EP0578575A1 - Device for operating a discharge tube with a high frequency high voltage signal - Google Patents

Abstract

The apparatus essentially includes an oscillator (1) with an output transformer (2), a complementary inductor (3), a capacitor (4) forming, with the inductors, a resonant circuit, with recovery diodes (4a), and a switching transistor (5) in which the emitter-collector space is arranged in parallel with the capacitor (4). The base of the transistor (5) is driven by a winding (25) formed on the transformer (2) through a resistor (9). A control transistor (8) is arranged to turn off the transistor (5) when the current supplied to the oscillator (1) through a resistor (7) reaches a threshold value. Thus, the switchings of the switching transistor (5) take place when the voltage between emitter and collector is substantially zero, which reduces the losses. A circuit (30) triggers the oscillation when a voltage is applied to the input of the direct current source (10). <IMAGE>

Description

The invention relates to an apparatus for supplying high voltage at high frequency to discharge tubes in a gaseous atmosphere, comprising a DC voltage source supplied from a distribution network, a resonant circuit, consisting of a capacitor in series with an inductor, connected between poles of the source, the inductor including a transformer primary coupled to a secondary connected to the discharge tube, and a switching transistor with an emitter-collector space mounted so as to modulate, with a appropriate phase, an injection of electrical charges into the resonant circuit in response to a signal applied to its base by a feedback circuit having a winding coupled to the inductor.

The gas discharge tubes in question are mainly intended for illuminated signs; they comprise, at the ends of a tube whose development can reach several meters, cold electrodes.

The current / voltage characteristics of such tubes are extremely different from the passive impedance characteristics, and impose special precautions for use. Schematically, if the voltage between electrodes is gradually increased starting from zero, practically no current flows until the voltage has reached a starting value. Then the discharge begins and the ionized gas lights up; the voltage between electrodes is established at a value, called discharge voltage, lower than the starting voltage and, as a first approximation independent of the current flowing through the tube but a direct function of the length thereof. The source must therefore have a high internal impedance to control the power in the discharge. Furthermore, if the electromotive force of the source is decreased below the discharge voltage, the tube ceases to be conductive.

The supply devices for these tubes almost always deliver alternating voltages, and at each half-cycle the tube describes the priming cycle, sustained discharge, extinction. This avoids asymmetrical aging of the tube in particular in the vicinity of the electrodes, and on the other hand, the limitation of the through current is easier, and obtaining a high supply voltage of the tube can be done by a transformer.

The relationship between the ignition voltage and the discharge voltage depends on many factors, the most important of which are the temperature of the gas before ignition, and the time between ignition and the previous extinction; it will be noted that this duration must drop to a few tens of microseconds to have a significant effect on the ratio of ignition voltage / discharge voltage.

In practice, it is commonly accepted that the starting voltage of a cold tube can reach 2.3 times the discharge voltage; more precisely it is expected, to be sure to obtain a priming of the tube under the most difficult environmental conditions, that the peak voltage delivered by the vacuum supply apparatus will be 2.3 times the nominal effective voltage of the tube Operating. The discharge voltage may deviate somewhat from this nominal voltage due to variations in the gas pressure, and therefore its temperature in the tube in operation, and variations specific to the tube (state of the electrodes - filling pressure, aging ).

Usually the discharge voltages are a few kilovolts, and the currents a fraction of an ampere. Typically, an appliance will provide 4,500 effective volts vacuum for a nominal discharge current of 100 mA.

• Des transformateurs à fréquence industrielle classiques sur noyau de tôles, comportant une fuite magnétique pour limiter le courant de décharge dans le tube ; ces appareillages sont lourds, encombrants, difficiles à protéger, et sources de pertes. Mais ils sont économiques.
• Des appareillages électroniques, fonctionnant à haute fréquence (entre 20 et 30 kHz), de taille et poids réduits, avec un bon rendement énergétique et de protection aisée. Ces appareillages sont plus coûteux et moins fiables que les transformateurs à fuite classiques, et en outre sont générateurs de parasites.

