Abstract:
A full-bridge inverter ballast for a fluorescent lamp comprises two pairs of switching transistors and is conditionally operable to self-oscillate in either of two modes: a first mode wherein one of the two pairs of switching transistors self-oscillates in manner of a half-bridge inverter and powers the thermionic cathodes of the fluorescent lamp, and a second mode wherein both pairs of transistors self-oscillate in manner of a full-bridge inverter and then powers the main gas discharge of the fluorescent lamp. The first mode gets initiated immediately upon applying power to the ballast, but the second mode does not get initiated until about 1.5 seconds after the initiation of the first mode. The way, the cathodes will have become fully thermionic prior to applying main power to the fluorescent lamp.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a bridge inverter ballast for a fluorescent lamp having thermionic cathodes that need to be pre-heated before the lamp is properly ready to be ignited. 
     2. Prior Art 
     A typical fluorescent lamp have thermionic cathodes that need to be brought to incandescence before the lamp is properly operable be ignited--that is, ready to receive its main operating power. 
     When, as frequently is desirable to do, such a lamp is powered by way of being parallel-connected with the tank capacitor of a high-Q resonant circuit that is series-excited by the voltage output of an inverter power supply, a problem arises: before the lamp is properly operative to absorb power, the high-Q resonant circuit is effectively unloaded and it therefore represents an effective short circuit across the inverter&#39;s voltage output. This not only causes an excessively large power drain from the inverter, but also causes excessively large-magnitude voltages to develop across the circuit elements of the resonant circuit. 
     One way of circumventing this problem is that of providing the requisite cathode heating power from a separate source of power, and to delay the turning-on of the inverter power supply until after the cathodes have reached incandescence. However, this approach requires the use of an extra power supply, and is therefore not as cost-effective as might be desired. 
     SUMMARY OF THE INVENTION 
     Objects of the Invention 
     A basic object of the present invention is that of providing a cost-effective inverter-type ballast for a fluorescent lamp. 
     A more specific object is that of providing a cost-effective inverter power supply operative to power a fluorescent lamp that is parallel-connected with a series-excited high-Q resonant L-C circuit, and wherein the lamp requires conditioning prior to being able to operate properly as a load. 
     This, as well as other objects, features and advantages of the present invention will become apparent from the following description and claims. 
     BRIEF DESCRIPTION 
     A full-bridge inverter comprises a first and a second pair of seires-connected switching transistors connected across a source of DC voltage. The inverter is conditionally operable to self-oscillate in either of two modes: a first mode wherein the first pair of switching transistors self-oscillates in manner of a half-bridge inverter and powers a first load, and a second mode wherein both pairs of transistors self-oscillate in manner of a full-bridge inverter and then powers a second load in addition to the first load. 
     The inverter&#39;s self-oscillation is accomplished by way of positive current feedback using saturable current transformers connected in circuit with the loads. The first load is connected between the center-junction of the first pair of series-connected transistors and the center-junction of two series-connected capacitors connected across the DC source. The second load is connected between the center-junctions of the two pairs of series-connected transistors. 
     The inverter is of such nature as to have to be triggered into oscillation. By triggering one of the transistors in the first pair of transistors, that first pair of transistors starts self-oscillating action and operates as a half-bridge inverter in combination with the series-connected capacitors. The other pair of series-connected transistors remains non-conductive and non-active until one of its transistors is properly triggered, after which point both pairs of transistors oscillate synchronously in ordinary bridge manner. 
     In the preferred embodiment, the first load amounts to only about 2 Watt and consists of the thermionic cathodes of a fluorescent lamp. The second load amounts to about 40 Watt and consists of the main gas discharge path of the fluorescent lamp. This main gas discharge path is connected across the tank capacitor in a series-connected L-C circuit; which series-connected L-C circuit is resonant at the inverter&#39;s oscillating frequency and connected between the center-junctions of the two pairs of transistors. 
