Supply circuit for discharge lamps with overvoltage protection

Abstract: The supply circuit for discharge lamps (L) comprises an inverter (3) for generating a supply voltage at a specified switching frequency (f c ) for a load circuit (15) comprising at least one lamp (L) and a resonant circuit (23, 25) in series with the electrodes (17, 19) of the said lamp. Associated with the load circuit is a control circuit (41) for turning off the supply to the load circuit in case of defective operation of the lamp (L). The control circuit (41) comprises a band-pass filter (35) centred on the switching frequency (f c ) of the inverter and whose input signal is dependent on the voltage at a point (P) of the load circuit; the output signal from the filter is sent to control means (37, 39) associated with the said inverter (3) so as to turn off the supply to the load circuit in case of defective operation.

Description: Field of the invention The present invention relates to a supply circuit with inverter for discharge lamps. More particularly the present invention relates to a supply circuit for discharge lamps with heated electrodes, in which an inverter comprising controlled breakers, turned on and off alternately, supply a load circuit having at least one lamp and an LC resonant circuit in series with the lamp. State of the art Systems for controlling the voltage on the electrodes of the lamp and which have the objective of modifying the behaviour of the load circuit or of turning off the supply thereto in case of defective operation of the lamp, are often used in supply circuits of the type mentioned above, with the objective of preventing excessively high voltages from arising between the electrodes. From EP-A-0 610 642 there is known a supply circuit with inverter for discharge lamps, in which associated with the load circuit is a control circuit comprising a voltage-dependent resistor (VDR) in series with a dissipative element. When the voltage at one terminal of the VDR exceeds a threshold value (which occurs for example in the case of failure of the lamp to light following a defect therein), the VDR becomes conducting with the consequence that the resonant circuit in series with the lamp receives an additional dissipative element. This modifies the quality factor of the circuit and hence reduces the voltage at the terminals of the lamp. Provision is further made for a timer circuit which turns off the supply to the load circuit should the overvoltage condition last for a time greater than a preset threshold value. From EP-A-0 113 451 there is known a different overvoltage control system, in which a voltage-dependent resistor (VDR) in series with a capacitor are inserted in parallel with a branch of the load circuit. In this case when a voltage difference greater than a specified threshold value is generated between the terminals of the VDR, it becomes conducting and inserts an auxiliary capacitor into the load circuit, modifying the frequency of resonance of the resonant circuit in series with the lamp. Traditional circuits for protection from overvoltages come into operation when the voltage between the electrodes of the lamp exceeds a threshold value. In the case of a defective lamp, and hence of the failure of this lamp to light, the voltage between the electrodes of the lamp reaches values of the order of 1000 V. Conversely, when the lamp is removed from the load circuit the potential difference between the electrodes is of the order of 700 V. The circuits currently available are unable to discriminate between these two voltage values, which may moreover vary from one instance to another of the circuit. Consequently they cut in anyway, turning off the supply, a prespecified time (of the order of 300 ms) having elapsed from the onset of the establishment of a situation of overvoltage between the electrodes of the lamp and are unable to distinguish between the two conditions of fault and lamp absent. It would, on the other hand, be appropriate to make provision for a circuit which is able to discriminate between a situation of actual defective operation and a situation of lamp absent in the load circuit, in such a way that the substitution of the lamp does not entail the disabling of the supply inverter of the load circuit. Thus, in currently known circuits the disabling of the supply inverter is permanent and hence requires the intervention of the operator in order to reactivate the supply for the lamp, even when the disabling has occurred through simple substitution of the lamp rather than through a defect in operation thereof. On the other hand, in case of overvoltage due to a defect in the operation of the lamp in the load circuit there flows a high current, which passes through the capacitor in parallel with the lamp. This anomalous condition may lead to the overstressing of the inverter and hence to damage thereto. Protection from overvoltage has the objective of preventing this consequence. When, conversely, the voltage in the load circuit increases on account of the lamp being absent, the current which flows in the circuit is practically zero and hence the inverter does not experience the dangerous stresses which occur under conditions of faulty lamp. Disabling of the inverter is therefore superfluous. From US-A-5,332,951 a ballast circuit for driving gas discharge lamps is known, which is provided with a band pass filter centred on the operating frequency of the inverter. The band pass filter is coupled between the output of the inverter and the inverter control. Its purpose is to provide protection