EP1424881B1 - Method and device for driving a fluorescent lamp - Google Patents

Abstract

The method involves monitoring for the rectifier effect in at least one lamp (FL1, 2) to detect the end of its operating life. The rectifier effect is monitored for by evaluating the d.c. voltage drop across the electrical connections of the at least one discharge lamp, the electrical power fed into an associated d.c./a.c. converter (T1,T2) or a proportional first parameter and a second parameter correlated with the lamp's burning voltage. AN Independent claim is also included for the following: (a) an operating device for at least one low pressure discharge lamp.

Description

The invention relates to a method for operating at least one low-pressure discharge lamp on an inverter according to the preamble of patent claim 1 and an operating device for at least one low-pressure discharge lamp according to claim 12.

I. State of the art

Such an operating method is disclosed, for example, in International Patent Application Publication No. WO 99/56506. This document describes the operation of a low-pressure discharge lamp on a circuit arrangement which has a half-bridge inverter with a load circuit connected thereto, in which the connections for the lamp are arranged. In order to detect the occurrence of the rectifier effect in the low-pressure discharge lamp, the voltage drop across the half-bridge capacitor is monitored, and when a predetermined upper limit value is exceeded or a predetermined lower limit value is exceeded, a shutdown device for the half-bridge inverter is activated.

The document DE 197 08 792 shows a method for operating at least one low-pressure discharge lamp according to the preamble of claim 1.

II. Presentation of the invention

It is the object of the invention to provide an operating method for at least one low-pressure discharge lamp, which enables a more reliable detection of the rectifier effect in the at least one low-pressure discharge lamp and in particular avoids disconnections of the operating device due to erroneous detection of the rectifier effect. In addition, it is the object of the invention to provide an operating device for at least one low-pressure discharge lamp for carrying out this method.

This object is achieved by the features of claim 1 and 12 respectively. Particularly advantageous embodiments of the invention are described in the dependent claims.

The method according to the invention for operating at least one low-pressure discharge lamp on an inverter is characterized in that for monitoring the occurrence of the rectifier effect in the at least one low-pressure discharge lamp, the DC voltage drop across the electrical terminals of the at least one low-pressure discharge lamp, the electric power fed into the inverter or one thereto proportional first size and a correlated with the burning voltage of the at least one low-pressure discharge lamp second variable are evaluated to define a criterion for the presence of the rectifier effect in the at least one low-pressure discharge lamp and thus a criterion for reaching the end of life of the at least one low-pressure discharge lamp. By monitoring and evaluating the aforementioned three variables, the occurrence of the rectifier effect can be determined with sufficient accuracy regardless of the lamp used and the current dimming position. The inventive method increases the reliability of the system consisting of the at least one low-pressure discharge lamp and the operating device, since the tolerance range for determining the end of life of the at least one low-pressure discharge lamp by means of the three aforementioned sizes can be specified more precisely and in this way a shutdown of the operating device due to a faulty Detection of the rectifier effect is avoided.

The second variable correlated with the burning voltage of the at least one low-pressure discharge lamp is advantageously the effective value of the alternating voltage component of the burning voltage of the at least one low-pressure discharge lamp. Instead, however, this second variable may also be a constant value which corresponds to the average value of the burning voltage characteristic of the lamp type of the at least one low-pressure discharge lamp. For a T5 fluorescent lamp with a power consumption of 80 watts is the aforementioned Mean value, for example, 145 V and for a T5 fluorescent lamp with a power consumption of 54 watts, the aforementioned mean, for example, 118 V. Instead of the fed into the inverter electrical power and a proportional size can be evaluated. In particular, the active component of the current flowing through the inverter is suitable for this purpose. Since the inverter is usually supplied with an approximately constant DC voltage, the active component of the current flowing through the inverter is proportional to the electrical power fed into the inverter. In order to determine the effective component of the aforementioned current, the voltage drop across a resistor is preferably evaluated, which is traversed by the entire current of the inverter during a switching phase of the inverter. This voltage drop is also proportional to the electrical power supplied to the inverter.

