BALLAST CIRCUIT

Abstract: An electronic ballast for powering at least two rapid start, parallel connected fluorescent lamps. Associated with each lamp is a serially connected choke and capacitor combination. A train of pulses of alternating polarity having a fundamental frequency, which is generated by an inverter, is applied to each serially connected combination. Each combination is characterized by a resonant frequency which is other than an odd harmonic of and at least two times the fundamental frequency. Reduction in filament heating, following lamp ignition, is provided through the addition of voltages applied to each filament which are substantially out of phase with each other.

Description: Ballast circuit. The invention relates to a ballast circuit for igniting and operating a lamp comprising - generating means for generating a voltage with a frequency fl, - a load circuit coupled to the generating means comprising a first series arrangement of first inductive means and first capacitive means, said first series arrangement having a first resonance frequency flO, wherein the dimensioning is such that nfl < flO < (n+l)fl wherein n is an even integer. Such a ballast circuit is known from EP 0583838 A2. The voltage with frequency fl generated by the known ballast circuit is substantially rectangular and has the same frequency fl during ignition and operation. In case for instance n = 2, the lamp is ignited by means of the third harmonic of the voltage with a frequency fl. After that the lamp is operated by means of the same voltage with frequency fl. For this reason the known ballast circuit is relatively simple and therefore relatively cheap. The known ballast circuit is however, designed for use with only one lamp. Conventional rapid start ballasts for powering two or more pre-heated, fluorescent lamps typically operate the lamps in series with a starting capacitor across all but one lamp. Use of a starting capacitor generally results in an increase in glow current when starting the lamp. This increase in glow current frequently lowers the life expectancy of the lamp. Conventional instant start ballasts for powering two or more fluorescent lamps typically operate the lamps in parallel. Operating conditions for each lamp are dependent on the operation of every other lamp. When any one of the parallel connected lamps fails to operate, there is a change in lamp load. A mismatch in the power outputted b the ballast and the lamp load results. Operation of each parallel connected lamp remaining in operation is adversely affected. In other words, the lamps do not operate independently of one another other. 2 Accordingly, it is desirable to provide a lamp ballast for powering two or more pre-heated, fluorescent lamps. The lamps should be operated independently of each other. Reduction in lamp life arising from glow currents should be minimized. The ballast should have safe open circuit (i.e., pre-ignition) voltage and current levels, with relatively low switching losses. The improved lamp ballast should operate at single frequency which is well below the resonant frequency of the series L-C circuit. A ballast circuit as mentioned in the opening paragraph according to the present invention is therefore characterized in that the load circuit comprises a second series arrangement of second inductive means and second capacitive means parallel to the first series arrangement, said second series arrangement having a second resonance frequency f20, the dimensioning being such that nfl < f20 < (n+l)fl. By choosing this relation between the operating frequency f 1 and the resonant frequencies, safe voltage and current levels can be maintained during pre-ignition. The generated signal results in safe non-resonant operation before lamp ignition as well as correct lamp current after ignition. Feedback circuitry for sensing ignition of the lamp load for switching to a different steady-state lamp operating frequency need not be provided. By providing an independent resonant series circuit associated with each lamp, each lamp can be operated independently of one another. Accordingly, and unlike - conventional instant start, parallel lamp operation, failure of one or more lamps does not adversely affect the performance of the ballast in properly powering the remaining lamp load. Good results have been obtained in case the dimensioning is such that 2fl < flO < 3fl and 2fl < f20 < 3fl. Preferably the voltage with frequency fl is substantially rectangular. Such a voltage is relatively easy to generate while its content of the harmonic with frequency (n+l)fl is high enough to ensure ignition. In a preferred embodiment of a ballast circuit according to the invention, the first capacitive means is coupled across a first lamp during operation and the second capacitive means is coupled across a second lamp. This allows a relatively simple configuration of the load circuit. Since ignition of the lamps is taking place under the influence of the harmonic with the frequency (n+l)fl, the capacities of the first and second capacitive means can be chosen relatively small so that so that the electrode heating currents 3 flowing through the first and second capacitive means are also relatively small. The life expectancy of the lamps is thereby increased. In a further preferred embodiment of a ballast circuit according to the invention, for operating at least two lamps, each lamp having a first and a second filament, the load circuit further comprises a transformer equipped with a secondary winding, a first choke, a second choke, a third choke, a