against the diode operation of the gas discharge lamps, and thus to avoid cross conduction of the transistors. This known device is not concerned with the problem of overvoltage protection. Summary of the invention The objective of the present invention is the construction of a supply circuit for discharge lamps, with an overvoltage protection system, not exhibiting the drawbacks of the traditional circuits briefly described above. More particularly the objective of the present invention is the construction of a supply circuit for discharge lamps with an overvoltage protection circuit which is able to discriminate between the conditions of failure to light on account of defective lamp and the conditions of lamp absent, and which cut off the supply only when necessary, i.e. in the case of defective lamp. Essentially, starting from a supply circuit of the type defined above, the objectives indicated above, and other objectives and advantages which will become clear to those skilled in the art by reading the text which follows, are achieved by making provision for the overvoltage control circuit to comprise a band-pass filter centred on the switching frequency of the inverter, the input signal of which is dependent on the voltage at a specified point of the load circuit and the output signal from which is sent to control means associated with the inverter so as to turn off the supply for the load circuit in the case of defective operation. The invention is based on the observation that, although the amplitude of the voltage between the electrodes of the lamp is of the same order of magnitude both in the case of defective operation and in the case of lamp absent, the waveform of the voltage signal is, conversely, qualitatively different in the two cases. In case of failure to light through a defect in the lamp, the voltage between its electrodes has a substantially sinusoidal profile with a frequency corresponding to the switching frequency of the inverter. In case of lamp absent, conversely, the waveform of the voltage signal at the terminals of the lamp exhibits, as well as a relatively limited component at the switching frequency, a strong signal content at the higher harmonics. By detecting this voltage signal and filtering it through a band-pass filter centred on the switching frequency, a signal, which will be a high signal when the overvoltage established between the electrodes is due to defective operation of the lamp inserted into the load circuit, is therefore obtained at the output of the band-pass filter. This signal will, conversely, be low when the overvoltage established between the electrodes is due to lamp absent. In this last case, in fact, the signal component at the switching frequency is of modest strength with respect to the components at the higher harmonics, which are blocked by the band-pass filter. The control circuit thereby becomes capable of discriminating between the conditions of defective operation and the conditions of lamp absent and will be able, with suitable logic, to intervene on the supply inverter, selectively disabling the operation thereof. Further and advantageous characteristics and embodiments of the circuit according to the invention are indicated in the appended claims and will be described in greater detail below. In particular, a voltage divider, to which the band-pass filter is linked, can be arranged in parallel with a branch containing at least one component of the load circuit (for example the inductive component). More particularly the voltage divider can be placed in parallel with a branch comprising one of the controlled breakers of the inverter and the inductive component of the resonant circuit in series with the electrodes of the lamp. The band-pass filter can consist, in a particularly simple embodiment, of an LC cell in parallel, with a resonant frequency corresponding to the switching frequency of the inverter. Brief description of the drawings The invention will be better understood by following the description and attached drawing, which shows a practical and non-limiting embodiment of the invention. More particularly in the drawing: Fig. 1 shows a functional diagram of a circuit according to the invention, Figs. 2 and 3 show the waveform of the voltage signal between the electrodes of the lamp, in case of defective lamp and in case of lamp absent respectively, Figs. 4 and 5 show graphs representing the frequency spectrum of the two signals of Figs. 2 and 3, and Fig. 6 shows a more detailed constructional diagram of the circuit diagrammatically indicated in Fig. 1. Detailed description of the preferred embodiment of the invention Represented in Fig. 1, diagrammatically and limited to the elements relevant in the present description, is a supply circuit for the discharge lamp L with a supply inverter. The label 3 generically indicates the supply inverter, which has two controlled breakers 5 and 7 in a half-bridge configuration. Diodes 9 and 11 are placed respectively in parallel with the controlled breakers 5 and 7. The label 13 generically and diagrammatically indicates the system for controlling the opening and closing of the breakers 5 and 7, of a type known per se. Arranged in parallel with the breaker 7 is a load circuit 15 which comprises, as well as the lamp L with its electrodes 17 and 19, a capacitor 21 in parallel with the lamp L and a resonant circuit in series with the electrodes 17 and 19, which in the example illustrated is shown diagrammatically with an inductive component 23 and a capacitive component 25. The circuit now described