For evaluating the above-mentioned variables, the product of the electrical power fed into the inverter and the quotient of the DC voltage drop across the electrical connections of the at least one low-pressure discharge lamp and the second variable correlated with the burning voltage of the at least one low-pressure discharge lamp are advantageously compared with a predetermined power value this product directly provides a measure of the asymmetry of the emission behavior of the lamp electrodes from the electrical power fed into the inverter and the quotient of the aforesaid voltages, and the result provides an electric power value that can be directly compared with the maximum allowed value in the inverter Supplement to standard IEC 61347-2-3 "Particular requirements for supplied electronic ballasts for fluorescent lamps" under Test 2 "Asymmetric Power Dissipation". This maximum value is 7.5 watts for T5 lamps and 5.0 watts for T4 lamps.

In order to avoid a division in the evaluation of the aforementioned variables, preferably the product of a predetermined power value and correlated with the burning voltage of the at least one low-pressure discharge lamp second size with the product of the fed into the inverter electrical Performance and the DC voltage drop across the electrical connections of the at least one low-pressure discharge lamp compared. As a given power value, the maximum allowable value of test 2 "Asymmetric Power Dissipation" cited above from the supplement to the standard IEC 61347-2-3 is used. Then this comparison is equivalent to the comparison described in the preceding paragraph.

The comparison is continuously repeated throughout the lamp operation with updated values of the three aforementioned quantities to avoid overheating of the lamp electrodes in the event of the occurrence of the rectifier effect. In order to enable a reliable detection of the rectifier effect and thus does not lead to a random, exceeding the permissible maximum value for switching off the at least one low-pressure discharge lamp, a counting operation is advantageously carried out depending on the result of the comparison and in the case of a counter overflow a status bit set or reset. The status of the status bit is thus an indicator of whether the at least one low-pressure discharge lamp has already reached its end of life.

The evaluation is advantageously carried out with the aid of a microcontroller in which a corresponding program for carrying out the comparisons has been implemented. The microcontroller can additionally take over the control of the driver circuits for the transistor switches of the inverter.

The electrical power supplied to the inverter is advantageously derived from the voltage drop across a voltage divider disposed in parallel with the input of the inverter and from the voltage drop across a resistor connected in series to an inverter transistor during a switching phase of the inverter and during the current through which at least one low-pressure discharge lamp flows is determined. The voltage drop across the aforementioned resistor can be used in addition to the brightness control of the at least one low-pressure discharge lamp. The same measured values can therefore be used for brightness control, for example with the aid of a microcontroller as well as for the detection of the end of life of the at least one low-pressure discharge lamp are evaluated.

• einen Halbbrückenwechselrichter, an den ein Lastkreis angeschlossen ist, in dem elektrische Anschlüsse für mindestens eine Niederdruckentladungslampe und mindestens ein Halbbrückenkondensator angeordnet sind,
• eine erste Messvorrichtung zur Messung einer ersten Spannung, die proportional zur in den Halbbrückenwechselrichter eingekoppelten elektrischen Leistung ist,
• eine zweite Messvorrichtung zur Messung einer zweiten Spannung, die proportional zu dem Spannungsabfall an dem mindesten einen Halbbrückenkondensator ist,
• eine dritte Messvorrichtung zur Messung einer dritten Spannung, die proportional zu dem Effektivwert der Brennspannung der mindestens einen Niederdruckentladungslampe ist,
• eine vierte Messvorrichtung zur Messung einer vierten Spannung, die proportional zur Versorgungsspannung des Halbbrückenwechselrichters ist,
• eine Auswertungseinheit, die mit den Ausgängen der Messvorrichtungen verbunden ist und einen programmgesteuert arbeitenden Mikrocontroller umfasst, und die zur Auswertung der ersten bis vierten Spannung sowie zur Steuerung des Halbbrückenwechselrichters in Abhängigkeit von dem Ergebnis der Auswertung dient.