There are two types of current equipment:

• Conventional industrial frequency transformers on a core of sheets, comprising a magnetic leak to limit the discharge current in the tube; these devices are heavy, bulky, difficult to protect, and sources of loss. But they are economical.
• Electronic devices, operating at high frequency (between 20 and 30 kHz), of reduced size and weight, with good energy efficiency and easy protection. These devices are more expensive and less reliable than conventional leakage transformers, and moreover generate parasites.

It will be noted that the discharge tubes with a gaseous atmosphere exhibit self-generating interference, due to the high voltages involved, and sudden current transitions on ignition. Leak transformers on the one hand create 100 Hz fundamental parasites, whose harmonic spectrum is very attenuated in the long radio wave range (which starts at 150 kHz), and their leakage inductance prevents return on the network of parasites created on their secondary.

However, the operating frequencies of electronic equipment are much closer to the radio frequency ranges. In addition, many electronic devices operate as a signal generator more or less close to square signals, with transformers whose primary is attacked by transistors controlled by a relaxer. The voltages and currents have spectra rich in harmonics. The document FR-A-2 503 522, for example, describes a device for supplying a discharge tube comprising a transistor mounted as a blocking oscillator.

Resonant circuit oscillators have been proposed, in particular for the supply of fluorescent tubes (see for example FR-A-2 578 709). Fluorescent tubes, which are discharge tubes with hot cathodes, have current / voltage characteristics similar to those of tubes with cold electrodes, and require, for their power supply, devices with high internal impedance also; however, the voltages used are much lower. These oscillators comprise a resonant circuit, consisting of a capacitor and an inductor, supplied through at least one transistor whose base receives a voltage derived from the voltage across the resonant circuit by a winding coupled to the inductor. The latter includes a transformer primary, the secondary of which is connected to the electrodes of the discharge tube. Proper biasing of the base of the transistor ensures that the current which powers the resonant circuit is injected during an appropriate fraction of the oscillation cycle.

These resonant circuit oscillators have a reduced harmonic spectrum, due to the overvoltage factor of the resonant circuit. The switching losses in the transistor are however relatively high, the switching not occurring at times when the voltage on the emitter-collector space is zero. The losses become prohibitive if one seeks relatively high output powers.

The object of the invention is to produce an apparatus for supplying discharge tubes in a gas, which, compared to existing apparatuses, produce parasites at a reduced level, whose energy efficiency is excellent, which has reliability. improved, which is a relatively low cost, and whose size and weight are reduced.

This objective is achieved by an apparatus for supplying high-voltage at high frequency to discharge tubes in a gaseous atmosphere, comprising a DC voltage source supplied from a distribution network, a resonant circuit consisting of a capacitor in series with an inductor connected between poles of the source, the inductor including an output transformer primary coupled to a secondary connected to the discharge tube, and a switch transistor mounted in sort of modulating, with an appropriate phase, an injection of electrical charges into the resonant circuit in response to a signal applied to its base by a feedback circuit having a winding coupled to the inductor, characterized in that, the emitter space -collector of the switch transistor being connected in parallel to the capacitor, with a recovery diode means mounted head to tail with the emitter-collector space, the feedback circuit comprises a pilot stage sensitive to the current delivered by the source to the resonant circuit , and mounted to block the switch transistor in response to a delivered current exceeding a fixed value.

The resonant circuit produces a sinusoidal alternating voltage whose harmonic spectrum is not very wide: the energy efficiency is excellent, because the commutations of the transistor, between the blocking state and the saturated state, and conversely occur, as we will see during the analysis of the operation, when the voltage between emitter and collector passes substantially zero. Thus, the losses of the transistors are reduced to a minimum, which increases the reliability of the assembly. The recovery diode means, which displaces the voltage excursion range across the resonant circuit capacitor, allows the switching transistor to drive this resonant circuit over the entire alternation, so that it is not necessary to use a pair of power transistors.

