CIRCUIT ARRANGEMENT

Abstract: A ballast for a compact fluorescent lamp includes a down-converter for operating the lamp with a DC current during stationary operation and a charge pump for igniting the lamp.

Description: Circuit arrangement. Description The invention relates to a circuit arrangement for igniting and supplying a lamp with a DC current, comprising - input terminals for connection to terminals of a power supply source supplying a DC voltage, - a DC/DC converter coupled to the input terminals and provided with - an inductive element, - a first unidirectional element, - a first switching element, - a control circuit coupled to a control electrode of the first switching element for rendering the first switching element conducting and non-conducting at a frequency f, - output terminals for connecting the lamp, and - an output capacitance shunting the lamp during operation, and comprising - a first branch connecting the output terminals during operation and comprising a second switching element and means for rendering the second switching element conducting and non-conducting. The invention also relates to a compact lamp. A circuit arrangement as described in the opening paragraph is known from US 5,581,161. In the known circuit arrangement, the DC/DC converter is a down-converter. Immediately after the known circuit arrangement is put into operation, both the first and the second switching element are conducting. During operation of the known circuit arrangement, both a first and a second lamp electrode form part of the first branch. The inductive element and the first branch convey a current during a first time interval, so that the electrodes of the lamp connected to the circuit arrangement are preheated. At the end of the first time interval, the second switching element is rendered non-conducting so that the first branch no longer conveys any current. The inductive element subsequently charges the output capacitance via the first unidirectional element, which output capacitance supplies the lamp with an ignition voltage. After igniting the lamp, during stationary operation, the control circuit renders the first switching element high-frequency conducting and non-conducting, and a lamp connected to the circuit arrangement is fed with a DC current supplied by the down-converter. Since the down-converter consists of only a small number of components, the known circuit arrangement can be manufactured relatively easily and is thus inexpensive. A drawback of the known circuit arrangement is, however, that the ignition voltage generated at the end of the first time interval has only a relatively small amplitude so that the lamp does not easily ignite. In the known circuit arrangement, the second switching element is constituted by a glow discharge starter. If the lamp does not ignite immediately, the glow discharge starter will successively become conducting and non-conducting a number of times, while an ignition voltage is generated across the output capacitance every time after the glow discharge starter has become non-conducting. It has been found in practice that, dependent on the specific properties of the relevant lamp, the ignition of the lamp is only successful after a number of attempts or, in the worst case, fails completely. It is an object of the invention to provide a relatively simple and, hence, inexpensive circuit arrangement for igniting and supplying a lamp with a DC current, with which the lamp can be ignited in an efficient and reliable manner. According to the invention, a circuit arrangement as described in the opening paragraph is therefore characterized in that the first branch is also provided with a second unidirectional element. Due to the presence of the second unidirectional element, the output capacitance is not discharged via the second switching element when this switching element becomes conducting again. Consequently, when the second switching element becomes nonconducting again, the inductive element charges the output capacitance to a higher voltage than was reached during the previous period of non-conductance of the second switching element. It is thus achieved that, after the second switching element has become conducting and non-conducting a relatively small number of times, a voltage with a relatively high amplitude can be obtained across the output capacitance. It has been found that lamps of various properties have a good ignition behavior if they are ignited by means of a circuit arrangement according to the invention. The first switching element is preferably rendered conducting and nonconducting by the control circuit in such a way that the inductive element conveys a continuous current prior to ignition of the lamp. Good results have also been found for embodiments of a circuit arrangement according to the invention in which the circuit arrangement is provided with a circuit component for decreasing the frequency f after ignition of the lamp. If the frequency at which the first switching element is rendered conducting