first inductive element being part of the first inductive means, a second inductive element being part of the second inductive means and a third inductive element being part of said secondary winding, during lamp operation the first filament of the first lamp being bridged by a series arrangement comprising the first choke and the first inductive element, the first filament of the second lamp being bridged by a series arrangement comprising the second choke and the second inductive element, and the second filaments of both lamps each being bridged by a series arrangement of the third chok and the third inductive element. By means of proper dimensioning it is realized that before ignition the voltages over the inductive elements and the voltages over the chokes are substantially in phase with each other while after lamp ignition the voltages over the inductive elements and the voltages over the chokes are substantially out of phase with each other. Filament heating following lamp ignition is therefore significantly lowered resulting in higher system efficiency. Good results have been obtained in case the third choke comprises two separate inductive elements. Preferably the generating means of a ballast circuit according to the invention comprise a DC-DC-converter and means for activating the DC-DC-converter a predetermined amount of time after the ballast circuit is switched on. The function of the DC-DC-converter is to generate a second DC-voltage with a relatively high amplitude out of a first DC-voltage with a relatively low amplitude. The DC-DC-converter makes it possible to increase the amplitude of the lamp currents during stationary operation. When the DC- DC-converter is activated immediately after the ballast circuit has been switched on, however, the electrode heating current has a relatively high amplitude influencing the life expectancy of the lamps in a negative way. By activating the DC-DC-converter only a predetermined amount of time after the ballast circuit has been switched on, it is possible to maintain the amplitude of the electrode heating current at a relatively low level while during stationary operation the lamp current can be increased to the desired level. For a fuller understanding of the invention, reference is had to the following description taken in connection with the accompanying drawings, in which: FIG. 1 is a circuit diagram of a ballast output circuit in accordance with the present invention; FIGS. 2(a), 2(b) and 2(c) are timing diagrams of a half-bridge inverter output voltage, output cuπent at its fundamental frequency and output current at its third harmonic, respectively, and FIG. 3 is a schematic diagram of a ballast circuit in accordance with the invention. The figures shown herein illustrate a prefeπed embodiment of the invention. In this preferred embodiment n was equal to 2. Those elements/components shown in more than one figure of the drawings have been identified by like reference numerals/letters and are of similar construction and operation. Referring now to FIGS. 1, 2(a), 2(b) and 2(c), a ballast output circuit 10 includes at least two serial connected combinations of an inductor L and a capacitor C connected across the output of a square wave generator 13. Square wave generator 13 is preferably, but not limited to, a half-bridge inverter generating a voltage E (i.e. the inverter output voltage). A lamp load 16 is connected across each capacitor C through a switch SW. Switches SW are shown merely for the purpose of simulating the pre-ignition and ignition states of the lamps. A current I flowing through each inductor L includes a fundamental frequency component I π and a third harmonic component of the fundamental frequency I 3fl . Other currents at higher odd harmonics are present but are significantly smaller. To facilitate further explanation regarding operation of ballast 10, reference shall be made hereafter to only one serially connected L-C combination, it being understood that each seri¬ ally connected L-C combination should be viewed in like manner. Square wave voltage 13 produces a sinusoidal wave at a fundamental frequency fj and odd harmonics of the fundamental frequency including a sinusoidal wave at a third harmonic 3f_. To achieve low switching losses within square wave generator 13 during pre-ignition of lamp load 16 (generally at trailing edges Er of voltage E), current I is preferably inductive (i.e., current lagging drive voltage) rather than capacitive (i.e. current leading drive voltage) during the voltage transitions of voltage E. For this reason the resonance frequency f 0 is chosen higher than 2^. To ensure that unsafe voltages and currents present at resonant frequency f 0 cannot occur, resonant frequency f 0 also should not be equal to the third harmonic frequency 3f x and , preferably, no other odd harmonics of voltage E. Therefore, in accordance with one embodiment of the invention, the values of inductor L and capacitor C should be chosen such that: 2f, < f 0 < 3f, By designing ballast circuit 10 such that resonant frequency f 0 is within the range of frequencies defined by the above equation, the unsafe voltages and currents which occur at resonant frequency f 0 during pre-ignition of lamp load 16 are avoided and total current delivered by square wave generator 13 remains inductive. There is no need to vary the frequency of voltage E between resonant frequency f 0 during pre-ignition of lamp load 16 and a different frequency immediately thereafter as in conventional ballast circuitry. Feedback circuitry designed to sense ignition of