CIRCUIT ARRANGEMENT

Abstract: The invention relates to a circuit arrangement for igniting and operating a high-pressure discharge lamp, provided with input terminals for connection of a supply source, output terminals for connection of the lamp, switching means for operating the lamp at a nominal lamp voltage Vla during stable lamp operation and for generating an open voltage Vo at the output terminals before the lamp has ignited, ignition means for generating an ignition voltage pulse when the voltage at the output terminals reaches a threshold value Vi, a control circuit for controlling the switching means. According to the invention, the circuit arrangement is characterized in that the control circuit comprises means for limiting the voltage at the output terminals to a value Vb such that the following is true: Vla < Vb < Vi.

Description: Circuit arrangement. The invention relates to a circuit arrangement for igniting and operating a high-pressure discharge lamp, provided with input terminals for connection of a supply source, output terminals for connection of the lamp, - switching means for operating the lamp at a nominal lamp voltage Via during stable lamp operation and for generating an open voltage Vo at the output terminals before the lamp has ignited, ignition means for generating an ignition voltage pulse when the voltage at the output terminals reaches a threshold value Vi, - a control circuit for controlling the switching means. Such a circuit arrangement is known from EP 0401931 = US 5,068,572). The known circuit arrangement is suitable for igniting and operating inter alia high-pressure sodium lamps and metal halide lamps. These lamps are in general provided with a discharge vessel in which an electric discharge is maintained during operation and which is enclosed with intervening space by an outer bulb. These lamps require a high ingition voltage pulse (a few kV and more). The ignition means become operative through the choice of the open voltage Vo such that Vo > Vi, and an ignition voltage pulse is generated. The lamp will ignite then, the voltage across the lamp dropping abruptly to around ten volts. As soon as a stable discharge has arisen in the lamp, the voltage across the lamp gradually rises up to the lamp voltage Via accompanying stable lamp operation. The circuit arrangement is so designed that the lamp voltage Via is lower than the voltage Vi, which in its turn is lower than the open voltage Vo. The control circuit of the circuit arrangement ensures that the switching means act as a controlled current generator in the stable operational state of the lamp. It is achieved thereby that no more ignition voltage pulses are generated by the ignition means as soon as the voltage at the connection terminals drops to the lamp voltage Via or lower. The generation of ignition voltage pulses for a longer period is undesirable because this forms a load on the circuit arrangement. It is often suggested in the literature, therefore, to limit the operauon of the ignition means by switching them off after a certain period has elapsed. This, however, has a number of disadvantages: - the comparatively high open voltage Vo remains at the output terminals if the lamp has not yet ignited, once the lamp has ignited, the ignition means will generate ignition voltage pulses the moment the lamp is extinguished, for example upon reaching the end of its operational life. Especially high-pressure sodium lamps and metal halide lamps show the characteristic that the lamp will ignite again upon the application of ignition voltage pulses at the end of its life after extinguishing and cooling-down. This gives rise to the characteristic flickering behavior of the lamp. This is unpleasant to observe, but most of all it is detrimental to the circuit arrangement, while it usually also leads to a considerable amount of interference radiation (radio interference, etc.). The invention has for its object to provide a measure for counteracting the above disadvantages. According to the invention, a circuit arrangement of the kind mentioned in the opening paragraph is for this purpose characterized in that the control circuit comprises means for limiting the voltage at the output terminals to a value Vb such that the following is true Via < Vb < Vi. The choice of the voltage at a level just above the nominal lamp voltage advantageously realizes that the generation of ignition voltage pulses is suppressed by one and the same mechanism both in the case of a badly starting lamp and in the case of a lamp which reaches the end of its life. A further advantage is that a discharge in the outer bulb of the lamp, either in the form of a glow discharge or in the form of an arc discharge, cannot be maintained when the discharge vessel of the lamp becomes leaky, so that hazardous situations are prevented. A particularly suitable embodiment of the means for limiting the voltage at the output terminals comprises a timer which after a preset time interval brings the control circuit into a state such that the voltage occurring at the output terminals is limited to the value Vb. Preferably, the timer is so connected that it is reset upon connection of a supply source. Preferably, a voltage buffer network is coupled to the timer in order to prevent the timer from being reset also in conditions where there is a short dip in the supply voltage delivered by the supply source. This network may be, for example, an RC network. A downconverter or Buck converter is very suitable for use as the controlled current generator. The above and further aspects of the invention will be explained in more detail below with reference to a drawing of an embodiment of a circuit according to the invention, in which Fig. 1 shows a circuit arrangement for igniting and operating a high- pressure discharge lamp, Fig. 2 shows switching means of the circuit arrangement of Fig. 1 in detail, Fig. 3 shows means for limiting the voltage at the output terminals of the circuit arrangement in more detail, and Fig. 4 shows an embodiment of the means of Fig. 3 in further detail. Fig. 1 shows a circuit arrangement for igniting and operating a high- pressure discharge lamp, provided with input terminals 1 for connecting a supply source, output terminals 2 for connecting the lamp 3, switching means II for operating the lamp at a nominal lamp voltage Via during stable lamp operation and for generating an open voltage Vo at the output terminals before the lamp has ignited. The circuit arrangement is also provided with commutator means III for periodically changing the polarity of the current through the lamp, and with a preconditioner I for generating a DC