and non-conducting is relatively high prior to ignition, the minimal value of the amplitude of this continuous current is relatively high. Consequently, the inductive element has a relatively large amount of energy at any moment, so that the voltage across the output capacitance increases to a relatively strong extent when the second switching element becomes nonconducting, even if the second switching element becomes non-conducting when the amplitude of the current through the inductive element is approximately minimal. However, after ignition of the lamp, a high value of the frequency at which the first switching element is rendered conducting and non-conducting may give rise to relatively high losses. Good results have been achieved with embodiments of a circuit arrangement according to the invention in which the DC/DC converter comprises a down-converter. The first and the second unidirectional element may be realized in a relatively simple and, hence, inexpensive manner if the first and the second unidirectional element comprise a diode each. In order to generate an ignition voltage with a relatively high amplitude, the second switching element of a circuit arrangement according to the invention must become successively conducting and non-conducting a number of times. It is possible to implement the second switching element as a semiconductor switching element and to realize the means for rendering this semiconductor switching element successively conducting and non-conducting by means of a second control circuit which is coupled to a control electrode of the semiconductor switching element. However, if the second switching element is implemented as glow discharge starter, a separate control circuit is not necessary for realizing the successive conducting and non-conducting states of the second switching element. The circuit arrangement thus comprises a relatively small number of components and can therefore be manufactured relatively easily and at low cost. To improve the ignition behavior of a lamp operated by means of a circuit arrangement according to the invention, it is possible to implement the circuit arrangement in such a way that the first branch comprises a lamp electrode during operation. When the first switching element is conducting, the current flowing through the first branch also flows through this lamp electrode so that it is pre-heated. Since the lamp is fed with a DC current after ignition, a considerable improvement of the ignition behavior can be realized if only the lamp electrode, constituting the cathode of the lamp during stationary lamp operation, is pre-heated. It has been found that, in dependence upon the specific properties of the lamp operated by means of a circuit arrangement according to the invention, it may be desirable to provide the circuit arrangement also with a voltage multiplier. This voltage multiplier is fed with an AC voltage and generates a voltage whose amplitude is several times the maximum amplitude of the AC voltage. This voltage generated by the voltage multiplier results in an improvement of the take-over behavior of the lamp. Take-over is herein understood to be the phase of the lamp operation between ignition and stationary operation of the lamp. It has also been found that a circuit arrangement according to the invention is particularly suitable for use in the ballast circuit of a compact lamp comprising - a light-transmissive discharge vessel having a filling of rare gas, and two electrodes, - a lamp housing secured to the discharge vessel, - a lamp base provided with electric contacts and secured to the lamp housing, and - a ballast circuit coupled between the contacts and the electrodes for generating a lamp current from a power supply voltage. More particularly, a circuit arrangement according to the invention may be eminently used in the ballast circuit of a compact fluorescent lamp, or a lamp whose discharge vessel filling contains mercury and a wall of the discharge vessel has a luminescent coating. These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. In the drawings: Fig. 1 shows diagrammatically a first embodiment of a circuit arrangement according to the invention, with a lamp connected thereto; Fig. 2 shows diagrammatically a second embodiment of a circuit arrangement according to the invention, with a lamp connected thereto, and Fig. 3 shows a compact fluorescent lamp according to the invention. In Fig. 1, the reference numerals 1 and 2 denote terminals for connection to an AC voltage source. Terminals 1 and 2 are connected to respective inputs of circuit component RM for generating a DC power supply voltage from an AC voltage supplied by the AC voltage source. Kl and K2 are input terminals for connection to the terminals of a power supply source supplying a DC voltage. In this embodiment, this power supply source is constituted by the AC voltage source together with the circuit component RM. Input terminals Kl and K2 are connected to respective output terminals of the circuit component