Abstract:
A high-frequency electronic ballast for fluorescent lamps has a first inverter for controllably providing heating power to the lamp cathodes and a second inverter for controllably providing main lamp operating power. The two inverters are separately and independently controllable, thereby: i) to permit adjustment of lamp current so as to provide full or reduced light output in accordance with requirements, ii) to permit cathode heating power to be removed under conditions of providing full light output, thereby to maximize efficiency, and iii) to permit cathode heating power to be restored under conditions of reduced light output, thereby to prevent premature lamp failure.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to dimmable- electronic ballasts for fluorescent lamps, particularly of a kind wherein cathode heating power is removed during periods of full light output and restored during periods of reduced light output. 
     2. Elements of Prior Art 
     Fluorescent lamp ballasts designed to permit wide-range control of light output provide cathode heating at all times; otherwise, during periods of reduced light output, lamp life would be seriously foreshortened. 
     Yet, as is well known, efficiency may be significantly improved by removing cathode heating power, especially during periods of full light output. 
     As is also well known, during periods of full light output, cathode heating power may be removed without suffering serious foreshortening of lamp life. 
     SUMMARY OF THE INVENTION 
     Objects of the Invention 
     An object of the present invention is that of providing an improved controllable ballast for fluorescent lamps. 
     These, as well as other objects, features and advantages of the present invention will become apparent from the following description and claims. 
     Brief Description 
     In its preferred embodiment, the present invention constitutes a power-line-operated ballast for two fluorescent lamps. This ballast comprises: i) a main self-oscillating half-bridge inverter whose high-frequency squarewave output voltage :s frequency-adjustable and connected with a series-resonant L-C circuit, and ii) an auxiliary controllable self-oscillating half-bridge inverter whose 30 kHz output voltage is connected with the primary winding of an auxiliary transformer. 
     The lamps are series-connected across the tank capacitor of the L-C circuit. A voltage-limiting Varistor is also connected across the tank capacitor. The lamps&#39; cathodes are connected with individual outputs of the auxiliary transformer. 
     When power is initially applied to the ballast, only the auxiliary inverter is initiated into oscillation, thereby providing heating power to the lamp cathodes. About 1.5 second later, the main inverter is also initiated into oscillation, and the lamps then ignite in ordinary Rapid-Start manner. 
     The frequency, and thereby the power output, of the main inverter is adjustable by application of an adjustable control voltage. Thus, the magnitude of the arc current as well as the light output level of the fluorescent lamps are therefore correspondingly adjustable. 
     The flow of lamp arc current is used to control operation of the auxiliary inverter. At full lamp current, the oscillation of the auxiliary inverter is inhibited, thereby eliminating the flow of cathode heating power. At reduced lamp current, the auxiliary inverter is reinitiated into operation, thereby restoring the provision of cathode heating power. 
     As a consequence, substantial energy savings are possible under conditions of full light output without giving rise to any significant reduction in lamp life; yet light dimming is permitted without suffering the substantial foreshortening of lamp life associated with not providing cathode heating power during periods of reduced light output. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 provides a basic electrical circuit diagram of the preferred embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Details of Construction 
     FIG. 1 schematically illustrates the electrical circuit arrangement of the preferred version of the present invention. 
     In FIG. 1, a source S of ordinary 277 Volt/60 Hz power line voltage is applied to power input terminals PITa and PITb; which terminals, in turn, are connected with a bridge rectifier BR. The DC output from bridge rectifier BR is applied to a B+ bus and a B- bus, with the B+ bus being of positive polarity. 
     A first filter capacitor FCa is connected between the B+ bus and a junction Jc; and a second filter capacitor FCb is connected between junction Jc and the B- bus. 
     A switching transistor Q1a is connected with its collector to the B+ bus and with its emitter to a junction Jlq. 
     A switching transistor Q1b is connected with its collector to junction Jlq and with its emitter to the B- bus. 
     A saturable current feedback transformer FTla has a primary winding FTlap and a secondary winding FTlas, which secondary winding is connected across the base-emitter junction of transistor Q1a. 
     A saturable current feedback transformer FTlb has a primary winding FTlbp and a secondary winding FTlbs, which secondary winding is connected across the base-emitter junction of transistor Q1b. 
     A resistor Rlta is connected between the B+ bus and a junction Jlta; a capacitor Clta is connected between junction Jlta and the B- bus; a Diac Dlta is connected between junction Jlta and the base of transistor Q1b; and a diode Dltb is connected with its anode to junction Jlta and with its cathode to junction. J1q. 
     Connected between junction Jc and Jlq, by way of primary windings FTlap and FTlbp of feedback transformers FTla and FTlb, is primary winding CHTp of a cathode heating transformer CHT; which cathode heating transformer has three secondary windings with output terminals a--a, b--b and c--c. 
     The circuit principally consisting of transistors Q1a and Q1b, feedback transformers FTla and FTlb, trigger elements Rlta, Clta, Dlta and Dltb, and cathode heating transformer CHT is referred to as cathode power inverter CPI. 
     A switching transistor Q2a is connected with its collector to the B+ bus and with its emitted to a junction J2q. 
     A switching transistor Q2b is connected with its collector to junction J2q and with its emitter to the B- bus. 
     A saturable current feedback transformer FT2a has a primary winding FT2ap and a secondary winding FT2as, which secondary winding is connected across the base-emitter junction of transistor Q2a. 
     A saturable current feedback transformer FT2b has a primary winding FT2bp and a secondary winding FT2bs, which secondary winding is connected across the base-emitter junction of transistor Q2b. 
