EP0288924B1 - Power supply device for a discharge lamp - Google Patents

Description

The present invention relates to a device for supplying a discharge lamp provided with first and second electrodes, said device comprising a first generator capable of supplying a voltage pulse capable of creating the initiation of the discharge in the lamp. and a second generator capable of maintaining a discharge current in the lamp.

Such an arrangement has already been proposed in document EP-A-0 152 026 (US-A-4 649 322). In this, the initiation of the discharge in the lamp is carried out by a first generator which supplies voltage pulses at predetermined periodic intervals. The light intensity of the lamp is controlled by a current source from a second generator which makes it possible to apply to the lamp a current for maintaining the discharge, the duration of application of which can be varied according to the light intensity that we wish to obtain. The arrangement in question further comprises a circuit which allows the application of the holding current in synchronism with the voltage pulse.

In addition to two modes of execution of the pulse generator, the cited document describes a way of making the generator for maintaining the discharge in the lamp. This holding generator, which is a current source, is supplied from a DC voltage source and essentially comprises a cascade of two transistors which drive continuously when a setpoint signal is sent to the input of the first transistor. The duration of application of the setpoint signal (which can be a video signal for example) conditions the period during which the current source conducts, period which can be of the order of 14 ms for a lamp giving its full brightness, period followed by a train of periods of similar duration if the lamp is to remain lit at this full brightness. In the case where the arrangement described had to be adapted to simply vary the light intensity of a fluorescent lighting lamp, for example by means of a manual control, a single pulse would be necessary, delivered by a pulse generator when the lamp is switched on, pulse followed by a direct current which is continuously maintained at the selected level.

This way of doing things is expensive in electrical energy which is dissipated in heat and that in pure loss. Indeed, it is said in the cited document that a supply voltage of 60 V DC makes it possible to ensure an arc voltage of around 40 V in the tube, which suggests that there is a drop in voltage of the order of 20 V which must be absorbed in the current generator. In reality we realize that the arc voltage can vary in large proportions (10 to 60 V), depending in this on the dynamic regime to which the lamp is subjected. The temperature also has an important influence on the value of this arc voltage. So, in the above-mentioned assembly, it is the current generator, formed of the two transistors which has been mentioned, which will absorb the existing difference between the supply voltage and the arc voltage, difference dissipated in pure loss as we said it.

Document US-A-3 890 537 describes a switching power supply (chopper) which serves as ballast for a gas discharge lamp.

In this document, a voltage source from the network is used to supply the lamp, a voltage source of which the two half-waves are rectified. No filter is provided after rectification. The cited power system provides, as is the case in the present invention, a switching generator provided with a transistor and a diode. However, the control of the current in the lamp is carried out in a completely different manner from that described in the present invention, in the sense that, in the cited document, each time that the maximum current is reached, the transistor switch is triggered, this switch being triggered again when a minimum current is reached. This results in a switching frequency which is essentially variable (between 10 and 40 kHz, depending on the text of the cited patent). Unlike this, the switching frequency of the present invention is fixed. If the transistor is cut off due to a maximum current in the lamp, its reclosing on the other hand is independent of this current. In the present invention, therefore, use is not made of a hysteresis comparator, as is the case with the cited invention.

In the present invention therefore, the lamp is supplied from a DC voltage and from a fixed frequency switching system. In the cited document, this voltage is neither rectified nor filtered and the frequency of the chopping is essentially variable. This would not be suitable for supplying the light points of a large matrix display panel, where it is necessary to control with precision the states of several light sources close to each other.

It is the aim of the present invention to remedy the drawbacks mentioned and to propose a device which is a stabilized current source, without own consumption, whatever the value of the load, load which is manifested here by the voltage of essentially variable arc presented by the lamp.

To achieve this goal, the second generator comprises a first electrical circuit comprising the putting in series of a first DC voltage source, a first switch and a second switch, said first and second switches being arranged in such a way that when the first is closed, the second is open and vice versa, and a second electrical circuit comprising the placing in series of an inductor and of said lamp, connected in parallel to said second switch, that said switches are actuated by a first control device supplied with an alternating signal of fixed period T₁ coming from an oscillator and that means are provided for measuring a value which is representative of the current flowing in the lamp, for comparing said representative value with a reference value supplied by a second DC voltage source U₃ and to provide an equality signal when said values are sensiblemen t identical, said first control device using said equality signal and disposing said first switch first in a closed state during a first period T _a which extends from the start of said fixed period T₁ until the appearance of said signal of equality, then in an open state for a second period T _b which ends with the end of said period T₁, said first switch being actuated according to a cyclic ratio T _a / T₁ controlling the current flowing in the lamp.

