FR2627342A1 - LUMINESCENT TUBE FEEDING DEVICE - Google Patents

Abstract

A luminescent tube feeder comprising a switching oscillator and means for regulating current in said tube. According to the invention, the primary winding 13a of the transformer 13 supplying the luminescent tube 14 is supplied by a switching oscillator 11 and regulation means 16 are interposed between a reference winding 13c of the transformer 13 and a saturation winding 12d of the transformer 12 of oscillator 11 so as to regulate the current in luminescent tube 14 by varying the oscillation frequency of switching oscillator 11. The same device may be suitable for luminescent tubes of different lengths.

Description

1 2627342

  The invention relates to a luminescent tube supply device provided with a switching oscillator, with semiconductors; it relates more particularly to a set of improvements to improve the reliability of this type of device, while allowing to use only one device model, for

  luminescent tubes of different lengths.

  Most luminescent tube supply devices include "leaky" power transformers operating at the frequency of the AC distribution network, ie, 50 or 60Hz. Such devices are heavy, bulky and bulky. Since there is no possibility to regulate the current in the luminescent tube, the manufacturers must provide one type of transformer for each tube model. This increases the manufacturing and storage costs. In addition, if the device runs empty (which can occur in particular if the luminescent tube is broken) the high voltage developed by the secondary winding of the transformer can cause a breakdown between turns and thus destruction of said transformer. More recently, it has been proposed electronic devices operating at a relatively high frequency (of the order of 20 kHz) allowing some current regulation but still using a "leaky" transformer at the output, with the disadvantages mentioned above. -above. This type of device has not reached so far, sufficient reliability, both from the point of view of regulation than'des

  risk of breakdown during empty operation.

  The invention proposes a new type of electronic device for the supply of a luminescent tube, having increased reliability, a saving in weight and space and a better current regulation. The proposed device is also remarkable in that it automatically adapts to any type of luminescent tube,

2 2627342.

  in particular of any length.

  In this spirit, the invention therefore relates to a luminescent tube supply device, characterized in that it comprises: a variable frequency switching oscillator comprising a first transformer provided with a primary winding and a winding of saturation, - a second transformer comprising a primary winding, one end of which is connected to said primary winding of said first transformer, a secondary winding intended to be connected to said light-emitting tube and a reference winding, and - regulation means arranged between said reference winding and said saturation winding for regulating the current flowing through said luminescent tube by varying the operating frequency of said oscillator. Preferably, the windings of said second transformer are high coupled (as opposed to the generally used leakage transformer) and the primary winding of this transformer is connected to the primary winding of said first transformer via a separate selfinduction. . This arrangement allows an effective protection of the transformer, in particular in the case of empty operation, thanks to the addition of a

  simple diode protection circuit.

  Moreover, in the known high-frequency power supply system, a so-called "bead" resonance phenomenon is often observed along the tube, that is to say a variation of luminous intensity along the tube, with appearance of bellies and nodes of brightness. Another object of the invention is to prevent the occurrence of such a phenomenon. For this purpose, the invention also relates to a device of the kind defined above and further comprising means for establishing different amplitude values between two consecutive alternations of said switching oscillator.

* 3 2627342

  According to a particularly simple embodiment, this means may consist of a Zener diode interposed between said saturation winding and said regulating means. Indeed, it has been found that the fact of creating an imbalance between the alternations of the switching oscillator results in the disappearance of the said oscillator.

  resonance phenomenon "in balls".

  The invention will be better understood and other advantages thereof will appear more clearly in the light of the

  following description of an embodiment of a

  device according to its principle given purely by way of example and with reference to the drawings in which: - Figure 1 is a block diagram of such a luminescent tube supply device; and Figure 2 is a variant of Box II of the

figure 1.

  The power supply device as shown schematically in FIG. 1 mainly consists of a variable-frequency switching oscillator 11, a first transformer 12 forming part of this oscillator, and a second transformer 13 capable of supplying a light-emitting tube 14. , a self-induction 15 and means of

  control 16 arranged between the transformers 12 and 13.

