Safety circuit for a ballast in a fluorescent lamp

Abstract: The invention relates to an electronic ballast for a low-pressure discharge lamp. The threshold value of a lamp voltage leading to the stoppage of an oscillator circuit (A1) in the ballast is altered proportionally to the frequency of a voltage being supplied to a lamp (LL). This eliminates the concurrent conduction of transistors included in the ballast.

Description: The present invention relates to a regulable or controllable electronic ballast for a low-pressure discharge lamp, especially a fluorescent lamp, the operation thereof being based on regulating the light output of a lamp by modifying the frequency of a voltage supplied to the lamp. The invention concerns a method and a circuit system for safeguarding such a ballast against stresses induced by an ageing or otherwise damaged lamp. The construction of electronic ballasts for low-pressure discharge lamps is known from the Applicant's patent No. FI 64487 as well as its theoretical foundations from the Applicant's patent No. FI 63314. On the other hand, the Applicant's patent No. FI 63146 discloses the construction and operation of a low-voltage controlled electronic ballast for producing a light output proportional to an external control signal. The construction and operation of a controllable ballast known from the Applicant's patent No. FI 63146 are described in more detail with reference to fig. 1 of the accompanying drawing, depicting one preferred embodiment for such a ballast. In order to simplify the representation, fig. 1 only illustrates structural components that are most vital in view of understanding the operation. The ballast shown in fig. 1 of the drawing includes a rectifier V1 connected to a supply voltage Uv, an equalizing capacitor C1, a high-frequency oscillator A1, as well as transistors TR1 and TR2 providing a half-bridge circuit. A lamp LL is connected as part of a resonance circuit, which consists of a capacitor C2 and a coil L1 and which is supplied by alternately conducting transistors TR1 and TR2. A pre-incandescence current for the cathodes of a lamp LL as the lamp is switched on is supplied over a capacitor C3. The oscillator Al is provided with a control circuit CO1 - CO2, over which the control signal has an effect on the oscillating frequency of the oscillator. Typically, this type of ballast includes a special safety feature, often referred to as a stand-by state, for abnormal operating conditions. One such feature is e.g. a mains undervoltage, which the oscillator A1 identifies over a circuit C3 and stops for protecting the ballast and the lamp. A problem with the above-described ballast and similar types of frequency-regulation based ballasts is the functioning of a circuit in a situation, in which the characteristics of a discharge lamp - particularly a fluorescent lamp - for one reason or another differ substantially from the nominal ratings thereof. Such a situation develops normally along the ageing of a lamp, whereby the emission medium serving as a coating for incandescable lamp cathodes gradually wears away and this deteriorates considerably e.g. the switch-on characteristics of a lamp. At this point, the lamp is said to assume a deactivated condition. Thus, the development of this condition is quite normal at a certain point over the life-span of a lamp, the time lapse for this phenomenon to develop depending upon factors relating to the design and manufacture of a lamp as well as upon the operating characteristics of a ballast. The ballasts shown in fig. 1 and described in the accompanying specification are characterized in that, in a deactivated lamp, the switch-on timing of a current passing through transistors TR1 and TR2 included in the oscillator circuit of a ballast changes in such a manner that said transistors begin to momentarily conduct simultaneously, which results in the passage of major current surges through the transistors. If the occurrence of this so-called co-conductivity is not somehow precluded, the consequence may be the failure of said half-bridge transistors TR1 and TR2 as a result of an excessive current or a thermal overload. Even in a less-than-serious situation, the consequence is a substantially increase in a voltage supplied to the lamp and hence also in the lamp power with respect to the nominal ratings, which usually destroys the lamp quickly and, if prolonged, may damage also damage the ballast as a result of excessive heating. In order to preclude the above-described overload condition, it is possible to design the half-bride transistors TR1 and TR2 with such a control circuit which synchronizes the switching-on of said transistors to the mid-point voltage of the half-bridge, i.e. to the voltage of a connection point between the emitter of TR1 and the collector of TR2. Thus, the mid-point voltage higher than a given set value precludes the switching of said transistors to a conducting state. This method requires circuit elements for measuring said mid-point voltage, for setting a set value, as well as for precluding an incorrect base control signal. Unfortunately, implemented on two transistors, this implicates quite a complicated circuit system, which adds to the complexity of a ballast and hence increases the implementation costs and, at the same time, undermines reliability of the apparatus. The method is techno-economically feasible mostly when the control of said half-bridge transistors is effected by using an integrated circuit and a significant number of the required circuit elements are included in said micro-circuit. A piece of prior art described in US patent 4.071.884 does not deal with electronic