• La figure 1a est un schéma général qui montre le principe de fonctionnement du dispositif d'alimentation d'une lampe à décharge selon l'invention,
• Les figures 1b et 1c montrent le cheminement du courant dans le montage de la figure la selon la position des interrupteurs I₁ et I₂,
• La figure 1d est un diagramme temporel simplifié expliquant le fonctionnement des schémas des figures 1a à 1c,
• La figure 2 est un schéma de détail d'alimentation d'une lampe à décharge selon un premier mode d'exécution de l'invention,
• La figure 3 est un diagramme temporel expliquant le fonctionnement du schéma de la figure 2,
• La figure 4 est un schéma de détail d'alimentation d'une lampe à décharge selon un troisième mode d'exécution de l'invention,
• La figure 5 est un diagramme temporel expliquant le fonctionnement du schéma de la figure 4.
• La figure 6 est un schéma général expliquant une variante possible du premier mode d'exécution de l'invention et dérivé du schéma de la figure 1a,
• La figure 7 est un schéma de principe exposant le fonctionnement du dispositif d'alimentation selon un deuxième mode d'exécution de l'invention,
• La figure 8 est un schéma de détail d'alimentation d'une lampe à décharge qui se réfère au schéma de principe de la figure 7 et
• La figure 9 est un diagramme temporel expliquant le fonctionnement du schéma de la figure 8.

The invention will now be understood with the aid of the description which follows and for the understanding of which reference will be made, by way of example, to the drawing in which:

• FIG. 1a is a general diagram which shows the operating principle of the device for supplying a discharge lamp according to the invention,
• FIGS. 1b and 1c show the path of the current in the assembly of FIG. La according to the position of the switches I₁ and I₂,
• FIG. 1d is a simplified time diagram explaining the operation of the diagrams of FIGS. 1a to 1c,
• FIG. 2 is a detailed diagram of the supply of a discharge lamp according to a first embodiment of the invention,
• FIG. 3 is a time diagram explaining the operation of the diagram in FIG. 2,
• FIG. 4 is a detailed diagram of the supply of a discharge lamp according to a third embodiment of the invention,
• FIG. 5 is a time diagram explaining the operation of the diagram in FIG. 4.
• FIG. 6 is a general diagram explaining a possible variant of the first embodiment of the invention and derived from the diagram of FIG. 1a,
• FIG. 7 is a block diagram showing the operation of the supply device according to a second embodiment of the invention,
• FIG. 8 is a detailed diagram of the supply of a discharge lamp which refers to the block diagram of FIG. 7 and
• FIG. 9 is a time diagram explaining the operation of the diagram in FIG. 8.

FIG. 1a is a general diagram which shows the basic principle on which the invention is based. A discharge lamp 1, which can be a fluorescent tube, is provided with two electrodes 2 and 3. A first generator or choke 4 supplies a voltage pulse capable of creating the initiation of the discharge in the lamp. We will see later that according to the embodiment of the invention the starter emits a single ignition pulse or on the contrary pulses repeated at predetermined periodic intervals. The FIG. 1a again shows a second generator capable of maintaining the discharge current in the lamp, the second generator which will be now described and which is the main object of the present invention.

The second generator comprises a first electrical circuit 5 which comprises the placing in series of a DC voltage source U₁, a first switch I₁ and a second switch I₂. The switches I₁ and I₂ are arranged in such a way that when the first is open, the second is closed and vice versa. This interdependence appears in Figure 1a by the dotted line 13 which connects the respective contact tabs of said switches. The diagram also shows that at the terminals of the second switch I₂ is connected a second electrical circuit 6 composed of the placing in series of an inductance L and of the discharge lamp 1.

The switch I₁ is actuated by a control device 7. This device is supplied on its input 8 by an alternating signal of period T₁ coming from an oscillator 9. We will see later that this signal is preferably chosen at high frequency for example between 150 and 600 kHz. This signal has its own period T₁ composed of alternation of duration T₂ at high level followed by alternation of duration T₃ at low level. The duty cycle of this signal is defined as being the ratio T₂ / T₁ The alternating signal of period T₁ is supplied by the oscillator 9 and the alternations T₂ and T₃ have a duration approximately equal.

FIG. 1a also shows that the supply device comprises means for measuring a value which is representative of the current flowing in the lamp, these means being symbolized by the loop 10 surrounding a conductor of the second electrical circuit 6. The value representative of this current is sent to a comparator 11 which compares said value with a reference value contained in a block 12. When said values are substantially identical, comparator 11 emits an equality signal which is introduced into the control device 7 by its input 14 and which will be used by said device to deliver to output 15 of the same device, in combination with the signal received on input 8, a control signal for switches I₁ and I₂. The operation of the device will now be explained with the aid of FIGS. 1b to 1d.

FIG. 1d shows the alternating signal of period T₁ present at the input 8 of the control device 7, signal coming from the oscillator 9. The signal of period T₁ is composed of a first high-level alternation T₂ followed by a second alternation at low level T₃. The control circuit 7 is arranged in such a way that when the signal at the input 8 passes from the low level to the high level, the switch I₁ closes and the switch I₂ opens, the switches remain in these same positions if the signal applied at 8 goes from high level to low level. In the graph in FIG. 1d, the closing of the switch I₁ is symbolized by the solid line 16. With I₁ closed and I₂ open the electrical circuits 5 and 6 appear as illustrated in FIG. 1b. The voltage source U₁ delivers a current i₁ in the inductance L and the lamp 1 via the switch I₁. Due to the presence of the inductance L and the resistance R of the lamp, the current i₁ will increase during a period T _a from a value close to zero to a value approximately similar to a reference value that is fixed (block 12 of Figure 1a). As soon as this value is reached, the comparator 11 supplies the input 14 of the control device with an equality signal 17 illustrated by FIG. 1d. This equality signal has the effect of opening the switch I₁ and closing the switch I₂. The situation of the electrical circuits 5 and 6 is then that presented in FIG. 1c. The electrical energy stored in the inductance L during the previous phase, then produces a current i₂ which, via the switch I₂, flows through the lamp 1. The inductance L then behaves like a generator. Contrary to the current practice of certain known power supplies, this inductor is not a current limiter but behaves like a current reservoir. The current i₂ will decrease during a period T _b until a new rise in the signal of period T₁ appears at the input 8 of the control device 7, a signal which again closes the switch I₁. At the end of period T _b a new cycle begins again and so on.