is linked via two contacts 27 and 29 to a rectifier bridge (not illustrated) which supplies the inverter 3 with a rectified voltage at a frequency of double the frequency of the supply network. Via the inverter 3 the load circuit comprising the discharge lamp L is supplied with a voltage typically of the order of 10,000-100,000 Hz. With the ignition of the lamp L, a high voltage of the order of 1000 V is established between its two electrodes 17 and 19. This voltage should persist for an extremely limited interval of time, of the order of tens of or a few hundred milliseconds, necessary for the lamp to fire. In case of defective operation of the lamp, ignition is delayed and the voltage between the electrodes 17 and 19 persists at the high value of around 1000 V in the presence of a flow of high current through a capacitor 21 in parallel with the lamp L. This entails heavy stressing of the inverter 3 and hence it is necessary to provide a control circuit which, depending on the voltage at the point P of the load circuit, makes provision for disabling the supply circuit in the case in which the overvoltage condition persists beyond a preset time threshold. When the lamp L is absent from the load circuit, a high voltage, of the order of 700 V, is again established at the point P. However, under these conditions the current through the load circuit is minimal and corresponds merely to the current through the stray capacitances of the circuit. Hence, in this case the supply circuit for the inverter 3 need not be disabled. The control circuit according to the invention, which will be described below, makes it possible to discriminate between these two conditions of overvoltage on the electrodes of the lamp and hence makes it possible to turn off the supply in the case of defective operation only and not in the case of lamp absent. Represented in Figs. 2 and 3 is the waveform of the voltage signal at the point P of the load circuit. Represented in Fig. 2 is a substantially sinusoidal waveform with a frequency corresponding to the frequency f c of switching of the inverter 3. This is the waveform of the voltage signal detectable at the point P in the case of a defect in the operation of the lamp L. The amplitude of the signal is around 1000 V. Conversely, in the case of lamp absent, the voltage signal at the point P takes the profile of Fig. 3 with an amplitude of around the same order of magnitude as the previous case (in the example around 700 V), but with a more complex harmonic content. Figs. 4 and 5 show the frequency spectrum of the two signals. As may be seen in Fig. 4, the waveform of Fig. 2 is practically a sinusoidal wave with a frequency f c , whereas the waveform of Fig. 3, to which the spectrum of Fig. 5 refers, has a modest harmonic content at the switching frequency f c and a large harmonic content at the higher harmonics. The control circuit according to the invention exploits this differing harmonic content of the voltage signals at the point P under the two conditions of defective lamp and absent lamp so as to discriminate the two cases of overvoltage and turn off the supply in the first case only. With this objective, a voltage divider 31, 33, at the intermediate point of which is linked a band-pass filter 35 centred on the switching frequency f c of the inverter 3, is connected to the point P of the load circuit. On account of the different harmonic content of the voltage signal in the case of defective lamp and of absent lamp, the output from the filter 35 centred on the frequency f c will be a sinusoidal signal of frequency f c with a large amplitude in the case of defective lamp and a very small amplitude in the case of lamp absent. This is represented qualitatively in the graph at the bottom of Fig. 1, where time is plotted along the abcsissa and the output voltage from the filter 35 along the ordinate and in which the labels V 1 and V 2 indicate the amplitudes of the signal output by the filter respectively in the case of defective lamp and in the case of absent lamp. The signal output by the filter 35 is sent to a level discriminator 37, which dispatches a high signal to a logic unit 39 when the amplitude of the signal output by the filter 35 is equal to V 1 and a low signal when the amplitude of the signal output by the filter 35 is equal to V 2 . The logic unit 39 is therefore able to discriminate between the two situations of absent lamp or of defective lamp and depending thereon will make provision to maintain the inverter 3 under supply conditions, when the lamp L is absent from the load circuit, whereas it will make provision to disable the inverter 3 when the overvoltage at the point P is due to a defect in the lamp. Shown in Fig. 6 is a practical embodiment of the control circuit represented at a functional level in Fig. 1 and generically labelled 41 therein. In Fig. 6 elements which are identical to or correspond to those of Fig. 1 are indicated with the same reference numerals and will not be described again. In this embodiment the band-pass filter 35 consists of an LC cell comprising a capacitor 51 and an inductor 53 in parallel which form a resonant circuit at the switching frequency f c of the inverter 3. The filter 35 is linked via a diode 55 to a capacitor 57, the latter being charged at the voltage corresponding to the peak voltage of the signal output by the filter 35. The capacitor 57 is linked to the logic unit 39 via a Zener diode 59, at the output of which will be present a low signal when the input voltage is less than the conduction voltage of the Zener diode and a