Abstract:
A ballast circuit for operating a lamp having preheatable electrodes. The frequency of a high frequency bridge inverter is controlled by a control circuit which has input connections to one of the lamp electrodes for monitoring the electrode temperature. The voltage across the lamp electrode controls oscillation frequency during preheating and ignition.

Description:
BACKGROUND OF THE INVENTION 
     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 U.S. Pat. No. 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 normal operation of the lamp, so that a certain amount of power is dissipated in the PTC. 
     BRIEF SUMMARY OF THE INVENTION 
     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 normal operation, do not fulfill a function but do dissipate power. As a result, the efficiency 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. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF 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. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1, K 1  and K 2  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 K 1  and K 2  are interconnected by means of a series arrangement of a first switching element Q 1  and a second switching element Q 2 , 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 Q 1  is shunted by a diode D 5  and the second switching element Q 2  is shunted by a diode D 6 . Control electrodes of the first switching element Q 1  and the second switching element Q 2  are connected to respective outputs of a circuit part SC. Input terminals K 1  and K 2  are also interconnected by means of a series arrangement of a capacitor C 2  and a capacitor C 3 . A common point of capacitor C 2  and capacitor C 3  is connected to a common point of the first switching element Q 1  and the second switching element Q 2  by means of a series arrangement of a first lamp electrode E 11  of lamp La, capacitor C 1 , a second lamp electrode E 12  of lamp La and a coil L 2 . This series arrangement forms a load branch. The first lamp electrode E 11  forms, in this example, a temperature-dependent impedance. Respective ends of the first lamp electrode E 11  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 E 11  jointly form a control circuit for rendering the switching means high-frequency conducting and non-conducting. Respective ends of capacitor C 3  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 K 1  and K 2  are connected to the poles of a supply voltage source, the circuit part SC renders the switching elements Q 1  and Q 2  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 E 11  is low. As a result, the impedance of lamp electrode E 11  is low and the voltage across lamp electrode E 11  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 E 11  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 C 1  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 E 11  increases too. As a result, both the impedance of lamp electrode E 11  and the amplitude of the voltage across lamp electrode E 11  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 C 1  to increase. When the temperature of lamp electrode E 11  has increased to a value suitable for emission, also the amplitude of the voltage across capacitor C 1  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 E 11  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. 
     K 1  and K 2  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 K 1  and K 2  are interconnected by means of a series arrangement of a first switching element Q 1  and a second switching element Q 2 . Input terminals K 1  and K 2  are also interconnected by means of a series arrangement of capacitor C 2  and capacitor C 3  and by means of a series arrangement of ohmic resistance  33  and ohmic resistance  34 . A common point B of capacitor C 2  and capacitor C 3  is connected to a common point A of the first switching element Q 1  and the second switching element Q 2  by means of a load branch, which is formed by a series arrangement of the first lamp electrode E 11  of lamp La, capacitor C 1 , second lamp electrode E 12  of lamp La and coil L 2 . Also in this example, electrode E 11  forms a temperature-dependent impedance. The first lamp electrode E 11  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 Q 1 . 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 K 2 . The base electrode of bipolar transistor  22  is connected to input terminal K 1  via ohmic resistance  23 . A collector electrode of bipolar transistor  22  is also connected to input terminal K 1  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 Q 2 . By means of diode  22   a , input terminal K 2  is also connected to the control electrode of the second switching element Q 2 . The common point A of the first switching element Q 1  and the second switching element Q 2  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 Q 1  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 E 11 . In this example, the first lamp electrode E 11 , zener diodes  26 ,  27 ,  29 ,  30 , coil  19 , capacitor  20 , ohmic resistances  23 ,  24  and  25 , bipolar transistor  22  and diodes  10  and  22 a 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 U.S. Pat. No. 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 U.S. Pat. No. 4,935,672. The only difference resides in that the circuit arrangement shown in U.S. Pat. No. 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 U.S. Pat. No. 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 K 1  and K 2 , 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 non-conducting 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 E 11  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 C 1  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 E 11  and the amplitude of the voltage across the first lamp electrode E 11  increase. This causes the value of the frequency f to decrease and the amplitude of the voltage across capacitor C 1  to increase. When the temperature of lamp electrode E 11  has increased to a suitable value for emission, also the amplitude of the voltage across capacitor C 1  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 E 11  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 C 1 -C 4 . 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.