Electronic ballast provided with a thermal safety circuit

Abstract: The invention relates to an electronic ballast provided with a thermal safety circuit, comprising a semi-bridge oscillator circuit (1, 2) and a transformer (3), which controls operation of the oscillator circuit and through which a high-frequency lamp current passes. The ballast is provided with a safety circuit (10), which prevents operation of the ballast as temperature and/or a voltage across the lamp exceeds a given value. The material and dimensioning for the core of the transformer (3) are selected in such a manner that the rise of the core temperature above a given temperature causes, as a result of saturation of the core, an increase in the operating frequency of the oscillator circuit. The safety circuit (10) includes elements for measuring a voltage existing across the lamp and elements for bringing the oscillator to an inactive or stand-by state as the measured voltage exceeds a given value.

Description: The present invention relates to a safety circuit for the electronic ballast of a low-pressure discharge lamp, especially a fluorescent lamp, intended to prevent destruction of the ballast at a high operating temperature. The structure of electronic ballasts for low-pressure discharge lamps is prior known from the Applicant's patent No. FI 64487 as well as theoretical principles from the Applicant's patent No. FI 63314. On the other hand, the Applicant's patent No. FI 63146 discloses the structure and operation of a low-voltage controlled electronic ballast for providing a light output proportional to an external control signal. The prior art is represented by a circuit for an electronic ballast set forth in the Applicant's patent No. FI 63148, a slightly simplified version of which is depicted in fig. 1. Between the poles of a source of direct voltage, which may be constituted by a rectifier connected to a source of alternating voltage, are connected two transistors 1 and 2 constituting a semi-bridge. One of the poles of a primary winding 4 of a transformer 3 is connected between the transistors 1 and 2 and the other pole is connected to a winding 7, which is in turn connected via a capacitor 11 to one of the electrodes of a lamp 8. The transformer 3 is provided with secondary windings 5 and 6 connected to the bases of the transistors 1 and 2 so as to receive oppositely phased control voltages. Thus, when one transistor is conducting, the other one is non-conducting and vice versa. In addition, parallel to the lamp 8 in series with the lamp electrodes is a capacitor 9. The circuit is intended to supply the lamp 8 with a high-frequency current inverted from direct voltage and, furthermore, to conduct the ignition of said lamp. This type of drive circuits connected as a semi-bridge are conventional in the electronic ballasts of discharge lamps. As one example, reference can be made to the Applicant's patent US 4,553,070. As prior art, reference can also be made to Patent publications US 4,237,403, US 4,277,728, and US 4,370,600. In view of a reliable operation and a long service life, it is beneficial to provide electronic ballasts with sufficient safety mechanisms against certain abnormal operating conditions. Such conditions include e.g. a deviation of the characteristics of a discharge lamp - especially a fluorescent lamp - from the standardized values thereof. Such a condition develops normally along with the ageing of a lamp as the emission material serving as a coating for the lamp cathodes gradually disappears, which has an adverse effect e.g. on the switch-on characteristics of a lamp. This effect is termed as deactivation of a lamp. The ageing of a lamp has also been discovered to result in changes relating to the burning voltage of a lamp. The cathodes of a discharge lamps may also break completely either as a result of ageing or structural fragility. Another condition different from normal circumstances is the under- or overvoltage condition of a supply line. In case a ballast is set at an operating temperature other than what is specified therefor, the result may be an excessive rise of the temperature existing within the ballast, which in the worst case may destroy electrical components or other materials of the ballast. For these reasons it is inevitable that the ballast be fitted with special safety features for preventing damage to the device. For example, the ballast can be set in such a safe condition in which the oscillation of an oscillator is stopped. This type of safe condition is often termed as a stand-by state. In this case, the ballast includes a special control block intended to identify the above-described abnormal operating conditions and to change the functional state of the ballast appropriately for protecting the electronic components. In addition, the circuit must be provided with elements, whereby the ballast can resume its normal functional state as the abnormal operating conditions are eliminated. Fig. 2 of the drawings depicts this type of ballast design, wherein the section for the above-discussed safety circuit is indicated by a block 10. This control block includes necessary electronic circuit elements for identifying said abnormal operating conditions by measuring certain electrical signals and temperature existing within the device. In practice, the measurement of temperature requires some circuit element, one electrcal parameter of which is proportional to temperature in a known fashion. Such components include e.g. thermistors, having a resistance which changes as a function of temperature. The temperature measuring components have a problem of being usually quite expensive, the site therefor within the device is strictly confined, and some have also a limited service life (e.g. the above-discussed thermistors). It is also possible to place such a temperature-proportional circuit element outside said safety circuit block 10, e.g. in series with the mains voltage feed of a ballast whereby, upon excessive heating of the device, its current supply can be cut off. Thus, a problem with the above-described thermal shields is that the circuit designs are either complicated or include expensive or unreliable circuit elements. An object of this invention is to provide thermal protection for the electronic mains ballast of a low-pressure discharge lamp, especially a fluorescent lamp, in a simpler and more reliable manner than what has been suggested in the prior art solutions. This object is achieved on the basis of the characterizing features set forth in the annexed claim 1. In the drawings, fig. 1 shows a prior known simplified circuit for an electronic ballast; fig. 2 shows the circuit of fig. 1 completed with a safety circuit of the invention. The invention will be described hereinbelow in more detail with reference to fig. 2 of the drawings. A transformer 3 is wound around a core made of a magnetic material, e.g. ferrite. One preferred structure is an annular transformer core or a toroid. In a solution of the invention, it is essential that the safety circuit 10 includes circuit elements for measuring a voltage operating across a lamp. This is depicted in fig. 2 by means of a connection, wherein a measuring signal is obtained from a connection point A common to a winding 7 and one of the cathodes of a lamp 8. The other cathode of the lamp 8 is in turn connected to an earthing point (-) of a direct-voltage source. Thus, in this circuit configuration, the voltage across a lamp is measurable with a measuring circuit from one end of the lamp only. The invention makes use of the magnetic properties of the core of said transformer 3. It is general knowledge that, with certain magnetic core materials, the highest acceptable induction density in the core becomes lower as temperatures rises, i.e. the core of a transformer saturates even with less intense magnetization. Thus, in said circuit arrangement of the invention, a transformer core, especially a toroid core, saturates with a lower winding current as temperature in the core, i.e. within a ballast, increases. Hence, the voltage of secondary windings 5 and 6, which produces a positive terminal current in switching transistors, runs out sooner. This results in an increase in the frequency of a semi-bridge oscillator circuit, which is shown in fig. 2 in the process of switching on the lamp 8. Thus, the voltage visible across the lamp 8 increases, which is identified in the safety block 10 as a function of frequency and voltage. The safety block 10 turns off the vibration of the oscillator circuit as the measuring value of a voltage across the lamp 8 exceeds a given predetermined threshold value. The oscillator can be stopped e.g. by blocking the passage of the terminal current of transistors 1 and 2. A circuit configuration appropriate for this is disclosed e.g. in Patent EP 0,146,683. As a result of the action of a safety circuit of the invention, the ballast assumes a shielded condition, in which there are hardly any power losses and the internal temperature of the device falls to a safe level so as to protect structural components of the device from destruction. A typical feature for the solution of the invention is that the measuring of temperature does not require separate circuit elements but, instead, it is possible to exploit the thermal properties of a component included integrally in the ballast assembly. Since the safety circuit 10 shall in any case include a measuring circuit for a voltage operating across the lamp 8, since the identification of a lamp voltage upon the ageing of a lamp must be carried out anyway for reasons of safety, the thermal protection of a ballast will be handled in a highly simple, reliable, and economical manner. It is essential for the operation of a solution of the invention that the transformer 3 be constructed such that the saturation properties of its core change as a function of temperature in a desired fashion. This can be contributed by properly selecting the shape of the magnetic material and the magnetic circuit of a transformer core. All structural components must be such a heat resistance and the circuit elements must be arranged such that damage to the materials is avoided prior to actuating the safety circuit. The solution makes use also of the frequency behaviour of the lamp voltage of low-pressure discharge lamps. It is totally clear for a skilled person that usefulness of the invention is not limited only to the circuit configuration shown in fig. 2 of the drawings. For example, said transistors 1 and 2 can be field effect transistors (MOSFET) fitted with an insulated grid. In addition, operation of the safety circuit 10 may differ from what is described above, e.g. the stoppage of oscillation is possible to effect with other embodiments as well. Likewise, it is possible to locate a capacitor 11 for the oscillating circuit also as depicted in fig. 3 of the drawings.