     A resistor R2ta is connected between the B+ bus and a junction J2ta; a capacitor C2ta is connected between junction J2ta and the B- bus; a Diac D2ta is connected between junction J2ta and the base of transistor Q2b; and a diode D2tb is connected with its anode to junction J2ta and with its cathode to junction J2q. 
     Connected between junction J2q and a junction Jx, by way of primary windings FT2ap and FT2bp of feedback transformers FT2a and FT2b, is an inductor L; and connected between junction Jx and junction Jc is a capacitor C, as well as a Varistor V. 
     The circuit principally consisting of transistors Q2a and Q2b, feedback transformers FT2a and FT2b, trigger elements R2ta, C2ta, D2ta and D2tb, inductor L, capacitor C, and Varistor V is referred to as main power inverter MPI. 
     A normally open bistable thermal switch TS is connected with its switched terminals across the base-emitter junction of transistor Q1b. Thermal switch TS is activated by a low-resistance heating means HM that is thermally connected therewith. 
     Two fluorescent lamps FL1 and FL2 are series-connected across junctions Jc and Jx by way of the low-resistance heating means HM. 
     Fluorescent lamp FL1 has thermionic cathodes TC1x and TC1y; and fluorescent lamp FL2 has thermionic cathodes TC2x and TC2y. The terminals of cathode TC1x are connected with terminals a--a of transformer CHT; the terminals of cathodes TC1y and TC2x are connected in parallel with terminals b--b of transformer CHT; and the terminals of cathode TC2y are connected with terminals c--c of transformer CHT. 
     Series-connected heating means HMa and HMb are electrically connected with control input terminals CITx and CITy, and thermally connected with saturable feedback transformers FT2a and FT2b, respectively. 
     Details of Operation 
     In their basic operation, half-bridge inverters CPI and MPI are substantially conventional. Their basic operation is explained in detail in conjunction with FIG. 8 of U.S. Pat. No. Re. 31,758 to Nilssen. 
     When power is initially applied to power input terminals PITa and PITb of FIG. 1, inverter CPI is triggered into oscillation within a very brief period, typically a few milliseconds long. The exact length of this period is principally determined by the values of resistor Rtla and capacitor Ctla. 
     As soon as inverter CPI starts to oscillate, cathode heating power begins to be supplied, by way of the three secondary windings on transformer CHT, to the cathodes of fluorescent lamps FL1 and FL2. After about 1.5 second, the cathodes are thermionic and the lamps are ready to be ignited. At that point in time, inverter MPI is triggered into oscillation. 
     The time at which inverter MPI is initially triggered into oscillation is principally determined by the values of resistor R2ta and capacitor C2ta; which time is chosen to be about 1.5 seconds after the initiation of inverter CPI. 
     Since inductor L and capacitor C are resonant at or near the oscillation frequency of inverter MPI, a relatively high-magnitude high-frequency sinusoidal voltage develops across capacitor C, thereby igniting the fluorescent lamps. Since the lamp cathodes are fully incandescent at this point in time, lamp ignition occurs almost immediately after inverter MPI starts oscillation. 
     After ignition, the magnitude of the current flowing through the lamps is determined by the exact value of the inverter&#39;s oscillation frequency; which, in turn, is determined by the temperature of the ferrite cores in the saturable feedback transformers FT2a and FT2b. This temperature, in turn, is determined by the amount of power provided to heating means HMa and HMb from control input terminals CITx and CITy. 
     Details in respect to the effect of core temperature on the inverter&#39;s oscillation frequency are provided in U.S. Pat. No. 4,513,364 to Nilssen. 
     Thus, by adjusting the magnitude of a control voltage provided at control input terminals CITx and CITy, a corresponding adjustment of the inverter&#39;s oscillation frequency results; which, in turn, provides for a corresponding adjustment of the light output from the fluorescent lamps. 
     The arc current flowing through the lamps is also flowing through the low-resistance heating means HM, thereby providing heat to thermal switch TS--the amount of heat being proportional to the square of the RMS magnitude of the lamp arc current. 
     At maximum flow of lamp arc current, which corresponds to maximum light output, the amount of heat generated by heating means HM is sufficient to cause thermal switch TS to close, thereby preventing inverter CPI from oscillating; which, in turn, removes the cathode heating power from the lamps cathodes. 
     At or below some given reduced flow of lamp arc current --which reduced flow would be the result of supplying at least a certain amount of power to the control input terminals--the amount of heat generated in heating means HM is insufficient to keep the thermal switch closed. Thus, at or below this predetermined degree of reduced lamp arc current, thermal switch TS opens, thereby to cause inverter CPI to start oscillating and to start providing cathode heating power again. 
     However, if at a later time the magnitude of the lamp arc current is brought back above the predetermined level, the thermal switch again closes, thereby again eliminating the supply of cathode heating power. 
     Additional Comments 
     a) More detailed information relative to a fluorescent lamp ballast wherein the fluorescent lamp is powered by way of a series-excited parallel-loaded L-C resonant circuit is provided in U.S. Pat. No. 4,554,487 to Nilssen. 
     One effect of such a ballasting arrangement is that of making the waveshape of the voltage provided across the output to the fluorescent lamps very nearly sinusoidal, even though the output from the inverter itself (MPI), at the input to the series-resonant L-C circuit, is basically a squarewave. 
     b) The thermal switch (TS) is of well known design. It is made to have two stable states and to switch between these two states in bi-stable manner. State No. 1, which is the state shown in FIG. 1 (open), represents the state- into which the switch will enter and wherein it will remain in the absence of adequate amount of power being applied to its built-in heating means (HM). State No. 2 represents the state (closed) into which the switch will enter and where it will remain in the presence of an adequate amount of power being applied to its heating means. 
     c) It is believed that the present invention and its several attendant advantages and features will be understood from the preceding 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.