Title of Invention	AN IMPROVED CIRCUIT DEVICE FOR OPERATING AT LEAST ONE DISCHARGE LAMP
Abstract	It is the object of the invention to provide a circuit arrangement for operating at least one discharge lamp with an improved switch-off device which does not have the disadvantages of the prior art. In particular, the aim is for the switch- off device to detect the occurrence of the rectifying effect of the at least one discharge lamp and to switch off the half-bridge inverter permanently in this case. This object is achieved according to the features of the invention.

Title of Invention

AN IMPROVED CIRCUIT DEVICE FOR OPERATING AT LEAST ONE DISCHARGE LAMP

Abstract

It is the object of the invention to provide a circuit arrangement for operating at least one discharge lamp with an improved switch-off device which does not have the disadvantages of the prior art. In particular, the aim is for the switch- off device to detect the occurrence of the rectifying effect of the at least one discharge lamp and to switch off the half-bridge inverter permanently in this case. This object is achieved according to the features of the invention.

Full Text	The invention relates to a circuit arrangement for operating at least one discharge lamp in accordance with the features of the invention. I.Prior art A circuit arrangement is disclosed, for example, in the Laid-Open Patent specification EP 0 753 987 Al. This circuit arrangement has a half-bridge inverter with a switch-off device which switches off the half-bridge inverter in the case of an anomalous operating state-for example in the case of a lamp which refuses to start or is defective. The switch-off device has a field effect transistor whose drain-source path is arranged in the control circuit of a half-bridge inverter transistor and switches the control circuit between a low-resistance and a high- resistance state. Upon the occurrence of an anomalous operating state, the switch off is performed synchronously with the blocking phase of that half-bridge inverter transistor in whose control circuit the field effect transistor is arranged. The switch-off device of this circuit arranged certainly switches the half-bridge inverter off reliably in the case of a lamp which refuses to start, but it reacts in general too insensitively to the occurrence of the so-called rectifying effect of the discharge lamp, which will be explained in more detail below. A possible cause of failure of discharge lamps, in particular of low-pressure discharge lamps, is occasioned by a reduction over the lifetime of the lamp in the ability of the lamp electrodes to emit. Since the loss of the ability to emit generally proceeds with varying intensity in the two lamp electrodes over the lifetime of the lamp, at the end of the lifetime of a discharge lamp operated with alternating current a preferred direction has been formed for the discharge current through the discharge lamp. The discharge lamp develops a current-rectifying effect in this case. This effect is designated as a rectifying effect of the discharge lamp. Owing to the occurrence of the rectifying effect in the discharge lamp, the lamp electrode incapable of emission is heated extremely, with the result that impermissibly high temperatures can occur which can even cause melting of the lamp bulb glass. Moreover, in the case of discharqe lamps which are operated on a half-bridge inverter, the rectifying effect of the discharge lamp causes a substantial deviation in the voltage drop across the coupling capacitor or the coupling capacitors from the normal value, which is usually half as large as the value of the input voltage of the half-bridge inverter. In the case of self-oscillating half-bridge inverters, this deviation in the voltage drop across the coupling capacitor or the coupling capacitors leads to stopping the oscillation of the half-bridge inverter, because the supply voltage of one of the two half-bridge branches is in this case too low to maintain the feedback. However, immediately after being interrupted the oscillation of the half-bridge inverter is set going again by the starting circuit of the half-bridge inverter if the switch-off device is not triggered. As a result, the discharge lamp affected by the rectifying effect is not reliably switched off, but flickers instead. II. Summary of the invention It is the object of the invention to provide a circuit arrangement for operating at least one discharge lamp with an improved switch-off device which does not have the disadvantages of the prior art. In particular, the aim is for the switch- off device to detect the occurrence of the rectifying effect of the at least one discharge lamp and to switch off the half-bridge inverter permanently in this case. This object is achieved according to the features of the invention. The circuit arrangement according to the invention, which has a half-bridge inverter with a downstream bad circuit, at least one coupling capacitor connected to the load circuit and the half-bridge inverter, as well as term rials for at least one discharge lamp and a switch-off device for switching off the half- bridge inverter upon the occurrence of an anomalous operating state, has according to the invention, means for monitoring the voltage drop across the at least one coupling capacitor and for activating the switch-off device as a function of the voltage drop detected across the at least one coupling capacitor. As already explained further above, the occurrence of the rectifying effect of the at least one discharge lamp causes a substantial deviation in the voltage drop across the at least one coupling capacitor from its normal valve, which is half as large as the input voltage of the half-bridge inverter. With the aid of the above named means according to the invention, the occurrence of the rectifying effect of the at least one discharge lamp is detected by using these means to monitor the voltage drop across the at least one. coupling capacitor, and activating the switch-off device when the voltage drop across the at least one coupling capacitor deviates substantially from its normal value. The above named means according to the invention advantageously have a first device for activating the switch-off device upon a predetermined upper limiting value of the voltage drop across the at least one coupling capacitor being reached, and a second device for activatinq the switch-off device upon a predetermined lower limiting value of the voltage drop across the at least one coupling capacitor being reached. The upper and the lower limiting value's must be preset so that a slight asymmetry in the case of the lamp electrodes does not already lead to activation of the switch-off device. For this reason, the upper limiting value is advantageously not less than 7 5 percent of the input or supply voltage of the half-bridge inverter, and the lower limiting value is advantageously at most 25 percent of the input or supply voltage of the half-bridge inverter. The first and/or second device advantageously have at least one electric component with a nonlinear current-voltage characteristic which is connected to the at least one coupling capacitor and to the at least one control input of the switch-off device. With the aid of such an electric component with a nonlinear current-voltage characteristic, the upper or lower limiting value of the voltage drop across the at least one coupling capacitor for which the switch-off device is activated by the first or second device can be preset to the desired level. Components from the group of diode, Zener diode, suppressor diode and varistor are advantageously suitable as electric components with a non-linear current-voltage characteristic. Furthermore, the switch-off device of the circuit arrangement according to the invention advantageously has at least two control or regulating inputs, specifically one each for the first device and the second device. A control input is advantageously additionally connected in parallel in terms of alternating current with the at least one discharge lamp, in order to monitor the voltage drop across the terminals for the at least one discharge lamp. In order to ensure that the half-bridge inverter is switched off as reliably and permanently as possible during a malfunction or upon the occurrence of the rectifying effect of the at least one discharge lamp, the switch- off device of the circuit arrangement according to the invention advantageously has a bistable switching device. A thyristor equivalent circuit constructed from two transistors is particularly well suited as bistable switching device, since said circuit already has available two separate control inputs which can be used by the first and the second device to activate the switch-off device. The first device advantageously comprises an electric component with a nonlinear