We have just described the general principle on which the supply device according to the invention is based. It is in fact a stabilized or controlled current source which delivers a current of constant value regardless of the load applied to it. As this load is a discharge lamp whose arc voltage, as we have seen, varies in large proportions, we will always be assured of a constant luminous flux and this without requiring consumption outside that which is necessary for produce this luminous flux. Indeed, the switches described operate by all or nothing and consume almost no clean energy.

In this arrangement therefore, the current delivered by the device of the invention remains constant whatever the value of the load. If this load is large (R small), the period T _a during which the switch is closed will also be small, while if this load is low (R large), this period T _{a will} lengthen, the duty cycle defined by the expression T _a / T₁ in fact controlling the current flowing in the lamp. The assembly also has the advantage of being resistant to short-circuits since, in this case, the period T _a would be reduced to an extremely short duration which could in no case damage the voltage source U₁.

The basic assembly has been explained by using two switches I₁, I₂ actuated by a control device. In practice, it will be possible to use a transistor working in commutation instead of the switch I₁, transistor controlled on its base by the signal coming from the output 15 of the device 7. In practice also, one will advantageously use a diode to replace the switch I₂, diode connected in such a way that it is non-conductive when the transistor is conductive. This diode has the advantage of being self-controlled by the very direction of the voltage present at its terminals. It is clear that the switch I₂ could also be a transistor controlled by the output signal of the device 7 and that the invention is not limited to the sole use of a diode.

To measure the current flowing in the lamp, a low value resistor placed in series in one of the circuits 5 or 6 of the supply device is advantageously used. For essentially practical reasons, this resistance will be placed in the first electrical circuit 5 and the voltage developed across its terminals will be measured, which voltage is representative of the current flowing in the lamp. Other means however could be implemented such as, for example, the use of a current transformer placed in the second electrical circuit 6.

We will now describe three practical embodiments of the invention, the first and the second applied to a single lighting lamp and the third to a lamp used to form one of the pixels of a matrix display board. . In both cases, we will explain what are the blocks of Figure 1a which served to explain the invention in principle.

1. First mode of execution

The diagram in FIG. 2 shows a first embodiment of the supply device according to the invention. The control device 7 is here a type D flip-flop (D-FF) whose D and reset terminals are connected to at least 12 volts of the logic supply. On its input 8, the flip-flop receives the alternating signal of period T₁, also called clock signal (Cl) or synchronization signal (Sync). The transistor Ti1 is controlled on its base by the output Q of the flip-flop. The collector of transistor Ti1 is connected to diode D1 and the emitter to the voltage source U₁ via a resistor RE. The voltage U _RE developed at the terminals of said resistor RE is compared to a reference voltage U₃ by means of a comparator 11 which is here a transistor Ti2 working in commutation. When the voltage U _RE is approximately equal to the voltage U₃, the transistor Ti2 emits an equality signal which acts directly on the input set 14 of the flip-flop. The operation of the assembly which has just been described will now be explained using the time diagram presented in FIG. 3.

On the input 8 of the flip-flop is applied the clock signal Cl, which appears on line a of the diagram. This signal oscillates between -12 V and 0 V (0 V symbolized by the sign 0̸), or between the logical values 0 and 1 respectively. This type of flip-flop (for example CMOS number 4013) has the particularity of having its output Q at the value carried by its input D when the signal Cl goes from 0 to 1 (arrows 18), the passage from 1 to 0 does not in no way changing the state of the Q output as long as the set and reset inputs are both at logic zero (-12 V). As input D is at logic value 0 (-12 V, line b of the diagram in Figure 3), output Q changes from 0 V to -12 V on each positive edge of signal Cl, which is shown at line e of the diagram, the rising edge 18 causing the falling edge 19 of the output Q (arrow 65).

The passage from 0 to -12 V of the output Q has the effect of placing the transistor Ti1 from the blocked state (switch I₁ open) to the conductive state (switch I₁ closed). A current i₁ begins to circulate in the circuit defined by figure 1b, current whose speed of growth is limited by the presence of the inductance L (see line f of the diagram of figure 3 which represents the current I₁ in the lamp 1 ).