high signal when the input voltage is greater than the conduction voltage. The conduction voltage of the Zener diode 59 lies between the values V 1 and V 2 indicated in the graph at the bottom of Fig. 1. In short, the signal at the output of the Zener diode 59 will be high only in the case of defective lamp and will remain low in the case of lamp absent or of normal operation of the load circuit. This signal is sent to the logic unit 39 which consists of a latch circuit with two inverting gates 61 and 63 in series with positive feedback. With this arrangement the output from the inverting gate 63 will remain at low level in the case of normal operation or of lamp absent whereas it will rise to high level and remain steady at this level, until the operator intervenes on the circuit, in the case of defective operation of the lamp. The signal output by the latch circuit is used to disable the inverter 3. With the objective of avoiding the intervention of the protection circuit with each attempted ignition of the lamp (even under conditions of lamp intact), there is provided a delay circuit 71 with a delay time of the order of 100-200 ms. In this way the voltage peak which occurs for very brief instants at the moment of ignition of the lamp L even when the latter is not damaged, does not prompt any disabling of the supply circuit. To achieve the disabling of the supply inverter of the lamp L via the signal output by the logic unit 39, this signal can be employed for example to short-circuit the base of a transistor. This can be the typical solution for a supply circuit of the self-oscillating type. When the supply to the lamp L is achieved via an integrated control circuit, the signal generated by the logic unit 39 can be applied to an enabling/disabling pin of the integrated circuit. It should be understood that the drawing shows merely one example given solely as a practical illustration of the invention, it being possible for this invention to vary in its forms and arrangements without however departing from the scope of the concept underlying the invention. The possible presence of reference numerals in the enclosed claims has the objective of facilitating the reading of the claims with reference to the description and drawing, and does not limit the scope of the protection represented by the claims.

Claims: A supply circuit for discharge lamps (L) comprising an inverter (3) with controlled breakers (5, 7) which are turned on and off alternately at a switching frequency (f c ) so as to generate a supply voltage at the said switching frequency (f c ) for a load circuit (15) arranged in parallel with one of the said controlled breakers and comprising at least one lamp (L) and a resonant circuit (23, 25) in series with the electrodes (17, 19) of the said lamp, there being associated with the said load circuit a control circuit (41) for turning off the supply to the said load circuit in case of defective operation of the lamp (L), characterized in that the said control circuit (41) comprises a band-pass filter (35) centred on the switching frequency (f c ) of the said inverter and whose input signal is dependent on the voltage at a point (P) of the load circuit, the output signal from the said filter being sent to control means (37, 39) associated with the said inverter (3) so as to turn off the supply to the load circuit in case of defective operation. Supply circuit according to Claim 1 characterized in that a voltage divider (31, 33), to which the said band-pass filter (35) is linked, is arranged in parallel with a branch containing at least one component (23) of the load circuit (15). Supply circuit according to Claim 2, characterized in that the said voltage divider is placed in parallel with a branch comprising one (7) of the said controlled breakers (5, 7) and the inductive component (23) of the resonant circuit in series with the electrodes of the lamp (L). Supply circuit according to one or more of the preceding claims, characterized in that the said band-pass filter (35) comprises an L-C cell in parallel, the frequency of resonance of which is substantially corresponding to the switching frequency (f c ) of the said inverter. Supply circuit according to one or more of the preceding claims, characterized in that the said control means comprise a level discriminator (37) and a latch circuit (39) whose output signal takes a high value in case of defective operation of the lamp, the said output signal turning off the supply to the load circuit. Supply circuit according to Claim 5, characterized in that the said level discriminator comprises a Zener diode (59) interposed between the said filter (35) and the said latch circuit (39), in such a way that the voltage input to the said latch circuit (39) is high if the voltage output by the band-pass filter (35) exceeds the conduction voltage of the Zener diode. Supply circuit according to Claim 5 or 6, characterized in that the said latch circuit comprises two inverting gates in series (61, 63) with positive feedback. Supply circuit according to one or more of Claims 5 to 7, characterized in that the said level discriminator (37) furthermore comprises a capacitor (57) which is charged by the voltage output by the band-pass filter (35). Supply circuit according to one or more of the preceding claims, characterized in that it comprises, downstream of the said band-pass filter (35), a delay timer circuit (71).

Inventors: CANOVA ANTONIO, CANOVA, ANTONIO
Applicants: MAGNETEK SPA, MAGNETEK S.P.A.
Application year: 1997
Main classification: H05B41
IPC codes: H05B41