current-voltage characteristic, and a diode connected in series therewith, the anode of the diode being connected to a lamp terminal and to the at least one coupling capacitor, while the cathode of this diode is connected to the electric component with the nonlinear current-voltage characteristic, this electric component being connected to the first control input of the switch-off device. The second device advantageously comprises the series circuit of at least one diode with at least one resistor, this series circuit being connected, on the other hand, to the at least one coupling capacitor and a lamp terminal, and being connected, on the other hand, to the second control input of the switch-off device. III. Description of the preferred exemplary embodiments The invention is explained below in more detail with the aid of two preferred exemplary embodiments. A sketch of the circuit arrangement in accordance with the first preferred exemplary embodiment is illustrated in Figure 1. This circuit arrangement serves to operate a fluorescent lamp. It has a freely oscillating half- bridge inverter which is fitted with two bipolar transistors Ql, Q2 and draws its input or supply voltage via the DC voltage terminals j1, j2. The DC voltage terminal j2 is connected to frame potential, and a voltage of approximately + 400 V is provided at the DC voltage terminal jl. This input or supply voltage is generated from the rectified AC supply voltage with the aid of an upstream step-up converter, for example (not shown in the figure). Furthermore, a radio interference suppression filter known per se (likewise not illustrated) is connected upstream of the supply voltage rectifier. Two bipolar transistors Ql, Q2 of the half-bridge inverter are provided in each case with a freewheeling diode D1, D2 which are connected in parallel with the collector-emitter path of the corresponding transistor Ql, Q2. Moreover, the two bipolar transistors Ql, Q2 respectively have an emitter resistor Rl, R2 and a base-emitter shunt resistor R3, R4. Furthermore, a capacitor Cl is arranged in parallel with the collector-emitter path of the transistor Ql. The two switching transistors Ql, Q2 of the half-bridge inverter are driven by means of an annular core transformer which has a primary winding Nl and two secondary windings N2, N3. The primary winding Nl is connected into the load circuit, designed as a series resonant circuit, of the half-bridge inverter. The load circuit is connected to the centre tap Ml between the bipolar transistors Ql, Q2 of the half-bridge inverter, and to the centre tap M2 between the two coupling capacitors C2, C3. The load circuit comprises the primary winding Nl, a resonance inductor L1, a resonance capacitor C4 and respectively two terminals for the two electrode filaments El, E2 of a fluorescent lamp LP. The resonance capacitor C4 is connected in parallel with the discharge path of the fluorescent lamp LP. The secondary windings N2, N3 are respectively arranged in the base-emitter junction of a bipolar transistor Ql, Q2, and connected to the base terminal of the relevant inverter transistor Ql, Q2 series, in each case via a base resistor R7, R8. The half-bridge inverter further has a starting device which essentially comprises the Diac DC, which is connected to the base terminal of the bipolar transistor Q2, and the starting capacitor C5, which is connected, on the one hand, to the terminal j2, which is at frame potential, and, on the other hand, via a resistor R9 and a rectifying diode D3 to the centre tap Ml of the half-bridge inverter, as well as the resistor R20 arranged in parallel with the starting capacitor C5. The starting circuit ensures the build-up of the half-bridge inverter after the circuit arrangement is switched on. The coupling capacitors C2, C3 each have a parallel resistor R5, R6. With the aid of the coupling capacitors C2, C3 and their parallel resistors R5, R6, a voltage drop which, in the ideal case, is half as large as the input or supply voltage of the half-bridge inverter provided at the terminals jl, j2, is generated at the centre tap M2 between the coupling capacitors C2, C3. In the ideal case, the voltage drop at the centre tap M2 and across the coupling capacitor C3 is therefore approximately + 2 00 V in the case of an input voltage of approximately + 400 V of the half-bridge inverter. In reality, the voltage drop at the centre tap M2 and across the coupling capacitor C3 deviates slightly from this ideal value. The circuit arrangement according to the invention also has a switch-off device which switches off the half-bridge inverter Ql, Q2 upon the occurrence of an anomalous operating state, that is to say in the case of a discharge lamp LP which refuses to start or is defective. The switch-off device essentially comprises a field effect transistor Tl whose drain-source path is connected in series with the emitter resistor R2 of the inverter transistor Q2, and a thyristor equivalent circuit A which is formed by the bipolar transistors Q3, Q4, as well as an error-signal monitoring unit which comprises the capacitors C8, C9, C10, the diodes D6, D7, D10, Dll and the resistors R10, R11, R17, R18. The thyristor equivalent circuit A has two control inputs. The first control input of the thyristor equivalent circuit is connected to the base terminal of the npn transistor Q4, while its second control input is connected to the base terminal of the pnp transistor Q3. The output of the thyristor equivalent circuit A at the collector terminal of the transistor Q4 is connected via a diode D9 to the gate of the field effect transistor Tl, the anode of the diode D9 being connected to the gate of the transistor Tl, and its cathode being connected to the collector of the transistor Q4. Furthermore, the gate terminal of the field effect transistor Tl is connected to the terminal j1 via the resistors R19, R6, R5 and via an electrode filament of the discharge lamp LP. Moreover, connected in parallel with the gate-source path of the field effect transistor Tl is a Zener diode D12 which serves as overvoltage protector for the transistor T1. The first control input of the thyristor equivalent circuit A is driven by means of the error-signal monitoring unit. With the aid of the RC integration element R17, C10, the rectifier diode D10 and the capacitor C9, the error-signal monitoring unit generates a smooth DC voltage which is present across the capacitor C10 and is proportional to the voltage drop across the discharge lamp LP. The abovementioned components are connected in parallel in terms of alternating current with the discharge path of the discharge lamp LP. The positive pole of the capacitor C10 is connected to a terminal of the electrode filament E2 of the discharge lamp LP via the components R10, C9, R17, and is connected to the first control input j3 of the thyristor equivalent circuit A via the components R10, R11, D6, D7. Moreover, by means of the CR series circuit C8, R10, which forms a differentiating element, the error-signal monitoring unit generates a synchronization signal which is obtained by differentiating the trapezoidal output voltage, present at the centre tap Ml, of the half-bridge inverter Ql, Q2. A square-wave voltage whose positive half wave is generated by the leading edge and whose negative half wave is generated by the trailing edge of the trapezoidal half-bridge inverter output voltage is therefore present at the resistor R10. The leading edge of the trapezoidal half-bridge inverter output voltage is produced by switching off the transistor Q2, while the trailing edge of the trapezoidal half-bridge inverter output voltage is produced by switching off the transistor Ql. Present at the centre tap ]5 of the differentiating element C8, R10 is a voltage which is composed additively of the voltage drop across the capacitor C10 and the voltage drop across the resistor R10. This voltage is fed to the first control input j3 of the thyristor equivalent circuit via the Zener diode D7. The error-signal monitoring unit and its cooperation with the thyristor equivalent circuit A and the field effect transistor Tl are described in detail in the Laid-Open Specification EP 0 753 987. Furthermore, the circuit arrangement illustrated in the figure has a first device V1 and a second device V2 for activating the switch-off device, which are connected to the first and, respectively, to the second control input of the thyristor equivalent circuit A. The first device V1 comprises the series circuit of a Zener diode D4 with a diode D5, the anode of the diode D5 being connected to the centre tap M2 between the coupling capacitors C2, C3 and to a terminal of the electrode filament El of the discharge lamp LP, and the cathode of the diode D5 being connected to the cathode of the Zener diode D4 . The anode of the Zener diode D4 is connected to the first