FR2696311A1 - Fluorescent lamp auto compensating ballast - supplies HF continuous voltage whilst current input from supply source is sinusoidal and in phase with supply voltage - Google Patents

Abstract

The ballast includes a self oscillating bridge composed of an active branch formed by transistors (1a,1b) and a passive branch formed by capacitors (16a,16b). The capacitive reactance is chosen so as to generate a high frequency voltage which, after smoothing by diodes (17,17b), compensates the rectified supply current whilst charging the switching capacitor (14) to a voltage near to that of the peak of the supply voltage. A fluorescent lamp (19) connected in parallel to a capacitor (4) is connected in series with an inductor (20). The energy supplied to the fluorescent lamp (19) is taken out from an output circuit at a higher frequency according to an energy transfer or voltage transfer mode. USE/ADVANTAGE - Low cost, small component auto-compensating ballast for standard luminaires.

Description

 The present invention relates to electronic devices suitable for supplying fluorescent lamps from an electrical distribution network.

 Many devices have been developed for this purpose. They meet the 3 categories described below, as well as their disadvantages.

 The first category of these devices offers suitable characteristics both with regard to the mains current which meets international standards and with regard to the current applied to the fluorescent lamp; however, this type of device has drawbacks such as volume and cost, factors making it unusable especially in the manufacture of a disposable fluorescent lamp.

 The second category of these devices offers a cost price compatible with said disposable fluorescent lamps while delivering a suitable lamp current.

However, this type of device has a major drawback with regard to the current taken from the network which does not comply with the prescriptions laid down by international standards.

 The third category of these devices has a low volume and a reduced cost price while meeting standards for the current taken from the network, but an important drawback: the current applied to the fluorescent lamp is affected by a rate very high modulation which shortens the lamp life and produces an unacceptable strobe effect.

 The first of these devices includes an active filter which corrects the current taken from the network, this determining the volume and the cost price.

 The second of these devices does not have any filter for correcting the current taken from the network, which implies for this current a high rate of harmonics, an unacceptable crest factor and a very low power factor.

The second of these devices does not use a smoothing capacitor to filter the rectified sector which ensures good characteristics of the current taken from the sector, but in return, a mediocre efficiency of the converter and a maximum modulation of the current envelope supplied to the lamp
The device according to the invention overcomes these drawbacks.

 According to a general embodiment, this device comprises a self-oscillator in a bridge of conventional structure using on the one hand an active branch comprising 2 transistors connected in series and on the other hand a passive branch comprising 2 capacitors connected in series .

 From the high frequency circuit connected between the midpoints of the aforementioned branches of the self-oscillating bridge is taken the energy ensuring the cyclic alternating conduction of said transistors and the output energy applied between the electrodes of the fluorescent lamp. A decoupling capacitor connected between the common polarized terminals of the self-oscillating bridge is charged by 2 rectifier bridges whose polarized outputs, suitably oriented, are mounted in series, the non-polarized input of the first being connected to the network while the the non-polarized input of the second is charged under a high frequency voltage recovered from the above-mentioned circuit connected between the midpoints of said branches of the self-oscillating bridge. This latter voltage is obtained thanks to the sufficiently large reactance offered by the above capacitors the passive branch of the self-oscillating bridge. This voltage is applied, via a galvanic isolation capacitor, to the rectifier bridge indicated above. The point common to the above rectifier bridges is decoupled by a capacitor, one terminal of which is connected to one of the common polarized terminals of the self-oscillating bridge. Between the terminals of this capacitor, an alternating voltage develops in phase with that of the rectified sector and having a substantially equal maximum amplitude, this ensuring almost perfect compensation for the current taken from the network, while the voltage across the above-mentioned decoupling capacitor of the common polarized terminals of the self-oscillator remains substantially constant. Under these conditions, the envelope of the output current applied between the electrodes of the fluorescent lamp also remains substantially constant, which results in the emission of a luminous flux without appreciable stroboscopic effect.

