Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Impedance or current regulator in the supply circuit
Reexamination Certificate
2003-06-11
2004-12-07
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Impedance or current regulator in the supply circuit
C315S2090SC, C315S244000, C315S291000, C315S307000, C315SDIG005
Reexamination Certificate
active
06828737
ABSTRACT:
TECHNICAL FIELD
The invention relates to an operating circuit for discharge lamps.
In this case, the invention relates to operating circuits which supply the discharge lamp with radio-frequency supply power which is obtained from a supply power via an oscillator circuit. In particular, but not necessarily, the invention relates to the situation where the supply power for the oscillator circuit is obtained from an AC voltage supply power which is rectified. Operating circuits such as these are in general use, in particular for low-pressure discharge lamps, and there is therefore no need to explain their details.
BACKGROUND ART
The oscillator circuit in this case supplies a so-called load circuit, in which the discharge lamp is connected, and through which a radio-frequency lamp current flows, which is produced by the oscillator circuit. The load circuit in this case defines a resonant frequency, which is influenced by various electrical parameters of the load circuit and also depends, inter alia, on the operating state of the discharge lamp. The aim is to operate the load circuit relatively close to the resonant frequency during continuous operation of the discharge lamp. This has the advantage of small phase shifts between the current and voltage, and hence of small reactive currents. This is beneficial for dimensioning of the components, particularly for a lamp inductor. Apart from this, the oscillator circuit which produces the radio-frequency supply power normally contains switching elements. When the phase shifts are low as a result of operation close to resonance, the switching losses in the switching elements are relatively small. This has advantages with regard to the efficiency of the operating circuit and with regard to the thermal load and the dimensioning of the switching elements.
Normally, one aim is to operate in the so-called inductive region, that is to say at an oscillator circuit operating frequency that is higher than the resonant frequency of the load circuit. However, in this case, it is necessary to avoid the operating frequency of the oscillator circuit becoming less than the resonant frequency since disturbing current spikes can be produced in the switching elements, and other difficulties can occur, in capacitive operation, that is to say when the operating frequency is less than the resonant frequency. In particular, incorrect synchronization between the switching times and the lamp inductor current during capacitive operation can lead to a pronounced positive current spike at the start of a lamp current half-cycle that is carried by a switching element. Thus, overall, it is desirable to operate as close as possible to the resonant frequency although, as far as possible, the frequency should not fall below the resonant frequency, or this should occur only to a restricted extent.
However, temperature changes and aging processes such as electrode wear, mercury diffusion in fluorescent substances and other aging phenomena as well as scatter between the individual examples of different individual discharge lamps result in fluctuations in the lamp impedance (with respect to continuous operation).
These lamp impedance fluctuations and the normal component tolerances mean that the operating circuits cannot easily be set relatively accurately to operation close to resonance. In fact, for safety reasons, a relatively large margin is maintained from the nominal resonant frequency, to take account of the fluctuations and tolerances as described. This results in higher component costs and an increased amount of space being required owing to correspondingly larger dimensioning and in reductions in efficiency.
Attempts have therefore already been made to equip operating circuits of the type described with detection circuits for identifying proximity to capacitive operation of the load circuit. By way of example, FIG. 5 in U.S. Pat. No. 6,331,755 illustrates a resistor RCS for measuring a lamp inductor current, and a comparator COMP for comparing this inductor current with a threshold value. The comparison is carried out on a switching-off flank of a switching transistor in a half-bridge oscillator circuit. The closer the operating frequency is to the resonant frequency and hence to capacitive operation, the smaller not only is a switching-on peak of the measurement voltage (at which the mathematical sign is reversed) across the resistor RCS, but the greater is the extent to which the measurement voltage falls, as well, at the end of the time for which said switching transistor is switched on. The threshold value therefore allows a limit state to be set, at which the circuit is switched off overall (shown on the right in
FIG. 6
in that document), when operation becomes too close to resonance.
DESCLOSURE OF THE INVENTION
Against the background of the cited prior art, the invention is based on the technical problem of further improving an operating circuit for a discharge lamp having an oscillator circuit and having a detection circuit for identifying proximity to capacitive operation of the load circuit.
The invention relates to an operating circuit of the described type, in which a regulation circuit is provided for regulating the load circuit, in particular the lamp power or the lamp current, to a nominal regulation value, and the operating circuit is designed to reduce the nominal regulation value in response to the detection circuit identifying proximity to capacitive operation.
Preferred embodiments are specified in the dependent claims.
According to the invention, the operating circuit is not switched off, as in the case of the prior art, when specific proximity to capacitive operation is identified but, at least normally, is still operated. Identification of proximity to capacitive operation is thus intended to lead to the method of operation being influenced such that this proximity is at least not increased any further, or is even reduced, in order to allow operation to continue. For this purpose, the nominal regulation value, that is to say by way of example the nominal power or current value, of a regulation circuit is reduced. The regulation circuit intrinsically has the purpose and advantage of reducing the influence on lamp operation of scatter between individual lamps and fluctuations which occur over time, such as temperature fluctuations or aging influences. In the invention, a regulation circuit furthermore offers a particularly advantageous and simple capability to prevent capacitive operation by influencing the nominal regulation value. In one preferred embodiment of the regulation circuit, changing the nominal regulation value can also be associated with indirectly influencing the operating frequency of the oscillator circuit, because the regulation circuit preferably influences the operating frequency, in order to regulate the load circuit. In plain words, the operating circuit according to the invention is thus designed not to excessively approach capacitive operation during continuous operation and to counteract any further approach if it becomes too close, but with lamp operation continuing. This is because it is more tolerable from the point of view of the invention for the discharge lamp to become slightly darker in situations such as this than for it to be switched off entirely.
The invention is preferably distinguished by the detection circuit identifying proximity to capacitive operation in a particularly advantageous form. To do this, the detection circuit detects the magnitude of fluctuations of the lamp current corresponding to the frequency of the supply power. If the oscillator circuit is supplied with a rectified AC supply power, the supply power of the oscillator circuit fluctuates with the fluctuations (which result from the AC frequency) of the rectified supply voltage (so-called intermediate circuit voltage). The intermediate circuit voltage is thus modulated at twice the frequency of the original AC voltage. The doubling of the frequency is a consequence of the rectification process. Theoretically, it is also feasible in th
Busse Olaf
Heckmann Markus
Alemu Ephrem
Patent Treuhand Gesellschaft fur elektrische Gluhlampen mbH
Wong Don
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