control input at the base terminal of the transistor Q4 of the thyristor equivalent circuit A via the resistors R10, R11, the diode D6, which is polarized in the same sense as the diode D5, and via a further Zener diode D7, which is polarized in the same sense as the Zener diode D4. The second device V2 comprises the series circuit of a diode D8 with a resistor R12. The cathode of the diode D8 is connected to the centre tap M2 and to the same terminal of the electrode filament El of the discharge lamp LP as the anode of the diode D5. The anode of the diode D8 is connected to the resistor R12, which is connected, for its part, to the second control input at the base terminal of the transistor Q3 of the thyristor equivalent circuit A. In addition to the transistors Q3, Q4, as further components the thyristor equivalent circuit A includes the capacitors C6, C7 and the resistors R13, R14, R15, R16. The mode of operation of a thyristor equivalent circuit constructed from two transistors is. described, for example, on pages 395 to 3 96 in the book entitled "Bauelemente der Elektronik und ihre Grundschaltungen" ["Electronic Components and their Basic Circuits"] by H. Hoger, F. Kahler, G. Weigt from the series entitled «Einfuhrung in die Elektronik" ["introduction to Electronics"], Vol. 1, published by H. Stam GmbH, 7th Edition. The mode of operation of the above described circuit arrangement is explained in more detail below for the case of normal operation, that is to say given an acceptably operating discharge lamp, and for the case of the anomalous operating state, that is to say a' discharge lamp which refuses to start, or in the case of the occurrence of the rectifying effect of the discharge lamp. A suitable dimensioning of the components used is specified in the table. Normal operation In the case of normal operation, after the discharge lamp or the circuit arrangement has been switched on the DC voltage supply for the half-bridge inverter Q1, Q2 builds up at the terminals jl, j2. The drain-source path of the field effect transistor T1, whose gate is connected via the resistors R19, R6, the electrode filament El and the resistor R5 to the terminal jl, which is at approximately + 400 V, is put into the low-resistance state. Furthermore, the starting capacitor C5 is charged via the resistor R5, the electrode filament El and the resistor R6 to the breakdown voltage of the Diac DC, which then generates trigger pulses for the base of the bipolar transistor Q2 and thereby causes the half-bridge inverter Q1, Q2 to build up. After the transistor Q2 has been turned on, the starting capacitor C5 is discharged via the resistor R9 and the diode D3 to such an extent that the Diac DC generates no further trigger pulses. Present at the two coupling capacitors C2, C3 in each case is half the input voltage of the half-bridge inverter Ql, Q2, with the result that the centre tap M2 between the coupling capacitors C2, C3 is located at an electric potential of approximately + 2 00 V. The two half-bridge inverter transistors Ql, Q2 switch in an alternating fashion, so that the centre tap Ml between the transistors Ql, Q2 is connected alternately to the positive pole jl (+ 400 V) and the negative pole j2 (frame potential) of the DC voltage supply of the half-bridge inverter. As a result, a medium-frequency alternating current whose frequency corresponds to the switching frequency of the half-bridge inverter flows in the load circuit between the centre taps Ml and M2. During the switching pauses, in which both transistors Ql, Q2 block, the load circuit current is maintained by means of the resonant inductor L1 and flows via one of the two freewheeling diodes D1, D2. Usually, the electrode filaments El, E2 of the fluorescent lamp LP have a heating current applied to them by means of a heating device (not illustrated) before the lamp is started, and are preheated in this way. In order to start the gas discharge in the discharge lamp LP, the starting voltage required for the purpose is provided at the resonance capacitor C4 by means of the method of resonant increase. That is to say, the switching frequency of the half-bridge inverter is approximated to the resonant frequency of the series resonance circuit L1, C4. After the lamp has been started, the resonance circuit L1, C4 is damped by the then conductive discharge path of the discharge lamp LP. The transistors Q3, Q4 of the thyristor equivalent circuit A are in the blocked state during normal operation, and the switch-off device is deactivated. Switching off the half-bridge inverter in the case of a discharge lamp which refuses to start (Anomalous operating state) Lacking the discharge lamp LP, the half-bridge inverter Q1, Q2 cannot build up, since the connection of the starting capacitor C5 to the voltage supply terminal j1 is led via the terminals for the electrode filament El. A discharge lamp LP which refuses to start, or a defective discharge lamp LP which has, for example, an operating voltage which has increased due to ageing, causes an increased voltage drop across the capacitor C10. The positive voltage peaks of the synchronization signal generated by the differentiating element C8, R10 are added at the tap j5 to the voltage of the capacitor C10. As a result, the threshold voltage of the Zener diode D7 is exceeded and the transistors Q3, Q4 of the thyristor equivalent circuit A are switched into the conductive state via the first control input j3. The gate of the field effect transistor T1 is now connected to the frame potential via the diode D9 and the conductive collector-emitter path of the bipolar transistor Q4. The drive signal is therefore extracted from the gate of the field effect transistor T1, and the drain-source path of the field effect transistor T1 goes over into the high-resistance or blocked state. Consequently, the half-bridge inverter Ql, Q2 is shut down, and cannot be restarted until the discharge lamp LP is switched on again or exchanged, since the thyristor equivalent circuit A is reset into the blocked state of the normal operation again only by interruption of the voltage supply. This switching off of the half-bridge inverter is performed synchronously with tne blocking phase of the transistor Q2. After the half-bridge inverter has been switched off, the capacitor C10 is discharged via its parallel resistor R18. Switching off the half-bridge inverter upon the occurrence of the rectifying effect of the discharge lamp (Anomalous operating state) The switch-off device Tl, A of the half-bridge inverter Ql, Q2 is activated either by means of the first device VI or by means of the second device V2 upon the occurrence of the rectifying effect in the discharge lamp LP. As already mentioned further above, the rectifying effect causes the discharge lamp LP to exert a rectifying effect on the medium-frequency load circuit current which flows between the centre taps Ml and M2. Depending on which current direction is preferred as a result of the rectifying effect of the discharge lamp LP, the voltage drop across the coupling capacitor C3 and the electric potential at the centre tap M2 are increased or decreased. The rectifying effect of the discharge lamp LP causes a deviation of the voltage drop across the coupling capacitor C3 from its normal value, which is approximately + 2 00 V. If the deviation of the voltage drop across the coupling capacitor C3 from its normal value has risen to virtually 100%, the switch-off device T1, A is activated by the first device VI or the second device V2. If the voltage drop across the coupling capacitor C3 is approximately + 4 00 V, which corresponds to the entire input voltage of the half-bridge inverter, the threshold voltage of the Zener diode D4 of the first device V1 is reached, and the capacitor C10 is charged to such an extent that the voltage drop across the capacitor C10 and the synchronization signal, added thereto at the tap j5, of the differentiating element C8, R10 reach the threshold, voltage of the Zener diode D7, and the transistors Q3, Q4 of the thyristor equivalent circuit A are switched into the conductive state via the first control input j3. As a result, the drain-source path of the field effect transistor Tl is blocked, and the half-bridge inverter Q1, Q2 is shut down synchronously with the blocking phase of the transistor Q2. If the voltage drop across the coupling capacitor C3 is very slight and the centre tap M2 is therefore virtually at frame potential, the pnp transistor Q3 of the thyristor equivalent circuit A is firstly turned on via the second control input j4, which is connected to the centre tap M2 by the series circuit D8, R12 of the second device V2, and subsequently the npn transistor Q4 is switched into the conductive state as well. Once again, the gate drive signal is extracted thereby from the field effect transistor Tl, with