Preferably the pilot stage comprises a resistor connected in series between a first pole of the source and the emitter of the switching transistor and a pilot transistor with its base connected by a resistance bridge to the emitter of the switching transistor, its emitter connected at the first pole of the DC source, and its collector at the base of the switching transistor. From this arrangement it follows that, when the current in the resistor connected in series between the first source pole and the emitter of the switching transistor reaches a determined value, the transistor pilot conducts and saturates, polarizing the base of the switch transistor so as to block the latter. Conversely, when the current in the resistor decreases below the determined value, the pilot transistor is blocked and the switching transistor conducts.

Preferably, the feedback circuit comprises a winding formed on the output transformer with one end connected to the emitter of the switch transistor, and a second end connected through a resistor at the base of this switch transistor. This arrangement makes it possible to return an image signal of the oscillating voltage in the output transformer to the base of the switching transistor, while allowing the switching transistor to be blocked by the pilot transistor.

As a variant, the maintenance winding is coupled to an inductor which completes the primary of the output transformer, and connected between the emitter of the switching transistor and a resistor connected to the base of this transistor, a transistor mounted as a phase corrector having its collector connected to the base of the switch transistor, its emitter connected to the emitter of this switch transistor and its driven base, through a capacitor by the junction of the winding and its series resistance. It will be understood that the phase shift between the voltage across the terminals of the complementary inductor and that which develops on the primary of the output transformer is compensated for by mounting the transistor as a phase corrector.

In preferred arrangement, the apparatus comprises a launching circuit coupled to the DC voltage source and capable of delivering on the base of the switching transistor a single pulse in response to the growth of a control voltage originating from the source between a value low and a trigger threshold.

If the voltage growth is too slow after switching on the switchgear, it is possible that a steady state will be established, without generation of oscillations. The impetus created by the launch circuit prohibits the establishment of this stable regime.

Preferably, the apparatus will include auxiliary circuits for various safeties.

Secondary characteristics, and the advantages of the invention will emerge moreover from the description which follows, by way of example, with reference to the single appended figure.

According to the embodiment chosen and shown, the apparatus for the high-voltage supply of gas discharge tubes comprises an oscillator 1 as a whole, supplied by a DC voltage source 10 as a whole, connected to a network 14 or mains electrical distribution, alternating voltage. This source 10 consists of a rectifier with four diodes in bridge 11, with a first continuous pole 11 a , negative, a second continuous pole 11 b , positive and two alternating poles 11 c and 11 d . A filtering and smoothing capacitor 12 is arranged between negative poles 11 a and positive 11 b , A suppressor filtering circuit 13 is disposed between sector 14 and bridge 11, and comprises a double-winding inductor and single core, and two capacitors mounted between alternative poles 11 c and 11 d and earth. Note that the negative pole or first continuous pole 11a is connected to an internal ground of the apparatus.

The oscillator 1 comprises an output transformer 2, with a primary with two terminals 23 and 24 respectively connected to the second pole 11 b of the DC voltage source 10, and one end of a complementary inductor 3, a high voltage secondary with a midpoint 20 connected to earth through a safety circuit which will be discussed later, and two extreme terminals 21 and 22, which will attack the two electrodes of a discharge tube, not shown.

The transformer 2 further comprises two auxiliary windings 25 and 26, the functions of which will be specified below.

Beyond the complementary inductance 3 in the direction of the first pole of the source, the circuit is divided into three parallel branches which meet on a common conductor 6. A first branch consists of a capacitor 4, intended to resonate with the transformer 2 and the complementary inductor 3, the second branch of a series 4 has two blocking recovery diodes from the inductor 3 to the common conductor 6, and the third branch of the emitter-collector space of a switching transistor 5, in series with a protective diode 5 a passing from the complementary inductance 3 to the collector of the switching transistor 5, the emitter of the latter being connected to the common conductor 6.

