CIRCUIT ARRANGEMENT

Abstract: In a ballast circuit for operating a lamp with a high-frequency current comprising a bridge inverter, the voltage across a lamp electrode is used to monitor the temperature of the electrode and to control the oscillation frequency of the bridge inverter during preheating and ignition.

Description: Circuit arrangement. The invention relates to a circuit arrangement for feeding a discharge lamp, comprising input terminals for the connection to a supply voltage source, switching means coupled to the input terminals for generating a high-frequency current from a supply voltage supplied by the supply voltage source, a control circuit coupled to the switching means for rendering the switching means high-frequency conducting and non-conducting, a temperature-dependent impedance for preheating electrodes of the discharge lamp. The invention also relates to a compact lamp. A circuit arrangement as mentioned in the opening paragraph is known from US 4,935,672. In the known circuit arrangement, the switching means form part of an inverter of the half-bridge type. A load branch, which during operation contains the lamp, is coupled to this half bridge. The temperature-dependent impedance is formed by a PTC, which shunts the lamp and is connected in series with the electrodes of the lamp. When the circuit arrangement is in operation, the switching means generate a high-frequency current through the load branch. Immediately after the circuit has been put into operation, the temperature of the PTC is relatively low. As a result, also the impedance of the PTC is relatively low. This causes a current with a relatively high amplitude to flow through the electrodes of the lamp, and the voltage across the lamp, which is equal to the voltage across the PTC, to be relatively low. In this stage of operation of the lamp, the electrodes of the lamp are preheated. Since the PTC carries a current, the temperature of the PTC increases and hence also the impedance of the PTC. As the impedance of the PTC increases, the amplitude of the current through the electrodes decreases, and the amplitude of the voltage across the lamp increases to a value at which the lamp ignites. The presence of the PTC in the known circuit arrangement thus causes the electrodes of the lamp to be preheated before the lamp is ignited. A drawback of the known circuit arrangement resides in that the PTC is a relatively expensive component which must be added to the circuit arrangement for preheating the electrodes. In addition, the PTC also carries a current during stationary operation of the lamp, so that a certain amount of power is dissipated in the PTC. It is an object of the invention to provide a circuit arrangement for feeding a lamp, which circuit arrangement can also be used to heat the electrodes of the lamp before the lamp is ignited, which circuit arrangement is, in addition, relatively inexpensive and has a relatively high efficacy. To achieve this, a circuit arrangement of the type mentioned in the opening paragraph is characterized in accordance with the invention in that the temperature-dependent impedance comprises, during operation of the lamp, one of the electrodes of the lamp and forms part of the control circuit. As the temperature-dependent impedance comprises an electrode of the lamp, the circuit arrangement is relatively inexpensive. In addition, the load branch of the circuit arrangement does not comprise components which, during stationary operation, do not fulfill a function but do dissipate power. As a result, the efficacy of a circuit arrangement in accordance with the invention is relatively high. Good results have been obtained with embodiments of a circuit arrangement in accordance with the invention, wherein the switching means comprise a series arrangement of two switching elements. A circuit arrangement in accordance with the invention can very suitably be used in the electronic ballast of a compact lamp comprising a light source provided with a gastight lamp vessel which allows passage of visible light, a housing which is secured to the light source and provided with a lamp cap, an electronic ballast which is electrically connected to the light source in order to feed the light source, which electronic ballast is situated in a space which is surrounded by the housing. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings: Fig. 1 and Fig. 2 show examples of a circuit arrangement in accordance with the invention to which a lamp is connected, and Fig. 3 shows an example of a compact lamp in accordance with the invention. In Fig. 1, Kl and K2 are input terminals which are to be connected to a supply voltage source. The example shown in Fig. 1 can suitably be fed by means of a direct voltage source. Input terminals Kl and K2 are interconnected by means of a series arrangement of a first switching element Ql and a second switching element Q2, which, in this example, form switching means for generating a high-frequency current from a supply voltage supplied by the supply voltage source. The first switching element Ql is shunted by a diode D5 and the second switching element Q2 is shunted by a diode D6. Control electrodes of the first switching element Ql and the second switching element Q2 are connected to respective outputs of a circuit part SC. Input terminals Kl and K2 are also interconnected by means of a series arrangement of a capacitor C2 and a capacitor C3. A common point