We will now observe the voltage U _RE at the terminals of the resistor RE and which is represented by line c of the diagram in FIG. 3. This voltage, initially equal to zero when the transistor Ti1 is non-conductive, will become increasingly more negative as soon as said transistor is conductive, and this until it becomes equal to the sum of the voltages represented by the reference voltage U₃ and the voltage V _BETi2 existing between the base and the emitter of transistor Ti2, ie - (U₃ + V _BETi2 ). From this instant (represented by point 64 of line c) the transistor Ti2, which is non-conductive as it was, becomes conductive and the reference voltage U₃, added to that existing between the collector and the emitter of Ti2 when it conduit, _i.e. U _CTi2 = - (U₃ + V _CETi2 ), is carried over to input 14 (set) of the flip-flop, which has the effect of changing said input set from -12 V to the indicated value (arrows 61). The signal U _CTi2 is given by line d of the diagram in FIG. 3.

The rising _edge of the value U _CTi2 , the final amplitude of which is close to that of a logic, has the effect of switching the flip-flop by its set input, of bringing its output Q to 0 V (arrow 62) and of render the transistor Ti1 nonconductive. The voltage U _RE then changes from the value indicated on line c to 0 V (arrow 63). From this moment, the energy stored in the inductance L produces a current i₂ which circulates in the circuit 6 (line f of the diagram of figure 3) and which decreases since no source of tension is more to him applied. This current i₂ will decrease until the transistor Ti1 becomes a new conductor, which takes place on the arrival of a new rising edge 18 presented by the signal T₁ at the input Cl of the flip-flop. The cycle which has just been described in detail then reproduces in the same way. It will be noted in passing that the voltage rise U _CTi2 is followed by a return to -12 V which has no effect on the operation of the device.

Thus the alternating signal of period T₁ applied to the input Cl of the flip-flop and composed of two equal half-waves T₂ and T₃, becomes, seen from the lamp 1, a signal of equal period T₁ but composed of two half-waves T _a and T _b , the respective durations of which vary from one another according to the current which is imposed on the lamp. The duty cycle T _a / T₁ then controls the current flowing in the lamp.

The diagram of FIG. 3 has been completed by a line g which represents the current I _D1 in the diode D1. It can be seen that during the conduction period T _a of the transistor Ti₁ no current flows in the diode while during the blocking period T _b of the same transistor, a current i₂ flows in the said diode.

The diagram in FIG. 3 also shows a current threshold I _lmin below which the current in the lamp does not fall. This comes from the fact that the inductance L is not completely discharged when the cycle T₁ starts again. This current explains the first voltage level at the terminals of the resistor RE and which is _equal to (I _lmin .RE).

To give a practical example now, it will be mentioned that the transistors are of the 2N5400 type and the diode of the 1N4148 type. The voltage source U₁ is 60 V and the reference voltage of 1.6 V. With a signal of period T₁ = 3.2 µs, a resistance RE of 27 ohms and an inductance of 800 µH, a peak current is measured 80 mA in the tube (equivalent to approximately 50 mA _eff ). It will be observed here that the inductance used is very small (a few mm³), which is another advantage of the device according to the invention. This is mainly due to the fact that the alternating signal of period T₁ is chosen at high frequency, for example greater than 150 kHz.

Figure 2 shows a reference voltage source U₃ crossed by an arrow. The latter indicates that the reference voltage can be adjusted, for example manually by means of a button, to adjust the light intensity emitted by the lamp. We understand that by varying this voltage, we move in the period T₁, the moment when the equality signal appears at the output of the transistor Ti2 and we consequently modify the duty cycle T _a / T₁ which controls the value of the current in the lamp. By decreasing the value of U₃ we decrease the current in the lamp and consequently its brightness.

The diagram in FIG. 2 also shows that the discharge lamp used, which is most often a fluorescent lamp, has a cold anode 2 and a hot cathode 3. This cathode is a filament supplied by a continuous source U₅. Considerations have been made in document EP-A-0152026 concerning this power supply and the reader will refer to it for more details.

To start the discharge in the lighting lamp 1, it suffices to send it a high-voltage pulse when the system is switched on. This pulse is supplied by the choke 4 shown in dotted lines in FIG. 2. This choke could be the one which will be described later on the subject of the third embodiment, but produced in such a way that it only provides a pulse high voltage when the lamp is turned on, instead of repeating pulses.

A possible solution for making the starter is shown in the block diagram of FIG. 6 which is a variant of the execution presented in FIG. 1a. The overvoltage pulse capable of creating the initiation of the discharge is produced by a third switch I₃ connected in parallel on the terminals 2, 3 of the lamp 1. This switch is controlled by a second control device 53, itself actuated by a first control device 7 already described with reference to FIG. 1a. We arrange so that when the supply device is switched on, this third switch is closed. As, at this time, the first switch I₁ is also closed, the inductance L stores energy as explained above. The opening of the switch I₃, synchronous with the opening of the switch I₁ by the fact of the interdependence of the first and second control devices 7 and 53, releases the energy stored in the inductance and creates the overvoltage requested at the lamp terminals. A detailed explanation of the operation of the starter will be given during the discussion which will be made about the second embodiment of the invention.

The lamp to be lit has been described in the diagrams 1a to 1c as having two cold electrodes 2 and 3. However, it is known that if one of the electrodes can be heated by means of a filament, the voltage necessary to initiate the discharge in the lamp is reduced by 1.5 to 2 times. It is also known that a heated electrode considerably increases the life of the lamp. For this we have shown in Figure 2 an electrode 3 provided with a filament supplied from a DC voltage source U₅. The second embodiment which will be explained now takes advantage of the supply device of the invention also to heat the filament.

