EP0247002A1 - Electronic power supply system for a fluorescent lamp provided with electrodes - Google Patents

Abstract

The electronic power supply system for fluorescent tubes with electrodes comprises two MOS power transistors (14) connected in series and, connected to the common point of the transistors (14), a series connection comprising a primary (13) of a transformer, whose two secondaries (15) drive the transistors, the lighting unit (s) (11) and a capacitor (10). The capacitor (10) is connected to the terminal opposite to that where is connected the transistor (14) which is attacked by a diac (18).

Description

The present invention relates to an electronic power supply system for fluorescent tubes with electrodes enabling almost instantaneous ignition as well as energy saving.

The majority of fluorescent tubes currently installed are powered by a system which has several disadvantages, the two main ones being a high energy consumption and rapid degradation of the tubes. Indeed, this assembly, comprising a reactance, uses the opening of a bimetallic strip to initiate the gas mixture located between the filaments of the tube. This ignition is rarely instantaneous, which requires several openings of the bimetallic strip. These repeated openings have the effect of causing the tube to flash, which is detrimental to the life of the latter. In addition, the reactance consumes a lot of energy, which results in heating of it. Finally, this reactance introduces a significant reactivity rate into the sector line, which increases consumption accordingly.

Research has been carried out with a view to achieving a system allowing the creation at the terminals of at least one tube of a high frequency alternating voltage, and this only by means of semiconductor components and transformers in order to consume very little energy and ensure instant ignition.

One orientation among others, illustrated for example in the presentation FR 2 520 575, uses two transistors connected in series amplifying a square signal supplied by an auxiliary frequency generator circuit and delivering it to a series oscillating circuit comprising a coil L1, a lighting unit and a charge capacitor C3.

This system was quickly improved by removing the related frequency generator circuit and replacing it with a transformer taking from the oscillating series circuit even a sinusoidal signal to drive the two transistors alternately. This simple design system, illustrated by the presentation FR 2 478 933, has two new drawbacks. On the one hand its starting is random, on the other hand short periods during which the transistors become simultaneously conducting are possible. During such periods, the lighting unit being no longer supplied with power goes out, and the transistors are traversed by a high intensity detrimental to their longevity.

Various patents providing solutions to the above problems were filed. We will note more particularly the presentations EP 0 171 108, DE 3 412 944 and WO 87/00719.

The certain starting of the circuit is resolved for example by sending a starting pulse to one of the transistors. This is achieved using a breakdown element, or diac, connected on the one hand to the base of the transistor whose emitter is connected to ground, on the other hand between a capacitor and a resistor mounted in series between the supply terminals and constituting a time base. Thus, once the voltage across the capacitor is greater than the sum of the base emitter voltage of the transistor and the breakdown voltage of the diac, the latter lets the starting pulse pass. But, given that the charge capacitor of the LC series circuit is generally connected to ground, it is imperative that the latter is previously charged before the transistor which short-circuits it becomes passing under the impulse of the diac. This therefore requires an additional system ensuring this preload during the rise in voltage of the time constant capacitor, which complicates the general circuit all the more. Thus, in the presentation EP 0 171 108, a resistor 51 short-circuits the transistor 11, in the presentation WO 87/00719, a special branch comprising a diode 17 and a parallel mounting of a capacitor 19 and a resistor 21 controls a switch 14 allowing the charging of the capacitor 15 through the capacitor 18, and in the description DE 3 412 944 a weak charging current passes through the diode 31 and the resistor 32.

The problem of the overlapping of the conduction time of the bipolar transistors is due to the fact that the time of transition from the saturated state to the blocked state is directly a function of the collector-emitter intensity. This phenomenon implies on the one hand a thermal instability in the transistors, on the other hand a quickly reached limit as regards the number of lighting unit with the basic system. To overcome this drawback, it is necessary to use more complex systems for synchronizing the state of the transistors of power, this in order to avoid any simultaneous current flow therein which may cause, in the worst case, a breakdown of these elements. This problem is solved in the European presentation using a complex control circuit including time unit elements such as the capacitors 33; in the presentation WO 87/00719 by arranging several transistors in parallel, each of them being controlled by its own secondary winding. The system presented in DE 3,412,944 can apparently only accommodate one lighting unit.

Furthermore, there is a device, described in the description FR 2 487 140, achieving certain goals in a more elegant manner but applied specifically to discharge lamps devoid of electrodes. This device generates a frequency of at least 0.5 Mhz necessary for discharge lamps without electrodes. This is not applicable to fluorescent tubes which work optimally with a frequency between 100 and 200 kHz. In addition, the shape of the alternating voltage is detrimental to the life of the windings. Furthermore, this system does not allow multiple lighting units to be connected simultaneously. Finally, the output of this device is only 87 1m / W while the device proposed by the applicant makes it possible to achieve a yield of 125 1m / W.

