CIRCUIT ARRANGEMENT

Abstract: The invention relates to a circuit arrangement for igniting and operating a high-pressure discharge lamp, provided with switching means, inductive means and rectifying means together forming a Buck converter, connected to input terminals for connection to a supply source and output terminals for connection of the lamp, comprising means for supplying the lamp with a currrent through periodic switching of the switching means alternately into a conducting and a non-conducting state by means of a switch-on and switch-off signal, respectively, which inductive means comprise a primary and a secondary winding, the secondary winding forming part of an intergration network for generating the switch-off signal. The secondary winding also forms part of a voltage divider network for generating the switch-on signal. A strongly simplified construction of the circuit arrangement can be realized as a result of this.

Description: Circuit arrangement. The invention relates to a circuit arrangement for igniting and operating a high-pressure discharge lamp, provided with switching means, inductive means and rectifying means together forming a Buck converter, connected to input terminals for connection to a supply source and output terminals for connection of the lamp, comprising means for supplying the lamp with a current through periodic switching of the switching means alternately into a conducting and a non-conducting state by means of a switch-on and switch-off signal, respectively, which inductive means comprise a primary and a secondary winding, the secondary winding forming part of an integration network for generating the switch-off signal. A circuit arrangement of the kind mentioned in the opening paragraph is known from EP-A-0 401 931 = USP 5,068,572. The known circuit arrangement is highly suitable for igniting and operating a high-pressure discharge lamp which forms part of a projection TV installation. The type of switch mode power supply known as Buck converter is also known under other designations such as downconverter, step-downconverter, inductor- coupled step-downconverter and direct-downconverter. An electrical separation in the form of a transformer may be present between input terminals and output terminals. The Buck converter operates in a self-oscillatory mode in the known circuit arrangement. It is possible with the known circuit arrangements to supply a substantially constant power to the connected lamp over a comparatively wide current and voltage range, so that a very constant luminous flux is generated by the lamp. The self-oscillatory mode is characterized by low switching losses in the periodic switching of the switching means, especially in the current-voltage range where lamp operation is stable. Preferably, the downconverter is so dimensioned that switching from the non-conducting to the conducting state takes place with a frequency above the limit of human hearing during stable lamp operation. This also has the result that the dimensions of the inductive means can remain comparatively small. For generating the switch-off signal, a comparison is made in the known circuit arrangement between a separately set control signal and a signal prevalent at an output of the integration network and proportional to the value of the current through the inductive means. The moment the signal at the output of the integration network becomes equal to the separately set control signal, the switch-off signal is generated, whereupon the switching means are switched from the conducting to the non-conducting state. In the known circuit arrangement, the integration network comprises capacitive means, and the voltage across the capacitive means forms the signal which is proportional to the current value through the inductive means. A controlled current source of comparatively simple construction is realized with the known circuit arrangement by which also the power in a connected load (the lamp) can be controlled. The self-oscillatory mode of the known circuit arrangement is characterized by the fact that the generation of the switch- on signal, and thus the process of switching of the switching means from the non-conducting to the conducting state take place in that the current through the primary winding of the induction means becomes zero and subsequently a tuned circuit comprising the primary winding of the inductive means is activated. This results in a voltage across the primary winding. The voltage thus generated is transformed to a further secondary winding of the inductive means. The further secondary winding in its turn forms a voltage source for generating the switch-on signal. This results in a complicated construction of the circuit arrangement, which is a disadvantage as regards manufacturing cost and as regards possibilities for miniaturization of the circuit arrangement. It is an object of the invention to provide a circuit arrangement with a measure for counteracting said disadvantage. According to the invention, the circuit arrangement of the kind mentioned in the opening paragraph is for this purpose characterized in that the secondary winding also forms part of a voltage divider circuit for generating the switch-on signal. It was found to be possible for the secondary winding to act as a voltage source both for generating the switch-off signal and for generating the switch-on signal. This leads to a strong simplification in the construction of the circuit arrangement, and thus to better possibilities for minitiaturization and cost reduction in manufacture. The measure described advantageously utilizes the fact that the switch-on and switch-off signals should be generated alternately. In an advantageous embodiment of the circuit arrangement according to the invention, the primary winding forms a tuned circuit with a capacitance which shunts the rectifying means of the Buck converter. In contrast to the known circuit arrangement in which the capacitor of the tuned circuit containing the primary winding is connected to one of the input terminals, the capacitance is connected to one of the output terminals of the circuit arrangement by way of a common ground point in the advantageous embodiment described. The result of this is that the maximum voltage across the capacitance remains strongly limited compared with the situation in the known circuit arrangement. This results in a further simplification in the construction of the circuit arrangement. It was found to be advantageous to supply the integration network with a signal generated by means of detection of a current flowing through the rectifying means while the switching means are in the non-conductive state, with the object of realizing a correct voltage distribution over the integration network at the start of the conducting period of the switching means. For this purpose, the circuit arrangement is provided with suitable detection means in a simple manner. A very simple realization of the detection means takes the form of an ohmic impedance. Preferably, the detection means are connected between the rectifying means and a direct electrical connection present between one of the input terminals and one of the output terminals. Detection means positioned in this way have the advantage that their position does not give rise to the supply of a high-frequency interference signal to the input terminals, and thus to the supply source. A further advantage is that the signal generated by the detection means is independent of the voltage delivered by the supply source. The supply of the integration network may as a result be realized in a very advantageous manner through shunting of the ohmic impedance by an op-amp of which an output is connected to the output of the integration network. This leads to a comparatively very simple circuit construction with the concomitant possibilities for minituarization. It has been ensured in the manner described that a signal proportional to the value of the current through the inductive means is present at the output of the integration network at all times. Aspects of the invention described above and further aspects will be described in more detail below with reference to a drawing of an embodiment. The drawing shows a diagram of a circuit arrangement for igniting and operating a high-pressure discharge lamp 7, provided with switching means 1, inductive means 2 and rectifying means 4, together forming a Buck converter, connected to input terminals 5 for connection to a supply source and output terminals 6 for connection of the lamp, comprising means 8 for supplying the lamp with a current through periodic switching of the switching means alternately into a conducting and a non-conducting state by means of a switch-on and switch-off signal, respectively, - which inductive means 2 comprise a primary and a secondary winding 21, 22, the secondary winding 22 forming part of an integration network 9 with an output 90 for generating the switch-off signal. The secondary winding also forms part of a voltage divider circuit 10 for generating the switch-on signal. The Buck converter is also provided with capacitive buffer means 3. A direct electrical connection 15 is present between one of the input terminals 5 and one of the output terminals 6. An ohmic impedance 13 is connected here between the direct connection 15 and the rectifying means 4. The ohmic impedance forms part of detection means 12 for detecting a current flowing through the rectifying means. The ohmic impedance is shunted by an op-amp 14 of which an output 140 is connected to the output 90 of the integration network 9. It is achieved thereby that the value of the current through the rectifying means is reliably detected and that the signal at the output of the integration network remains proportional to the value of the current through the inductive means. The circuit arrangement in addition comprises a capacitance 11 which together with the primary winding 21 forms a tuned circuit the moment the rectifying means 4 pass no current anymore with the switching means 1 being in the non-conducting state. The voltage thus arising across the primary winding induces a voltage in the secondary winding 22, which in its turn forms a voltage source for generating the switch-on signal through voltage divider circuit 10. In an alternative embodiment, a converter portion (not shown) is included between input terminals 5 and switching means 1 for converting the voltage of the supply source to a DC voltage suitable for operating the switching means. The converter portion comprises inter alia a comparatively large capacitor C connected between the input terminals. The tuned circuit in this case comprises the capacitor C and a capacitor 11 ' connected across the switching means. Capacitance 11 is not necessary in this embodiment. The switch-off signal generated in the integration network 9 is sent to a control circuit 100, as is the switch-on signal generated in the voltage divider circuit 10. The control circuit 100 provides the actual control necessary for switching the switching means. The means 8 containing the lamp comprise a commutator circuit in many practical cases because a current of periodically changing polarity flows through the lamp during lamp operation. The means 8 in general also comprise an igniter circuit for generating a voltage pulse for igniting the lamp. Both the commutator circuit and the ingiter circuit will generally form part of the circuit arrangement according to the invention. The commutator means may be omitted in cases where the lamp is suitable for DC operation. In a practical realization of the circuit arrangement described, the latter is suitable for operating a high-pressure metal halide lamp of the UHP type, make Philips. The practical realization of the circuit arrangement is suitable for connection to a 220 V, 50 Hz supply source. For this purpose, the circuit arrangement is provided with a circuit portion, which is known per se and not shown in the drawing, arranged between the input terminals and the switching means for converting the AC voltage connected to the input terminals into a DC voltage suitable for operating the switching means. The converter portion comprises between the input terminals a capacitor C of 47 μF. A MOSFET, type IRF840, make International Rectifier, serves as the switching means 1. The inductive means 2 are formed by a transformer with a ferrite core, a primary winding of 100 turns and a secondary winding of 30 turns. The capacitive buffer means have a capacitance value of 0.82 μF. The rectifying means 4 are formed by a type BYV29F500 diode, make Philips. The switching means 1 are shunted by the capacitor 11' of 560 pF. The detection means 12 in the practical embodiment comprise an ohmic impedance of 0.15 Ω shunted by an operational amplifier of the CA 3140 M96 type. The amplification factor which is set in dependence on the value of ohmic impedance 13, on the dimensions of the components of the integration network, and on the winding ratio between primary and secondary winding of the inductive means 2, is 100/30. The practical realization of the circuit arrangement described is highly suitable for use in a projection TV installation. An alternative possible application is, for example, the operation of a discharge lamp in a motorcar lantern system.