     As long as the first load requires but a modest amount of power compared with that of the second load, the total power-handling capability of the dual-mode inverter is effectively determined by the sum total power-handling capabilities of all the four transistors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 provides a schematic diagram of the electrical circuitry of the preferred embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Details of Construction 
     FIG. 1 shows a source of voltage S, which represents an ordinary 120 Volt/60 Hz electric utility power line. Connected across the output terminals of S, by way of a bridge rectifier BR, is a dual-mode inverter power supply DMIPS. The positive and negative output terminals of bridge rectifier BR provide a DC voltage between a B+ bus and a B- bus, respectively. 
     A first capacitor C1 is connected between the B+ bus and a junction JC; a second capacitor C2 is connected between junction JC and the B- bus. 
     The collector of a first transistor Qa1 is connected with the B+ bus; and the emitter of this same transistor is connected with a junction JQa. The collector of a second transistor Qa2 is connected with junction JQa; and the emitter of this same transistor is connected with the B- bus. 
     The collector of another first transistor Qb1 is connected with the B+ bus; and the emitter of this same transistor is connected with a junction JQb. The collector of another second transistor Qb2 is connected with junction JQb; and the emitter of this same transistor is connected with the B- bus. 
     The terminals of a secondary winding CTa1s of a first current transformer CTa1 are connected between the base and emitter of transistor Qa1; and the terminals of a secondary winding CTa2s of a second current transformer CTa2 is connected between base and emitter of transistor Qa2. 
     The terminals of a secondary winding CTb1s of another first current transformer CTb1 are connected between the base and emitter of transistor Qb1; and the terminals of a secondary winding CTb2s of another second current transformer CTb2 is connected between base and emitter of transistor Qb2. 
     The primary windings of current transformers CTa1 and CTa2 are connected in series between a junction JXa and junction JQa; and the primary windings of current transformers CTb1 and CTb2 are connected in series between a junction JXb and junction JQb. 
     The terminals of a primary winding PTp of a power transformer PT are connected between junctions JXa and JC. The terminals of a first secondary winding PTas of power transformer PT are connected with the terminals of a first thermionic cathode TCa of fluorescent lamp FL; and the terminals of a second secondary winding PTbs of power transformer PT are connected with the terminals of a second thermionic cathode TCb of fluorescent lamp FL. 
     A resistor Ra is connected between the B+ bus and a junction Ja. A capacitor Ca is connected between junction Ja and the B- bus. A Diac Da is connected between junction Ja and the base of transistor Qa2. 
     A resistor Rb1 is connected between the B+ bus and a junction Jb1. A capacitor Cb1 is connected between junction Jb1 and the B- bus. A diode Db1 is connected with its anode to junction Jb1 and with its cathode to the base of an auxiliary transistor AQ. The collector of auxiliary transistor AQ is connected with the B+ bus; and the emitter of auxiliary transistor AQ is connected with a junction Jb2 by way of a resistor Rb2. A capacitor Cb2 is connected between junction Jb2 and the B- bus; and a Diac Db2 is connected between junction Jb2 and the base of transistor Qb2. 
     An inductor L is connected between junction JXb and a junction Jo; and a capacitor C is connected between junction Jo and junction JXa. Thermionic cathode TCa of fluorescent lamp FL is connected with junction Jo; and thermionic cathode TCb of fluorescent lamp FL is connected with junction JXa. A Varistor V is connected between junctions JXa and Jo. 
     The assembly principally consisting of transistors Qa1 and Qa2 and current transformers CTa1 and CTa2 is referred to as the first half-bridge inverter. The assembly principally consisting of transistors Qb1 and Qb2 and current transformers CTb1 and CTb2 is referred to as the second half-bridge inverter. The B+ bus and the B- bus--as well as junction JC, which effectively constitutes a center-tap for the DC supply--are common to the two half-bridge inverters. 
     Details of Operation 
     The operation of the dual-mode self-oscillating bridge inverter and ballast circuit of FIG. 1 is explained as follows. 