the result that the drain-source path thereof goes over into the blocked state and the half-bridge inverter Ql, Q2 is shut down. The thyristor equivalent circuit A is not reset into the blocking state, nor is the switch-off device deactivated until there is an interruption in the current such as is caused, for example, by exchanging the discharge lamp LP or by renewed switching on. A second preferred exemplary embodiment of the invention is illustrated in Figure 2. This second exemplary embodiment differs from the first exemplary embodiment described in more detail above only in additional components R21, D13 and D14. In the remaining components, the second exemplary embodiment corresponds to the first exemplary embodiment. For this reason, identical reference symbols have also been selected for identical components in Figures 1 and 2. The emitter of the transistor Q3 is connected to the voltage supply terminal jl via the resistor R21. In the case of an anomalous operating state, an additional holding current is provided for the thyristor equivalent circuit A with the aid of this resistor R21. The resistor R21 is dimensioned such that the thyristor equivalent circuit A receives approximately 50 to 80 percent of its holding current via the resistor R21. The remainder of the holding current component, which is required to maintain the switched-on state of the thyristor equivalent circuit A, is provided via the resistor R5, the electrode filament El of the low-pressure discharge lamp LP, the resistor R6, and via the diode D14 polarized in the forward direction. Permanent switching on of the thyristor equivalent circuit A is ensured by the additional holding current flowing via the resistor R21, even in the case when the potential at the nodal point M2 - caused by the occurrence of the rectifying effect - is virtually at frame potential. The diode D14 serves to decouple the Diac DC from the thyristor equivalent circuit A. The anode of the diode D14 is connected to a nodal point arranged between the Diac DC and the resistor R6, while the cathode of the diode D14 is connected to the emitter of the transistor Q3. The additional Zener diode D13 protects the thyristor equivalent circuit A against overvoltages. For this purpose, the anode of the Zener diode D13 is connected to the voltage supply terminal j2, and its cathode is connected to the emitter of the transistor Q3. The mode of operation of this circuit arrangement corresponds to that of the first exemplary embodiment. Upon the occurrence of an anomalous operating state, the thyristor equivalent circuit A is reset to the blocking state by exchanging the lamp LP, since the DC connection to the resistor R6 in the case of the electrode filament El is interrupted by taking out the lamp LP, and the holding current still remaining, and flowing via the resistor R21 is insufficient to leave the thyristor equivalent circuit A in the switched on state. The invention is not limited to the exemplary embodiments described in more detail above. For example, the invention can also be applied to half-bridge inverters which have only one coupling capacitor. Furthermore, the . invention can be used not only in the case of self-oscillating half-bridge inverters, but also in the case of externally controlled half-bridge inverters. Furthermore, the upper limiting value and the lower limiting value of the voltage drop across the coupling capacitor C3, at which the switch-off device is activated, can be set to other values by suitable dimensioning of the components. Table: Dimensioning of the components illustrated in the figures in accordance with the preferred exemplary embodiments Table: Dimensioning of the components illustrated in the figures in accordance with the preferred exemplary embodiments (continuation) We Claim 1. An improved circuit device for operating at least one discharge lamp, comprising: - a half-bridge inverter (Ql, Q2)havtng a downstream load circuit (N1, L1, C4); - at least one coupling capacitor (C3) connected to the load circuit (N1, L1, C4) and to the half-bridge inverter (Ql, Q2); - a switch-off device (Tl, A) for switching off the half-bridge inverter (Q1, Q2) on occurrence of an anomalous operating state of the discharge lamp; - the load circuit (N1, L1, C4) having terminals for the at least one discharge lamp (LP), characterized in that means (V1, V2) are disposed for monitoring the voltage drop across the at least one coupling capacitor (C3) and for activating the switch-off device (T1, A) as a function of the voltage drop detected across the at least one coupling capacitor (C3) 2. The circuit device as claimed in claim 1, wherein the means (V1, V2) comprises a first series circuit (vl) for activating the switch-off device (tl, A) upon a predetermined upper limiting value of the voltage drop across the at least one coupling capacitor (C3) being reached; and a second series circuit (V2) for activating the switch-off device (T1, A) upon a predetermined lower limiting value of the voltage drop across the at least one coupling capacitor (C3) being reached. 3. The circuit device as claimed in claim 1, wherein two coupling capacitors (C2, C3) having a centre tap (M2) in-between are provided, and wherein the load circuit (N1, L1, C4) is connected to the centre tap (M2) between the coupling capacitors (C2, C3). 4. The circuit device as claimed in claim 2, wherein - the switch-off device (Tl, A) has at least one control input (j3), and wherein the first (V1) and the second device (V2) have at least one electric component (D4) with a nonlinear current-voltage characteristic, which is connected to the at least one coupling capacitor (C3) and to the at least one control input (j3) of the switch-off device (T1, A). 5. The circuit device as claimed in claim 2, wherein the upper limiting value is greater than or equal to 75 percent of the input voltage or supply voltage of the half-bridge inverter (Q1, Q2). 6. The circuit device as claimed in claim 2, wherein the lower limiting value is less than or equal to 25 percent of the input voltage or supply voltage of the half-bridge inverter (Q1, Q2). 7. The circuit device as claimed in claim 4, wherein the switch-off device (T1, A) has at least two control inputs (j3, J4), and wherein the first series circuit (V1) serving to drive the first control input (j3), and the second series circuit (V2) serving to control the second control input (j4). 8. The circuit device as claimed in claim 4, wherein the at least one electric component (D4) with a nonlinear current-voltage characteristic is a component selected from the group of a diode, Zener diode, suppressor diode and varistor. 9. The circuit device as claimed in claim 4, wherein at least one diode (D5) is connected in series with the at least one electric component (D4) with a nonlinear current-voltage characteristic. 10. The circuit device as claimed in claim 7, wherein the switch-off device (T1, A) has a bistable switching means (A). 11. The circuit device as claimed in claims 7 and 9, wherein - the first series circuit (V1) comprises an: electric component (D4' with a nonlinear current-voltage characteristic, and a diode (D5) connected in series therewith, the anode of the diode (D5) being connected to a terminal for the at least one discharge lamp (LP) and to the at least one coupling capacitor (C3), and the cathode of the diode (D5) being connected to the electric component (D4) with a nonlinear current voltage characteristic, wherein - the electric component (D4) with a nonlinear current-voltage characteristic of the first series circuit (V1) is connected to the first control input (j3) of the switch-off device (Tl, A), and wherein the second switch circuit (V2) comprises at least one diode (D8) with at least one resistor (R12), and being connected to the at least one coupling capacitor (C3), a terminal for the at least one discharge lamp (LP) and to the second control input (j4) of the switch-off device (T1, A). 12. The circuit device as claimed in claim 7, wherein a control input (j3) of the switch-off device (T1, A) is connected in parallel in terms of alternating current with the at least one discharge lamp (LP) and monitors) the voltage drop across the terminals for the at least one discharge lamp (LP). 13. The circuit device as claimed in claim 10, wherein the bistable switching means (A) is a thyristor equivalent circuit (Q3, Q4). It is the object of the invention to provide a circuit arrangement for operating at least one discharge lamp with an improved switch-off device which does not have the disadvantages of the prior art. In particular, the aim is for the switch- off device to detect the occurrence of the rectifying effect of the at least one discharge lamp and to switch off the half-bridge inverter permanently in this case. This object is achieved according to the features of the invention.