ballasts for a discharge lamp but, instead, with high-voltage direct-current converter. This prior art invention relates to the regenerative regulation of an output voltage, wherein a measuring of a load voltage is used as a basis for controlling the oscillator and amplifier of a converter. In the US publication, the reference voltage in a reference voltage device is stable in terms of operation or at least it is not changed on the basis of any feature of the measuring signal and the frequency of the measuring signal is of no significance. On the other hand, the Japanese publication 64-31393 describes a safety circuit connection based on the measurement of a lamp voltage, wherein the measured voltage is compared to a stable set value, and this comparison is used as a basis for switching, whenever necessary, the oscillator of a ballast off the oscillating state. The reference potential E is stable and the measuring signal is independent of the frequency. An object of this invention is to introduce a method and a circuit configuration for eliminating in a simple and reliable manner the simultaneous conduction, i.e. a so-called co-conductivity phenomenon, of transistors included in an electronic ballast. This object is achieved by means of a ballast as set forth in the annexed claim 1. Preferred embodiments of the invention are set forth in the non-independent claims. The invention will now be described in more detail with reference made to the accompanying drawings, in which: Fig. 1 depicts the above-described prior art; Fig. 2 shows one embodiment of the invention in a ballast, whose basic configuration was explained in fig. 1 and in the specification relating thereto; Fig. 3 shows an alternative implementation for a lamp circuit; Fig. 4 shows alternative ways of implementing a measuring circuit; and Fig. 5 depicts the variation of a safety-feature activating voltage as a function of the light intensity level produced by a lamp. In the description of basic technology relating to a controllable electronic ballast (drawing Fig. 1), it was noted that the ballast must usually be provided with a separate safety feature for certain special conditions, such as an over- or undervoltage in the mains supplying a ballast, the apparatus becoming overheated, a missing lamp or breaking of a lamp's cathodes. Such a safety feature is often referred to as a stand-by feature. In such special conditions or situations, it is often absolutely necessary to stop the oscillation of an oscillator in a ballast for precluding the failure of the ballast. In a procedure of the invention, depicted in drawing Fig. 2, this safety-circuit feature inherent in a ballast is exploited by readjusting, in the case of a deactivated lamp, the threshold level of a lamp voltage required for activating a stand-by safety feature. The drawing Fig. 3 illustrates an alternative circuit embodiment with respect to a lamp circuit. In a method of the invention, this is performed by measuring the frequency of a voltage being supplied to a lamp LL and by using the obtained measuring result to alter a voltage threshold leading to the activation of a safety circuit. Usefulness of the invention is based on the fact that a control signal proportional to the frequency of a lamp can be produced by means of an extremely simple circuit system by utilizing the frequency-dependency of the impedance of a capacitive circuit element C6. It should be noted that several circuit elements included in the measuring signal circuit are found as such in the basic circuit of a ballast and, thus, can be directly exploited. The frequency of a voltage operating over a lamp is measured with a measuring circuit consisting of a resistance R1 and a capacitor C6. Resistances R2 and R3 are used for setting a suitable voltage level for the measured signal. The measuring signal is guided over a capacitor C7 to a rectifier D1-D2 and further over an equalizing capacitor C8 to a difference element A2, one terminal thereof being set with a reference voltage Vref. When the measuring signal level in the positive terminal of the difference element A2 exceeds the set value Vref, an output C04 switches its logic state and stops a ballast oscillator A1. Alternative implementations for a measuring circuit are illustrated in the drawing Fig. 4. When the frequency of a voltage supplied to a lamp is used as a basis for altering the stand-by level of a ballast by delivering a measuring signal directly into an existing safety circuit, as shown in the drawing Fig. 2, the protection against the effects of a deactivated lamp is achieved by effectively utilizing the basic qualities of a ballast without expensive and sophisticated circuit structures. The method is also highly suitable to be implemented with separate components since the number of required components is very low. The drawing Fig. 5 depicts how the activating voltage for a safety feature of the invention varies in the case of a regulable or controllable ballast as a function of the light intensity level produced by a lamp (LL). Thus, a graph 1 represents the behaviour of a lamp voltage in a conventional ballast circuit and a graph 2 in a circuit of the invention. It is obvious for a person skilled in the art that the invention is by no means limited to the described embodiments but is just as well applicable to a number of other ballast circuits based on frequency control, e.g. to multi-lamp ballast versions. Especially the technology relating to switching transistors usable in a half-bridge does not affect the usefulness of the described invention, i.e. this can be achieved by using e.g. a bipolar transistor and a variety of field effect transistors.