The present invention aims to obviate the above-mentioned drawbacks by proposing an electronic power supply system for fluorescent tubes with electrodes delivering a high frequency voltage across said tubes, allowing instantaneous ignition. of these, having a very good yield and providing a reduction in energy consumption compared to the traditional assembly of 45 to 50%.

To this end, the system according to the present invention comprises a power supply connecting to the mains and producing a DC voltage on its output terminals 1A and 1B, a time constant connected between said output terminals made up of a circuit. series of a resistor and a capacitor, two power transistors connected in series between said output terminals, and, connected to the common point of the transistors, a series connection comprising the primary of a transformer, the unit or units lighting and a capacitor. The two secondary transformers drive the transistors. The system further includes a bilateral breakdown effect conduction element, or diac, connected on one side between the resistor and the time constant capacitor and on the other side on the control branch of the transistor, the one of the other branches is connected to the output terminal 1B. This system is characterized in that the transistors are of the MOS type and that the capacitor in series with the lighting unit (s) is connected to the output terminal 1A so that it does not start to charge until the passage in the saturated state of the transistor under the impulse of the diac.

Other advantages and characteristics will appear on reading the description which follows, illustrated by the drawing given by way of nonlimiting example schematically representing the system according to the invention.

As shown in the attached figure, the power supply system for fluorescent electrode tubes according to the invention comprises a power supply 1 connecting to the mains and producing a DC voltage on its terminals so 1A and 1B.

This supply 1 comprises in known manner between its input terminals a diode bridge 2 rectifying the alternating voltage of the network and between the output terminals of said diode bridge, a capacitor 3 preferably of the electrochemical type filtering the rectified voltage. Between the input terminals of the power supply 1 are mounted two capacitors 4 connected in parallel between the earth and one of the said input terminals. These capacitors 4 make it possible to eliminate the high frequency parasites generated by the system. On the other hand, a fuse 5 for protection against a possible short circuit is mounted on one of the power supply input terminals. On the other input terminal of the power supply is mounted a resistor 6 limiting the input intensity in the power supply.

Between terminals 1A and 1B is mounted the time constant 7 constituted by the series connection of a resistor 8 and a capacitor 9.

The ignition system for fluorescent electrode tubes according to the invention comprises a capacitive element 10 connected to the positive terminal 1A of the supply 1 and in series with at least one lighting unit 11, two groups of controls 12 of l state of charge of the capacitive element 10 through the lighting unit (s) and a transformer including the primary 13 is connected between the lighting unit (s) 11 and the control groups 12 and, depending on the load of the capacitive element 10, controlling the control groups 12. The capacitive element 10 consists of a capacitor of known type.

Advantageously, the control circuit of each MOS transistor 14, connected in series between the source S and the gate G of this transistor 14, comprises the secondary 15 of the transformer, a resistor 16 connected in series with the said secondary, and a Zener diode 17 mounted in parallel with the secondary series mounting 15 / resistor 16.

The Zener diode 17 mounted between the source S and the gate G and in parallel with the secondary circuit 15 / resistor 16 protects the transistor 14 by preventing too high a bias voltage from being received on the gate G of the transistor.

The secondary 15 of each of the control groups 12 come from the transformer and are mounted in phase opposition on the same core as the primary 13. A control group 12, called the first group, is mounted between the primary 13 of the transformer by the drain D of transistor 14 and terminal 1B of power supply 1 by source S of said transistor. The other group 12, called the second group, is mounted between the capacitive element 10 by the drain D of the transistor 14 and the primary 13 of the said transformer by the source S of the transistor.

In other words, two MOS type power transistors 14 are connected in series between the terminals 1A and 1B of the power supply. The transistor of the first command group 12 is connected to terminal 1B by its source S. The drain of the transistor 14 of the first command group 12 is connected to the source of the transistor 14 of the second command group. The drain D of the transistor of the second group of commands is connected to the terminal 1A of the power supply. Connected in the middle of this series connection, ie at the common point of the transistors 14, a series branch comprises in order the primary 13 of the transformer, the lighting unit or units 11 and a charge capacitor 10, the latter being connected at terminal 1A of the power supply.

Thus, the circuit comprises a first series branch connected between the terminals of the power supply and comprising the transistor 14 of the first group of commands, the primary 13 of the transformer, the lighting unit or units 11 and the charge capacitor 10. This series connection is short-circuited between the primary 13 of the transformer and the capacitor 10 by the transistor 14 of the second group of commands.