Full Text

The invention relates to a circuit arrangement for operating at least one
discharge lamp in accordance with the features of the invention.
I.Prior art
A circuit arrangement is disclosed, for example, in the Laid-Open Patent
specification EP 0 753 987 Al. This circuit arrangement has a half-bridge inverter
with a switch-off device which switches off the half-bridge inverter in the case of
an anomalous operating state-for example in the case of a lamp which refuses to
start or is defective. The switch-off device has a field effect transistor whose
drain-source path is arranged in the control circuit of a half-bridge inverter
transistor and switches the control circuit between a low-resistance and a high-
resistance state. Upon the occurrence of an anomalous operating state, the
switch off is performed synchronously with the blocking phase of that half-bridge
inverter transistor in whose control circuit the field effect transistor is arranged.
The switch-off device of this circuit arranged certainly switches the half-bridge
inverter off reliably in the case of a lamp which refuses to start, but it reacts in
general too insensitively to the occurrence of the so-called rectifying effect of the
discharge lamp, which will be explained in more detail below.
A possible cause of failure of discharge lamps, in particular of low-pressure
discharge lamps, is
occasioned by a reduction over the lifetime of the lamp
in the ability of the lamp electrodes to emit. Since
the loss of the ability to emit generally proceeds with
varying intensity in the two lamp electrodes over the
lifetime of the lamp, at the end of the lifetime of a
discharge lamp operated with alternating current a
preferred direction has been formed for the discharge
current through the discharge lamp. The discharge lamp
develops a current-rectifying effect in this case. This
effect is designated as a rectifying effect of the
discharge lamp. Owing to the occurrence of the
rectifying effect in the discharge lamp, the lamp
electrode incapable of emission is heated extremely,
with the result that impermissibly high temperatures
can occur which can even cause melting of the lamp bulb
glass.
Moreover, in the case of discharqe lamps which are
operated on a half-bridge inverter, the rectifying
effect of the discharge lamp causes a substantial
deviation in the voltage drop across the coupling
capacitor or the coupling capacitors from the normal
value, which is usually half as large as the value of
the input voltage of the half-bridge inverter. In the
case of self-oscillating half-bridge inverters, this
deviation in the voltage drop across the coupling
capacitor or the coupling capacitors leads to stopping
the oscillation of the half-bridge inverter, because
the supply voltage of one of the two half-bridge
branches is in this case too low to maintain the
feedback. However, immediately after being interrupted
the oscillation of the half-bridge inverter is set
going again by the starting circuit of the half-bridge
inverter if the switch-off device is not triggered. As
a result, the discharge lamp affected by the rectifying
effect is not reliably switched off, but flickers
instead.
II. Summary of the invention
It is the object of the invention to provide a circuit arrangement for operating at
least one discharge lamp with an improved switch-off device which does not
have the disadvantages of the prior art. In particular, the aim is for the switch-
off device to detect the occurrence of the rectifying effect of the at least one
discharge lamp and to switch off the half-bridge inverter permanently in this
case.
This object is achieved according to the features of the invention.
The circuit arrangement according to the invention, which has a half-bridge
inverter with a downstream bad circuit, at least one coupling capacitor
connected to the load circuit and the half-bridge inverter, as well as term rials for
at least one discharge lamp and a switch-off device for switching off the half-
bridge inverter upon the occurrence of an anomalous operating state, has
according to the invention, means for monitoring the voltage drop across the at
least one coupling capacitor and for activating the switch-off device as a function
of the voltage drop detected across the at least one coupling capacitor.
As already explained further above, the occurrence of the rectifying effect of the
at least one discharge lamp causes a substantial deviation in the voltage drop
across the at least one coupling capacitor from its normal valve, which is half as
large as the input voltage of the half-bridge inverter. With the aid of the above
named means according to the invention, the occurrence of the rectifying effect
of the at least one discharge lamp is detected by using these means to monitor
the voltage drop across the at least one.
coupling capacitor, and activating the switch-off
device when the voltage drop across the at least one
coupling capacitor deviates substantially from its
normal value.
The above named means according to the invention
advantageously have a first device for activating the
switch-off device upon a predetermined upper limiting
value of the voltage drop across the at least one
coupling capacitor being reached, and a second device
for activatinq the switch-off device upon a
predetermined lower limiting value of the voltage drop
across the at least one coupling capacitor being
reached. The upper and the lower limiting value's must
be preset so that a slight asymmetry in the case of the
lamp electrodes does not already lead to activation of
the switch-off device. For this reason, the upper
limiting value is advantageously not less than
7 5 percent of the input or supply voltage of the
half-bridge inverter, and the lower limiting value is
advantageously at most 25 percent of the input or
supply voltage of the half-bridge inverter.
The first and/or second device advantageously have at
least one electric component with a nonlinear
current-voltage characteristic which is connected to
the at least one coupling capacitor and to the at least
one control input of the switch-off device. With the
aid of such an electric component with a nonlinear
current-voltage characteristic, the upper or lower
limiting value of the voltage drop across the at least
one coupling capacitor for which the switch-off device
is activated by the first or second device can be
preset to the desired level. Components from the group
of diode, Zener diode, suppressor diode and varistor
are advantageously suitable as electric components with
a non-linear current-voltage characteristic.
Furthermore, the switch-off device of the circuit
arrangement according to the invention advantageously
has at least two control or regulating inputs,
specifically one each for the first device and the
second device. A control input is advantageously
additionally connected in parallel in terms of
alternating current with the at least one discharge
lamp, in order to monitor the voltage drop across the
terminals for the at least one discharge lamp. In order
to ensure that the half-bridge inverter is switched off
as reliably and permanently as possible during a
malfunction or upon the occurrence of the rectifying
effect of the at least one discharge lamp, the switch-
off device of the circuit arrangement according to the
invention advantageously has a bistable switching
device. A thyristor equivalent circuit constructed from
two transistors is particularly well suited as bistable
switching device, since said circuit already has
available two separate control inputs which can be used
by the first and the second device to activate the
switch-off device. The first device advantageously
comprises an electric component with a nonlinear
current-voltage characteristic, and a diode connected
in series therewith, the anode of the diode being
connected to a lamp terminal and to the at least one
coupling capacitor, while the cathode of this diode is
connected to the electric component with the nonlinear
current-voltage characteristic, this electric component
being connected to the first control input of the
switch-off device. The second device advantageously
comprises the series circuit of at least one diode with
at least one resistor, this series circuit being
connected, on the other hand, to the at least one
coupling capacitor and a lamp terminal, and being
connected, on the other hand, to the second control
input of the switch-off device.
III. Description of the preferred exemplary embodiments
The invention is explained below in more detail with
the aid of two preferred exemplary embodiments. A
sketch of the circuit arrangement in accordance with
the first preferred exemplary embodiment is illustrated
in Figure 1. This circuit arrangement serves to operate
a fluorescent lamp. It has a freely oscillating half-