lamp load 16 for determining when to vary the frequency of voltage E from resonant frequency f 0 to a different operating frequency can be eliminated. In accordance with one preferred embodiment of the invention, a safer, simpler circuit is provided by maintaining resonant frequency f 0 within the boundaries defined by eq. 8. A ballast circuit 20 in accordance with the invention is shown in FIG. 3. The elements within ballast 20 shown in dashed lines include an electromagnetic interference (EMI) suppression filter 23, a full wave rectifier 30, a preconditioner 40 and a half bridge circuit 80. An A.C. source 21 nominally at 120 volts, 60 hertz is connected to a line (L) side input and a neutral (N) side input of ballast 20. The A.C. voltage (Vm) of 120 volts, which is referred to herein for exemplary purposes only and is not limited thereto, is applied to EMI suppression filter 23. Filter 23 filters high frequency components inputted thereto lowering conducted and radiated EMI. The output of filter 23 is supplied via terminals 24 and 25 to full wave rectifier 30 which includes diodes Dl, D2, D3 and D4. The anode of diode Dl and cathode of diode D2 are connected to terminal 24. The anode of diode D3 and cathode of diode D4 are connected to terminal 25. The cathodes of diodes Dl and D3 are connected to input terminal 31 of the preconditioner 40. The anodes of diodes D2 and D4 are connected to a ground bus rail 32 forming also a further input terminal of preconditioner 40. The preconditioner 40 is a boost converter having output terminals 41 and 42. The boost converter includes a choke L3, a preconditioner transistor Ql, a diode D5, an 6 electrolytic capacitor CE and preconditioner control 50. A series arrangement of choke L3 and diode D5 connects input terminal 31 to output terminal 41 and the anode of electrolytic capacitor CE. A common terminal of choke L3 and diode D5 is connected to a first main electrode of transistor Ql. A further main electrode of transistor Ql is connected to input terminal 32, output terminal 42 and the cathode of electrolytic capacitor CE. An output terminal of preconditioner control 50 is connected to a control electrode of transistor Ql. The rest of the components of the ballast circuit shown in Fig. 3 together form an inverter circuit 80. Output terminals 41 and 42 are connected by means of a series arrangement of switching elements Q6 and Q7 and also by means of a series arrangement of capacitors C5 and C6. A control electrode of switching element Q6 is connected to an output terminal of level shifter 60. A control electrode of switching element Q7 is connected to an output terminal of half bridge drive 70. A common terminal A of switching elements Q6 and Q7 is connected to a common terminal B of capacitors C5 and C6 by means of a load circuit, comprising a transformer T4. Transformer T4 includes a primary winding 71 and a secondary winding 73 having a winding section 75 and a winding section 77. Winding sections 75 and 77 are connected together at a tap 79 of secondary winding 73. Primary winding 71 of transformer T4 is connected between terminal A and terminal B. One end of winding section 77 is connected to a junction joining together a pair of DC blocking capacitors Cll and C12. Capacitors Cll and C12 block DC currents in the event that Lamp 1 and Lamp 2 begin to act as rectifiers and thereby prevent transformer T4 from saturating, respectively. A pair of ballasting/current limiting chokes L4 and L5 are serially connected to capacitors Cll and C12, respectively. Choke L4 includes two sections 96 and 97 joined together at a tap 85. Choke L5 includes two sections 98 and 99 joined together at a tap 87. A pair of auxiliary windings 91 and 93 and a resistor R27 are serially connected between tap 79 of secondary winding 73 of transformer T4 and to a junction joining together a filament LF2 of Lamp 1 and a filament LF4 of Lamp 2. Filaments LF2 and LF4 are connected in parallel. Auxiliary windings 91 and 93 are coupled to chokes L4 and L5, respectively. A capacitor C15 is connected at one end to a junction joining together a resistor R25 and tap 85 of choke L4. The other end of capacitor C15 is connected to a junction joining together winding section 75, filaments LF2 and LF4 and a capacitor C16. Capacitor C16 is connected at its other end to a junction joining together a resistor R26 and tap 87 of choke L5. An auxiliary winding 95, coupled to secondary winding 73, is connected between winding section 97 of choke L4 and a filament LFl of Lamp 1. Winding section 97, auxiliary winding 95, filament LFl and resistor R25 are serially connected together so as to form a closed path for controlling the heating of filament LFl. Similarly, an auxiliary winding 101, coupled to secondary winding 73, is connected between winding section 99 of choke L5 and a filament LF3 of Lamp 2. Winding section 99, auxiliary winding 101, filament LF3 and resistor R26 are serially connected together so as to form a closed path for controlling the heating of filament LF3. Winding section 75, auxiliary windings 91 and 93, resistor R27 and filament LF2 are serially connected together so as to form a closed path for controlling the heating of filament LF2. Similarly, winding section 75, auxiliary windings 91 and 93, resistor R27 and filament LF4 are serially connected together so as to form a closed path for controlling the heating of filament LF4. Resistors R25, R26 and R27 serve to limit current flow in the event of a short circuit across filaments LFl, LF3 and LF2/LF4, respectively. In the event of a momentary short, these