RM. Input terminal Kl is connected to input terminal K2 by means of a series arrangement of diode Dl and switching element SI, constituting, in this embodiment, first unidirectional means_and a first switching element, respectively. A control electrode of switching element S 1 is coupled to an output of a circuit component SC which constitutes a control circuit for rendering the switching element SI conducting and non-conducting. Diode Dl is shunted by a series arrangement of glow discharge starter S2, a first lamp electrode Ell of lamp LA and coil LI. In this embodiment, glow discharge starter S2 constitutes a second switching element together with means for rendering the second switching element conducting and non-conducting. Coil LI constitutes an inductive element. Glow discharge starter S2 is shunted by a series arrangement of diode D2 and capacitor Cl, constituting second unidirectional means and an output capacitance, respectively. A common point of diode D2 and capacitor Cl is connected to a first end of a second lamp electrode E12. A second end of second lamp electrode E12 is connected to an output terminal K4 for connecting the lamp. A second output terminal K3 for connecting the lamp is shown diagrammatically as a common point of the first lamp electrode Ell and coil LI. Switching element SI, circuit component SC, diode Dl, coil LI, capacitor Cl and output terminals K3 and K4 jointly constitute a DC/DC converter of the down-converter type. The embodiment shown in Fig. 1 operates as follows. If terminals 1 and 2 are connected to terminals of an AC voltage source, the circuit component RM generates a DC power supply voltage from an AC voltage supplied by the AC voltage source, which DC power supply voltage is present between input terminals Kl and K2. As long as the lamp is not yet ignited, the glow discharge starter successively becomes alternately conducting and non-conducting, while switching element SI is rendered conducting and non-conducting by the circuit component SC at the frequency f. When the glow discharge starter and the switching element SI are conducting, a current flows from input terminal Kl to input terminal K2 via the glow discharge starter S2, the first lamp electrode Ell, coil LI and switching element SI. When the glow discharge starter S2 is conducting and switching element SI is non-conducting, a current flows from a first end of coil LI via diode Dl, the glow discharge starter and first lamp electrode Ell to a second end of coil LI. The frequency f and the duty cycle with which the circuit component SC renders the switching element SI conducting are chosen to be such that the current in coil LI is a continuous current with a relatively high minimal amplitude. After the glow discharge starter S2 has become nonconducting, a current flows from a first end of coil LI to a second end of coil LI via diode Dl, diode D2, capacitor Cl and first lamp electrode Ell. Capacitor Cl is charged by this current. When the glow discharge starter S2 becomes subsequently conducting again, capacitor Cl is not discharged thanks to the presence of diode D2. When the glow discharge starter S2 subsequently becomes non-conducting again, capacitor Cl is further charged to a higher voltage. This cycle is repeated until the voltage across capacitor Cl is high enough to ignite the lamp. After ignition of the lamp, means (not shown in Fig. 1) ascertain that the lamp conveys a current, whereafter the control circuit SC decreases the frequency f at which the first switching element is rendered high-frequency conducting and non-conducting. The lamp is subsequently fed with a DC current. The operating voltage of the lamp La has such a value that no glow discharge is realized by this voltage in the glow discharge starter, so that the glow discharge starter S2 remains non-conducting after ignition of the lamp La. It is possible to measure the lamp current with means which are not shown in Fig. 1, compare it with a reference value and, in dependence upon the result of the comparison, adjust the frequency and/or the duty cycle of the switching element SI via the circuit component SC. In this way, the amplitude of the DC current with which the lamp is fed is controlled at a substantially constant value. The embodiment shown in Fig. 2 differs from that in Fig. 1 only in that a voltage multiplier (SV) constituted by capacitors C2, C3, C4 and C7 and diodes D5, D6, D7 and D9 has been added. A first and a second output terminal of the voltage multiplier are connected to input terminal Kl and output terminal K4, respectively, of the down-converter. The first output terminal of the voltage multiplier is connected to the second output terminal by means of a series arrangement of diodes D5, D6, D7 and D9. Capacitor C7 shunts diodes D7 and D9. Capacitor C2 shunts diodes D6 and D7. Terminal 1 is connected via capacitor C4 to a common point of diodes D5 and D6. Terminal 2 is connected via capacitor C3 to a common point of diodes D6 and D7. The structure of the circuit arrangement of Fig. 2 is