 This device therefore provides the desired function, current taken from the network satisfying the prescriptions decreed by the standards in such matter and low rate of modulation of the current applied between the electrodes of the fluorescent lamp. In addition, this device makes it possible to produce, at low cost, a product of very reduced volume.

 This device is applicable to the construction of disposable electronic modules intended to power fluorescent lamps called "compact", monoblock luminaires called "brights-sticks", standard luminaires with one or more fluorescent lamps.

 Figure 1, attached, illustrates a particular embodiment of the device according to the invention.

 This embodiment, in accordance with what has been described above, comprises a self-oscillating bridge composed of a first active branch constituted by the transistors la and lb connected in series, on the other hand of a passive branch constituted by the capacitors 16a and 16b connected in series. The common polarized terminals of this self-oscillating bridge are connected to the terminals of the decoupling capacitor 14 which is charged by the polarized outputs, connected in series, of two rectifier bridges, the first bridge 13 has its non-polarized input charged by the sector through the deworming inductor 9, while the second bridge, constituted by suitably polarized diodes 17a and 17b, has its non-polarized input charged by a voltage recovered at high frequency transmitted by the galvanic isolation capacitor 15.

 Said voltage recovered at high frequency is taken from the midpoint of the said passive branch of the oscillator bridge. Between the midpoint of this passive branch and the other midpoint belonging to the above-mentioned active branch, is connected a high frequency circuit from which the energy is taken, on the one hand, ensuring the control of the alternating cyclic conduction. transistors la and lb and on the other hand the output energy applied to the fluorescent lamp 19. In order to apply a suitable amplitude voltage to the unpolarized input of the doubling bridge comprising the diodes 17a and 17b, the sum of the reactances of the capacitors 16a and 16b is determined so that the sum of the currents flowing through the said capacitors develops between their common midpoint and one of the said common polarized terminals, the desired voltage amplitude. The integration of this high frequency voltage thus rectified takes place between the terminals of the capacitor 18 in the form of a low frequency alternating voltage, the peak amplitude of which will be substantially equal to the peak amplitude of the voltage. sector rectified by the rectifier bridge 13. According to this embodiment of the device, object of the invention, the above high-frequency circuit is a series circuit comprising the inductor 20, the capacitor 4 connects in parallel to the fluorescent lamp 19 via the cathodes emissive of the latter which it provides heating and the primary 7b of the control transformer 7 whose primaries 7a and 7c, suitably oriented, respectively control the alternating cyclic conduction of the transistors la and lb. The parallel circuits comprising resistors 2a and 2b respectively as well as diodes 3a and 3b have the role of shaping the control current of said transistors in order to minimize the switching losses.

 The diodes 5a and 5b ensure the double circulation of alternating current at high frequency between the midpoint of said active branch of one self-oscillator and its common polarized terminals.

 The initiation of the operation of the self-oscillator is ensured by a conventional trigger circuit comprising the resistor 6, the capacitor 12, the diac 11 and the inhibition diode 10.

 When this device is energized, the capacitor 14 is charged under a voltage which is substantially continuous and substantially equal to the peak voltage of the sector applied between the terminals A and 8, this, on the one hand thanks to the current rectified by the bridge 13 and on the other hand thanks to the additional voltage developed between the terminals of the capacitor 18. Under these conditions, the mains current rectified by the bridge 13 is compensated by the current rectified by the diode bridges 17a and 17b, it therefore affects a substantially sinusoidal envelope in phase with the mains voltage.

 Because the voltage between the terminals of the capacitor 14 is substantially continuous, the envelope of the high frequency output voltage also remains substantially continuous, this corresponding to a low rate of modulation of the current passing through the fluorescent lamp 19, which, therefore, will produce a luminous flux presenting only a negligible stroboscopic effect.

 With regard to this result, the current taken from the network has a very reduced harmonic ratio, a crest factor close to 1.414 and a power factor approaching 1 unit.

 Figure 2, appended, illustrates another embodiment of the device according to the invention.

 In the latter case, the capacitor 4 no longer ensures the heating of the cathodes of the lamp 19, but is simply connected in parallel with the lamp 19.

