EP0848581A1

EP0848581A1 - Cathode filament heating circuit for a low-pressure discharge lamp

Info

Publication number: EP0848581A1
Application number: EP97660141A
Authority: EP
Inventors: Harri c/o Oy Helvar Naakka
Original assignee: Helvar Oy AB
Current assignee: Helvar Oy AB
Priority date: 1996-12-12
Filing date: 1997-12-10
Publication date: 1998-06-17

Abstract

The invention relates to a filament heating circuit for the cathodes of a low-pressure discharge lamp, comprising a filament heating current capacitor (11) connected in series with electrodes (6, 7) of a lamp (3) in parallel with a lamp. In the filament heating current circuit there is connected one or more components (12, 13), which constitute a resonance circuit or affects on the operation of the main resonance circuit and are dimensioned to change the filament heating current passing through the electrodes (6, 7) as the frequency of a lamp current changes.

Description

The present invention relates to a cathode filament heating circuit for a low-pressure discharge lamp, whereby the power used for heating can be regulated to a level required by any given operating condition of the lamp.

It is generally known that heating of the cathodes of low-pressure discharge lamps during the ignition or switch-on sequence of a lamp has a positive contribution to the service life and operating features of the lamp. With an appropriately dimensioned filament power, the service life of a lamp can be prolonged by reducing the stress on the lamp cathodes and especially on the emissive material layer serving as a coating thereof. At the same time, this enables the ignition of a lamp without flicker with a voltage across the lamp being lower than what is required if heating is not applied. The requirements set on electronic ballasts regarding the preheating of cathodes during the switch-on sequences of a lamp are set forth in the international standard IEC 929. The purpose of these requirements is to make sure that fluorescent lamps according to standards IEC 81 and IEC 901 reach the service life specified therefor. Deviation from the standardized requirements may also cause other adverse effects, such as blackening of the glass bulb of a lamp in areas next to the cathodes as material emitting from the cathodes accumulates on the inner surface of the glass bulb.

It is also prior known that said heating of lamp cathodes is inevitable whenever it is desirable to adjust the light output of a lamp. Sufficient cathode heating is a way of making sure that a sufficient number of charge carriers are emitted from the cathodes to sustain discharge in a lamp as the current passing therethrough is diminishing. The optimal filament heating level is also necessary for maintaining the service life of a lamp on an acceptable level. The basic technology regarding controllable ballasts is described e.g. in the Applicant's patent application FI-955695.

Heating of the cathodes of a lamp has an adverse effect on the energy efficiency of the assembly consisting of a lamp and a ballast as the heating of cathodes consumes energy. Traditionally, this aspect has not been considered especially important, since the electronic ballasts operating at a high frequency have had a significant advantage over magnetic ballasts in terms of energy efficiency. However, it has become necessary to improve the efficiency of electronic ballasts in an effort to achieve a better saving of energy for cutting down the equipment restitution time.

On the other hand, the electric power spent for the heating of cathodes takes up a substantial share of the nominal output of low-power fluorescent lamps. This aspect is particularly evident in connection of new T-5 lamps 16 mm in diameter. As a result of the above considerations, various alternatives are searched in designing ballasts for optimizing the amount of energy spent on cathode heating.

An object of this invention is a filament circuit for a low-pressure discharge lamp, especially a fluorescent lamp, whereby the power used for incandescence can be regulated as required by the operating condition of a lamp for improving the energy efficiency but without jeopardizing the favourable operating conditions of a lamp.

In the drawings,

figs. 1-6: illustrate solutions of the prior art;
figs. 7-10: show examples of filament circuit designs of the invention; and
figs. 11-13: show examples of filament circuit designs of the invention, wherein the power for cathode filament heating is obtained through windings of a transformer.

The prior art is represented by an electronic ballast circuit shown in fig. 1. The electric power delivered from a rectifier 1 is used for driving a high-frequency oscillator 2, the high-frequency alternating current supplied thereby passing through a series circuit constituted by a winding 4, a capacitor 5, and a lamp 3. Parallel to the lamp 3 in series with

electrodes

6 and 7 is connected a series circuit constituted by a temperature-dependent resistance 9 and a capacitor 8.