Between the gate G of the transistor 14 of the first group 12 and the time constant 7 is mounted a bilateral conduction element with breakdown effect or diac 18. This diac 18 is connected on one side between the resistor 8 and the capacitor 9 of the time constant 7 and on the other side on the gate G of the transistor 14 of the first group 12.

When the supply 1 is switched on, the capacitor 9 of the time constant 7 charges through the resistor 8 until the voltage at terminals of said capacitor 9 reaches the threshold voltage of diac 18 plus the low voltage existing between the gate and the source of transistor 14. Once this voltage is reached, diac 18 becomes conducting and said voltage is present on gate G of transistor 14 of the first group 12 which makes this transistor on. The conduction of this transistor causes the capacitor 10 to charge through the primary 13 of the transformer and the lighting unit (s).

Charging the capacitor 10 causes a voltage pulse across the transformer. This voltage pulse transmitted by the primary 13 to the secondary 15 causes the change of polarization of the gates G of the transistors 14 of the first and second group 12. This change of polarization makes the transistor 14 of the first group 12 blocked and passing the transistor 14 of the second group 12.

The on state of the transistor of said second group causes the discharge of the capacitor 10 through the primary 13 and the lighting unit or units 11. The voltage pulse due to the discharge of the capacitor 10 is transmitted to the secondary 15 in the primary 13, which again causes the polarization of the gates G of the transistors 14 to change. Thus, the system enters into oscillations at a frequency F. The frequency F of the system, according to the invention, is relatively low since it is between 100 and 200 khz. This low frequency therefore imposes a value of the capacitor 9 of the time constant 7 relatively high.

The use of MOS transistors switching at low voltage has made it possible to reduce the number of turns required in the transformer. By way of nonlimiting example, the primary of the transformer consists of a single turn on a toroidal core with a diameter of 12 mm and each secondary consists of two turns on the same core. This reduction in the number of turns reduces the electrical losses by heating, the phase shift induced between the voltage and the current in the series circuit, which consequently makes it possible also to reduce the dimensions of the induction coils 19. In addition, the external dimensions of the transformer are reduced to a cylinder 12 mm in diameter and 5 mm high.

Advantageously, the transformer primary consists of at most five turns and each transformer secondary consists of at most ten turns.

According to a preferred embodiment shown in the attached figure, the system supplies two lighting units 11, but it goes without saying that this number is in no way limiting and that the system can receive several lighting units 11 or else a alone. Each lighting unit is mounted parallel between the primary 13 of the transformer and the capacitor 10. According to a preferred embodiment, each lighting unit 11 comprises a coil 19, a fluorescent tube 20 with electrodes 21, and a capacitor 22 mounted in series between the two electrodes or filaments 21.

The coil 19 of the lighting unit 11 allows the creation of a high voltage and keeps the tube 20 in the lit state. The capacitor 22 connecting the filaments 21 of the same tube 20 recovers part of the oscillation and allows the filaments 21 to remain slightly hot. This last effect ensures a wide passage in the tube to the electric current.

The high frequency alternating signal present on the primary 13 of the transformer makes it possible to operate a large number of tubes. The load present at the output of the system according to the invention does not affect the operation and the frequency of said system. Preferably, the tubes 20 are mounted two by two at the outlet of the system.

In order to allow possible instantaneous re-ignition of the lighting unit (s) 11, or else re-ignition of the oscillation of the system during micro-cuts in the supply due to irregularities in the source voltage, the system according to the invention , includes means allowing the rapid discharge of the capacitor 9 of the time constant 7 during the conduction of the transistor 14 of the first group 12, that is to say the source S of which is connected to the output terminal 1B.

This means, according to a preferred embodiment, consists of a diode 23 connected on the anode side to the time constant 7 and on the cathode side to the drain D of the transistor 14, the source S of which is connected to the output terminal 1B, ie at common point of the two transistors 14; and a resistor 24 connected between terminal 1A of supply 1 and the anode of diode 23.

During the conduction of the transistor 14 of the first group 12, the diode 23, polarized in the passing direction, causes the rapid discharge of the capacitor 9, discharge which is limited only by the weak internal resistance of the transistor 14. In order to synchronize the conduction of the diode 23 with the conduction of the transistor 14 of the first group 12, the resistor 24 polarizes the said diode in the opposite direction (anode voltage less than cathode voltage). The usefulness of this arrangement is explained by the fact that the switching time of the MOS transistors is less than that of the silicon diodes in general. So that the conduction of the diode 23 is simultaneous with that of the power transistor 14, the cathode of the diode 23 is brought to a positive potential with respect to its anode, this having the effect of blocking the latter. During the conduction of the transistor 14, this blocking disappears suddenly and the diode 23 enters into immediate conduction and in phase with the transistor 14. The effect of this particular arrangement has in fact a role of accelerator of conduction of the element 23 The presence of this diode 23 / resistor 24 assembly is therefore essential for the proper functioning of the system according to the invention because of the low frequency generated by said system.