resistors limit the total current flowing through the auxiliary windings and thereby protect the serially connected windings from being damaged. In the event of an extended short circuit across the filament, the associated resistor will fail open without overheating or otherwise presenting a fire hazard to other components within ballast 20. Choke L4 and capacitor C15 form a tuned resonant circuit. Similarly, choke L5 and capacitor C16 form a tuned resonant circuit. Each resonant circuit is tuned to the same resonant frequency. By selecting the component values, this tuned resonant frequency is for this embodiment of a ballast circuit according to the invention about 2.5 times the operating frequency of the inverter. The values for capacitor C15 and C 16 are . chosen such that safe open circuit operation is provided, that is, within the range of resonant frequencies defined by the equation 2fl < fO < 3fl. Accordingly, no additional circuits to protect ballast 20 are required. When the ballast 20 is tumed-on rectifier 30 rectifies the low frequency supply voltage present between terminals L and N. Electrolytic capacitor CE is charged and half bridge drive 70 and level shifter 60 render switching elements Q6 and Q7 alternately conductive and non-conductive. As a result an alternating current flows through primary winding 71. Pre-heating of filaments LFl, LF2, LF3 and LF4 occurs for approximately the first 750 milliseconds after ballast 20 is tumed-on. Preconditioner control 50 is turned-on only after this 750 millisecond delay period by delay means not shown in Fig.3. These delay means can be realized in many different ways. The delay means can for instance be realized by means of a capacitor that is charged through a resistor after the ballast is turned-on, and means for activating the preconditioner control when the voltage over the capacitor has reached a predetermined level, the preconditioner control is turned on. In accordance with the invention, it should be understood that the pre-heating period is not fixed to about 750 milliseconds and can be any suitable period for operating a rapid start type of fluorescent lamp. After the delay period preconditioner control 50 renders transistor Ql alternately conductive and non-conductive so that the voltage over electrolytic capacitor CE is boosted. Auxiliary windings 91 and 93 are wound such that the voltages developed across auxiliary windings 91 and 93 are substantially in phase with and substantially add to the voltage developed across winding section 75 during this pre-heating period. Similarly, auxiliary windings 95 and 101 are wound such that the voltages developed across auxiliary windings 95 and 101 are substantially in phase with and substantially add to the voltages developed across winding sections 97 and 99 during this pre-heating period, respectively. The voltages developed across these auxiliary windings during the pre-heating period are relatively small in view of their relatively small number of turns. Once lamp ignition has occurred, the voltages developed across auxiliary windings 91 and 93, 95 and 101 are substantially out of phase with and substantially subtract from (cancel) the voltage developed across winding sections 75, 97 and 99, respectively. The voltage across each lamp filament is significantly reduced. A reduction (cut back) in filament heating results thereby improving system efficiency. When ballast circuit 20 is first turned on, prior to preconditioner control 50 being turned on, the input voltage of approximately 120 volts results in a peak voltage of approximately 170 volts peak to peak being applied across primary winding 71 of transfor- mer T4 which is stepped up to approximately 400 volts peak to peak across secondary winding 73. After approximately 750 milliseconds, preconditioner control 50 is activated. A regulated D.C. voltage of approximately 235 volts across capacitor CE is produced and a voltage of approximately 560 volts peak to peak across secondary winding 73 is generated. The voltage across secondary winding 73 is sufficient for igniting Lamp 1 and Lamp 2. Once Lamp 1 and Lamp 2 are ignited (i.e. during steady-state lamp operation), the voltage across each filament is substantially reduced. This reduction in filament voltage and consequential reduction in filament heating is based on the out-of-phase voltages of auxiliary windings 91, 93, 95 and 101 substantially cancelling the voltages which would 9 otherwise be applied across the filaments. The voltage across each filament can be viewed as the sum of a first voltage and a second voltage wherein the first voltage and second voltage are substantially in phase with each other prior to lamp ignition and are substantially out of phase with each other following lamp ignition. The first voltages are produced by winding sections 75, 97 and 99. The second voltages are produced by auxiliary windings 91, 93, 95 and 101. Following ignition, each lamp voltage (i.e. voltage across Lamp 1 or Lamp 2) drops to approximately +. 