connected between the base of transistor Qa and a junction Ja. A capacitor Ca is connected between junctions Ja and J5. A Zener diode Za is connected with its anode to junction Ja and with its cathode to junction J5. An auxiliary transistor AQa is connected with its collector to junction Ja and with its emitter to junction J5. A resistor Ra is connected between the B+ bus and the base of transistor Qa. The base of auxiliary transistor AQa is designated a. 
     Secondary winding SCTbs of saturable current transformer SCTb is connected between the base of transistor Qb and a junction Jb. A capacitor Cb is connected between junction Jb and the B- bus. A Zener diode Zb is connected with its anode to junction Jb and with its cathode to the B- bus. An auxiliary transistor AQb is connected with its collector to junction Jb and with its emitter to the B- bus. A resistor Rb is connected between junction J5 and the base of transistor Qb. The base of auxiliary transistor AQb is designated b. 
     Tank inductor TI has a secondary winding SWt, which has a center tap CTt connected with the collector of a control transistor CQ. The emitter of control transistor CQ is connected with the B- bus. 
     The terminals of secondary winding SWt are connected with the cathodes of two diodes D1 and D2; whose anodes are connected with the terminals of a secondary winding SWc of a control transformer Tc; which secondary winding has a center tap CTc connected with the B- bus. 
     A fluorescent lamp FL has two thermionic cathodes TCx and TCy; which has power input terminals x--x and y--y, respectively. One of the power input terminals of cathode TCx is connected with junction J1 by way of primary winding PWc of control transformer Tc. One of the power input terminals of cathode TCy is connected with junction J2. 
     Power input terminals x--x and y--y of cathodes TCx and TCy are connected with power output terminals x--x and y--y of secondary windings SWx and SWy of auxiliary inductor AI, all respectively; which secondary windings have series-connected capacitors Cx and Cy, also respectively. 
     A resistor R1 is connected between the B+ bus and a junction J6; and a capacitor C1 is connected between junction J6 and the B- bus. A resistor R2 and a Diac D4 are connected in series between junction J6 and the base of control transistor CQ. A resistor R3 is connected between the base of transistor CQ and the B- bus. 
     A resistor R4 is connected between junction J6 and the collector of a transistor Qc, whose emitter is connected with the B- bus. A resistor R5 is connected between the base of transistor Qc and the B- bus. A resistor R6 is connected between the base of transistor Qc and a junction J7. A capacitor C2 is connected between junction J7 and the B- bus. A diode D3 is connected with its anode to the anode of diode D2 and with its cathode to junction J7. 
     Control transformer Tc also has two secondary windings SWa and SWb. The terminals of secondary winding SWa are connected between base a of transistor AQa and junction J5; the terminals of secondary winding SWb are connected between the B- bus and base b of transistor AQb. 
     2. Details of Operation 
     The operation of the circuit arrangement schematically illustrated by the drawing may be explained as follows. 
     In the arrangement of the drawing, ordinary 277 Volt/60 Hz power line voltage is provided from the power line (PL) and is rectified and filtered by conventional rectifier and filter means RFM such as to provide a DC voltage between the B+ and the B- buses, with the B+ bus carrying the positive polarity. 
     The half-bridge inverter, which principally consists of capacitors FCa and FCb, transistors Qa and Qb, and saturable current feedback transformers SCTa and SCTb, is self-oscillating and functions in a substantially ordinary manner, such as for instance described in conjunction with FIG. 8 of U.S. Pat. No. Re. 31,758 to Nilssen. 
     The output of the half-bridge inverter is provided to and between junctions J1 and J4; between which junctions are connected in series: tank capacitor TC, tank inductor TI, and auxiliary inductor AI. 
     Auxiliary inductor AI is tuned to about 70 kHz by way of capacitors Cx and Cy; which two capacitors are connected with the secondary windings of the auxiliary inductor as well as with the loads connected to the output of these secondary windings. Thus, only when lamp cathodes TCx and TCy are indeed connected with the two secondary windings is the auxiliary inductor tuned to about 70 kHz. As a result, at about 70 kHz, the auxiliary inductor appears like a parallel-resonant circuit as viewed from between junctions J2 and J3. 