of capacitor C2 and capacitor C3 is connected to a common point of the first switching element Ql and the second switching element Q2 by means of a series arrangement of a first lamp electrode Ell of lamp La, capacitor Cl, a second lamp electrode E12 of lamp La and a coil L2. This series arrangement forms a load branch. The first lamp electrode Ell forms, in this example, a temperature-dependent impedance. Respective ends of the first lamp electrode Ell are connected to, respectively, a first and a second input of the circuit part SC. In this example, the circuit part SC and the first lamp electrode Ell jointly form a control circuit for rendering the switching means high-frequency conducting and non-conducting. Respective ends of capacitor C3 are connected to, respectively, a third and a fourth input of the circuit part SC. The operation of the circuit arrangement shown in Fig. 1 is as follows. If the input terminals Kl and K2 are connected to the poles of a supply voltage source, the circuit part SC renders the switching elements Ql and Q2 alternately high- frequency conducting and non-conducting with a frequency f. As a result, a high-frequency alternating current, also with a frequency f, flows in the load branch. Immediately after the circuit arrangement has been put into operation, the temperature of lamp electrode Ell is low. As a result, the impedance of lamp electrode Ell is low and the voltage across lamp electrode El 1 has a relatively small amplitude. This voltage is present between the first and the second input of circuit part SC. If the amplitude of the voltage across the first lamp electrode Ell is relatively low, the circuit part SC sets the frequency f with which the switching elements are rendered conducting and non-conducting to a relatively high value. Since the value of f is relatively high, the voltage across capacitor Cl has a relatively small amplitude, so that the lamp La does not ignite at the voltage across capacitor C 1. As the time during which the current flows in the load branch increases, however, the temperature of the lamp electrode Ell increases too. As a result, both the impedance of lamp electrode Ell and the amplitude of the voltage across lamp electrode Ell increase. As a result of the higher amplitude of the voltage between the first and the second input of the circuit part SC, the circuit part SC sets the frequency f to a lower value. This decrease of the frequency f causes the amplitude of the voltage across capacitor Cl to increase. When the temperature of lamp electrode Ell has increased to a value suitable for emission, also the amplitude of the voltage across capacitor Cl has increased to such a level that the lamp ignites at this voltage. It is thus achieved that the lamp does not ignite until after the lamp electrodes are sufficiently preheated. During stationary operation of the lamp, the temperature of the lamp electrode Ell remains approximately constant, so that the same applies to the frequency f. In the example shown in Fig. 2, components and circuit parts which correspond to components and circuit parts of the example shown in Fig. 1 bear the same reference numerals. Kl and K2 are input terminals to be connected to a supply voltage source. Also the example shown in Fig. 2 can suitably be fed by means of a direct voltage source. Input terminals Kl and K2 are interconnected by means of a series arrangement of a first switching element Ql and a second switching element Q2. Input terminals Kl and K2 are also interconnected by means of a series arrangement of capacitor C2 and capacitor C3 and by means of a series arrangement of ohmic resistance 33 and ohmic resistance 34. A common point B of capacitor C2 and capacitor C3 is connected to a common point A of the first switching element Ql and the second switching element Q2 by means of a load branch, which is formed by a series arrangement of the first lamp electrode Ell of lamp La, capacitor Cl, second lamp electrode E12 of lamp La and coil L2. Also in this example, electrode Ell forms a temperature-dependent impedance. The first lamp electrode Ell is shunted by a series arrangement of a coil 19 and a capacitor 20. Coil 19 is shunted by a series arrangement of zener diodes 30 and 29 and ohmic resistance 28. Capacitor 20 is shunted by a series arrangement of zener diodes 26 and 27 and ohmic resistance 25. A common point of zener diode 26 and ohmic resistance 25 is connected to a control electrode of the first switching element Ql. A common point P of coil 19 and capacitor 20 is connected to a cathode of diode 10. An anode of diode 10 is connected to a base electrode of bipolar transistor 22. An emitter electrode of bipolar transistor 22 is connected to input terminal K2. The base electrode of bipolar transistor 22 is connected to input terminal Kl via ohmic resistance 23. A collector electrode of bipolar transistor 22 is also connected to input terminal Kl by means of ohmic resistance 24. The collector electrode of bipolar transistor 22 is directly connected to a control electrode of the second switching element Q2. By means of diode 22a, input terminal K2 is also connected to the control electrode of the second switching element Q2. The common point A of the first switching element Ql and the second switching element Q2 is connected, via capacitor 35, to a common point of ohmic resistance 33 and ohmic resistance 34. The common point of ohmic resistance 33 and ohmic resistance 34 is also