Inverter for the supply of discharge lamps with heated electrodes, with resonant circuit

Abstract: An inverter for the supply of one or more discharge lamps (11; 111A, 111B) with heated electrodes (13, 15), comprises: a load circuit with a series resonant circuit (17, 19, 21, 117A, 117B; 119; 121) and at least one lamp (11, 111A, 111B), two controllable switches (7, 9; 107, 109) and a control circuit (25) for the said switches, causing the said switches to conduct alternately. A normally isolated dissipative component (33; 133), which is connected, when the voltage across the load circuit exceeds a given value, to the load circuit, thus modifying its figure of merit to limit the maximum voltage without varying the resonant frequency of the load circuit.

Description: The invention relates to an inverter for the supply of one or more discharge lamps with heated electrodes, of the type stated in the preamble of the principal claim. An inverter of this type is described, for example, in Patent EP-B-O, 113, 451. In this patent, a load circuit, comprising a series resonant circuit and a discharge lamp, is associated with a special circuit to prevent the appearance of unacceptable excess voltages across the lamp, particularly in the starter transient. For this purpose, a voltage-dependent resistor and a capacitor are connected together in series, in parallel with a branch comprising the inductance of the resonant circuit. With this arrangement, when the voltage across the lamp exceeds a certain limit value, depending on the variable resistor, the latter becomes conducting. This produces a variation in the resonant frequency of the load circuit, changing it from the supply frequency and consequently limiting the voltage across the lamp. The present invention relates to a circuit of the type mentioned initially, with a different system of limiting the voltage applied to the lamp. The invention is defined in Claim 1. It is substantially based on the concept of using a resonant load circuit in which it is possible to connect a dissipative component, which has no effect on the resonant frequency of the said load circuit, but changes its figure of merit. The inverter according to the invention is therefore substantially characterised by a normally isolated dissipative component, which is connected, when the voltage across the load circuit exceeds a given value, to the load circuit, thus modifying its figure of merit to limit the maximum voltage without varying the resonant frequency of the load circuit. In a practical embodiment of the inverter according to the invention, there is placed, in a shunt in parallel with a branch containing at least one of the components of the load circuit, a voltage-dependent resistor, in series with a dissipative component of the resistive type, the said voltage-dependent resistor becoming conducting at a limit value of the voltage applied across it. From NL-A-9000175 a supply circuit with an oscillator for a gas discharge lamp is known, wherein a voltage divider is connected to the resonant load circuit. In one embodiment disclosed therein the voltage divider includes a varistor. The voltage at the tap of the voltage divider is used to control the voltage applied to the load. However, in this known device the voltage applied to the load is limited by varying the duty cycle of the circuit via a thyristor controlled by the voltage at the tap of the voltage divider. Voltage limitation is therefore not based on a dissipative effect and on a consequent reduction of the figure of merit of the resonant load circuit. Further advantageous embodiments of the inverter according to the invention are indicated in the attached claims. The invention will be more clearly understood by an examination of the description and the attached drawing, which shows a practical and non-restrictive embodiment of the invention. In the drawing, Fig. 1 is a simplified diagram of the circuit, limited to the characteristic parts; Fig. 2 shows the variation in the figure of merit of the resonant circuit and the effect of the voltage limiting device; and Fig. 3 is a highly schematic view of a version with two lamps. With reference to Fig. 1 initially, the inverter according to the invention, indicated as a whole by 1, has two terminals 3 and 5, for connection to a source of direct voltage. This may be taken from a power supply circuit (not shown, and known per se), which is connected to an alternating voltage mains and which may comprise, for example, a filter, a rectifier and a DC/DC converter for the correction of the power factor. The two terminals 3 and 5 are connected to two transistors 7 and 9, acting as switches controlled in the way described below. A discharge lamp 11, equipped with two heated electrodes 13 and 15, is connected in a shunt in parallel with the transistor 9. The electrodes 13 and 15 are connected together by a capacitor 17. The lamp 11 is connected to the emitter of the transistor 9 through a further capacitor 19, and to the collector of the said transistor through an inductance 21 in series with the primary winding 23 of a saturable transformer. The circuit comprising the lamp 11, the capacitor 17, the capacitor 19, the inductance 21 and the primary winding 23 forms a resonant load circuit whose resonant frequency remains unchanged even during transients, as will be made clear by the following description, and is determined by the value of the inductances and capacitances present in it. The two transistors 7 and 9 are caused to conduct alternately by means of a control system 25 of a type known per se, comprising two secondary windings, indicated by 27 and 29, of the saturable transformer, whose primary winding is indicated by 23. The secondary windings 27 and 29 supply the control voltages of the transistors 7 and 9. The operating frequency of the inverter described up to this point is close to (and slightly greater than) the resonant frequency of the LC circuit comprising the components 17, 19 and 21, but is such that the voltage across the lamp 11 is kept below a safety value. The parameters of the individual circuit components are calculated for this purpose. However, excessive voltages may occur during the starter transient or in the case of defective operation of the lamp. In order to avoid this phenomenon, the circuit according to the invention has a variable resistor (VDR) 31, which becomes conducting only when a voltage above a threshold value is present across it. The variable resistor 31 is connected in series with a resistor 33 and forms a shunt in parallel with the branch comprising the inductance 21, the primary winding 23 and the switch 9. When the voltage across the lamp 11 and consequently across the variable resistor 31 exceeds the threshold value, the variable resistor becomes conducting, and consequently the load circuit connected to the switches 7 and 9 is no longer formed by the components 11, 17, 19, 21, and 23 alone, but also includes the additional dissipative member formed by the resistor 33 (in addition to the resistor 31 itself). This reduces the figure of merit of the load circuit and consequently decreases the value of the voltage across the lamp 11, but does not affect the resonant frequency of the circuit, which remains unchanged. The damping provided by the resistive component is such that the voltages concerned are kept below the safety values even when the operating frequency is very close to the resonant frequency. Fig. 2 shows the variation in the figure of merit Qo in the absence of the dissipative component formed by the dissipative shunt 31, 33, and the variation in the figure of merit Q1 when the resistor 31 is conducting. The horizontal axis shows the frequency in kHz, while the vertical axis shows the ratio between the voltage across a reactive component of the oscillating circuit and the supply voltage. The numerical values shown are for guidance only. In the embodiment illustrated in Fig. 1, the supply system also has a monitoring device comprising: a peak-to-peak detector indicated as a whole by 36, to which are connected a resistor 37, transistors 39, 41, and 43, and a diode 45. The monitoring circuit locks the inverter when the dissipative component 33 remains in the circuit for a period greater than a limit period determined by the time constants of the peak-to-peak detector circuit. When the voltage across the load increases, the voltage across the resistor 37 also rises. When a given threshold value is exceeded, the transistors 39 and 41 become conducting. Consequently the transistor 43 also becomes conducting. This causes the control circuit to be disabled in two ways: 1) through the diode 45, the potential of the base circuit of the transistor 9 of the inverter is decreased to a value which causes it to cut off; 2) through the transistor 43, the diac 47, which is used to trigger the starting of the inverter, is kept in the off state. It is to be understood that the drawing shows only one example provided solely as a practical demonstration of the invention; this invention may be varied in its forms and arrangements without thereby departing from the scope of the guiding principle of the invention. Any reference numbers in the attached claims has the purpose of facilitating the reading of the claims with reference to the description and to the drawing, and does not restrict the scope of the protection represented by the claims. In particular, the lamp 11 and the series resonant circuit may be connected in a shunt in parallel with the transistor 7, in other words in the upper part of the device, instead of in a shunt in parallel with the transistor 9. The inverter may also supply more than one lamp at the same time, as shown schematically in Fig. 3 for two lamps 111A and 111B. In this case, a single capacitor 119, common to the two lamps 111A and 111B, is used, with a variable resistor, 131A and 131B respectively, for each lamp. In this case, the variable resistors are connected in series to a common dissipative component 133, corresponding to the resistor 33. The diagram in Fig. 3 also shows the two switches 107 and 109, corresponding to the transistors 7 and 9; the two impedances 121A and 121B of the resonant circuits of the two lamps; the primary winding 123, corresponding to the winding 23; and the capacitors 117A and 117B which connect the electrodes of each lamp.