  The electrical energy is provided here by the alternative layout network connected to the VAC terminals. This voltage, for example 220 volts, is rectified by a conventional bridge rectifier 18, developing between the conductors 19 and 20 a rectified voltage, the shape of which is wave is substantially that of a succession of half-sinusoids. It should be noted that the capacitors C1 and C2 connected in series between the conductors 19 and 20 have, here, a relatively low capacity, so that the rectified voltage does not undergo "filtering" strictly speaking. In the remainder of the text, reference will be made to the term "source of supply voltage" VR, any continuous voltage, possibly rectified and unfiltered.

4 2627342

  (That is to say of the waveform indicated above) capable of supplying the oscillator 11. This voltage is therefore applied between the conductors 19 and 20. The device could also operate from a stable DC voltage, ie a filtered rectified voltage or

  a voltage delivered by a storage battery.

  The switching oscillator 11 comprises, in addition to the transformer 12, two oscillating half-bridges. The first half-bridge consists of semiconductor components, in this case transistors Q1, Q2 and more particularly MOS-type field effect transistors. The second half bridge consists of the two capacitors C1, C2 mentioned above. In the half-bridge of semiconductors, the transistors Q1 and Q2 are mounted with their circuits

  drain-source in series, between the conductors 19 and 20.

  Each of the control inputs of the transistors Q1, Q2 (in this case their "grids") are connected to respective secondary windings 12b, 12c of the transformer 12. The primary winding 12a of this same transformer is connected by one of its ends at the midpoint of the half bridge of

  transistors, that is to say between the two transistors Q1, Q2.

  The primary winding 13a of the second transformer 13 is connected between the two half-bridges via the series connection of the primary winding 12a (which has only a few turns) and the self-induction 15. More precisely, a first end of the primary winding 13a is connected to the primary winding 12a through the selfinduction 15 while a second end of the primary winding 13a is connected to the midpoint of the capacitor half bridge C1, C2. A starting network of the oscillator 11 comprises a resistor R1, a diode D1, a diac 25 and a capacitor C3. The resistor R1, connected to the conductor 19 forms a series branch with the capacitor C3 connected to the conductor 20. The diode D1 is connected between the midpoint of this branch and the midpoint of the half-bridge of transistors. Diac 25 is connected between the "gate" of transistor Q2 and the midpoint of

said series branch R1, C3.

2627342

  At start-up, the resistor R1 charges the capacitor C3 until the voltage at the terminals of the latter is equal to the ignition voltage of the diac 25, which transmits the energy accumulated in the capacitor C3 to the "gate of the transistor Q2, which triggers the first conduction of the transistor and the self-oscillation of the switching oscillator 11. In operation, the diode D1 bypasses the capacitor C3 at each conduction of the transistor Q2,

  to avoid inadvertent priming of the diac 25.

  According to an advantageous characteristic of the invention, the second transformer 13 is not a "leaked" transformer. In other words, the coupling between its windings, in particular the primary winding 13a and the secondary winding 13b is high. The luminescent tube 14 is provided to be directly connected to the terminals of this secondary winding 13b. In fact, the combination of the transformer 13 and the selfinduction 15 advantageously replaces a "leaked" transformer, as will be seen below. As a result, the switching oscillator 11 can be completed by a half-bridge of diodes D2, D3 in series, connected in reverse to the rectified voltage source, that is to say between the conductors 19 and 20. The midpoint of this half-bridge of diodes is connected to the link established between the self-induction 15 and said first end of the primary winding 13a of the second transformer 13.

  role of this half-diode bridge will be explained later.

  According to an important characteristic of the invention, said first transformer 12 comprises a saturation winding 12d, said second transformer 13 comprises a reference winding 13c and the regulation means 16 are arranged between said reference winding 13c and said saturation winding 12d , so as to regulate the current flowing through the luminescent tube 14, by varying the operating frequency of the switching oscillator 11. In the example described, these regulating means

6 2627342

  comprise first rectifying means constituted here by a simple diode D4 connected to the reference winding 13c and ensuring a single-wave rectification, and second rectifying means 26, constituted here by a bridge of four diodes ensuring a double rectification -alternance. This dicode bridge is connected to the terminals of the saturation winding 12d and a charging circuit 30 is established between the rectified voltage terminals of said rectifying means 26. This charging circuit 30 comprises in particular a semiconductor component such as a transistor Q3 developing between its terminals a voltage representative of the DC voltage produced by said first rectifying means. In the example, this semiconductor component is a field effect transistor and the drain-source connection of this transistor is part of said charging circuit. So it's between the source and the drain of this

  transistor that develops said representative voltage.