     Tank inductor TI is tuned to series-resonate with tank capacitor TC at about 30 kHz; which is to say that the total impedance between junctions J1 and J4 appears substantially like a series-resonant circuit at about 30 kHz. 
     At 30 kHz, the impedance of auxiliary inductor AI is inductive and relatively small, and is at that frequency simply considered as a small part of tank inductor TI. 
     At 70 kHz, the impedance of tank capacitor TC is capacitive and relatively small, whereas the impedance of tank inductor TI is inductive and relatively high. 
     Thus, when the inverter oscillates at 70 kHz, its output voltage is applied by way of high-impedance tank inductor TI to the parallel-resonant circuit represented by auxiliary inductor AI; which parallel-resonant circuit then operates to power the two thermionic cathodes of fluorescent lamp FL. During this mode, the power provided to these two cathodes is about two watts, and the magnitude of the current then drawn from the inverter output is quite small. As a result, the magnitude of the 70 kHz voltage resulting across tank capacitor TC is very small. 
     On the other hand, when the inverter oscillates at 30 kHz, essentially no power is provided to the thermionic cathodes. However, at that frequency, the resonant series-tuned L-C circuit then loading the inverter&#39;s output causes a 30 kHz voltage of very large magnitude to develop across the tank capacitor. The magnitude of this 30 kHz voltage is so large as to cause the fluorescent lamp to ignite; whereafter the magnitude of the 30 kHz voltage across the tank capacitor will be determined by the current-voltage characteristices of the fluorescent lamp. In reality, at the 30 kHz series-resonance, the output provided from the output terminals to the fluorescent lamp (i.e., from junctions J1 and J2) will essentially be a 30 kHz constant-magnitude current. 
     The frequency of inverter oscillation is determined by the saturation characteristics of saturable current transformers SCTa and SCTb in conjunction with the magnitude of the voltage presented to their secondary windings SCTas and SCTbs. 
     The magnitude of the voltage presented to secondary windings SCTas and SCTbs will be determined by the base-emitter voltage of transistors Qa and Qb in combination with the magnitude of the voltage present at junctions Ja and Jb as referenced to the emitters of transistors Qa and Qb, respectively. 
     With no control signals provided to the bases a and b of auxiliary transistors AQa and AQb, the magnitude of the voltage at junctions Ja and Jb will be determined by the Zener voltages of Zener diodes Za and Zb; which Zener voltages are chosen to be about 4.0 Volt each. However, when sufficient control current is provided to each of bases a and b, transistors AQa and AQb become conductive and therefore operative to shunt Zener diodes Za and Zb, thereby to cause the magnitudes of the voltages at junctions Ja and Jb to become very low (about 1.0 Volt each). 
     Thus, absent control currents at bases a and b, the inverter will oscillate at about 70 kHz; whereas, with control currents, the inverter will oscillate at about 30 kHz. 
     When the inverter is initially powered-up, no lamp current is flowing through the primary winding of control transformer Tc and control transistor CQ is non-conductive; which means that no control currents are provided to bases a and b of transistors AQa and AQb. Thus, when initially powered-up, the inverter will initiate oscillations at a frequency of about 70 kHz. 
     However, after about one second, capacitor C1 will have reached a voltage high enough to cause Diac D4 to break down; which, in turn, causes capacitor C1 to discharge into the base of control transistor CQ, thereby causing this transistor to become conductive. 
     Control transistor CQ will remain conductive for a period of about 10 milli-seconds; and, during this period, current from secondary winding SWt of tank inductor TI will flow through secondary winding SWc of control transformer Tc; thereby--via secondary windings SWa and SWb on control transformer Tc--providing control currents to bases a and b of transistors AQa and AQb; thereby causing capacitors Ca and Cb to discharge to a voltage level of about 1 Volt; thereby, in turn, to cause the inverter&#39;s oscillating frequency to become about 30 kHz. 