Claims: CLAIMS: 1. A circuit arrangement for igniting and operating a high-pressure discharge lamp, provided with switching means, inductive means and rectifying means together forming a Buck converter, connected to input terminals for connection to a supply source and output terminals for connection means comprising the lamp for supplying the lamp with a current through periodic switching of the switching means alternately into a conducting and a non¬ conducting state by means of a switch-on and switch-off signal, respectively, which inductive means comprise a primary and a secondary winding, the secondary winding forming part of an integration network for generating the switch-off signal, characterized in that the secondary winding also forms part of a voltage divider circuit for generating the switch-on signal. 2. A circuit arrangement as claimed in Claim 1, characterized in that the primary winding forms a tuned circuit with a capacitance which shunts the rectifying means of the Buck converter. 3. A circuit arrangement as claimed in Claim 1 or 2, characterized in that the circuit arrangement is provided with detection means for detecting a current flowing through the rectifying means. 4. A circuit arrangement as claimed in Claim 3, characterized in that the detection means generate a signal for supplying the integration network during the period in which the switching means are non-conducting. 5. A circuit arrangement as claimed in Claim 3 or 4, characterized in that a direct electrical connection is present between one of the input terminals and one of the output terminals, and in that the detection means are connected between said direct connection and the rectifying means. 6. A circuit arrangement as claimed in Claim 3, 4 or 5, characterized in that the detection means comprise an ohmic impedance. 7. A circuit arrangement as claimed in Claim 6, characterized in that the ohmic impedance is shunted by an op-amp of which an output is connected to the output of the integration network.

Inventors: BLANKERS HENDRIK JAN, BLANKERS, HENDRIK, JAN
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1996
Main classification: H05B41
IPC codes: H05B41