Electric lamp and discharge devices: systems – Current and/or voltage regulation
Reexamination Certificate
2003-02-06
2004-07-06
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Current and/or voltage regulation
C315S224000, C315S307000
Reexamination Certificate
active
06759813
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an operating circuit for a discharge lamp, in particular a low-pressure discharge lamp.
Such operating circuits regularly have AC voltage generators with the aid of which an AC voltage with a specific frequency is applied to electrodes of the discharge lamp in order, on the one hand, to ignite the discharge lamp and, on the other hand, to maintain the operation after the ignition. The details of such operating circuits have been known in general for a long time and need not be explained in detail here.
PRIOR ART
In particular, it is already known to design the operating circuits such that they carry out the ignition operation with the aid of a resonant peak in a resonant circuit which is connected to the discharge lamp. For this purpose, the frequency for ignition is set to a value in the vicinity of the resonance of the resonant circuit (which value is mostly increased with reference to the continuous operating frequency to be applied later). The resonant circuit generates the voltage amplitudes required for the ignition by resonance effects.
It is already known, furthermore, to build into operating circuits for discharge lamps safety shutdown devices which are designed so as to shut down the supply power in the event of impermissibly high supply currents of the AC voltage generator. The aim of this is to avoid damage to the operating circuit and the lamp and/or to avoid risks.
SUMMARY OF THE INVENTION
Starting from the prior art as outlined, the invention is based on the problem of specifying an operating circuit improved with regard to the ignition of the discharge lamp.
To this end, according to the invention there is provided for a discharge lamp an operating circuit which has an AC voltage generator which can generate an AC supply power with different frequencies for the purpose of igniting and operating the discharge lamp, the operating circuit being designed in order to ignite the discharge lamp at a frequency at which a resonant peak of a resonant circuit connected to the discharge lamp occurs, and having a safety shutdown device which shuts down the supply power at a threshold value in the region of impermissibly high supply currents of the AC voltage generator.
By contrast with the prior art, the invention does not proceed from a permanently prescribed ignition frequency, but gradually varies the frequency, starting from an initial, presumably excessive frequency, to lower frequencies, the described safety shutdown device monitoring the supply current of the AC generator. If adequate excitation of the resonant circuit and ignition of the lamp occur during the reduction of the frequency, the resonant frequency of the resonant circuit is sharply reduced because of the change in the electric properties of the discharge lamp. An impermissibly high supply current will not then normally occur. However, should an excessively high supply current occur nevertheless as operation proceeds, it can certainly be provided according to the invention that the safety shutdown device then responds again. However, this then would essentially have nothing to do with the ignition procedure as such which is addressed by the invention.
On the other hand, should impermissibly high supply currents occur during the lowering of frequency owing to an excessively close approach to the resonant frequency of the resonant circuit (in which case the discharge lamp would then not yet have been ignited), the safety shutdown device then responds according to the invention.
It is essential for the operating circuit to be designed such that it does not now stop operating completely, for example, but rather starts a new attempt at ignition with a frequency (shutdown frequency in what follows) which is increased with reference to the frequency at which the shutdown has been performed. The invention does not stipulate in general how this is performed in detail. A variety of preferred variants are still to be outlined below. However, it is essential that the operating circuit is initially remote once again from the shutdown frequency.
The operating circuit according to the invention is therefore capable of, as it were, “feeling its way” to the resonance of the resonant circuit, the safety shutdown circuit preventing damage. This has a consequence that fluctuations in the resonant frequency do not lead to ignition interference. It is thereby possible, in particular, to greatly reduce the sensitivity of the operating circuit to interference and temporal or temperature-dependent variations in the characteristics of the various circuit constituents. This enhances the reliability, for one thing, but also has marked economic advantages owing to the reduced requirements when selecting components.
Conventional low-pressure discharge lamps are regularly connected in series with a so-called lamp inductor which is required because of the negative values, occurring in these lamps, in the derivative of the current-voltage characteristic. However, the invention is certainly not restricted to such discharge lamps. It does, however, offer great advantages in the use of lamp inductors. Specifically, one difficulty in the prior art consists in that the lamp inductor becomes saturated at specific current values. The saturation of the lamp inductor can lead to an instability in the operating circuit which cannot be tolerated. This is the main reason for the use of safety shutdown devices in conventional low-pressure discharge lamps.
The saturation current of a lamp inductor is, however, relatively strongly dependent on temperature, and so the lamp inductor must be of relatively large design as a precaution in view of possible high temperatures. The point is that it would be necessary on the other hand to generate relatively high ignition voltages with the operating circuit in order to ignite the discharge lamp given a conventionally permanently prescribed ignition frequency, in order to ensure reliable ignition even at low temperatures. The high ignition voltages are necessarily associated with relatively high lamp inductor currents. This leads overall to the fact that excessively large lamp inductors are built up for reasons of reliability and safety while not being necessary at all for most applications. This is firstly associated with disadvantages regarding overall size and overall weight of the operating circuit, and on the other hand also with cost disadvantages, of course.
Comparatively smaller lamp inductors can be constructed, however, when the function of the safety shutdown device is integrated, according to the invention, to a certain extent in the normal ignition procedure. To be precise, in the case of the invention the occurrence of safety shutdown devices in the ignition procedure does not form a problematical exceptional case which entails absence of successful ignition, but forms a procedure that is integrated in to the normal functional cycle of the operating circuit.
The operating circuit is advantageously designed such that once a shutdown has been performed following a response of the safety shutdown device, it gradually lowers the supply frequency again. In this case, it proceeds from a starting frequency that is increased with reference to the shutdown frequency. The ignition procedure then therefore has a basically iterative character.
There are two respectively advantageous preferred variants in this case:
Firstly, the operating circuit can be designed such that during the renewed attempt at ignition it lowers the frequency only down to a minimum frequency that is slightly increased with reference to the last shutdown frequency. However, in this case the safety shutdown device remains active, and therefore shuts down again, if appropriate. The minimum frequency is therefore not necessarily reached. Should a shutdown have occurred once again, the procedure is repeated, that is to say the frequency is gradually lowered again—specifically, if no shutdown occurs, once again only down to a frequency that is slightly increased with reference to the last, that
Busse Olaf
Orth Klaus
Patent-Treuhand-Gesellschaft für elektrische Glühlampen mbH
Vu Jimmy T
Wong Don
LandOfFree
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