Electric lamp and discharge devices: systems – High frequency starting operation for fluorescent lamp
Reexamination Certificate
1998-11-17
2001-06-12
Lee, Benny (Department: 2817)
Electric lamp and discharge devices: systems
High frequency starting operation for fluorescent lamp
C315SDIG005, C315S219000, C315S224000
Reexamination Certificate
active
06246173
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an operating circuit for a load in which an oscillator circuit produces a high-frequency power to supply the load. For this purpose, the oscillator circuit itself is supplied with a supply power, for example a rectified mains power. Here, oscillator circuits with voltage-controlled switching elements, for example field-effect transistor half-bridges, are considered specifically. Such operating circuits are principally applied in electronic ballasts for low-pressure gas discharge lamps.
An aspect of such operating circuits which is important for this invention results from the need to initiate the free-running oscillation of the oscillator when operation is started. Frequently, in order to produce the free-running oscillation, coupled controlled transformers, for example, are used to drive the switching elements of the oscillator. However, the positive feedback effect is produced only in the actual oscillation mode and at the beginning has to be produced only, as it were, by an external impetus.
A known solution for a starting circuit which produces this impetus when the supply power is switched on can be found in the German Application DE 195 48 506 A1. Here, after the supply power is switched on, a capacitor is charged via a resistor until the breakdown voltage of a diac is reached. Its breakdown discharges a portion of the charge stored in the capacitor into a drive circuit of a field-effect transistor of a half-bridge oscillator. Further details can be found in the quoted document.
SUMMARY OF THE INVENTION
This invention is based on the technical problem of specifying an operating circuit of the type described at the beginning, with an improved starting circuit.
For this purpose, the invention provides an operating circuit for a load, in particular a low-pressure gas discharge lamp, with a free-running oscillator with voltage-controlled switching elements for producing a high-frequency output power for the load from a supply power which is characterized by a starting circuit for initiating the free-running oscillation with a starting capacitor connected between a drive circuit at a control terminal of a switching element and a reference potential of the switching element.
Firstly, the invention is based here on the recognition that the diac used in the prior art constitutes a significant disadvantage of the conventional solution. It has become apparent, namely, that diacs exhibit a failure rate which is above average in comparison with other components used in the operating circuits, and thus lead to unnecessarily high failure rates of the electronic ballasts or other operating circuits equipped with them.
For this reason, the solution according to the invention dispenses with the use of a diac. Instead, a capacitor is provided which is referred to here as a starting capacitor. The starting capacitor has the function, given increasing charging by its connection to a reference potential of a switching element, for example to a power supply branch, of enabling a drive circuit of the oscillator or one of its switching elements to bring about a first switching process of the respective switching element, after the power supply has been switched on. Here, it is necessary to consider the fact that in the case of voltage-controlled switching elements this does not require very high currents but rather merely a certain voltage. Voltage-controlled switching elements have basically one defined threshold voltage in terms of their switching-on or switching-off procedure. The breakdown procedure of the diac which the invention is, as explained, intended to avoid can therefore be replaced by approaching and exceeding the voltage threshold value of the switching element. This procedure is brought about directly or indirectly by the starting capacitor which charges up when the power supply is switched on.
This may take place, for example, by virtue of the fact that the potential of a relatively complex drive circuit, of which more details will be given below, is displaced in its entirety by the starting capacitor. However, it is not absolutely necessary to provide a relatively complex drive circuit, instead, in the simplest case, it is also possible here to provide just one connection point of a positive feedback device, of whatever kind, for the free-running oscillation (for example the secondary winding of a control transformer) as “drive circuit” in the minimal sense, with the result that the voltage applied to the starting capacitor acts virtually directly on the control terminal of the voltage-controlled switching element. These simple “minimum versions” of a drive circuit could be considered in the case of the control transformer when, for example, the primary winding already has applied to it the correct phase delay for driving the switching elements.
At any rate, in accordance with the invention, the use of a starting capacitor is proposed as a simple and cost-effective component, making the diac with the described disadvantages superfluous.
In preferred applications of the invention, the oscillator is a half-bridge circuit such as is known in electronic ballasts. Preferred examples of voltage-controlled switching elements are, in particular, field-effect transistors, especially MOSFETs or else IGBT (“Insulated Gate Bipolar Transistor”).
Preferably, the starting capacitor is charged easily by means of a charging resistor which is connected between the starting capacitor and a potential which is suitable for charging, for example a power supply branch, and which should be dimensioned with relatively high impedance so that the starting circuit causes little disruption, that is, no asymmetry effects, during the oscillation mode.
In order to avoid a stable state of the entire operating circuit, otherwise possible under certain circumstances, after the power supply is switched on, it may, for example, be advantageous to select the connection point used for charging the starting capacitor such that a certain oscillation or fluctuation of the potential is present at it. For this purpose, it is possible, for example in the case of a mains connection of the operating circuit via a rectifier, to connect the charging resistor to an AC voltage terminal on the mains input side of the rectifier. However, it is also possible to find other solutions here which are appropriate particularly if there is no mains AC voltage at all, for example when the circuit is being operated from a battery supply. Then, the aforesaid fluctuation or oscillation can be produced specifically, for example by means of a bistable component such as a flip-flop. However, the electronic low-pressure gas-discharge-lamp ballasts which are particularly considered for application in the operating circuit according to the invention are, as a rule, configured for mains operation.
Furthermore, it may be advantageous also to provide, in addition to the charging resistor, a discharging resistor parallel to the starting capacitor. The discharging resistor supports its discharging, in particular given a standing oscillation of the oscillator. In order to increase the operational reliability, it is particularly appropriate if, in the case of mains operation, a rectifier and a large electrolytic capacitor which has been charged by the latter is used. This aspect relates to those operating phases in which the rectifier does not charge the electrolytic capacitor and there is thus no potential connection between the rectified side and the mains side. In this “floating” state of the rectified side, the influence of displacement capacitances in the rectifier may disrupt the potential conditions at a charging resistor connecting the mains side and the rectified side and thus disrupt its discharging function.
A further advantageous refinement of the invention consists in a discharging diode for cyclically discharging the starting capacitor in the oscillation mode. This discharging diode is connected in such a way that during the actual starting process it initially blocks and thus permits
Fischer Klaus
Schmitt Harald
Bessone Carlo S.
Glenn Kimberly E
Lee Benny
Patent-Treuhand-Gesellschaft Fuer Elektrische Gluehlampen MBH
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