The operating device according to the invention for at least one low-pressure discharge lamp has the following features:

• a half-bridge inverter, to which a load circuit is connected, in which electrical connections for at least one low-pressure discharge lamp and at least one half-bridge capacitor are arranged,
• a first measuring device for measuring a first voltage that is proportional to the electrical power coupled into the half-bridge inverter,
• a second measuring device for measuring a second voltage that is proportional to the voltage drop across the at least one half-bridge capacitor,
• a third measuring device for measuring a third voltage which is proportional to the effective value of the operating voltage of the at least one low-pressure discharge lamp,
• a fourth measuring device for measuring a fourth voltage, which is proportional to the supply voltage of the half-bridge inverter,
• an evaluation unit, which is connected to the outputs of the measuring devices and includes a programmatically operating microcontroller, and which is used to evaluate the first to fourth voltage and for controlling the half-bridge inverter in dependence on the result of the evaluation.

The operating device described above enables the implementation of the operating method according to the invention.

III. Description of the Preferred Embodiment

Figur 1

Eine Schaltskizze der Schaltungsanordnung des erfindungsgemäßen Betriebsgerätes zur Durchführung des erfindungsgemäßen Betriebsverfahrens in schematischer Darstellung

Figur 2

Ein Flussdiagramm des erfindungsgemäßen Betriebsverfahrens

The invention will be explained in more detail below with reference to a preferred embodiment. Show it:

FIG. 1

A circuit diagram of the circuit arrangement of the operating device according to the invention for carrying out the operating method according to the invention in a schematic representation

FIG. 2

A flow chart of the operating method according to the invention

The inventive operating device schematically illustrated in FIG. 1 is an electronic ballast for operating two parallel-connected low-pressure discharge lamps, in particular T5 fluorescent lamps FL1, FL2. This ballast in particular also allows brightness regulation of the fluorescent lamps FL1, FL2.

The ballast has two mains voltage terminals 1, 2, a downstream mains voltage rectifier GL, which also includes a filter circuit and optionally a boost converter and at the voltage output, the supply voltage for the downstream half-bridge inverter is provided. The half-bridge inverter has two half-bridge transistors T1, T2, at whose center tap M a load circuit designed as a series resonant circuit is connected, which comprises the resonance inductor L1 and the resonant capacitor C1. Parallel to the resonance capacitor C1, a parallel circuit consisting of two fluorescent lamps FL1, FL2 is arranged. This parallel circuit has two half-bridge capacitors C2, C3, which are each arranged in series with one of the fluorescent lamps FL1 and FL2. In addition, in each branch of the parallel circuit, a winding N1 or N2 of a balancing transformer L2 is connected, which serves to balance the lamp currents in the two branches. The high-potential terminal A2 of the first half-bridge capacitor C2 is connected through the winding N2 of the transformer L2, the electrode E2 of the first fluorescent lamp FL1 and the resistor R1 to the positive DC voltage output of the mains voltage rectifier GL. Similarly, the high potential terminal A3 of the second half-bridge capacitor C3 is connected across the winding N1 of the transformer L2, the electrode E4 of the second fluorescent lamp FL2 and the resistor R2 to the positive DC output of the mains voltage rectifier GL. The low potential Terminals of the half-bridge capacitors C2, C3 are respectively connected to the negative DC voltage output of the mains voltage rectifier GL and the ground potential. The terminal A1 of the resonance capacitor C1 is connected to the electrode E1 of the first fluorescent lamp FL1 and the electrode E3 of the second fluorescent lamp and connected via the resonance inductor L1 to the center tap M of the half-bridge inverter. The other terminal of the resonant capacitor C1 is connected to the negative DC voltage output of the mains voltage rectifier GL and the ground potential. In addition, the terminal A1 is connected via the electrode E1 and the resistor R3 to the positive DC voltage output of the mains voltage rectifier GL. The heating device H shown only schematically in FIG. 1 is inductively coupled to all the electrodes E1, E2, E3, E4 of the two fluorescent lamps FL1, FL2 and serves to heat the lamp electrodes before the gas discharge is ignited or else during the dimming operation of the lamps. Details of this heating device H are described, for example, in the published patent application EP 0 748 146 A1. The resistors R0, R1, R2 and R3 serve to adjust the potentials at the taps A1, A2 and A3. In particular, by means of the abovementioned resistors, the corresponding electrical voltages can build up on the capacitors C1, C2 and C3 immediately after switching on the operating device and before igniting the gas discharge in the lamps FL1, FL2.