     Within a few milli-seconds after power is applied to dualmode inverter power supply DMIPS, trigger pulses starts being provided to the base or transistor Qa2 by way of the trigger circuit consisting of resistor Ra, capacitor Ca and Diac Da. These trigger pulses initiate conduction of transistor Qa2; which, in turn, starts a cycle of positive feedback by way of saturable current transformers Qa1 and Qa2, thereby initiating the series-connected transistors Qa1 and Qa2--as combined with power transformer PT and center-tapped capacitors C1 and C2--into self-oscillation in manner of an ordinary half-bridge inverter; the operation of which is explained in detail in U.S. Pat. No. Re. 31,758 and 4,506,318 to Nilssen. 
     The output from this first half-bridge inverter, which is a substantially squarewave voltage at a frequency of about 30 kHz, exists without affecting the other pair of series-connected transistors Qb1 and Qb2 (i.e, the second half-bridge inverter) --as long as these Qb1/Qb2 transistors remain non-conductive. And, of course, these other transistors do remain non-conductive for as long as no current is provided to their bases; which is for as long as no trigger pulse is applied to the base of transistor Q2b. 
     However, some time after trigger pulses started to be provided to the base of transistor Qa1, trigger pulses start to be provided at the base of transistor Qb2 as well; which means that transistor-pair Qb1/Qb2 starts to get involved in the positive feedback cycle and thereby in the inverter action. Thus, from that point and forward, a 30 kHz squarewave output voltage is provided between junctions JXa and JXb. Of course, the output voltage between JXa and JC remains substantially unaffected. 
     The time required before trigger pulses starts being provided to the base of transistor Qb2 depends on the time it takes to charge capacitor Cb1 to the point where the voltage across it reaches a magnitude high enough to cause auxiliary transistor AQ to start to conduct. Thus, by adjusting the capacitance value of Cb1 and/or the resistance value of Rb1, it is possible to select substantially any desirable amount of delay between the onset of oscillation of the half-bridge inverter consisting of transistors Qa1/Qa2, capacitors C1/C2 and power transformer PT, and the onset of oscillation of the full-bridge inverter consisting of transistors Qa1/Qa2, Qb1/Qb2 and the main load circuit consisting of the resonant L-C circuit and fluorescent lamp FL. 
     For the preferred embodiment of FIG. 1, the delay time was chosen to be about 1.5 seconds; which represents the length of time it takes for thermionic cathodes TCa and TCb to reach full thermionic emission. 
     Additional Comments 
     (a) For more information in respect to the operation of a full-bridge self-oscillating inverter, reference is made to U.S. Pat. No. 4,502,107 to Nilssen. 
     (b) For detailed information in respect to the operation of a series-excited L-C circuit powering a gas discharge lamp, reference is made to U.S. Pat. No. 4,538,095 to Nilssen. 
     (c) The four saturable current transformers CTa1, CTa2, CTb1 and CTb2 are nominally of identical construction. 
     (d) By placing a control transistor in shunt across the base-emitter junction of transistor Qb2, the oscillation of the full-bridge inverter can be controlled OFF and ON (i.e., stopped and started) by causing that transistor to constitute a substantial short circuit or a substantial open circuit, respectively. The control input would be the base of the control transistor. Of course, even if the full-bridge oscillation were to be disabled in this manner, the half-bridge oscillation would continue unaffectedly. 
     (e) The full-bridge inverter of FIG. 1 actually consists or two half-bridge inverters, either of which can be made to operate independently of the other, as long as the other is maintained in a non-operative state. When both half-bridge inverters oscillate, they are bound by their common feedback current to operate in synchronism and out-or-phase with one another. If required, some other load may be connected between the JXb/JC junctions--in direct correspondence with the load (namely power transformer PT) connected between the JXa/JC junctions. 
     In other words, the JC junction constitutes a common output terminal for the two half-bridge inverters. Thus, the output from the one half-bridge inverter is from junction JXa with respect to the common terminal JC; and the output from the other half-bridge inverter is from junction JXb with respect to the same common terminal JC. 
     When both half-bridge inverters are in operation, the output from the then resulting full-bridge inverter is simply between junction JXa and JXb--with the common terminal JC only being operative to provide a return path for any unbalanced AC in the total load. (Of course, such unbalanced AC would result from any load connected with power transformer PT.) 
     (f) 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 presently preferred embodiment.