On the other hand, taking into account the high voltages transmitted to the control circuit of the transistors 14 by the primary of the transformer 13, and the speed of switching of the MOS transistors, the winding (s) 19 of the lighting unit (s) 11 receive a tension alternative having a rectangular signal which causes unnecessary heating of these coils 19 and thereby a drop in efficiency. In order to avoid this drawback, a series connection of capacitor 25 / resistor 26 is connected between the drain D of the transistor 14 of the first group 12 and the terminal 1A of the supply 1. Thus, during the conduction of the transistor 14 of the first group 12, the capacitor 25 charges through the resistor 26 and therefore introduces a slight damping on the rising edge of the signal, as well as on the falling edge when the capacitor 10 discharges. The signal therefore has a more acceptable shape for the winding 19 of the lighting unit or units. In addition, this capacitor 25 / resistor 26 assembly makes it possible to increase the system efficiency by approximately 10%, and to eliminate the phenomenon of dark spots on fluorescent tubes 20 due to poor distribution of the ionized charges. By way of example, the efficiency of the system according to the invention with two 50 W high brightness tubes 20 brings this to 125 lm / W.

The table below gives an average value of the electronic components used in the system according to the invention and creating a frequency F of approximately 125 kHz.

The system according to the present invention can also supply discharge lamps (mercury vapor) under the same economy conditions as for fluorescent tubes with electrodes.

It appeared on reading the description that this system requires only a few electronic components, which reduces the risk of system breakdowns in general. This reliability can be further improved by selecting the electronic components themselves and by assembling the circuit on a fully automated machine. In addition, this system can advantageously be made in small dimensions: two power MOS transistors, a power supply capacitor, a small transformer (12 mm x 5 mm) and an electronic chip bringing together the rest of the components, all included in a volume less than 40 mx 20 mm x 20 mm.

Thus, the system according to the invention allows the instantaneous ignition of fluorescent tubes with electrodes, to increase the lifetime of these, to produce an excellent efficiency and to consume very little electric current. It goes without saying that the present invention can receive any arrangement and any variant in the field of technical equivalents without departing from the scope of this patent.

Claims (5)

1. Electronic supply system for fluorescent electrode tubes comprising
- a power supply (1) connecting to the mains and producing a DC voltage on its output terminals (1A) and (1B),
- a time constant (7) connected between said output terminals consisting of a series arrangement of a resistor (8) and a capacitor (9),
- two power transistors (14) connected in series between said output terminals,
- connected to the common point of the transistors 14, a series connection comprising a primary (13) of a transformer, the two secondary (15) of which control the transistors (14), the lighting unit or units 11 and a capacitor ( 10),
- a bilateral conduction element with breakdown effect, or diac (18) connected on one side between the resistor (8) and the capacitor (9) of the time constant (7) and on the other side on the branch for controlling the transistor (14) one of the other branches of which is connected to the output terminal (1B).
characterized in that the transistors are of the MOS type and that the capacitor (10) is connected to the output terminal (1A) so that it does not start to charge until the transistor goes to saturated state ( 14) at the instigation of the diac (18). 2. System according to claim 1, characterized in that the control circuit of each MOS transistor (14), connected in series between the source S and the gate G of this transistor (14), comprises the secondary (15) of the transformer, a resistor (16) connected in series with said secondary and a Zener diode (17) connected in parallel with the secondary series connection (15) / resistor (16). 3. System according to claim 2, characterized in that the primary of the transformer consists of five turns at most and each secondary of the transformer consists of ten turns at most. 4. System according to claim 1, characterized in that it comprises means allowing an instantaneous re-ignition or a re-ignition of the oscillation of said system constituted
- a diode (23) connected on the anode side to the time constant (7) and on the cathode side to the drain D of the transistor (14) whose source (S) is connected to the output terminal (1B),
- a resistor (24) connected to the output terminal (1A) and the cathode of the diode (23). 5. System according to claim 1, characterized in that it comprises a series connection of the capacitor (25) / resistor (26) connected between the output terminal (1A) of the power supply (1) and the drain D of the transistor (14) whose source (S) is connected to the output terminal (1B), in order to supply a voltage in a form other than rectangular to the lighting unit (s).

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