The base of the switching transistor 5 is connected to a feedback circuit which consists of the auxiliary winding 25 of the output transformer 2, one end of which is connected to the common conductor 6, while the other is connected to the base of the transistor 5 through a resistor 9. Furthermore, the common conductor 6 is connected to the first pole 11 a of the bridge 11 of the continuous source 10 by a resistor 7, which will be traversed by the current delivered to the oscillator 1.

A pilot transistor 8 is arranged with its emitter at the first pole 11 a , its collector at the base of the switching transistor 5, while its base is attacked by a voltage derived by a bridge of resistors 8 a of the voltage developed on the resistance 7 by the current in oscillator 1.

To describe the operation of the oscillator, we will start from an initial instant when the switching transistor 5 is on, the capacitor 4 discharged, the current supplied to the oscillator is zero and therefore the pilot transistor 8 blocked. In all that follows, the protection diode 5 a will not be mentioned, since its role is only to prevent the transistor 5 from being subjected to a reverse voltage. Furthermore, given that, when the secondary of the transformer 2 flows through the tube to discharge, the impedance of the primary 23-24 is approximately ohmic, the complementary inductance 3 will form the essential of the resonant inductance with the capacitor 4.

At the original instant, as defined above, the DC source voltage 11 is almost entirely applied to the inductance of the resonant circuit (formed from the leakage inductance of the transformer primary 2 and the additional inductance 3). The current flowing through oscillator 1 increases linearly over time; in parallel, the auxiliary winding 25, seat of an increasing voltage, injects into the base of the switching transistor 5, through the resistor 9, charges which keep this transistor in saturation.

When the current in the resistor 7 reaches a selected threshold value, the pilot transistor 8 becomes conductive, and brings the base of the switching transistor 5 to the potential of the first pole 11 a , less than the potential of the common conductor 6, so that the transistor switch 5 hangs. The current supplied by the source 10 is diverted in the capacitor, and all of the capacitor 4 and the inductors resonates. The current in the inductor decreases with the charge of the capacitor, then cancels and reverses to discharge the capacitor through the recovery diodes 4 a . The voltage across the inductor describes a half-sine wave, approximately.

Although the current in the resistor 7 is canceled with the current in the inductor, the negative voltage induced in the winding 25, image of the voltage across the inductors takes over from the saturation of the pilot transistor 8 to maintain the transistor switch 5 in the off state, until the capacitor 4 is discharged. At this time the oscillator 1 has returned to its original state, and the transistor 5 is turned on, the emitter-collector voltage being zero, then a new cycle occurs.

If the winding 25 were coupled to the inductor 3 to benefit from a better decoupling with respect to the load by the discharge tube, there would be a phase-shifting transistor, with its collector at the base of the transistor switch 5, its transmitter connected to the common conductor 6, and its base attacked by the junction between the winding 25 and the resistor 9, through a capacity forming time constant with a resistance between base and emitter. The phase shift obtained would compensate for the voltage phase shift across the inductor 3 and the transformer primary 2.

We admitted an original state, where the switching transistor 5 is saturated, and the capacitor 4 discharged. However, when the apparatus is energized, the high continuous voltage increases relatively slowly at the terminals of the smoothing / filtering capacitor 12. There can then occur a state of equilibrium where the transistors 5 and 8 are both partially on. , under a voltage between emitter and notable collector. A launch circuit 30 has therefore been provided as a whole. This circuit 30 comprises a transistor 32 with its emitter at the first pole 11 a , its base driven by an alternating pole 11 c of the bridge 11 through a circuit 33 with time constant, and its collector connected by a resistor 34 to this same pole alternative 11 c . In parallel with the emitter-collector space of the transistor 32 is arranged a capacitor 35, and the collector of the transistor 32 is connected to the base of the switching transistor 5 through a diac 31. This launching circuit operates as follows:
on the alternating pole 11 c is born a tension, compared to the first continuous pole 11 a which comprises a continuous component equal to the rectified half-voltage, and an alternating component of amplitude equal to the continuous component.