Claims: An electronic ballast for a low-pressure discharge lamp (LL), the ballast comprising an oscillator circuit (A1) and a safety circuit determining a threshold value of lamp voltage leading to the stoppage of the oscillator circuit (A1), characterized in that the threshold value of the lamp voltage is altered proportionally to the frequency of the voltage being supplied to the lamp (LL). An electronic ballast as set forth in claim 1, characterized in that the lamp voltage leading to the stoppage of the oscillator circuit increases as the frequency being supplied to the lamp (LL) increases. An electronic ballast as set forth in claim 1, characterized in that the lamp voltage leading to the stoppage of the oscillator circuit increases as the light intensity produced by the lamp (LL) diminishes. A ballast as set forth in any of claims 1-3, characterized in that a measuring circuit for the lamp voltage includes a capacitive circuit element. A ballast as set forth in any of claims 1-3, characterized in that a measuring circuit for the lamp voltage comprises a series connection for a capacitor and a resistance. A ballast as set forth in any of claims 1-3, characterized in that a measuring circuit for the lamp voltage is provided with a connection in parallel for a capacitor and a resistance in series with a resistance. A ballast as set forth in any of claims 1-3, characterized in that a measuring circuit for the lamp voltage is provided with a series connection for a capacitor and a resistance in parallel with at least one resistance and this combination in series with at least one resistance. A ballast as set forth in any of claims 1-3, characterized in that a measuring circuit for the lamp voltage is provided with a connection in parallel for a capacitor and a resistance in series with a resistance and connected in series with this combination is a resistance. A ballast as set forth in any of claims 1-3, characterized in that a measuring circuit for the lamp voltage is provided with a resistance which is in series with a series connection for a capacitor and a resistance. A ballast as set forth in any of claims 1-3, characterized in that a measuring circuit for the lamp voltage comprises a series connection for a coil and a resistance. A ballast as set forth in any of claims 1-3, characterized in that a measuring circuit for the lamp voltage comprises a series connection for a coil and a resistance in series with a resistance. A ballast as set forth in any of claims 1-3, characterized in that a measuring circuit for the lamp voltage comprises a series connection for a coil and a resistance in parallel with at least one resistance and in series with this combination is connected at least one resistance. A ballast as set forth in any of claims 1-3, characterized in that a measuring circuit for the lamp voltage comprises a connection in parallel for a coil and a resistance in series with a resistance and in series with this combination is connected at least one resistance. A ballast as set forth in any of claims 1-3, characterized in that a measuring circuit for the lamp voltage comprises a connection in parallel for a coil and a resistance in series with at least one resistance. A ballast as set forth in any of claims 1-14, characterized in that measuring of the lamp voltage is effected from one of the cathodes of the lamp (LL). A ballast as set forth in any of claims 1-14, characterized in that measuring of the lamp voltage is effected from between one of the cathodes of the lamp (LL) and a coil (L1) included in the lamp circuit. A ballast as set forth in any of claims 1-14, characterized in that measuring of the lamp voltage is effected from between one of the cathodes of the lamp (LL) and a capacitor (C2) included in the lamp circuit, said capacitor being connected over a cathode of the lamp (LL).

Inventors: AHONEN ERKKI, AHONEN, ERKKI
Applicants: HELVAR OY, OY HELVAR
Application year: 1996
Main classification: H05B41
IPC codes: H05B41