Circuit Arrangement for Igniting and Operating Gas-Discharge Lamps

The invention relates to a circuit arrangement for igniting and operating gas-discharge lamps, particularly metal-vapor lamps.

An almost endless variety of circuits for ballasts of gas-discharge lamps are known for a long time. Every one of these circuits employs for ignition the glow starter arranged in parallel with the lamp and, for current limitation during operation, the choke which is connected in series with the gas-discharge lamp. In these arrangements with glow starter and choke, no flicker-free instantaneous start is effected and, furthermore, lamp burning is not stable. The influence of mains voltage fluctuations on the lighting current of the lamp is likewise not inconsiderable. Since the glow starter comprises a mechanical switch which is a thermostatic bimetal switch, the risk of failure of the latter is relatively high.

Electronic ballasts have been developed to avoid these disadvantages. A circuit of this type is described in German Patent No. 31 52 342. Such circuit consists of a current source, an ignition circuit, a control element and an auxiliary voltage source. However, such a ballast requires a great deal of components and is therefore also quite susceptible to failure. Furthermore, it is conceived particularly for fluorescent lamps which require a lower ignition voltage than metal-vapor lamps. This ballast is thus entirely unsuitable for metal-vapor lamps.

The object of the invention is thus to provide an electronic ballast particularly for metal-vapor lamps requiring a high ignition voltage in the range of 2 kV to 5 kV.

The object is achieved by the invention. This is characterized in that a buffer capacitor, preferably an electrolytic capacitor, is connected to a rectified voltage, preferably the mains voltage rectified by means of a bridge rectifier, a choke-capacitor series circuit being in parallel with the buffer capacitor, whereby an electronic switch, preferably a transistor or thyristor, and a fast recovery diode forming part of the electronic switch are connected in parallel with the capacitor, and that the gas-discharge lamp provided with a parallel capacitor is connected by means of a further choke-capacitor series circuit to the parallel circuit consisting of capacitor, electronic switch and fast recovery diode. In this new type of ballast, the ignition circuit and the supply circuit are combined and can be considered as one unit. Therefore, a separate ignition circuit is also not required. Furthermore, the circuit operates independently of the supply voltage. Moreover, the gas-discharge lamp is always supplied with constant power. This even then when the lamp burning voltage changes, such burning voltage increasing with the age of the lamp.

An embodiment of the invention is characterized in that a further choke is provided in series with the parallel circuit consisting of capacitor, electronic switch and fast recovery diode. In this manner, the structural size of the two already existing chokes can be kept small.

It is advantageous that the choke of the further choke-capacitor series circuit is arranged in series with the parallel circuit consisting of capacitor, electronic switch and fast recovery diode. By virtue of this arrangement, the ballast according to the invention is excellently suitable for gas-discharge lamps with an ignition voltage less than 2 kV.

The invention will now be described in still greater detail in conjunction with the drawing.

FIG. 1 shows the basic circuit of the electronic ballast,

FIG. 2 shows the augmented circuit having three chokes, and

FIG. 3 shows the circuit for lamps with an ignition voltage of less than 2 kV.

In all three Figures there is a direct current voltage applied at the terminals 1, 2, which is normally the mains voltage rectified by means of a bridge rectifier. At this direct current voltage there is placed a buffer or smoothing capacitor C4.

This is generally an electrolytic capacitor having a high capacitance value. In the circuit depicted in FIG. 1, a choke-capacitor series circuit L1, C1 is connected in parallel with the buffer capacitor C4. An electronic switch S, which is customarily a transistor or thyristor, as well as a fast recovery diode D are connected in parallel with the capacitor C1 of this series circuit consisting of L1, C1. Furthermore, in the circuit of the electronic ballast according to FIG. 1 there is arranged a second choke-capacitor series circuit L2, L3 in parallel with the capacitor C1, this second choke-capacitor series circuit being connected in series with the : gas-discharge lamp L. Furthermore, the gas-discharge lamp L comprises a parallel capacitor C2.

The circuit depicted in FIG. 2 is basically identical with the circuit in FIG. 1. However, there is additionally provided a third choke L3 which is arranged in series with the capacitor C1.

A further variant of the embodiment of the electronic ballast according to the invention is illustrated in FIG. 3. In this variant the choke L2 series-connected to the capacitor C3 has been eliminated. Consequently, only the two chokes L1, L3 are provided in this circuit

The circuits apparent from the three Figures are based on a series resonance circuit, whereby in each case there are provided a main oscillating circuit and a load circuit. The main oscillating circuit depicted in FIG. 1 consists of the choke L1 and the capacitor C1. On the other hand, the main oscillating circuit in FIGS. 2 and 3 is structured by the two chokes L1, L3 and the capacitor C1. The choke L2 and the two capacitors C2, C3 form the load circuit in FIGS. 1 and 2, while in FIG. 3 the choke L3 is part of the load circuit, since the choke L2 does not exist. The gas-discharge lamp L represents the consumer. The electronic switch S has to definedly excite the respective series resonance circuit. The smoothing capacitor C4 has to be dimensioned such that it always makes available the supply voltage which is required for the resonance circuit and is above all intermittence-free. For a stable oscillation behavior of the series resonance circuit, the inductances of the chokes L1, L2 as well as the capacitances of the capacitors C1, C2 must be in a predeterminate relationship to one another. The choke L2 in FIG. 1 and FIG. 2 serves for adapting the voltage ratios in the load circuit and at the gas-discharge lamp L, as well as for compensating the phase position of the main oscillating circuit. In the circuit of the ballast according to FIG. 3, the choke L3 takes over the above task of the choke L2.

The additional choke L3 in FIG. 2 is required for fine tuning of the series resonance circuit in unstable consumers.

In the circuits of the three Figures there has been selected in each case the negative potential as the common reference point. The positive potential could also be considered to be the common reference point, whereby the function of the circuit is perfectly preserved.

Finally, the mode of operation of the circuit during ignition and operation will now be described.

Prior to ignition or at the moment of ignition the gas-discharge lamp represents a high-ohmic load. In this operating instance the circuits operate as step-up transducers or as high-voltage ignition generators, whereby several kV high voltage are generated depending upon dimensioning.

The frequency determining components in the series resonance circuit at the moment of ignition are the capacitances of the capacitors C1, C2, C3 in all three circuits, plus the inductance of the choke L2 in FIG. 1, the inductances of the chokes L2, L3 in FIG. 2 and the inductance of the choke L3 in FIG. 3. The frequency of these resonance circuits upon ignition is a multiple of the frequency of the respective main oscillating circuit. By appropriate dimensioning of the capacitor C2 as a function of the capacitor C1 there is generated a high voltage of several kV at the gas-discharge lamp L as well as at the capacitor C2 itself, whereby the gas-discharge lamp L is ignited.

When the gas-discharge lamp L ignites as a result of a high voltage generated at the capacitor C2, this high voltage will abruptly break down at this capacitor C2 and at the gas-discharge lamp L. The oscillating-circuit frequency thereby decreases to that of the main oscillating circuit. Then only the operating voltage of the gas-discharge lamp L lies at the capacitor C2. The circuits in the three Figures now operate as step-down transducers, whereby the maximum oscillating-circuit voltage is divided between the components in the load circuit. The required energy for operating the gas-discharge lamp L is transferred from the main oscillating circuit to the load circuit and made available to the lamp L.