2. Second mode of execution

The basic diagram is shown in FIG. 7. In this diagram, we recognize the holding current generator formed by the first 5 and second 6 electrical circuits described above. The lamp 1 is equipped with a first cold electrode 2 and a second electrode provided with a filament 56. The second generator of this assembly, formed by circuits 5 and 6 will be used both for heating the filament and for maintaining of discharge into the lamp.

For this purpose, the second electrical circuit 6 includes the series inductance L, the first cold electrode 2 and a first terminal 54 of the filament 56. This second circuit 6 is connected in parallel to the second switch I₂ . FIG. 7 also shows a third switch I₃ connected on the one hand to the cold electrode 2 and on the other hand to a second terminal 55 of the filament 56. The third switch I₃ is actuated by a second control device 53, itself - even actuated by the first control device 7. The second device 53 is arranged so that when the supply device is switched on (by a general switch not shown) the third switch I₃ closes. The filament 56 is then supplied with energy by the second generator 5,6 according to the same fundamental principle explained above. The feeding of the filament takes place during a period of predetermined duration T _d fixed for example by a time constant provided by the block 90 acting on an input of the second control device 53. This heating period will last as long as it takes to make the filament glow, for example a second. When the heating period that has been set has elapsed, the third switch opens, this opening taking place the first time that the first switch I₁ goes from the closed state to the open state after the period of predetermined duration T _d . This change of state is in the form of a logic signal at the output 15 of the first control device 7. This same logic signal acts on the second control device 53 and opens the switch I₃. As it turns out that when the first switch is opened, the energy stored in inductance L is maximum (see point 64 in Figure 3c, corresponding to a maximum current i₁ in the lamp according to Figure 3f) , the opening of the third switch I₃, which is synchronous to the first, causes an overvoltage in the lamp, overvoltage which creates the initiation of the discharge. Then from this the third switch I₃ remains open and the lamp 1 is supplied with holding current by the second generator 5, 6.

Figure 8 is a detailed diagram of the second embodiment explained above in principle. The new elements added to those of FIG. 2 will be described here. The third switch I₃ is a second transistor Ti3 which is controlled by the signal present at the output Q 57 of the control device 53 which is a second type D flip-flop The output Q 15 of the first flip-flop 7 is connected to the input Cl of the second flip-flop 53. The input D 58 of the second flip-flop is connected to the 0 volt of the logic supply by the through a resistor R₃ and a capacitor C is connected between this input D and the -12 volts of the logic supply. The set and reset terminals of the second flip-flop are also connected to -12 volts. An inverting amplifier in the form of a transistor Ti4 is interposed between the output Q 57 and the base of the transistor Ti3. Its purpose is to amplify the signal present at the Q output and to invert it at the same time. The second transistor Ti3 has its collector connected to the cold electrode 2 of the lamp and its emitter connected to the second terminal 55 of the filament 56 of the same lamp.

To explain the operation of the circuit in Figure 8, we refer to the time diagram in Figure 9.

When the system is switched on, for example by means of a switch (not shown), the input D 58 of the flip-flop 53 is found at logic level 0 (-12 V). The output Q 57 of the flip-flop 53 is also at level 0, the transistor Ti4 conducts and provides a base current at transistor Ti3 which also conducts. The filament 56 is then energized and is supplied by the same second generator 5,6 which has been described above (see FIG. 9a). The current I _f in the filament is made up of a succession of currents I _f1 supplied by the circuit 5 and currents I _f2 supplied by the circuit 6 (see beginning of FIG. 9d). The lamp 1 is then short-circuited by Ti3 and the voltage U _l between the terminals 2 and 55 is zero (see the beginning of FIG. 9f). After switching on the system, the input D 58 of the flip-flop 53 is brought gradually from -12 V to 0 V and this during a period of predetermined duration T _d which is fixed by the time constant R₃C and which is calculated sufficient to bring the filament to incandescence (see beginning of Figure 9b). At the end of period T _d , input D 58 of the second flip-flop is at level 1 (0V). From this moment we understand that the next rising edge 69 applied to the input Cl of the second flip-flop (and coming from the output Q 15 of the first flip-flop 7) switches the output Q 57 of said second flip-flop (arrow 65) which changes to 1 (0V). At this instant the transistor Ti3 opens and the current I _f in the filament 56 is interrupted (arrow 66). The opening of the transistor Ti3 causes an overvoltage 80 (FIG. 9f, arrow 68) at the terminals of the lamp, overvoltage due to the energy stored in the inductance L and which is released to create the ignition of the arc. The switching of the output Q 57 of the second flip-flop which brings about the opening of the transistor Ti3 also leads the second generator 5,6 to supply the terminals 2,56 of the lamp with a current I _l (FIG. 9c, arrow 67) formed as already described by an alternation of two currents I _l1 and I _l2 Following the overvoltage pulse 80, a holding voltage U _{l is} then established at the terminals of the lamp (end of FIG. 9f).