220 volts peak with the remainder of the voltage of secondary winding 73 across choke L4 or choke L5, respectively. The number of lamps connected in parallel can be varied as desired with the value of each serially connected choke being chosen so as to provide the desired lamp current during steady-state operation of the lamp. As now can be readily appreciated, by maintaining the inverter fundamental sinusoidal frequency fj well below resonant frequency f 0 of the series L-C output circuit, the undesirable and unsafe high voltages and current levels produced in conventional ballast circuits during pre-ignition of a lamp are avoided. More particularly, by choosing the values of chokes L4 and L5 and capacitors C15 and C16, respectively, such that the L4,C15 resonant frequency f 0 and L5,C16 resonant frequency is defined by the equation 2f t < f 0 < 3fj, the voltage level across chokes L4 and L5 and capacitors C15 and C16 and current flow therethrough will be safe and well defined during pre-ignition. The invention provides rapid start, parallel and independent lamp operation. Unlike conventional rapid start operation in which the lamps are serially connected, the invention avoids the need for starting capacitors and thereby reduces the level of glow current produced during lamp starting. A much longer lamp life is provided. Unlike conventional instant start, parallel lamp operation, failure of one or more lamps does not adversely affect the performance of any lamps remaining in operation. In particular, each lamp operates independently of one another by providing an independent resonant series circuit (e.g. choke L4 and capacitor C15) associated with each lamp. A ballast in accordance with the invention can operate four rapid start fluorescent lamps in parallel. In the event that one, two or three of these four lamps fail, a ballast in accordance with the invention would continue to operate the remaining lamp(s) as though designed for three, two or one lamp operation. In other words, a change in lamp load does not adversely affect performance of the ballast in properly powering the remaining lamp load. More generally in a ballast circuit according to the invention the resonant frequency f 0 can range from 10 approximately at least n times the inverter fundamental frequency f_ of the square wave generated by the square wave generator to n+ 1 times fi (n is an even integer) but should exclude those frequencies equal to a higher odd harmonic of the fundamental frequency f,. Unsafe operation (i.e., resonant operation of the series L-C output circuit) of ballast circuit 20 is thereby prevented. As now can be readily appreciated, the generated voltage (i.e. voltage E of FIG. 1) is at a frequency which is far less than the resonant frequency of the series connected L-C circuit and therefore provides safe open circuit (pre-ignition) voltages and current levels. The frequency of this generated signal need not be changed following pre- ignition since it is never at or near resonant frequency f 0 of the series connected L-C circuit. Feedback circuitry for sensing ignition of lamp load LL for switching to a different steady- state lamp operating frequency need not be provided. By eliminating the need to operate at resonant frequency f 0 of each series L-C circuit during pre-ignition of the lamp load, the value and resulting size of the capacitor for the series connected L-C circuit can be far smaller than normally used in a conventional series connected L-C circuit.

Claims: CLAIMS: 1. A ballast circuit for igniting and operating a lamp comprising - generating means for generating a voltage with a frequency fl, - a load circuit coupled to the generating means comprising a first series arrangement of first inductive means and first capacitive means, said first series arrangement having a first resonance frequency flO, wherein the dimensioning is such that nfl < flO < (n+l)fl wherein n is an even integer, characterized in that the load circuit comprises a second series arrangement of second inductive means and second capacitive means parallel to the first series arrangement, said second series arrangement having a second resonance frequency f20, the dimensioning being such that nfl < f20 < (n+l)fl. 2. A ballast circuit as claimed in claim 1, wherein the dimensioning is such that 2fl < flO < 3fl and 2fl < f20 < 3fl. 3. A ballast circuit as claimed in claim 1 or 2, wherein the voltage with frequency fl is substantially rectangular. 4. A ballast circuit as claimed in one or more of the previous claims, wherein during operation the first capacitive means is coupled across a first lamp and the second capacitive means is coupled across a second lamp. 5. A ballast circuit as claimed in one or more of the previous claims for operating at least two lamps, each lamp having a first and a second filament, wherein the load circuit further comprises a transformer equipped with a secondary winding, a first choke, a second choke, a third choke, a first inductive element being part of the first inductive means, a second inductive element being part of the second inductive means and a third inductive element being part of said secondary winding, during lamp operation the firs filament of the first lamp being bridged by a series arrangement comprising the first choke and the first inductive element, the first filament of the second lamp being bridged by a series arrangement comprising the second choke and the second inductive element, and the second filaments of both lamps each being bridged by a series arrangement of the third cho and the third inductive element. 6. A ballast circuit as claimed in claim 5, wherein the third choke comprises two separate inductive elements. 7. A ballast circuit as claimed in one or more of the previous claims, wherein the generating means comprise a DC-DC-converter and means for activating the DC-DC-converter a predetermined amount of time after the ballast circuit is switched on.

Inventors: ERHARDT ROBERT A, MATTAS CHARLES B, ERHARDT, ROBERT, A., MATTAS, CHARLES, B.
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1996
Main classification: H05B41
IPC codes: H05B41