voltage for supplying the switching means II. The commutator means also comprise ignition means (not shown in any detail) for generating an ingition voltage pulse when the voltage at the output terminals reaches a threshold value Vi. The circuit arrangement is further provided with a control circuit IV for controlling the switching means, and with means V for limiting the voltage at the output terminals to a value Vb such that it is true that Via < Vb < Vi. In an advantageous embodiment, the switching means are constructed as a downconverter or Buck converter as shown in detail in Fig. 2. In Fig. 2, A and B are connection points between the switching means and the preconditioner I, and C and D are connection points with the commutator means III. The downconverter comprises a controlled semiconductor switch 10, a self-inductance 11, and a freewheel diode 12. The converter is further provided with a capacitor 13 for reducing ripple on the voltage across the connection points C, D. A control electrode 101 of switch 10 is connected to an output 41 of the control circuit IV. The control circuit has an input 42 for detecting a signal Sv which is proportional to the voltage at the output terminals and an input 43 for detecting a signal Si which is proportional to the lamp current and is formed over a small preferably ohmic impedance Z. The signal Sv is compared with a reference voltage Vref. The result of this comparison is conducted to a gate controller 45. The signal Si is conducted through an integrator 44 A to Pulse Width Modulator (PWM) 44, which in its turn sends a switching signal to the gate controller 45. The control signal generated in the gate controller 45 is conducted to the control electrode 101 through output 41. The reference voltage Vref forms part of the means for limiting the voltage at the output terminals to a value Vb such that it is true that Via < Vb < Vi. The means V are shown in more detail in Fig. 3. The reference voltage Vref is formed at connection point 52 from an auxiliary voltage Vh by means of a voltage divider network 50. A switch 51 is included in a branch of the voltage divider network 50 which is connected to connection point 52 at one side. The switch 51 shunts a portion of the impedance belonging to the voltage divider network. In the embodiment shown, the switch is accommodated in a branch between connection point 52 and ground. When the switch is closed, the result will be a lower voltage at connection point 52 compared with the situation of an opened switch. When the switch is placed between the voltage Vh and connection point 52, a reduction in the voltage at connection point 52 will take place when the switch 51 is opened. A low voltage at connection point 52 means a low value for the reference voltage Vref, which results in the gate controller 45 controlling the semiconductor switch such that the voltage at the output terminals of the circuit arrangement is limited to a comparatively low value. An embodiment of the means V as shown in Fig. 3 is depicted in further detail in Fig. 4. The switch is formed by a series arrangement of a diode 55, a timer 500, a voltage buffer network 53, and a connection point 54 for connecting a supply voltage for supplying the timer. The switch 51 operates as follows. The moment a supply voltage is connected to connection point 54, a voltage will be applied to input terminal 501 of timer 500, whereby the timer is reset. The result of this is that a high signal is present at an output terminal 502 of the timer. This corresponds to the state of a comparatively high voltage at connection point 52. After a period for which the timer was set has elapsed, the timer switches over and the voltage at output terminal 502 becomes low, so that the voltage at connection point 52 also becomes low. In a practical realization of a circuit arrangement according to the embodiment described, the circuit arrangement is suitable for igniting and operating a metal halide lamp, for example of the CDM 35W type, make Philips, with a power rating of 39 W. The lamp has a rated lamp voltage Via of 90 V. When the supply source is connected, for example a voltage source of 220 V, 50 Hz, the preconditioner supplies a DC voltage of 400 V to the switching means II. The open voltage at the output terminals lies between 310 and 380 V when the lamp has not ignited. The threshold value Vi of the voltage at the ouφut terminals at which the ignition means start generating ignition pulses is 240 V. The timer 500 is formed by a type HEF 4541 integrated circuit. The voltage buffer network 53 comprises a parallel arrangement of a 100 nF capacitor and a zener diode with a zener voltage of 15 V. The rectified supply source voltage of 220 V serves as the supply voltage for the timer circuit. To limit the current, a resistor of 300 kO is connected between connection point 54 and the voltage buffer network. Diode 55 is of the BAV 103 type, make Philips. The value of Vref is 3.3 V when the timer is reset. When the timer switches over after at most 20 min, Vref drops to 1.5 V, which results in a limitation of the voltage at the output terminals to a value Vb of at most 200 V. It is clear that the condition Via < Vb < Vi is thus complied with. The timer is so set that it switches over after a minimum of 10 min and a maximum of 20 min. This time is preferably chosen to be so long to provide the possibility of hot re-ignition of the lamp. The controlled semiconductor switch of the switching means is formed by a MOSFET, type STP4NA60FI, make SGS-Thomson. The gate controller is an integrated circuit, type IR2117, make Int. Rectifier. The PMW 44 is an integrated circuit, type L6560A, make SGS-Thomson.

Claims: 2 U CLAIM: 1. A circuit arrangement for igniting and operating a high-pressure discharge lamp, provided with input terminals for connection of a supply source, output terminals for connection of the lamp, - switching means for operating the lamp at a nominal lamp voltage Via during stable lamp operation and for generating an open voltage Vo at the output terminals before the lamp has ignited, ignition means for generating an ignition voltage pulse when the voltage at the output terminals reaches a threshold value Vi, - a control circuit for controlling the switching means, characterized in that the control circuit comprises means for limiting the voltage at the output terminals to a value Vb such that the following is true Via < Vb < Vi.

Inventors: DEURLOO OSCAR JAN, LAMMERS FRANCISCUS HUBERTUS TH, DEURLOO, OSCAR, JAN, LAMMERS, FRANCISCUS, HUBERTUS, THEODORUS
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1997
Main classification: H05B41
IPC codes: H05B41