  Said first rectifying means, that is to say the diode D4, is connected to charge a capacitor C4 at the terminals of which is connected a potentiometer P1 whose cursor is connected to the control electrode (here the "gate") of the transistor Q3. Therefore, part of the voltage developed across the capacitor C4 is taken by the potentiometer P1 to drive the transistor Q3. In addition, the drain-source connection of the latter is connected in series with a Zener diode Z1 which is also part of said charging circuit 30. Therefore, at each alternation of the oscillator 11, the voltage "seen" by the saturation winding 12d depends on the voltage developed across the charging circuit defined above. Another Zener diode Z2 is connected in parallel on the drain-source connection of the transistor Q3 and a capacitor C5 of low value, whose role is to eliminate the parasites. The role of the Zener diode Z1 is to limit the frequency of the oscillator 11 when the device operates empty, that is to say when the transistor Q3 is saturated. Zener diode Z2 determines a high limit of saturation voltage for

7 262734Z

  the winding 12d in the case where the transistor Q3 is blocked, which may, for example, be caused by a

short circuit operation.

  It should be noted that in the diagram, the suitable direction of the windings of the windings of the transformer 12, relative to each other, is indicated conventionally.

by points.

  Finally, it should be noted that the device described comprises means for establishing values of different amplitudes between two consecutive alternations of said switching oscillator 11, that is to say to create a difference between the corresponding alternations

  respectively to the conductions of transistors Q1 and Q2.

  In the example described, it is a simple zener diode Z3 interposed between the saturation winding 12d and the second rectifying means 26. In this way, the voltage "seen" by the saturation winding 12d is different from one alternation to another according to the meaning of the

  circulation of the current in Zener diode Z3.

  The operation of the device which has just been described is as follows: As soon as the device is connected to the AC distribution network VAC, the switching oscillator 11 goes into auto-oscillation, the transistors Q1 and Q2 being periodically switched under the control of the windings 12b and 12c. Given the assembly, the current flowing through the primary winding 12_ also flows through the primary winding 13a and the self-induction 15. In accordance with the principle of the invention, the frequency of the oscillator 11 will stabilize around a value for which the luminescent tube 14 will pass through a suitably selected current, with regulation of the latter. Indeed, in operation, the luminescent tube 14 behaves substantially like a Zener diode "imposing" the voltage at its terminals, that is to say at the terminals of the secondary winding 13b. This tension depends on the characteristics

  the luminescent tube 14 and in particular its length.

8a 2627342.

  It is therefore important to regulate the current flowing through the tube 14

  so that it works in optimal conditions.

  Indeed, if the current is too weak, the lighting is bad and if it is too strong, the life of the tube is abbreviated. The regulation means 16 perform the regulation of the desired current. Indeed, the self-induction 15 limits the variation of current in the primary winding 13a of the transformer 13. However, the voltage across the primary winding 13a is imposed by the tube 14 and the entire voltage of the Available power is shared between the primary winding 13a and the self-induction 15. Therefore, if the voltage tends to increase across the primary winding 13a, then the voltage across the self-induction 15 tends to decrease. The growth of the current at each alternation therefore also tends to decrease. If we want to maintain the peak current in the tube 14 has a constant value, we must drive longer at each alternation, that is to say that the frequency of the oscillator 11 must fall. This is precisely what the regulation means 16 achieve. Indeed, if the voltage across the winding 13a increases, so is the voltage across the winding 13c. The voltage across the capacitor C4 then increases and the voltage between drain and source of the transistor Q3 decreases. As a result, the saturation winding 12d "sees" a voltage decrease across it. Under these conditions, the magnetic circuit of the transformer 12 will take longer to saturate,

  causing a decrease in the operating frequency.