connected to the control electrode of the first switching element Ql by means of a series arrangement of a breakdown element 32 and ohmic resistance 31. The control voltages with which the first and the second switching element are rendered conducting and non-conducting are derived, in this example, from the voltage across the first lamp electrode Ell . In this example, the first lamp electrode Ell, zener diodes 26, 27, 29, 30, coil 19, capacitor 20, ohmic resistances 23, 24 and 25, bipolar transistor 22 and diodes 10 and 22a jointly form a control circuit for rendering the switching means high-frequency conducting and non-conducting. Ohmic resistances 31, 33 and 34 and breakdown element 32 and capacitor 35 jointly form a starter circuit to start the oscillation in the circuit arrangement immediately after a supply voltage source has been connected. The operation of the starter circuit corresponds to the operation of the starter circuit of the circuit arrangement shown in Fig. 2 of United States Patent Specification US 4,935,672. The operation of the control circuit also corresponds to that of the control circuit of the circuit arrangement shown in Fig. 2 of US 4,935,672. The only difference resides in that the circuit arrangement shown in US 4,935,672 uses a part of the ballast coil instead of the first lamp electrode to generate control voltages for the first and the second switching element. For more detailed information about the operation of the starter circuit and the control circuit reference is made to US 4,935,672. The operation of the example shown in Fig. 2 is as follows. If a direct voltage source is connected to input terminals Kl and K2, the starter circuit causes the circuit arrangement to start oscillating, and the control circuit renders the first and the second switching element alternately high-frequency conducting and nonconducting with a frequency f. As a result, an alternating current with a frequency f flows in the load branch. Immediately after the circuit arrangement has been put into operation, the temperature of the first lamp electrode Ell is relatively low. As a result, the impedance of the first lamp electrode is relatively low and the amplitude of the voltage across the first lamp electrode is also relatively low. Due to this low amplitude of the voltage across the first lamp electrode, the frequency f has a relatively high value and the amplitude of the voltage across capacitor Cl is relatively low. The temperature of the first lamp electrode increases as the time during which current flows through the first lamp electrode is longer. As a result, also the impedance of the first lamp electrode Ell and the amplitude of the voltage across the first lamp electrode Ell increase. This causes the value of the frequency f to decrease and the amplitude of the voltage across capacitor Cl to increase. When the temperature of lamp electrode Ell has increased to a suitable value for emission, also the amplitude of the voltage across capacitor Cl has increased to such a level that the lamp ignites at this voltage. It is thus achieved that the lamp does not ignite until after the lamp electrodes are preheated sufficiently. During stationary operation of the lamp, the temperature of lamp electrode Ell remains approximately constant, so that the same applies to the frequency f. In Fig. 3, reference numeral 8 refers to a part of a gastight lamp vessel which passes visible light. Reference numeral 6 refers to the wall of a housing connected to the lamp vessel 8 and provided with a lamp cap 3, a circuit arrangement B in accordance with the invention being present in a space 7 surrounded by the housing. The circuit arrangement is diagrammatically represented by the components P and C1-C4. Reference numeral 9 refers to electric connections between the circuit arrangement and (not shown) electrodes in the lamp vessel. E refers to connection wires between the circuit arrangement and electric contacts 1 and 2 arranged on the lamp cap.

Claims: CLAIMS: 1. A circuit arrangement for feeding a discharge lamp, comprising input terminals for the connection to a supply voltage source, switching means coupled to the input terminals for generating a high-frequency current from a supply voltage supplied by the supply voltage source, - a control circuit coupled to the switching means for rendering the switching means high-frequency conducting and non-conducting, a temperature-dependent impedance for preheating electrodes of the discharge lamp, characterized in that the temperature-dependent impedance comprises, during operation of the lamp, one of the electrodes of the lamp and forms part of the control circuit. 2. A circuit arrangement as claimed in claim 1 , wherein the switching means comprise a series arrangement of two switching elements. 3. A compact lamp comprising a light source provided with a gastight lamp vessel which allows passage of visible light, a housing which is secured to the light source and provided with a lamp cap, an electronic ballast which is electrically connected to the light source in order to feed the light source, which electronic ballast is situated in a space which is surrounded by the housing, characterized in that the electronic ballast is a circuit arrangement as claimed in claim 1.

Inventors: SIMPELAAR BENNIE I P, SIMPELAAR, BENNIE, I., P.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2000
Main classification: H05B41
IPC codes: F21S2, F21Y103, H05B41