bridge inverter which is fitted with two bipolar
transistors Ql, Q2 and draws its input or supply
voltage via the DC voltage terminals j1, j2. The DC
voltage terminal j2 is connected to frame potential,
and a voltage of approximately + 400 V is provided at
the DC voltage terminal jl. This input or supply
voltage is generated from the rectified AC supply
voltage with the aid of an upstream step-up converter,
for example (not shown in the figure). Furthermore, a
radio interference suppression filter known per se
(likewise not illustrated) is connected upstream of the
supply voltage rectifier.
Two bipolar transistors Ql, Q2 of the half-bridge
inverter are provided in each case with a freewheeling
diode D1, D2 which are connected in parallel with the
collector-emitter path of the corresponding transistor
Ql, Q2. Moreover, the two bipolar transistors Ql, Q2
respectively have an emitter resistor Rl, R2 and a
base-emitter shunt resistor R3, R4. Furthermore, a
capacitor Cl is arranged in parallel with the
collector-emitter path of the transistor Ql. The two
switching transistors Ql, Q2 of the half-bridge
inverter are driven by means of an annular core
transformer which has a primary winding Nl and two
secondary windings N2, N3. The primary winding Nl is
connected into the load circuit, designed as a series
resonant circuit, of the half-bridge inverter. The load
circuit is connected to the centre tap Ml between the
bipolar transistors Ql, Q2 of the half-bridge inverter,
and to the centre tap M2 between the two coupling
capacitors C2, C3. The load circuit comprises the
primary winding Nl, a resonance inductor L1, a
resonance capacitor C4 and respectively two terminals
for the two electrode filaments El, E2 of a fluorescent
lamp LP. The resonance capacitor C4 is connected in
parallel with the discharge path of the fluorescent
lamp LP. The secondary windings N2, N3 are respectively
arranged in the base-emitter junction of a bipolar
transistor Ql, Q2, and connected to the base terminal
of the relevant inverter transistor Ql, Q2 series, in
each case via a base resistor R7, R8. The half-bridge
inverter further has a starting device which
essentially comprises the Diac DC, which is connected
to the base terminal of the bipolar transistor Q2, and
the starting capacitor C5, which is connected, on the
one hand, to the terminal j2, which is at frame
potential, and, on the other hand, via a resistor R9
and a rectifying diode D3 to the centre tap Ml of the
half-bridge inverter, as well as the resistor R20
arranged in parallel with the starting capacitor C5.
The starting circuit ensures the build-up of the
half-bridge inverter after the circuit arrangement is
switched on.
The coupling capacitors C2, C3 each have a parallel
resistor R5, R6. With the aid of the coupling
capacitors C2, C3 and their parallel resistors R5, R6,
a voltage drop which, in the ideal case, is half as
large as the input or supply voltage of the half-bridge
inverter provided at the terminals jl, j2, is generated
at the centre tap M2 between the coupling capacitors
C2, C3. In the ideal case, the voltage drop at the
centre tap M2 and across the coupling capacitor C3 is
therefore approximately + 2 00 V in the case of an input
voltage of approximately + 400 V of the half-bridge
inverter. In reality, the voltage drop at the centre
tap M2 and across the coupling capacitor C3 deviates
slightly from this ideal value.
The circuit arrangement according to the invention also
has a switch-off device which switches off the
half-bridge inverter Ql, Q2 upon the occurrence of an
anomalous operating state, that is to say in the case
of a discharge lamp LP which refuses to start or is
defective. The switch-off device essentially comprises
a field effect transistor Tl whose drain-source path is
connected in series with the emitter resistor R2 of the
inverter transistor Q2, and a thyristor equivalent
circuit A which is formed by the bipolar transistors
Q3, Q4, as well as an error-signal monitoring unit
which comprises the capacitors C8, C9, C10, the diodes
D6, D7, D10, Dll and the resistors R10, R11, R17, R18.
The thyristor equivalent circuit A has two control
inputs. The first control input of the thyristor
equivalent circuit is connected to the base terminal of
the npn transistor Q4, while its second control input
is connected to the base terminal of the pnp transistor
Q3. The output of the thyristor equivalent circuit A at
the collector terminal of the transistor Q4 is
connected via a diode D9 to the gate of the field
effect transistor Tl, the anode of the diode D9 being
connected to the gate of the transistor Tl, and its
cathode being connected to the collector of the
transistor Q4. Furthermore, the gate terminal of the
field effect transistor Tl is connected to the terminal
j1 via the resistors R19, R6, R5 and via an electrode
filament of the discharge lamp LP. Moreover, connected
in parallel with the gate-source path of the field
effect transistor Tl is a Zener diode D12 which serves
as overvoltage protector for the transistor T1. The
first control input of the thyristor equivalent circuit
A is driven by means of the error-signal monitoring
unit.
With the aid of the RC integration element R17, C10,
the rectifier diode D10 and the capacitor C9, the
error-signal monitoring unit generates a smooth DC
voltage which is present across the capacitor C10 and
is proportional to the voltage drop across the
discharge lamp LP. The abovementioned components are
connected in parallel in terms of alternating current
with the discharge path of the discharge lamp LP. The
positive pole of the capacitor C10 is connected to a
terminal of the electrode filament E2 of the discharge
lamp LP via the components R10, C9, R17, and is
connected to the first control input j3 of the
thyristor equivalent circuit A via the components R10,
R11, D6, D7. Moreover, by means of the CR series
circuit C8, R10, which forms a differentiating element,
the error-signal monitoring unit generates a
synchronization signal which is obtained by
differentiating the trapezoidal output voltage, present
at the centre tap Ml, of the half-bridge inverter Ql,
Q2. A square-wave voltage whose positive half wave is
generated by the leading edge and whose negative half
wave is generated by the trailing edge of the
trapezoidal half-bridge inverter output voltage is
therefore present at the resistor R10. The leading edge
of the trapezoidal half-bridge inverter output voltage
is produced by switching off the transistor Q2, while
the trailing edge of the trapezoidal half-bridge
inverter output voltage is produced by switching off
the transistor Ql. Present at the centre tap ]5 of the
differentiating element C8, R10 is a voltage which is
composed additively of the voltage drop across the
capacitor C10 and the voltage drop across the resistor
R10. This voltage is fed to the first control input j3
of the thyristor equivalent circuit via the Zener diode
D7. The error-signal monitoring unit and its
cooperation with the thyristor equivalent circuit A and
the field effect transistor Tl are described in detail
in the Laid-Open Specification EP 0 753 987.
Furthermore, the circuit arrangement illustrated in the
figure has a first device V1 and a second device V2 for
activating the switch-off device, which are connected
to the first and, respectively, to the second control
input of the thyristor equivalent circuit A. The first
device V1 comprises the series circuit of a Zener diode
D4 with a diode D5, the anode of the diode D5 being
connected to the centre tap M2 between the coupling
capacitors C2, C3 and to a terminal of the electrode
filament El of the discharge lamp LP, and the cathode
of the diode D5 being connected to the cathode of the
Zener diode D4 . The anode of the Zener diode D4 is
connected to the first control input at the base
terminal of the transistor Q4 of the thyristor
equivalent circuit A via the resistors R10, R11, the
diode D6, which is polarized in the same sense as the
diode D5, and via a further Zener diode D7, which is
polarized in the same sense as the Zener diode D4. The
second device V2 comprises the series circuit of a
diode D8 with a resistor R12. The cathode of the diode
D8 is connected to the centre tap M2 and to the same
terminal of the electrode filament El of the discharge
lamp LP as the anode of the diode D5. The anode of the
diode D8 is connected to the resistor R12, which is
connected, for its part, to the second control input at
the base terminal of the transistor Q3 of the thyristor
equivalent circuit A. In addition to the transistors
Q3, Q4, as further components the thyristor equivalent
circuit A includes the capacitors C6, C7 and the
resistors R13, R14, R15, R16. The mode of operation of
a thyristor equivalent circuit constructed from two