The control of the half-bridge transistors T1, T2 takes place with the aid of the programmatically operating microcontroller MC and the driver circuits TR for the transistors T1, T2. By alternately switching the transistors T1, T2, the center tap M is alternately connected to the negative and the positive DC output of the mains voltage rectifier GL. Since the half-bridge capacitors C2, C3 are charged to half the supply voltage of the half-bridge inverter flows during lamp operation between the taps M and A2 and A3, a high-frequency alternating current whose frequency is determined by the switching clock of the transistors T1, T2. For igniting the gas discharge in the fluorescent lamps FL1, FL2, the switching clock of the half-bridge transistors T1, T2 is changed such that the frequency of the alternating current in the load circuit is close to the resonance frequency of the series resonant circuit L1, C1. As a result, a sufficiently high voltage is generated at the resonance capacitor C1 in order to ignite the gas discharge in the fluorescent lamps FL1, FL2. After igniting the gas discharge in the fluorescent lamps FL1, FL2, the series resonant circuit L1, C1 is attenuated by the parallel connection of the fluorescent lamps FL1, FL2. The brightness control of the fluorescent lamps FL1, FL2 also takes place by changing the frequency of the alternating current in the load circuit and in the parallel connection of the fluorescent lamps FL1, FL2. To regulate the brightness or to regulate the power of the fluorescent lamps FL1, FL2, the resistor R4 is arranged in series with the half-bridge transistor T2 so that its connection A4 can be connected to the center tap M via the switching path of the transistor T2 and its other connection to the ground potential and to the ground terminal negative DC voltage output of the mains voltage rectifier GL is connected. Thus, while the half-bridge transistor T2 is conductive, the entire current of the load circuit and the parallel connection of the fluorescent lamps FL1, FL2 flows through the resistor R4. At the terminal A4, the voltage drop across the resistor R4 is measured with the aid of the low-pass filter R5, C4 connected thereto. The voltage drop U 1 at the center tap A5 of the low-pass filter R5, C4, which is proportional to the active component of the current through the half-bridge inverter transistor T2, is supplied to the corresponding terminal A5 of the microcontroller MC for evaluation and in particular also for the brightness control of the fluorescent lamps FL1, FL2. Parallel to the DC voltage output of the mains voltage rectifier GL, the voltage divider R6, R7 is arranged with the capacitor C5 connected in parallel with the resistor R7. At the tap A6 between the resistors R6, R7, which is connected to the corresponding terminal A6 of the microcontroller MC, the voltage U2 is measured, which is proportional to the supply voltage of the half-bridge inverter. Parallel to the half-bridge capacitor C3, the voltage divider R8, R9 is arranged with the parallel to the resistor R9 connected capacitor C6. At the tap A7 between the resistors R8, R9, which is connected to the corresponding terminal A7 of the microcontroller MC, the voltage U3 is measured, which is proportional to the voltage drop across the half-bridge capacitor C3. Similarly, parallel to the half-bridge capacitor C2 of Voltage divider R10, R11 arranged with the parallel to the resistor R11 connected capacitor C7. At the tap A8 between the resistors R10, R11, which is connected to the corresponding terminal A8 of the microcontroller MC, the voltage U4 is measured, which is proportional to the voltage drop across the half-bridge capacitor C2. The terminal A1 of the resonant capacitor C1 is connected to the ground potential via the capacitor C8, the resistor R12 and the reverse-biased diode D1. A tap between the resistor R 12 and the cathode of the diode D1 is connected to the ground potential via the forward-biased diode D2 and the resistor R13. Parallel to the resistor R13, the capacitor C9 is connected. The connected to the cathode of the diode D2 terminal A9 of the resistor R13 is connected to the corresponding terminal A9 of the microcontroller MC. At the terminal A9, the voltage U5 is measured, which is to a good approximation proportional to the rms value of the AC voltage component of the burning voltage of the parallel-connected fluorescent lamps FL1, FL2.