 The said cathodes are heated according to the so-called "rapid-start" mode by means of a secondary winding 20b torque to the inductor 20 from which it is galvanically isolated, and by a second secondary winding 20a which can be formed from '' a tap made on the output of the main inductor 20.

The rest of the circuit remains in accordance with the embodiment described above
FIG. 3, attached hereto, illustrates another embodiment of the device according to the invention.

 In the latter case, the output energy applied to the fluorescent lamp 19 is no longer as in the previous examples using the "energy transfer" mode, but according to the "voltage transfer" mode which uses a transformer 23 whose the primary 23c is, alone, connected in the high frequency circuit mounted between the midpoints of the two branches of the self-oscillating bridge.

 A secondary winding 23d galvanically isolating the lamp 19 from the rest of the circuit, supplies the latter with its operating energy. The secondary cathode heating windings 23e and 23f ensure the operation of said lamp according to the "rapid-start" mode.

 An optional capacitor 4 is connected in parallel with the lamp 19.

 It is obvious that in the latter case, it is possible to replace the heating provided by the windings 23e and 23f by the heating caused by the current passing through the capacitor 4 connected in series with the cathodes of the lamp 19, as described in the figure. 1.

 According to this embodiment, the sum of the unit reactances specific to the capacitors 16a and 16b is replaced by the reactance of the single capacitor 16, which produces the same effects.

 FIG. 4, attached hereto, illustrates an embodiment in accordance with the modes described above, but differs therefrom by the mode of control of the alternating cyclic conduction of the transistors la and lb which no longer uses a control transformer independent but at the two secondary windings 23a and 23b couples to the transformer 23. These suitably oriented windings fulfill the same function as the windings 7a and 7b, described above.

 In addition, this circuit differs from the circuits described above, by the triggering mode of the self-oscillator which is constituted by the series circuit comprising the voltage threshold element 21 mounts in series with the capacitor 22, which series circuit is connected between the common polarized terminals of the self-oscillator through the emitter base space of the transistor lb or through the control winding 23b.

 Finally, this circuit differs from the previous ones by the resistance with a positive temperature coefficient 8 connected in parallel with the capacitor 18, its role being to limit, when the device is powered up, the voltage across the terminals of the capacitor 14, as long as the lamp 19 has not started, this only occurs after a suitable time for preheating the cathodes. This PTC resistor 8 could also be connected in parallel with the capacitor 16 and would operate in this case, in alternating current.

 Figure 5, attached, illustrates a last embodiment according to the invention.

 The device described here represents the application of the invention to electronic ballasts capable of fulfilling all the required functions, security in the event of deactivation of the lamps, regulation of the lamp current in the event of variation of the mains voltage, security in the event of over - accidental tension of the sector, etc ...

 The first difference between the circuits described above, concerns the replacement of the diodes 17a and 17b by the use of one half of the rectifier bridge 17, the other half of this bridge having its non-polarized input connected to the output of the inductor. 20, common to one of the terminals of the lamp 19, by means of the low-value capacitor 37. This arrangement ensuring advantageous shaping of the alternating voltage available between the terminals of the capacitor 18.

 Another difference concerns the primary winding of the inductor 20 which is wound here in two-wire corresponding to the two windings 20d and 20e whose inputs are respectively connected to the terminals of the resistor 38, itself connected in series with the transistors la and 1b while the outputs of said two-wire winding are connected together and connected to one of the terminals of the lamp 19. This particular assembly, associated with the resistor 38 ensures the suppression of the overcurrents which pass through the transistors during the switching operations carried out by those -this.

 Another difference with the previous circuits relates to the reactance divider bridge comprising the capacitor 24 associated with the capacitor 15. In this arrangement, the capacitor 24 takes a fraction of the high frequency voltage developed between the terminals of the capacitor 16 to reapply it, after rectification by the diodes 25a and 25b suitably oriented, on the terminals of the capacitor 14. Under these conditions, it is possible in any case to limit the peak voltage between the terminals of the capacitor 18 to a value substantially equal to the voltage of sector peak, despite possible variations in lamp impedance 19.