transistors is. described, for example, on pages 395 to
3 96 in the book entitled "Bauelemente der Elektronik
und ihre Grundschaltungen" ["Electronic Components and
their Basic Circuits"] by H. Hoger, F. Kahler, G. Weigt
from the series entitled «Einfuhrung in die Elektronik"
["introduction to Electronics"], Vol. 1, published by
H. Stam GmbH, 7th Edition.
The mode of operation of the above described circuit
arrangement is explained in more detail below for the
case of normal operation, that is to say given an
acceptably operating discharge lamp, and for the case
of the anomalous operating state, that is to say a'
discharge lamp which refuses to start, or in the case
of the occurrence of the rectifying effect of the
discharge lamp. A suitable dimensioning of the
components used is specified in the table.
Normal operation
In the case of normal operation, after the discharge
lamp or the circuit arrangement has been switched on
the DC voltage supply for the half-bridge inverter Q1,
Q2 builds up at the terminals jl, j2. The drain-source
path of the field effect transistor T1, whose gate is
connected via the resistors R19, R6, the electrode
filament El and the resistor R5 to the terminal jl,
which is at approximately + 400 V, is put into the
low-resistance state. Furthermore, the starting
capacitor C5 is charged via the resistor R5, the
electrode filament El and the resistor R6 to the
breakdown voltage of the Diac DC, which then generates
trigger pulses for the base of the bipolar transistor
Q2 and thereby causes the half-bridge inverter Q1, Q2
to build up. After the transistor Q2 has been turned
on, the starting capacitor C5 is discharged via the
resistor R9 and the diode D3 to such an extent that the
Diac DC generates no further trigger pulses. Present at
the two coupling capacitors C2, C3 in each case is half
the input voltage of the half-bridge inverter Ql, Q2,
with the result that the centre tap M2 between the
coupling capacitors C2, C3 is located at an electric
potential of approximately + 2 00 V. The two half-bridge
inverter transistors Ql, Q2 switch in an alternating
fashion, so that the centre tap Ml between the
transistors Ql, Q2 is connected alternately to the
positive pole jl (+ 400 V) and the negative pole j2
(frame potential) of the DC voltage supply of the
half-bridge inverter. As a result, a medium-frequency
alternating current whose frequency corresponds to the
switching frequency of the half-bridge inverter flows
in the load circuit between the centre taps Ml and M2.
During the switching pauses, in which both transistors
Ql, Q2 block, the load circuit current is maintained by
means of the resonant inductor L1 and flows via one of
the two freewheeling diodes D1, D2. Usually, the
electrode filaments El, E2 of the fluorescent lamp LP
have a heating current applied to them by means of a
heating device (not illustrated) before the lamp is
started, and are preheated in this way. In order to
start the gas discharge in the discharge lamp LP, the
starting voltage required for the purpose is provided
at the resonance capacitor C4 by means of the method of
resonant increase. That is to say, the switching
frequency of the half-bridge inverter is approximated
to the resonant frequency of the series resonance
circuit L1, C4. After the lamp has been started, the
resonance circuit L1, C4 is damped by the then
conductive discharge path of the discharge lamp LP. The
transistors Q3, Q4 of the thyristor equivalent circuit
A are in the blocked state during normal operation, and
the switch-off device is deactivated.
Switching off the half-bridge inverter in the case of a
discharge lamp which refuses to start
(Anomalous operating state)
Lacking the discharge lamp LP, the half-bridge inverter
Q1, Q2 cannot build up, since the connection of the
starting capacitor C5 to the voltage supply terminal j1
is led via the terminals for the electrode filament El.
A discharge lamp LP which refuses to start, or a
defective discharge lamp LP which has, for example, an
operating voltage which has increased due to ageing,
causes an increased voltage drop across the capacitor
C10. The positive voltage peaks of the synchronization
signal generated by the differentiating element C8, R10
are added at the tap j5 to the voltage of the capacitor
C10. As a result, the threshold voltage of the Zener
diode D7 is exceeded and the transistors Q3, Q4 of the
thyristor equivalent circuit A are switched into the
conductive state via the first control input j3. The
gate of the field effect transistor T1 is now connected
to the frame potential via the diode D9 and the
conductive collector-emitter path of the bipolar
transistor Q4. The drive signal is therefore extracted
from the gate of the field effect transistor T1, and
the drain-source path of the field effect transistor T1
goes over into the high-resistance or blocked state.
Consequently, the half-bridge inverter Ql, Q2 is shut
down, and cannot be restarted until the discharge lamp
LP is switched on again or exchanged, since the
thyristor equivalent circuit A is reset into the
blocked state of the normal operation again only by
interruption of the voltage supply. This switching off
of the half-bridge inverter is performed synchronously
with tne blocking phase of the transistor Q2. After the
half-bridge inverter has been switched off, the
capacitor C10 is discharged via its parallel resistor
R18.
Switching off the half-bridge inverter upon the
occurrence of the rectifying effect of the discharge
lamp (Anomalous operating state)
The switch-off device Tl, A of the half-bridge inverter
Ql, Q2 is activated either by means of the first device
VI or by means of the second device V2 upon the
occurrence of the rectifying effect in the discharge
lamp LP. As already mentioned further above, the
rectifying effect causes the discharge lamp LP to exert
a rectifying effect on the medium-frequency load
circuit current which flows between the centre taps Ml
and M2. Depending on which current direction is
preferred as a result of the rectifying effect of the
discharge lamp LP, the voltage drop across the coupling
capacitor C3 and the electric potential at the centre
tap M2 are increased or decreased. The rectifying
effect of the discharge lamp LP causes a deviation of
the voltage drop across the coupling capacitor C3 from
its normal value, which is approximately + 2 00 V. If
the deviation of the voltage drop across the coupling
capacitor C3 from its normal value has risen to
virtually 100%, the switch-off device T1, A is
activated by the first device VI or the second device
V2.
If the voltage drop across the coupling capacitor C3 is
approximately + 4 00 V, which corresponds to the entire
input voltage of the half-bridge inverter, the
threshold voltage of the Zener diode D4 of the first
device V1 is reached, and the capacitor C10 is charged
to such an extent that the voltage drop across the
capacitor C10 and the synchronization signal, added
thereto at the tap j5, of the differentiating element
C8, R10 reach the threshold, voltage of the Zener diode
D7, and the transistors Q3, Q4 of the thyristor
equivalent circuit A are switched into the conductive
state via the first control input j3. As a result, the
drain-source path of the field effect transistor Tl is
blocked, and the half-bridge inverter Q1, Q2 is shut
down synchronously with the blocking phase of the
transistor Q2.
If the voltage drop across the coupling capacitor C3 is
very slight and the centre tap M2 is therefore
virtually at frame potential, the pnp transistor Q3 of
the thyristor equivalent circuit A is firstly turned on
via the second control input j4, which is connected to
the centre tap M2 by the series circuit D8, R12 of the
second device V2, and subsequently the npn transistor
Q4 is switched into the conductive state as well. Once
again, the gate drive signal is extracted thereby from
the field effect transistor Tl, with the result that
the drain-source path thereof goes over into the
blocked state and the half-bridge inverter Ql, Q2 is
shut down. The thyristor equivalent circuit A is not
reset into the blocking state, nor is the switch-off
device deactivated until there is an interruption in
the current such as is caused, for example, by
exchanging the discharge lamp LP or by renewed
switching on.
A second preferred exemplary embodiment of the
invention is illustrated in Figure 2. This second
exemplary embodiment differs from the first exemplary
embodiment described in more detail above only in
additional components R21, D13 and D14. In the
remaining components, the second exemplary embodiment
corresponds to the first exemplary embodiment. For this
reason, identical reference symbols have also been
selected for identical components in Figures 1 and 2.
The emitter of the transistor Q3 is connected to the