The voltage applied to the terminals A5, A6, A7, A8 and A9 voltages U1 to U5 are converted by analog-to-digital converters into digital values and evaluated by the microcontroller MC using a program implemented in the microcontroller, via the driver circuit TR by a corresponding control of the half-bridge transistors T1, T2 to ensure a brightness control of the fluorescent lamps FL1, FL2 and a detection of the end of life of the lamps FL1, FL2. The end of life of the lamps FL1, FL2 is detected by monitoring the occurrence of the rectifier effect in the fluorescent lamps FL1 FL2. For this purpose, the DC voltage _drop U _dc1 or U _dc2 via the electrical connections of the fluorescent lamps FL1, FL2 and the rms value U _{ac of} the AC voltage _component of the _burning voltage of the parallel-connected fluorescent lamps FL1, FL2 by means of the microcontroller MC in the half-bridge _inverter evaluated. The electric power P fed to the half-bridge inverter is proportional to the product of the voltages applied to the terminals A5 and A6. It is calculated from the voltages U1 and U2 to: $$P=\mathrm{<figure-callout\; id="1"\; label="U"\; filenames="imgf0002.png"\; state="\{\{state\}\}">U</figure-callout>}1\cdot \mathrm{<figure-callout\; id="2"\; label="U"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">U</figure-callout>}2\cdot \frac{R6+R7}{R4\cdot R7}$$

The DC voltage _drop U _dc1 across the electrical terminals of the fluorescent lamp FL1 is calculated from the difference of half the supply voltage of the half-bridge inverter and the voltage drop across the half-bridge capacitor C2 and can therefore be determined from the voltages U2 and U4. $$U_{dc1}=\frac{1}{2}\cdot \mathrm{<figure-callout\; id="2"\; label="U"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">U</figure-callout>}2\cdot \frac{R6+R7}{R7}-U4\cdot \frac{\mathrm{<figure-callout\; id="10"\; label="R"\; filenames="imgf0001.png"\; state="\{\{state\}\}">R</figure-callout>}10+R11}{R11}$$

Analogously, the DC voltage _drop U _dc2 across the electrical connections of the fluorescent lamp FL2 is calculated from the difference of half the supply voltage of the half-bridge inverter and the voltage drop across the half-bridge capacitor C3 and can therefore be determined from the voltages U2 and U3. $$U_{dc2}=\frac{1}{2}\cdot \mathrm{<figure-callout\; id="2"\; label="U"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">U</figure-callout>}2\cdot \frac{R6+R7}{R7}-U3\cdot \frac{R\mathrm{8th}+R9}{R9}$$

The effective value U _{ac of} the alternating voltage component of the burning voltage of the parallel-connected fluorescent lamps FL1, FL2 is calculated from the voltage U5 measured at the terminal A9 with sufficient accuracy to: $$U_{ac}=2\cdot k\cdot U5\cdot \frac{R12+R13}{R13}$$

berechnet werden. Die Werte der Leistungen P1 bzw. P2 können unmittelbar mit dem in dem "Test 2: Asymmetric Power Dissipation" der Ergänzung zu der Norm IEC 61347-2-3 aufgeführten maximal zulässigen Grenzwert P_max von 7,5 Watt für die Lampenleistung bei T5-Lampen verglichen werden, um das Ende der Lebensdauer der beiden Leuchtstofflampen FL1, FL2 zu überwachen.The constant k is the form factor of the voltage U5. For a rectangular voltage it has the value 1 and for a sinusoidal voltage it has the value 1.11. From the above variables P, U _ac , and U _dc1 , or U _dc2 can for both fluorescent lamps FL1 and FL2, the power P1 or P2 by means of the formula $$P$$$$1=P\cdot \frac{\left|U_{dc1}\right|}{U_{ac}}\mathrm{respectively},\mathrm{<figure-callout\; id="2"\; label="P"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">P</figure-callout>}2=P\cdot \frac{\left|U_{dc2}\right|}{U_{ac}}$$

be calculated. The values of powers P1 and P2 can be directly compared with the maximum permissible limit P _max of 7.5 watts for the lamp power at T5- in the "Test 2: Asymmetric Power Dissipation" of the supplement to the standard IEC 61347-2-3. Lamps are compared to monitor the end of the life of the two fluorescent lamps FL1, FL2.