This voltage, indicating the presence of DC voltage on oscillator 1, will charge the capacitor 35 through the resistor 34, while the transistor 32 remains blocked, the voltage applied to its base increasing more slowly through the time constant circuit 33. When the charging voltage of the capacitor 35 reaches the breakdown voltage of the diac 31, the latter transmits a positive pulse on the basis of the transistor switch 5, and the oscillation cycle begins, as described above. Then the transistor 32 is unblocked due to the growth of its basic polarization, the capacitor 35 remains discharged, as long as the control voltage remains.

Several safety circuits are provided on the switchgear. All these circuits lead to the gate of a MOS transistor 15, the source of which is connected to the common conductor, 6, and the drain at the base of the switching transistor 5. The presence of a positive voltage on the gate of the MOS 15 causes the blocking of transistor 5, and stopping operation of oscillator 1.

A first safety device 40 intervenes if the secondary load 21, 22 of the output transformer 2 disappears, which results in overvoltages in this transformer 2. The auxiliary winding 26, one end of which is connected to the common conductor 6, attacks by its other end a detection circuit consisting of a diode 42 and a capacitor 43 in parallel with a resistor. A diac 41 is disposed between the preceding detection circuit and the gate of the MOS 15.

If the secondary load of the transformer 2 disappears, the voltage detected by the diode 42 exceeds the breakdown voltage of the diac 41, and the MOS 15 conducts.

The safety circuit 50 has a double function: on the one hand, to stop the oscillation as soon as the mains voltage disappears, and on the other hand, to cause a tripping if leaks at the secondary of the transformer 2 appear.

Stopping the oscillator as soon as the mains voltage disappears is useful for enabling rapid switch-on and switch-off, making it possible to attract public attention more easily on the message of the illuminated sign.

For this, the control voltage taken from the alternating pole 11 c polarizes, through a resistor 54, the base of a transistor 52, called standby transistor, whose emitter-collector space is in parallel on a capacitor 53, capable of being charged through a resistor 53 a by the high voltage across the capacitor 12. A diac 51 connects the collector of the transistor 52 to the gate of the MOS transistor 15.

As long as the control voltage polarizes the base of the transistor 52, the latter conducts and prevents the charging of the capacitor 53. If the mains voltage, and therefore the control voltage disappear, the transistor 52 is blocked, the capacitor 53 charges and reaches the breakdown voltage of diac 51, causing tripping.

Furthermore, between midpoint 20 of the secondary of the transformer 2 and the earth, a detector circuit 50 ′ has been arranged composed of two diodes 59 and 58 in opposite directions, with interposition between cathode of diode 58 and earth of a resistor 57 shunted by a capacity. If a leak appears between one of the terminals 21 or 22 of the output transformer 2 and the earth, a voltage will appear at the terminals of the resistor 57.

At the terminals of the resistor 57 is disposed a light-emitting diode 56, optically coupled to a phototransistor 55 placed in the safety circuit 50, with its emitter-collector space in parallel with the emitter space base of the transistor 52. When the light emitted by the light-emitting diode 56 strikes the phototransistor 55, the latter conducts and cancels the witness voltage applied, through the resistor 54, to the base of the transistor 52. Thus, a leakage to earth of the secondary of the output transformer 2 controls the disjunction under the same conditions as a mains voltage cut.

Of course, the invention is not limited to the example described, but embraces all of the variant embodiments thereof, within the scope of the claims.

In particular, the auxiliary circuits provided for launching 30 and the safety devices 40, 50, 50 'could take any suitable form adapted to the desired functions.