voltage supply terminal jl via the resistor R21. In the
case of an anomalous operating state, an additional
holding current is provided for the thyristor
equivalent circuit A with the aid of this resistor R21.
The resistor R21 is dimensioned such that the thyristor
equivalent circuit A receives approximately 50 to
80 percent of its holding current via the resistor R21.
The remainder of the holding current component, which
is required to maintain the switched-on state of the
thyristor equivalent circuit A, is provided via the
resistor R5, the electrode filament El of the
low-pressure discharge lamp LP, the resistor R6, and
via the diode D14 polarized in the forward direction.
Permanent switching on of the thyristor equivalent
circuit A is ensured by the additional holding current
flowing via the resistor R21, even in the case when the
potential at the nodal point M2 - caused by the
occurrence of the rectifying effect - is virtually at
frame potential. The diode D14 serves to decouple the
Diac DC from the thyristor equivalent circuit A. The
anode of the diode D14 is connected to a nodal point
arranged between the Diac DC and the resistor R6, while
the cathode of the diode D14 is connected to the
emitter of the transistor Q3. The additional Zener
diode D13 protects the thyristor equivalent circuit A
against overvoltages. For this purpose, the anode of
the Zener diode D13 is connected to the voltage supply
terminal j2, and its cathode is connected to the
emitter of the transistor Q3. The mode of operation of
this circuit arrangement corresponds to that of the
first exemplary embodiment. Upon the occurrence of an
anomalous operating state, the thyristor equivalent
circuit A is reset to the blocking state by exchanging
the lamp LP, since the DC connection to the resistor R6
in the case of the electrode filament El is interrupted
by taking out the lamp LP, and the holding current
still remaining, and flowing via the resistor R21 is
insufficient to leave the thyristor equivalent circuit
A in the switched on state.
The invention is not limited to the exemplary
embodiments described in more detail above. For
example, the invention can also be applied to
half-bridge inverters which have only one coupling
capacitor. Furthermore, the . invention can be used not
only in the case of self-oscillating half-bridge
inverters, but also in the case of externally
controlled half-bridge inverters. Furthermore, the
upper limiting value and the lower limiting value of
the voltage drop across the coupling capacitor C3, at
which the switch-off device is activated, can be set to
other values by suitable dimensioning of the
components.
Table: Dimensioning of the components illustrated in
the figures in accordance with the preferred
exemplary embodiments
Table: Dimensioning of the components illustrated in
the figures in accordance with the preferred
exemplary embodiments (continuation)
We Claim
1. An improved circuit device for operating at least one discharge lamp,
comprising:
- a half-bridge inverter (Ql, Q2)havtng a downstream load circuit
(N1, L1, C4);
- at least one coupling capacitor (C3) connected to the load circuit
(N1, L1, C4) and to the half-bridge inverter (Ql, Q2);
- a switch-off device (Tl, A) for switching off the half-bridge inverter
(Q1, Q2) on occurrence of an anomalous operating state of the
discharge lamp;
- the load circuit (N1, L1, C4) having terminals for the at least one
discharge lamp (LP),
characterized in that means (V1, V2) are disposed for monitoring the
voltage drop across the at least one coupling capacitor (C3) and for
activating the switch-off device (T1, A) as a function of the voltage
drop detected across the at least one coupling capacitor (C3)
2. The circuit device as claimed in claim 1, wherein the means (V1, V2)
comprises a first series circuit (vl) for activating the switch-off device (tl,
A) upon a predetermined upper limiting value of the voltage drop across
the at least one coupling capacitor (C3) being reached; and a second
series circuit (V2) for activating the switch-off device (T1, A) upon a
predetermined lower limiting value of the voltage drop across the at least
one coupling capacitor (C3) being reached.
3. The circuit device as claimed in claim 1, wherein two coupling capacitors
(C2, C3) having a centre tap (M2) in-between are provided, and wherein
the load circuit (N1, L1, C4) is connected to the centre tap (M2) between
the coupling capacitors (C2, C3).
4. The circuit device as claimed in claim 2, wherein
- the switch-off device (Tl, A) has at least one control input (j3), and
wherein the first (V1) and the second device (V2) have at least one
electric component (D4) with a nonlinear current-voltage
characteristic, which is connected to the at least one coupling
capacitor (C3) and to the at least one control input (j3) of the
switch-off device (T1, A).
5. The circuit device as claimed in claim 2, wherein the upper limiting value
is greater than or equal to 75 percent of the input voltage or supply
voltage of the half-bridge inverter (Q1, Q2).
6. The circuit device as claimed in claim 2, wherein the lower limiting value is
less than or equal to 25 percent of the input voltage or supply voltage of
the half-bridge inverter (Q1, Q2).
7. The circuit device as claimed in claim 4, wherein the switch-off device (T1,
A) has at least two control inputs (j3, J4), and wherein the first series
circuit (V1) serving to drive the first control input (j3), and the second
series circuit (V2) serving to control the second control input (j4).
8. The circuit device as claimed in claim 4, wherein the at least one electric
component (D4) with a nonlinear current-voltage characteristic is a
component selected from the group of a diode, Zener diode, suppressor
diode and varistor.
9. The circuit device as claimed in claim 4, wherein at least one diode (D5) is
connected in series with the at least one electric component (D4) with a
nonlinear current-voltage characteristic.
10. The circuit device as claimed in claim 7, wherein the switch-off device (T1,
A) has a bistable switching means (A).
11. The circuit device as claimed in claims 7 and 9, wherein
- the first series circuit (V1) comprises an: electric component (D4'
with a nonlinear current-voltage characteristic, and a diode (D5)
connected in series therewith, the anode of the diode (D5) being
connected to a terminal for the at least one discharge lamp (LP)
and to the at least one coupling capacitor (C3), and the cathode of
the diode (D5) being connected to the electric component (D4)
with a nonlinear current voltage characteristic, wherein
- the electric component (D4) with a nonlinear current-voltage
characteristic of the first series circuit (V1) is connected to the first
control input (j3) of the switch-off device (Tl, A), and wherein
the second switch circuit (V2) comprises at least one diode (D8)
with at least one resistor (R12), and being connected to the at least
one coupling capacitor (C3), a terminal for the at least one
discharge lamp (LP) and to the second control input (j4) of the
switch-off device (T1, A).
12. The circuit device as claimed in claim 7, wherein a control input (j3) of the
switch-off device (T1, A) is connected in parallel in terms of alternating
current with the at least one discharge lamp (LP) and monitors) the
voltage drop across the terminals for the at least one discharge lamp (LP).
13. The circuit device as claimed in claim 10, wherein the bistable switching
means (A) is a thyristor equivalent circuit (Q3, Q4).
It is the object of the invention to provide a circuit arrangement for operating at
least one discharge lamp with an improved switch-off device which does not
have the disadvantages of the prior art. In particular, the aim is for the switch-
off device to detect the occurrence of the rectifying effect of the at least one
discharge lamp and to switch off the half-bridge inverter permanently in this
case.
This object is achieved according to the features of the invention.

Documents:

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Patent Number

225000

Indian Patent Application Number

IN/PCT/1999/0119/KOL

PG Journal Number

44/2008

Publication Date

31-Oct-2008

Grant Date

29-Oct-2008

Date of Filing

14-Dec-1999

Name of Patentee

PATENT-TREUHAND-GESELLSCHAFT FUR ELEKTRISCHE GLUHLAMPEN MBH

Applicant Address

HELLABRUNNER STR.1, D - 81536 MUNCHEN

Inventors:

#	Inventor's Name	Inventor's Address
1	SCHEMMEL, BERNHARD	VEILCHENWEG 11, D-82234 WESSLING
2	RUDOLPH, BERND	KARL-WITTHALM-STR.21, D-81375 MUNCHEN
3	WEIRICH, MICHAEL	RATHAUSSTR.42, D-82008 UNTERHACHING

PCT International Classification Number

HO5B 41/28

PCT International Application Number

PCT/DE99/01011

PCT International Filing date

1999-04-01

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	198 19 027.1	1998-04-29	Germany