An advantage offered by the above-described circuit is the optimal behaviour of cathode heating during and after the switch-on sequence of a lamp. After the switch-on sequence of a lamp, upon assuming a normal burning condition, the filament current decreases whereby the power consumption of the circuit decreases to improve the efficiency of an entire ballast. This is depicted in fig. 2, which basically illustrates the behaviour of a cathode current and a lamp voltage starting from the switch-on moment. On the other hand, a problem with such a circuit is that the temperature-dependent resistance will have such a high voltage load that setting up the circuit requires the use of special components. This naturally increases the implementation costs of a circuit.

Patent publication EP 185179 as well as the works "Betriebsgeräte und Schaltungen fuer elektrische Lampen", (C.H. Sturm & E. Klein, Siemens AG, 1992, page 131) and "Application Report: Electronic Ballasts for Fluorescent Lamps using BUL770/791 Transistors", (Texas Instruments, 1992, p. 9) disclose a solution, wherein parallel to a voltage-dependent resistance is connected a capacitor to achieve a significant reduction in the voltage stress of said resistance. This circuit configuration is depicted in fig. 3 and the basic behaviour of the cathode current of a lamp as well as the lamp voltage is shown in fig. 4. A drawback with this circuit is that, after the switch-on moment, the filament current increases along with the lamp voltage and, after the lamp has been switched on, it remains at a reasonably high level resulting in unnecessary consumption of power and, thus, decreases the efficiency of a ballast. In addition, incandescence does not work if one of the lamp cathodes is broken as these are connected in series with the filament circuit and in this situation the ignition of the lamp is impossible.

The Applicant's patent FI-95527 discloses a circuit arrangement, wherein a series circuit constituted by one of the electrodes of a lamp and a positive temperature-coefficient dependent resistance is connected in parallel with the lamp, and in parallel with a series circuit constituted by the other electrode of the lamp and said resistance is connected a capacitor, as shown in fig. 5. In this circuit, the filament current of a cathode 6 decreases substantially after the switch-on sequence, which brings down the consumption of power by the cathode and in the voltage-dependent resistance. The reduced power loss improves the efficiency of a ballast and increases the service life of a voltage-dependent resistance since, in principle, said component ages along with the passage of service time. Also, incandescence works even if the lamp cathode 6 is broken, enabling the ignition of a lamp in this situation as well. A possible problem could be that the filament circuit does not operate symmetrically relative to the lamp, which may result in the previously discussed hazard of the blackening effect for the ends of a lamp.

A common feature for all of the above-described circuit configurations is that said circuits are based on the use of a temperature-dependent resistance. A major drawback with this type of component is that, in principle, the component is ageing, i.e. it has a limited service life. Furthermore, it is quite expensive and unreliable. None of the above-described circuit configurations is as such suitable for use in controllable or dimmable ballasts, since it is inevitable that the filament power of cathodes can be varied as desired according to variations of the current passing through the lamp.

An object of the invention is to provide an improved circuit solution, whereby the filament heating level of cathodes can be varied as desired according to variations in a current passing through a lamp without having to resort to the use of a temperature-dependent resistance.

This object is achieved by means of a circuit system as set forth in the appended claim 1 or 5.

In a cathode filament circuit of the invention, the filament current passing through the cathodes is regulated by the application of a resonance principle. A circuit arrangement according to an embodiment of the invention is depicted in fig. 7. The purpose of a capacitor 11 is to provide a sufficient pre-incandescence heating for cathodes during a switch-on sequence, the filament current passing primarily through a circuit constituted by said cathodes and capacitor. The series circuit constituted by a winding 12 and a capacitor 13 is connected in parallel with each electrode of the lamp 3, in series with the filament current capacitor 11. The

circuit elements

12 and 13 develop a resonance circuit at a certain frequency, whereby it is possible to regulate the current passing through the cathodes by changing between

poles

14 and 15 the frequency of a supply voltage operating across the lamp. With such a simple method, it is possible to reduce the filament current to a desired level after the ignition of a lamp in non-controllable or controllable ballasts, which results in a desired saving in the energy required for heating the cathodes. The circuit can be used in a variety of ballast designs, the only requirement being the accurate control over the frequency of a voltage to be supplied to the lamp. Said regulation of the heating is achieved in practice without expensive or unreliable circuit elements.

Figs. 8-10 illustrate other feasible embodiments for the invention described in this patent application. The operating principle is always the same, but the resonance circuit is set up in a number of ways.

In the case of fig. 8, the resonance circuit is completed with a winding 12', which is connected in series with a filament current capacitor 11 in parallel with a lamp.

In the case of fig. 9, the resonance circuit is constituted by a series circuit, which comprises a winding 12 and a capacitor 13 and is connected in series with a filament current capacitor 11 in parallel with a lamp 3.