 Another difference with the previous circuits relates to the semiconductor 33 mounted in parallel with the capacitor 16 and whose instantaneous conduction is controlled by the threshold element 34. The tilting voltage of the latter is taken from the potentiometer 35 or a resistive bridge whose input E is connected to a terminal having a reference voltage, such as terminal I, terminal H or output terminal G of a 20g winding coupled to inductor 20. This assembly allows slaving the power applied to the lamp 19 to a pre-established average value, without altering the power factor of the current taken from the network by the device.

 Another difference with the previous circuits relates to a safety device making it possible to inhibit the operation of the self-oscillator in the event of a problem affecting the output load, in particular in the event of deactivation of the lamp 19.

This circuit comprises the semiconductor 27, associated with the suitably polarized diode 26, the conduction of which is caused by the voltage threshold element 28 associated with the capacitor 30 and with the bridge of resistors 31 and 32 whose input F is connected to one of the H terminals,
I, etc ... carrying the reference voltage. In this way, as soon as this reference voltage exceeds a predetermined threshold, the conduction of the semiconductor 27 takes place and, deriving an alternation circulating in the above-mentioned circuit at high frequency, stably inhibits the operation of the auto- oscillator which can only start again after having disconnected and reconnected the sector between sockets A and B, this allowing the trigger circuit to supply a new trigger pulse.

 The same function can be ensured by the semiconductor 27 by making its connection at other points of the high frequency circuit as well as to the control circuit of one of the transistors la or lb.

Claims (18)