Thus in this second embodiment, the same second generator is used, the main object of the present invention, to supply the filament of the lamp first for a certain time, then to maintain the arc current in this lamp. This system leads to the use of means which are much less expensive and cumbersome than the well-known heavy ballast which must be used today for the supply of fluorescent tubes used for lighting.

Finally, it will be noted that FIG. 8 shows a variable reference voltage source U₃ which can be used to vary the light intensity of the lamp. This tension could be removed if one does not want such a feature. At this time we would connect the emitter of transistor Ti2 directly to the + of the source U₁.

2. Third mode of execution

This third embodiment will be used to preferably supply discharge lamps forming the pixels or elementary light points making up a matrix display table. The board can display still or moving images, in color or in black and white. One way of supplying the lamps has been explained in the document cited in the preamble to this description and which bears the number EP-A-0152026 (US-A-4 649 322), a supply which has the disadvantage of being expensive in energy consumed and in joule losses, as already mentioned. Also we will replace the current source of the cited document by that which is the subject of the present invention.

To do this, reference is made to FIG. 4 which presents a detailed diagram of the supply device according to this third embodiment of the invention. We recognize in this diagram the holding current generator formed by the first 5 and second 6 electrical circuits described in detail above.

In this third embodiment, where the light intensity of the lamp is adjusted as a function of a set signal (for example a video signal), the discharge lamp receives at predetermined periodic intervals T _r voltage pulses creating initiating the lamp discharge. These high voltage pulses are supplied by the generator 4. Two embodiments of this generator have been described in detail in the document EP-A-0152026. Here we will briefly recall the operation of one of them, mentioning that the other could also be suitable here.

The generator 4 is composed of a DC voltage source U₄, a coil 20, a switch 21 and a capacitor 22. In such a system, the energy accumulated in the coil 20 in the form of current during the conduction of the switch 21 is restored in the form of voltage across the capacitor 22 when the switch 21 opens. The value of the accumulated energy is determined by the voltage U₄, the inductance of the coil 20 and the accumulation period t₁ - t₀, t₀ representing the closing time and t₁ the opening time of the switch 21. The opening and closing signals of the switch 21 are sent by line 32. The overvoltage pulses are applied to the lamp via a diode 24 and a resistor 25. Diode 24 prevents the current source, supplied by circuits 5 and 6, from supplying another lamp via the common line of the overvoltage generator, if generator 4 is used for several tubes at the same time. The purpose of the resistor 25 is to limit the arc current in the tube from the moment it is started. This device ensures the lighting of several lamps by means of a single generator. Otherwise, since the lamps have different starting characteristics, only the lamp requiring the lowest voltage pulse would light up. Indeed, the voltage present at the terminals of the tube once the arc is established is significantly lower than the voltage necessary to cause it. An important current would then arise if no precautions were taken. This current would prevent, on the one hand, the priming voltage from reaching sufficient values to ignite the other tubes and could, on the other hand, lead to the destruction of the first primed tube.

The electrical circuit 6 further comprises a diode 31 which prevents the overvoltage pulse supplied by the generator 4 from going up to the source of current for maintaining the discharge.

In synchronism with each overvoltage pulse, the lamp is supplied with a discharge maintenance current, the duration of which will depend on a reference signal carrying information indicating the level of light flux which must be reached by the lamp. at some point. This system, based on the time of application of the current and not on its amplitude, is described in detail in the document EP-A 0 152 026 cited above. We can therefore refer to it for other desirable information.

As in the first embodiment, the second generator according to the invention comprises a first electrical circuit 5 comprising the series connection of a DC voltage source U₁, of a first switch (replaced in FIG. 4 by the transistor Ti1) and a second switch (replaced in the same figure by a diode D1 connected in such a way that it is non-conductive when the transistor Ti1 is conductive) and a second electrical circuit 6 comprising the placing in series of an inductance L and of the lamp 1, second circuit connected in parallel to the diode D1. A control device (this is the flip-flop 7) actuates the system. The flip-flop 7 is supplied on its input Cl by an alternating signal of period T₁ = T₂ + T₃ coming from an oscillator. The oscillator of FIG. 4 is presented at 70 and drives a frequency divider 71 on its input Cl. The output Q₁ provides the desired signal T₁ which happens to be, in this example, the frequency of the oscillator 70 divided by two .

In the first embodiment, the output of the device 7 (Q) permanently supplied a signal T₁ = T _a + T _b since the input D of the flip-flop was permanently at -12 V from the supply of the logic. In this third embodiment, on the contrary, the signal T₁ = T _a + T _b only appears periodically (T _r ) and for a duration Tc which is a function of the reference signal of which we have spoken above. The signal of duration T _c is applied to the input D of the flip-flop 7 and is included in the limits 0 ≦ T _c ≦ T _r . When the signal of duration T _c is present at the input D, the current source formed by the circuits 5 and 6 behaves as in the first embodiment: here we find indeed the same means for measuring the representative value of the current flowing in the lamp 1 (RE, 10), to compare (11, Ti2) this representative value with a reference value (U₃, 12) and to provide an equality signal (set) when these values are substantially identical with, as a result, a current flow (i₁, i₂) in two phases of respective durations T _a and T _b as already explained.