In the case of fig. 10, the resonance circuit is constituted by a parallel circuit, which comprises a winding 12 and a capacitor 13 and is connected in series with a filament current capacitor 11 in parallel with a lamp 3.

In the cathode filament heating circuit according to figs. 11-13 as well, the filament heating current passing through cathodes is regulated by a resonance principle. An example of the circuit system for bringing about this function is depicted in fig. 11 of the drawing. In this circuit, the coil 4 of fig. 1 is represented by a transformer 17, consisting of a primary winding L1 and at least two secondary windings L2 and L3. The coil 17 and the capacitor 11 constitute a main resonance circuit for the ballast, which determines the oscillation frequency. The function of a capacitor 19 is the separation of a direct voltage. The energy required for

heating cathodes

6 and 7 is received from the secondary windings L2 and L3 of the transformer 17. A capacitor 13 and an additional winding 12 as well as a capacitor 16 and an additional winding 18, respectively, constitute a resonance circuit at a given frequency, whereby it is possible to regulate a current passing through the cathodes by changing the frequency of a supply voltage between

poles

14 and 15. In this simple manner it is possible to reduce the filament heating current to a desired level after the ignition in undimmable or dimmable ballasts. This achieves a desired saving in the energy required for heating the cathodes. The circuit can be used in a variety of ballast configurations, the only condition being an accurate control over the frequency of a voltage supplied to a lamp. Such filament heating regulation function can be achieved in practice without expensive or unreliable circuit elements.

Fig. 12 of the drawing illustrates another possible embodiment for a transformer-equipped circuit. The operating principle is similar to that of fig. 11, but the resonance circuit is implemented without a separate coil in the cathode filament heating circuit (coils 12 and 18 in fig. 11). In this case, the transformer 17 is implemented in such a way that the stray inductance of its windings provides an inductance of required magnitude for creating a resonance circuit. Thus, of course, the implementation costs of a circuit go down dramatically.

A transformer-equipped circuit solution can also be implemented as depicted in fig. 13 of the drawing. Here, the location of a capacitor 19 is changed in such a manner that one cathode head 15 of a lamp will be connected to the earth potential of the transistor bridge of a chopper circuit.

The key idea of figs. 11-13 with respect to figs. 7-10 is that the energy to be used for cathode heating is obtained through the windings of a transformer included in the main resonance circuit and resonance is utilized in said secondary circuits of a transformer for regulating the filament power as desired.

Claims

A filament circuit for the cathodes of a low-pressure discharge lamp, comprising a filament current capacitor (11) connected in series with electrodes (6, 7) of a lamp (3) in parallel with a lamp, characterized in that in series with the filament current capacitor (11) is connected one or more components (12, 13; 12'), which constitute a resonance circuit and are dimensioned to change a filament current passing through the electrodes (6, 7) as the frequency of a lamp current changes.
A filament circuit as set forth in claim 1, characterized in that the resonance circuit is constituted by a series circuit, comprising the winding and the capacitor (12, 13), and that such a series circuit is connected in parallel with each electrode (6, 7) of the lamp, in series with the filament current capacitor (11).
A filament circuit as set forth in claim 1, characterized in that the resonance circuit includes the winding (12') or a series circuit which is constituted by the winding (12) and the capacitor (13) and connected in series with the filament current capacitor (11) in parallel with the lamp.
A filament circuit as set forth in claim 1, characterized in that the resonance circuit includes a parallel circuit which is constituted by the winding (12) and the capacitor (13) and connected in series with the filament current capacitor (11) in parallel with the lamp.
A cathode filament heating circuit for a low pressure discharge lamp, said lamp being connected to a main resonance circuit constituted by a capacitor (11) and a coil (L1), said coil (L1) comprising a winding for a transformer provided with secondary windings (L2, L3), characterized in that cathodes (6, 7) are connected to receive the energy used for cathode heating through said secondary windings (L2, L3), the resonance of the main resonance circuit (L1, 11) or a change in the frequency of a supply voltage being used for regulating the filament heating power.
A filament heating circuit as set forth in claim 5, characterized in that the cathodes (6, 7) are connected by way of a capacitor (12, 16) to a common circuit, each through its own secondary winding (L2, L3).
A filament heating circuit as set forth in claim 6, characterized in that the cathode filament heating circuit includes an additional winding (13, 18) which, together with the capacitor (12, 16), constitutes a resonance circuit at a given frequency.