 Another difference with the previous circuits relates to the deworming filter 9 which in the present case is no longer connected in the unpolarized input circuit of the rectifier bridge 13, but in its polarized output circuit which has the effect of reinforcing the high frequency filtering operated by the capacitor 18.  The last difference with the previous circuits concerns the control of the alternating cyclic conduction of the transistors la and lb, which in this embodiment using the "energy transfer" mode, is ensured by the windings 20c and 20f coupled to the inductor 20.  The device according to the invention, as just described allows the realization of electronic ballasts suitable for equipping fluorescent lights, disposable or not, this for a low cost price, a small volume and in accordance with relative international standards aware of the network.  As is obvious and as it follows from the above, the invention is in no way limited to the modes of application more particularly envisaged, it embraces, on the contrary, all variants.  1. Device powered by the electrical distribution network comprising a high frequency self-oscillator affecting a bridge structure comprising a first active branch provided with transistors la and lb connected in series and a passive branch provided with capacitors 16a and 16b mounted in series, the common polarized terminals of said bridge being decoupled by a capacitor 14 which is continuously charged by a series circuit comprising the polarized output circuit of a first rectifier bridge whose non-polarized input circuit is connected to said network and comprising the polarized output of a second rectifier bridge whose non-polarized input circuit is charged by a voltage of suitable amplitude taken from the energy at high frequency converted by said self-oscillator, this device being intended for supplying the fluorescent lamps, characterized in that by suitably choosing the value of the sum of the re actuations specific to capacitors 16a and 16b at the average operating frequency of the self-oscillator, this making it possible to develop, between the midpoint of the above-mentioned passive branch and one of the common polarized terminals of the self-oscillator, a voltage at high frequency which is applied to the unpolarized input circuit of said second rectifier bridge comprising the diodes 17a and 17b suitably polarized, this making it possible to obtain between the terminals of the high frequency decoupling capacitor 18 a low frequency voltage of maximum amplitude substantially equal to the peak voltage supplied by said network, this having the effect of compensating for the rectified mains current charging the decoupling capacitor 14, the power supply for the aforesaid fluorescent lamp 19 being taken from the frequency output circuit high depending on the "energy transfer" mode or the "voltage transfer" mode.  2. Device according to Claim 1, characterized in that the capacitors 16a and 16b are replaced by a single capacitor 16 connected to one of the common polarized terminals of the self-oscillator, this capacitor offering a reactance equal to the sum of the reactances of the first two.  3. Device according to Claims 1 and 2, characterized in that the fluorescent lamp 19 connected in parallel with the capacitor 4 is mounted in series with the inductor 20.  4. Device according to any one of Claims 1 and 2, characterized in that the fluorescent lamp 19 is connected between the terminals of a secondary winding 23d belonging to the isolation transformer 23 whose primary is connected between the active branch and the passive branch of the self-oscillating bridge.  5. Device according to any one of Claims 1 to 4, characterized in that the control of the alternating cyclic conduction of the transistors la and lb is ensured by the windings 7 a and 7b of the control transformer 7 whose primary is connected in series with inductance 20 or with primary 23c of transformer 23.  6. Device according to any one of Claims 1 to 4, characterized in that the alternating cyclic control of the conduction of the transistors la and lb is ensured by the windings 20c and 20f or the windings 23 a and 23b respectively coupled to the inductor 20 and transformer 23.  7. Device according to Claim 5, characterized in that the heating of the cathodes of the fluorescent lamp 19 is ensured by the secondary windings 20a and 20b couples to the inductance 20.  8. Device according to Claim 6, characterized in that the heating of the cathodes of the fluorescent lamp 19 is provided by the secondary windings 23e and 23f coupled to the transformer 23.  9. Device according to any one of Claims 1 to 8, characterized in that the capacitor 18 is shunted by a resistor with a positive temperature coefficient 8 intended to limit the overvoltage between the terminals of the capacitor 14, when the device is started.  10. Device according to any one of Claims 1 to 9, characterized in that the trigger circuit of the self-oscillator is constituted by the series circuit comprising a voltage threshold element 21 and a capacitor 22, this series circuit being connected between the common polarized terminals of the self-oscillator through the base-emitter space of transistor lb or through the control winding of this transistor.  11. Device according to any one of Claims 1 to 10, characterized in that a galvanic isolation capacitor 15 is mounted in series with the high frequency circuit connected between the midpoint of the said active branch and the point middle of the above passive branch.  12. Device according to any one of Claims 1 to 11, characterized in that between the base-emitter space of each transistor la and lb and the relative control winding, is connected in series with a parallel circuit comprising respectively on the one hand the resistor 2a and the diode 3a and on the other hand the resistor 2b and the diode 3B, said diodes being polarized so that the switching losses of said transistors are limited.  13. Device according to any one of Claims 1 to 12, characterized in that a capacitor 24 whose reactance is combined with that of the capacitor 15 so that the maximum amplitude of the voltage developed between the terminals of the capacitor 18 does not exceed the peak voltage of the sector, said capacitor 24 being connected to the bridge of diodes 25a and 25b, suitably polarized, which allow restitution of the energy in question between the terminals of capacitor 14.  14. Device according to any one of Claims 1 to 13, characterized in that the winding of the inductor 20 or that the primary of the transformer 23 is wound in bi-wire whose inputs are connected respectively to each terminal d '' a resistor 38 itself mounted in series with the transistors la and lb, its  outputs taking place on a single output connected to the above high frequency output circuit.  15. Device according to any one of Claims 1 to 14, characterized in that the diodes 17a and 17b are replaced by half of the rectifier bridge 17, the other half of said bridge being charged by the capacitor 37 whose free terminal is connected to the terminal common to the lamp 19 and to the inductor 20.  16. Device according to any one of Claims 1 to 15, characterized in that a semiconductor 33 connecting between the terminals of the capacitor 16b or of the capacitor 16, can temporarily cancel the voltage existing between their terminals, this under l action of a device sensitive to the rise in a circuit voltage taken for reference.  17. Device according to any one of Claims 1 to 16, characterized in that a semiconductor 27 connects between the output of the inductor 20 or the output of the primary of the transformer 23 and the common polarized terminal of the self-oscillator, allow to stably inhibit the operation of the auto-oscillator by means of a device sensitive to the rise in a circuit voltage taken for reference.  18. Device according to any one of Claims 1 to 17, ca racter i se in that the high frequency deworming inductor 9 is connected in series with the polarized output circuit of the mains rectifier bridge 13.