We will now explain, also with the aid of the diagram in FIG. 5, how it is done, according to a possible embodiment, to ensure synchronization of the starting signal and the signal of duration T _c for maintaining running through the lamp. The arrangement comprises the combination of the oscillator 70, the divider 71 and three monostable circuits 40, 41 and 42 of the type 555 well known in the art.

We start from a high frequency oscillator 70. This feeds the frequency divider 71 (of the MC 14020 type) at the output Q₁ from which we find the signal for the period T₁ supplying the flip-flop 7 (Figure 5a). A signal at a much lower frequency, here equal to the frequency of the oscillator divided by 2¹³, is taken from the output Q₁₃ of the divider. Let T _{r be} the periodicity of this last signal (Figure 5b). This period T _r represents the repetition rate of the overvoltage pulses.

In the particular case where the device described finds its application in the reproduction of animated images from a video signal for example, it will be understood that an image point must be able to be refreshed, or, in other words, must be able to be capable of receiving new information at least every 1/25 of a second in 50 Hz networks (1/30 of a second in 60 Hz networks), which leads to a repetition of overvoltage pulses every 40 ms . However, this periodicity will be reduced to a third of this value, that is to say 13.33 ms, to avoid above all the blinking of the image.

The signal of period T _r attacks the input 2 of a monostable circuit 40 which only engages on the falling edge of the signal of period T _r to provide on its output 3 a short pulse 50 whose width depends on the values qu 'we give to R₀ + R′₀ and C₀. This width can be varied by adjusting R₀ (Figure 5c). The pulses 50 in turn control the circuit 41 which is also a monostable which engages on the falling edge of the pulse 50 and lengthens said pulse by an amount imposed by the values given to R₁ + R′₁ and C₁. It can be adjusted by varying R₁. The pulse 51 which results therefrom and which is represented in FIG. 5d is collected at the output 3 of the circuit 41 and controls by line 32 the switch 21 of the generator 4. The pulse of width t₁ has been generated in this way - t₀ necessary to create the overvoltage pulse capable of creating the ignition of the arc in the lamp, pulse which is represented at 80 on line 5g and which is repeated with the periodicity T _r . The pulses 51 in turn control the circuit 42 which is still a monostable which engages on the falling edge of the pulse 51 and lengthens said pulse by an amount imposed by the values given to R₂ + R′₂ and C₂. The pulse 52 of duration T _c which results therefrom, and which is shown in FIG. 5e, is collected at the output 3 of the circuit 42 and controls, via the inverter 81, the input D of the flip-flop 7, this last commander, as we have seen, the holding current source formed by circuits 5 and 6. The signal present at input D is shown in FIG. 5f. The pulse 52, or its inverse present at the input D, is none other than the setpoint signal of duration T _c , produced in this example by the circuit 42, circuit which operates in synchronism with the ignition generator 4 .

It should also be mentioned in connection with FIG. 4 the presence of the circuit comprising the transistor 60 which aims at resetting the monostable 42 as soon as a new pulse 50 appears at the output 3 of the circuit 40, this to avoid any overlap. from pulse 50 to pulse 52 which would not be finished.

FIG. 5g shows the voltage U _l which appears at the electrodes of the lamp and which is the result of the combination of diagrams 5b to 5f. Thus, the overvoltage pulse 80 coincides with the falling edge of the pulse 51 and the modulation voltage 82 (or of maintaining the arc) coincides with the pulse 52.

The embodiment of Figure 4 allows to vary the light intensity by means of a potentiometric adjustment (R₂) which is here the setpoint signal itself. It is clear that this adjustment would be carried out in a completely different manner if the reference signal were to be information delivered by a television camera for example. In this case, the camera presents an analog signal at its output which is transformed into a digital signal by a converter. We generally find at the output of the converter 2⁵ = 32 possible tones, one of these tones corresponding to the light intensity of the point analyzed at a specific time. These 32 tones result, in an exemplary embodiment, from the combination of 128 elementary slices of equal duration to take account of the sensitivity curve of the eye (see on this subject the document EP-A-0 152 025 already cited ). The digital information is then sent to a counter which will output a signal at its output, the duration of which will correspond to the light intensity analyzed at that time. This signal will finally command a holding current source as explained above.

To give an example of the different signals involved in this third embodiment, we will cite:

Oscillator 70: 614.4 kHz

Divider 71, Q₁ output: 307.2 kHz
T₁ = 3.2 µs, T₂ = T₃ = 1.6 µs
0 ≦ T _a ≦ 3.2 µs

Divider 71, Q₁₃ output: 75 Hz = 614.6 kHz: 2¹³
T _r = 13.33 ms
0 ≦ T _c ≦ 13.33 ms

Finally, it will be noted that the reference voltage U₃ may be adjustable, which will make it possible to adapt the brightness emitted to the ambient light.