     With the inverter frequency at 30 kHz, the magnitude of the voltage provided between the lamp&#39;s cathodes becomes large enough to cause lamp ignition within the 10 milli-second period; which, in turn, gives rise to the flow of lamp current; which lamp current flows through primary winding PWc of control transformer Tc, thereby continuing to provide control currents to bases a and b of transistors AQa and AQb; thereby continuing to maintain the inverter&#39;s oscillation frequency at 30 kHz. 
     On the other hand, if the fluorescent lamp were to fail to ignite within the 10 milli-second time-window during which the control transistor CQ be conductive, control currents to bases a and b would not be sustained; thereby causing the inverter to revert to its 70 kHz oscillating frequency. 
     In short, with a properly operational fluorescent lamp connected, the ballast arrangement of the drawing operates as follows. 
     (1) Upon initial connection to the power line, the inverter starts oscillating at a 70 kHz frequency; which, via a 70 kHz resonating circuit associated with auxiliary inductor AI, therefore causes cathode heating power to be provided to the thermionic cathodes of the fluorescent lamp. 
     (2) After about one second, at which time the cathodes are fully thermionic, control transistor CQ suddenly becomes conductive and thereafter remains conductive for a period of about 10 milli-seconds. With transistor CQ conductive, control current is provided to auxiliary transistors AQa/AQb; which then become conductive, thereby to cause a reduction in the magnitudes of the voltages across capacitors Ca/Cb; which, in turn, causes the frequency of inverter oscillation to reduce to 30 kHz and to remain at 30 kHz for at least 10 milli-seconds. 
     (3) With the inverter oscillating at 30 kHz, series-resonance occurs between tank capacitor TC and tank inductor TI (including the net inductance of auxiliary inductor AI and its associated circuitry); which series-resonance, due to so-called Q-multiplication effects, results in a high-magnitude 30 kHz voltage developing across the tank capacitor. 
     (4) The high-magnitude 30 kHz voltage developing across the tank capacitor is applied across the fluorescent lamp and, because its cathodes are already thermionic, causes it to ignite immediately. The resulting lamp current will then, via control transformer Tc, continue to provide control current to auxiliary transistors AQa/AQb; thereby, even after the initial 10 millisecond period, ensuring that the inverter&#39;s frequency of oscillation remains at 30 kHz. 
     (5) When the inverter is operating at 30 kHz, essentially no power is being delivered to the cathodes of the fluorescent lamp, thereby providing for improved energy efficiency as compared with the situation where cathode power be supplied on a continuous basis. 
     (6) If the lamp were to be removed or if lamp current otherwise were to fail to flow, control current would cease to be provided to auxiliary transistors AQa/AQb, thereby causing the inverter&#39;s oscillation frequency to revert to 70 kHz. Thus, as long as no lamp current is flowing, the inverter will alternate between two modes: a first mode of oscillating at 70 kHz and a second mode of oscillating at 30 kHz, spending about one second (1000 milli-seconds) at 70 kHz for each 10 milliseconds at 30 kHz. 
     3. Additional Comments 
     (a) To protect against possible self-destruction of the inverter circuit (which might occur if the circuit were to operate for a period of time without being connected with a properly functioning lamp load), it may be advantageous to connect a voltage-limiting means, such as a Varistor, in parallel with the tank capacitor. 
     (b) For further details relative to the biasing arrangement used in connection with main inverter transistors Qa/Qb, reference is made to FIG. 3 of U.S. Pat. No. 4,307,353 to Nilssen. 
     (c) By providing for additional levels of adjustment for the magnitude of the bias voltage (i.e., the voltage across capacitors Ca/Cb), corresponding adjustment of the magnitude of lamp current may be attained, thereby to provide for lamp dimming. 
     (d) It is believed that the present invention and its several attendant advantages and features will be understood from the preceeding description. However, without departing from the spirit of the invention, changes may be made in its form and in the construction and interrelationships of its component parts, the form herein presented merely representing the preferred embodiment.