  The diodes D2 and D3 make it possible to avoid overvoltages during a vacuum operation (that is to say for example when the tube 14 is broken) by returning the energy accumulated in the transformer 13 to one of the capacitors C1 or C2. For example, if it is the transistor Q2 that drives, the energy accumulated when it blocks, has

9 2627342

  tendency to generate an overvoltage between the primary winding 13a and the self-inductance 15. However, this overvoltage is eliminated by the fact that the energy is returned to the capacitor C1 by the diode D2. Under these conditions, the voltage excursion across the primary winding is limited to that of the AC distribution network and the transformer 13 is of course calculated to support such a voltage. It will be understood that this series connection of the primary winding 13a of a high-coupled transformer 13 with a self-inductance 15 is the equivalent of a "leaked" transformer, the self-inductance 15 materializing, so to speak, the However, by using a "leak-free" transformer and a separate inductor, it is possible to connect the diode half-bridge D2, D3, that is to say the elimination of overvoltages in operation. In addition, the fact of separating the transformer 13 from the "leakage choke" 15 makes it possible to control the transformation ratio of the transformer 13. It is thus possible to make a secondary 13b called "jacks" comprising selected numbers of turns between a common terminal a and terminals b, c, d, for the supply of different luminescent tubes requiring different currents.So, if we consider that the luminescent tubes are classified in three main families according to the current admissible (25mA, 50mA, 100mA) it is possible, from a single transformer 13 and therefore a single model of feeding device, to

  cover the entire range of luminescent tubes.

  Figure 2 illustrates a variant in which the

  transistor Q3 is replaced by an adjustable Zener diode Z4.

  The control input of this diode is connected to the cursor of the potentiometer P1 while a resistor R1 establishes a feedback between the common point of the diodes Z1 and Z2 and said control input of the diode Z4. The latter is equivalent to an operational amplifier with integrated reference.

2627342

Claims (9)

  1- luminescent tube supply device, characterized in that it comprises: - a switching oscillator (11), variable frequency comprising a first transformer (12) provided with a primary winding (12a) and a saturation winding (12d), - a second transformer (13), comprising a primary winding (13a) whose end is connected to said primary winding (12a) of said first transformer, a secondary winding (13h) intended to be connected to said tube a light source (14) and a reference winding (13c), and - regulating means (16) arranged between said reference winding and said saturation winding for regulating the current flowing through said light-emitting tube (14) by varying the frequency of operation of said oscillator.  2- Device according to claim 1, characterized in that the windings of said second transformer are high coupling and in that its primary winding is connected to the primary winding (12a) of said first transformer via a self-winding induction (15). 3- Device according to claim 1 or 2, characterized in that the aforementioned control means (16) comprise first rectifying means (D4) connected to said reference winding, second rectifying means (26) connected to said saturation winding and a load circuit (30) of said second rectifying means comprising a semiconductor component (Q3) developing at its terminals a voltage representative of the voltage produced by said first means of recovery.   4- Device according to claim 3, characterized in that said charging circuit (30) comprises an assembly 11i 2627342 in series of a first Zener diode (Z1) and the component semiconductor (Q3) supra.   - Device according to claim 3 or 4, characterized in that said semiconductor component is a transistor (Q3) whose control input is connected to said first means of recovery.   6. Device according to claim 5, characterized in that said transistor (Q3) is a field effect transistor whose drainsource connection is in series with said Zener diode (Z1).   7- Device according to claim 5 or 6, characterized in that a second Zener diode (Z2) is   connected in parallel with said transistor (Q3).   8- Device according to one of the claims   characterized in that it comprises means (Z3) for establishing different amplitude values between two   consecutive alternations of said switching oscillator.   9- Device according to all of claims 3   and 7, characterized in that this means consists of a zener diode (Z3) interposed between said saturation winding   (12d) and said second rectifying means (26).   - Device according to one of the claims   characterized in that said oscillator comprises two oscillating half-axes, respectively a half-bridge of semiconductor components (Q1, Q2) connected to the terminals of a supply voltage source (19, 20), whose control inputs are connected to secondary windings (12h, 12c) of said first transformer (12) and a capacitor half-bridge (C1, C2) connected across said supply voltage source (19, 20) and whose mid-point is connected to a second end of   the primary winding (13a) of said second transformer.   11- Device according to all of claims 2   and 10, characterized in that it comprises a half-bridge of 35-diodes (D2, D3) in series, connected in reverse to said source of supply voltage (19, 20) and whose midpoint is connected to the link established between the 12 2627342   self-induction (15) and said first end of   the primary winding (13a) of said second transformer.   12- Device according to one of the claims   wherein said secondary winding (13b-) of said second transformer comprises a selected number of turns between a common terminal (a) and terminals (b, c, d) for feeding different luminescent tubes.