Claims (9)

1. Power supply device for a discharge lamp (1) having first (2) and second (3, 56) electrodes, said device comprising a first generator (4) capable of providing a voltage pulse adapted to trigger discharge in the lamp and a second generator adapted to maintain a discharge current in the lamp, characterized by the fact that the second generator includes a first electric circuit (5) so arranged as to couple in series a first DC voltage source (U₁), a first switch (I₁) and a second switch (I₂), said first and second switches being arranged in a manner such that when the first switch is closed the second is open and vice versa, and a second electric circuit (6), so arranged as to couple an inductance (L) and said lamp in series, connected in parallel across said second switch, that said switches are operated by a first control means (7) energized by an alternating signal having a fixed period T₁ provided by an oscillator (9) and that means (10, 11, 12) are provided to measure a value representative of the current flow in the lamp, in order to compare said representative value with a reference value provided by a second DC voltage source (U₃) and to furnish a signal indicating equality when said values are substantially identical, said first control means employing said equality signal and placing said first switch initially in a closed state during a first time period T_a which extends from the beginning of said fixed period T₁ until appearance of said equality signal, then in an open state during a second time period T_b which ends at the end of said fixed period T₁, said first switch being operated in accordance with a cyclic relationship T_a/T₁ controlling current flow in the lamp.
2. Power supply device according to claim 1, characterized by the fact that the first switch (I₁) comprises a transistor (Ti1) controlled by the first control means (7), that the second switch (I₂) comprises a diode (D1) connected so as to be non conductive when said first switch is closed, that the means (10) for measuring the value representative of the current flow in the lamp are formed by a resistance (RE) arranged in series in said first electric circuit (5), that the first control means (7) is a first D type flip-flop energized at its clock input (8) by the alternating signal of period T₁ and that the transistor (Ti1) is controlled on its base by the Q output (15) of said flip-flop, the collector and the emitter of said transistor being connected respectively to the diode (D1) and the first voltage source (U₁) via said resistance (RE), the voltage (U_RE) developed at the terminals of said resistance being compared with said second DC voltage source (U₃) by means of a comparator (Ti2), the equality signal from the said comparator acting on the set input (14) of said flip-flop.
3. Power supply device according to claim 2, characterized by the fact that said second DC voltage source (U₃) is adjustable.
4. Power supply device according to claim 1, characterized by the fact that the first generator which provides a voltage pulse adapted to trigger discharge in the lamp (1) includes a third switch (I₃) connected in parallel to the lamp electrodes (2, 3) and operated by a second control means (53) itself operated by said first control means (7), said second control means being arranged so that said third switch is closed at the switching on of said power supply device then opens at the first occasion that said first switch (I₁) passes from the closed state to the open state.
5. Power supply device according to claim 4, characterized by the fact that the discharge lamp includes a first cold electrode (2) and a second electrode (3) provided with a filament (56) having first (54) and second (55) terminals, that said second electric circuit (6) is arranged as to couple said inductance (L), said first cold electrode (2) and said first terminal (54), that said third switch is connected on the one hand to said first cold electrode (2) and on the other hand to said second terminal (55), that said second generator (5, 6) is adapted to heat the filament during a period of predetermined duration T_d, then to maintain the discharge current in the lamp and that said second control means (53) are arranged in a manner such that said third switch (I₃) closes with the switching on of said power supply device and then opens after said predetermined period T_d, said opening taking place at the occasion that said first switch (L₁) passes from the closed state to the open state after said period of predetermined duration.
6. Power supply device according to claim 5, characterized by the fact that it comprises the arrangement set forth in claim 2, that the third switch (I₃) is a second transistor (Ti3) controlled by the second circuit means (53) being a second D type flip-flop fed at its clock input (Cl) by the signal present on the Q output of said first flip-flop, said period of predetermined duration T_d being present in the form of a signal corresponding to the D input (58) of said second flip-flop (53) and that said second transistor (Ti3) being controlled by the signal present at the Q output (57) of said second flip-flop via an amplifier inverter (Ti4), the collector and emitter of said second transistor being connected respectively to the first cold electrode (2) and to the second terminal (55) of said filament (56) of said lamp.
7. Power supply device according to claim 1, characterized by the fact that the first generator (4) may furnish at predetermined periodic intervals T_r voltage pulses adapted to trigger discharge in the lamp, that the second generator (5, 6) may furnish the lamp with a discharge maintaining current synchronized with each voltage pulse and that said signal of duration T₁ is applied during a duration T_c which is a function of an instruction signal, said duration of application being within the limits 0 ≦ T_c ≦ T_r.
8. Power supply device according to claim 7, characterized by the fact that the first switch (I₁) is a transistor (Ti1) controlled by the first circuit means (7), that the second switch (I₂) is a diode (D1) connected so as to be non conductive when said first switch is closed, that the means (10) for measuring the value representative of the current flow in the lamp are formed by a resistance (RE) arranged in series in the first electric circuit (5), that the first control means (7) is a D type flip-flop energized on its clock input (8) by the alternating signal of period T₁ and on its D input by the instruction signal of duration T_c and that the transistor (Ti1) is controlled at its base by the Q output (15) of said flip-flop, the collector and emitter of said transistor being connected respectively to the diode (D1) and the first DC voltage source (U₁) via said resistance (RE), the voltage (U_RE) developed with the second DC voltage source (U₃) by means of a comparator (Ti2), the equality signal from said comparator acting on the set input (14) of said flip-flop.
9. Power supply device according to claim 8, characterized by the fact that said second DC voltage source (U₃) is adjustable.

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