Claims: An electronic ballast provided with a thermal safety circuit, comprising a semi-bridge oscillator circuit (1, 2) and a transformer (3), which controls operation of the oscillator circuit and through which a high-frequency lamp current passes, said ballast being provided with a safety circuit (10), which prevents operation of the ballast as temperature and/or a voltage across the lamp exceeds a given value, characterized in that the material and dimensioning for the core of the transformer (3) are selected in such a manner that the rise of the core temperature above a given temperature causes, as a result of saturation of the core, an increase in the operating frequency of the oscillator circuit, and that the safety circuit (10) includes elements for measuring a voltage existing across the lamp and elements for bringing the oscillator to an inactive or stand-by state as the measured voltage exceeds a given value. A ballast as set forth in claim 1, characterized in that the transformer (3) is a toroid, provided with secondary windings (5, 6) which control the transistors (1, 2) of the semi-bridge oscillator circuit and that the safety circuit (10) brings these transistors to a non-conducting state as the measuring value of a voltage existing across a lamp (8) exceeds a given predetermined threshold value. A ballast as set forth in claim 2, characterized in that the safety circuit (10) is provided with a voltage measuring point (A) which is located between an electrode of the lamp (8) and a coil (7) included in the lamp circuit. A ballast as set forth in claim 1 or 2, characterized in that one of the electrodes of the lamp is connected directly to the earth potential of a current source.

Inventors: NAAKKA HARRI, NAAKKA, HARRI
Applicants: HELVAR OY AB
Application year: 1997
Main classification: H05B41
IPC codes: H05B41