Claims: Inverter for the supply of one or more discharge lamps (11) with heated electrodes (13, 15), comprising: aresonant load circuit including a series resonant circuit (13, 15, 17, 19, 21) and at least one lamp (11), two controllable switches (7, 9) and a control circuit (25) for the said switches, causing the said switches to conduct alternately, characterised by a dissipative component (33; 133), which, under normal operating conditions of said load circuit, is electrically isolated from the load circuit, and which, when the voltage across the load circuit exceedes a given value, is electrically connected to the load circuit, the connection of the dissipative component (33; 133) to the resonant load circuit causing a reduction of the figure of merit of said resonant load circuit, said reduction of the figure of merit directly causing a limitation of the maximum voltage without varying the resonant frequency of the load circuit. Inverter according to Claim 1, characterised in that there is placed, in a shunt in parallel with a branch containing at least one of the components of the said load circuit, a voltage-dependent resistor (31; 131A, 131B) in series with a dissipative component of the resistive type (33; 133), the said voltage-dependent resistor becoming conducting at a limit value of the voltage applied across it. Inverter according to Claim 2, characterised in that the said shunt is placed in parallel with a branch comprising at least the inductive component (21; 121) of the load circuit. Inverter according to Claim 2 or 3, characterised in that the shunt is placed in parallel with the inductive component (21; 121) and with one (9; 109) of the controllable switches. Inverter according to one or more of Claims 1 to 4, characterised in that it comprises at least two lamps (111A, 111B) and a resonant circuit with a common dissipative component (133) normally isolated from the load circuit. Inverter according to Claims 2 and 5, characterised in that it comprises, for each lamp, a corresponding voltage-dependent resistor, the said voltage-dependent resistor being in series with the said common dissipative component. Inverter according to one or more of the preceding claims, characterised in that it comprises a voltage monitoring circuit which cuts off the control circuit (25) of the switches (7, 9; 107, 109) when the dissipative component (33; 133) is connected for a period greater than a predetermined time interval. Inverter according to Claim 7, characterised in that it comprises semiconductor means (39, 41, 43, 45) which, when the voltage across the load circuit exceeds a predetermined value for a period greater than a predetermined time interval, become conducting and cut off the switches (7; 9) and the inverter starting means (47).

Inventors: CANOVA ANTONIO, CANOVA, ANTONIO
Applicants: MAGNETEK SPA, MAGNETEK S.P.A.
Application year: 1993
Main classification: H05B41
IPC codes: H05B41