similar to that shown in Fig. 1 and corresponding components and circuit sections have been denoted by the same reference symbols. The operation of the embodiment shown in Fig. 2 corresponds to the operation of the embodiment shown in Fig. 1. The voltage multiplier generates an output voltage at a high amplitude from the AC voltage, which output voltage is present between input terminal Kl and output terminal K4. In the same manner as described above for the embodiment shown in Fig. 1, capacitor Cl is charged to a voltage at which the lamp ignites. Immediately after breakdown of the plasma in the lamp La, the high output voltage of the circuit section SV becomes available for generating a relatively high current through the lamp and thus favorably influencing take-over by the lamp. After the lamp has been ignited, the circuit arrangement shown in Fig. 2 operates in substantially the same way as the one shown in Fig. 1. Since the lamp voltage is relatively low during stationary operation and the lamp also takes up a relatively high current, the voltage multiplier SV is rendered substantially inactive during stationary lamp operation and thus does not generate a high output voltage during this phase of the lamp operation. In Fig. 3, the reference numeral 8 denotes a light-transmissive discharge vessel provided with a filling containing mercury and a rare gas, and two electrodes (not shown). A luminescent coating is provided on the wall of discharge vessel. The reference numeral 6 denotes a lamp housing secured to the discharge vessel 8, and the reference numeral 3 denotes a lamp base provided with electric contacts (1 and 2) and secured to the lamp housing, and circuit sections P and C1-C4 diagrammatically represent a ballast circuit B according to the invention, coupled between the contacts (1, 2) via the conductors E and the electrodes (via conductors 9) for generating a DC lamp current from a power supply voltage.

Claims: CLAIMS: 1. A circuit arrangement for igniting and supplying a lamp (LA) with a DC current, comprising - input terminals (Kl, K2) for connection to terminals of a power supply source supplying a DC voltage, - a DC/DC converter coupled to the input terminals and provided with - an inductive element (LI), - a first unidirectional element (Dl), - a first switching element (SI), - a control circuit (SC) coupled to a control electrode of the first switching element for rendering the first switching element conducting and non-conducting at a frequency f, - output terminals (K3, K4) for connecting the lamp, and - an output capacitance (Cl) shunting the lamp during operation, and comprising - a first branch (S2, D2) connecting the output terminals during operation and comprising a second switching element (S2) and means (S2) for rendering the second switching element conducting and non-conducting, characterized in that the first branch is also provided with a second unidirectional element (D2). 2. A circuit arrangement as claimed in claim 1, wherein the DC/DC converter comprises a down-converter. 3. A circuit arrangement as claimed in claim 1 or 2, wherein the first and the second unidirectional element comprise a diode (Dl, D2) each. 4. A circuit arrangement as claimed in claim 1, 2 or 3, wherein the second switching element and the means for rendering the second switching element conducting and non-conducting comprise a glow discharge starter (S2). 5. A circuit arrangement as claimed in any one of the preceding claims, wherein the first branch comprises a lamp electrode (Ell) during operation. 6. A circuit arrangement as claimed in any one of the preceding claims, wherein the circuit arrangement is also provided with a voltage multiplier (SV). 7. A circuit arrangement as claimed in any one of the preceding claims, wherein the first switching element is rendered conducting and non-conducting by the control circuit in such a way that the inductive element conveys a continuous current prior to ignition of the lamp. 8. A circuit arrangement as claimed in any one of the preceding claims, wherein the circuit arrangement is provided with a circuit component for decreasing the frequency f after ignition of the lamp. 9. A compact lamp comprising - a light-transmissive discharge vessel (8) having a filling of rare gas, and two electrodes, - a lamp housing (6) secured to the discharge vessel, - a lamp base (3) provided with electric contacts (1, 2) and secured to the lamp housing, and - a ballast circuit (B; C1-C4, P) coupled between the contacts and the electrodes for generating a lamp current from a power supply voltage, characterized in that the ballast circuit comprises a circuit arrangement as claimed in any one of the preceding claims. 10. A compact lamp as claimed in clajm 9, wherein the compact lamp is a compact fluorescent lamp.

Inventors: EBERSON ETIENNE N K P M, MARINUS ANTONIUS A M, EBERSON, ETIENNE, N., K., P., M., MARINUS, ANTONIUS, A., M.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1999
Main classification: H05B41
IPC codes: H02M3, H02M7, H05B41