Claims (11)

Apparatus for the high-frequency high-voltage supply of discharge tubes in a gaseous atmosphere, comprising a direct voltage source (10) supplied from a distribution network (14), a resonant circuit consisting of a capacitor ( 4) in series with an inductor (3, 2) connected between poles (11 a , 11 b ) of the source (10), the inductor including a primary (23-24) of output transformer (2) coupled to a secondary (21-22) connected to the discharge tube, and a switch transistor (5) mounted so as to modulate, with an appropriate phase, an injection of electric charges into the resonant circuit (2, 3, 4) in response to a signal applied to its base by a feedback circuit having a winding (25) coupled to the inductor (2, 3), characterized in that, the emitter-collector space of the switching transistor (5) being connected in parallel to the capacitor (4), with diode means (4 a ) for recovering mounted head to tail with the emitter-collector space, the feedback circuit comprises a pilot stage (7, 8) sensitive to the current delivered by the source (10) to the resonant circuit, and mounted to block the switching transistor (5) in response to a delivered current exceeding a fixed value. Apparatus according to claim 1, characterized in that the pilot stage comprises a resistor (7) connected in series between a first pole (11 a ) of the source (10) and the emitter of the switching transistor (5) and a transistor pilot (8) with its base connected, through a resistance bridge (8a) to the emitter of the switching transistor (5), its emitter connected to the first pole (11 a ) of the direct current source (10), and its collector at the base of the switch transistor (5). Apparatus according to claim 2, characterized in that the feedback circuit comprises a winding (25) formed on the output transformer (2), with one end connected to the emitter of the switching transistor (5), and a second end connected, through a resistor (9), to the base of this switching transistor. Apparatus according to claim 2, characterized in that the feedback circuit comprises a maintenance winding coupled to an inductor (3) completing the primary of the output transformer, and connected between the emitter of the switching transistor (5) and a resistor connected to the base of this transistor, a transistor mounted as a phase corrector being arranged with its emitter connected to that of the switching transistor (5), its collector connected to the base of this transistor, and its base connected, through a capacitor, at the junction between maintenance winding and resistance. Apparatus according to any one of claims 1 to 4, characterized in that it comprises a starting circuit (30) coupled to the source (10) of direct voltage and capable of delivering on the base of the switching transistor a single pulse in response to the growth of a control voltage (11 c ) originating from the source (10), between a low value and a triggering threshold. Apparatus according to claim 5, characterized in that the starting circuit (30) comprises a capacitor (35) charged by said control voltage (11 c ) through a resistor (34) which defines, together with the capacitor (35), a first time constant, a diac (31) sensitive to the charging voltage of the capacitor (35) in connection between the latter and the base of the switching transistor (5), and a transistor (32) with its emitter-collector space in parallel on the capacitor (35) and driven on its base by said control voltage (11c) through an RC time constant (33) greater than the first. Apparatus according to any one of claims 1 to 6, characterized in that it comprises a circuit of disjunction comprising a MOS transistor (15) with a source space drain in parallel on the base emitter space of the switching transistor (5), this transistor MOS (15) being made passing by a disjunction signal applied to its grid. Apparatus according to claim 7, characterized in that the circuit breaker comprises a winding (26) coupled to the output transformer (2) and charging a capacitor (43) through a diode (42), and a diac (41) sensitive to the charging voltage of the capacitor (43) connecting it to the gate of the MOS transistor (15). Apparatus according to one of claims 7 and 8, characterized in that the circuit breaker comprises an auxiliary circuit (50) sensitive to a control voltage (11 c ) from the source (10) and able to deliver on the gate of the MOS transistor (15) a disjunction signal in response to the disappearance of the control voltage. Apparatus according to claim 9, characterized in that the auxiliary circuit (50) comprises a standby transistor (52) with its emitter-collector space in parallel on a capacitor (53) charged by the DC voltage from the second pole (11 b ) through a resistor (53 a ), the standby transistor (52) being turned on by applying the control voltage (11 c ) to its base, the collector of the standby transistor (52) being connected to the gate of the MOS transistor (15) through a diac (51). Apparatus according to claim 10, characterized in that the secondary of the output transformer (2) has a midpoint (20) connected to the ground through a detector circuit (50 '), the detector circuit (50') comprising a diode electroluminescent (56) sensitive to the detected voltage, and optically coupled to a phototransistor (55) in parallel on the emitter space base of the standby transistor (52).

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