erfüllt ist.So that in the microcontroller MC no division must be performed, is cyclically checked to monitor the end of life of the fluorescent lamps FL1, FL2 during lamp operation, whether the condition $$P\cdot \left|U_{dc1}\right|<P_{\mathrm{Max}}\cdot U_{ac}\mathrm{respectively},P\cdot \left|U_{dc2}\right|<P_{\mathrm{Max}}\cdot U_{ac}$$

is satisfied.

The method for monitoring the end of life of the two T5 fluorescent lamps FL1, FL2 will now be described in more detail with reference to the flowchart shown in FIG.

At the beginning of the cyclical process, the electrical power consumption P of the half-bridge inverter is determined by means of the program implemented in the microcontroller MC from the measured values U1 and U2 updated during each cycle of the method according to the formula (1). Subsequently, the product P _{max.U ac is} calculated from the measured value U5 likewise updated during each cycle of the method according to the formula (4). Thereafter, the status of the status bit S0 is checked, indicating whether the lamp FL1 has been checked during the last cycle performed, in which case to proceed with the test of the lamp FL2. If the status bit S0 is not set, that is, the lamp FL1 was not checked during the last cycle, the status bit S0 is set and then, according to the formula (2), from the measured values of the voltages U2 also updated during each cycle of the method and U4 determines the DC voltage component across the terminals of the lamp FL1 and according to the formula (6a) the product of the amount of this DC voltage _component U _dc1 and the power consumption P of the half-bridge _inverter formed. After that it is checked whether the condition (6a) is satisfied, that is, whether the value of the product P · IU _dc1 | is less than the value of the product P _max · U _ac .

If this condition (6a) is not met, the count Z1 of a first counter is increased by the value of one. Thereafter, it is checked whether the count Z1 of the first counter has the value zero and thus a counter overflow, which occurs at the value 256, has taken place. If so, the status bit S1 is set, which indicates the end of the life of the lamp FL1, and the current cycle of the method is ended. If the counter reading Z1 of the first counter is greater than zero, the current counter reading Z1 is stored and the current cycle is exited.

If the condition (6a) is satisfied, it is checked whether the count Z1 is zero and leave the current cycle in this case. If the count Z1 was greater than zero, the count Z1 is decreased by one and then checked again if it is still greater than zero. If the counter reading is now equal to zero, the status bit S1, which indicates the occurrence of the end of life of the lamp FL1, is deleted or reset and the counter reading Z1 is stored. Otherwise, only the counter Z1 is stored. Then in both cases the current cycle is left.

Completely analogous to this is the monitoring of the other fluorescent lamp FL2. If the fluorescent lamp FL1 was checked during the last cycle of the monitoring process, the status bit S0 is set and the program or algorithm branches into the branch for monitoring the lamp FL2, as shown in the flowchart of FIG.

If the status bit S0 is set, that is, the lamp FL1 was checked during the last cycle, the status bit S0 is reset and then, according to the formula (2), from the measured values of the voltages U2 and U3 also updated during each cycle of the method determines the DC voltage component across the terminals of the lamp FL2 and according to the formula (6b), the product of the amount of this DC voltage _component U _dc2 and the power consumption P of the half-bridge _inverter formed. Thereafter, it is checked if the condition (6b), that is, whether the value of the product P · | U _dc2 | is less than the value of the product P _max · U _ac .

If this condition (6b) is not met, the counter reading Z2 of a second counter is increased by the value 1. Thereafter, it is checked whether the count Z2 of the second counter has the value zero and thus a counter overflow, which occurs at the value 256, has taken place. If so, the status bit S2 is set, which indicates the end of the life of the lamp FL2, and the current cycle of the method ends. If the count Z2 of the second counter is greater than zero, the current count Z2 is stored and leave the current cycle.

If the condition (6b) is satisfied, it is checked whether the count Z2 is zero and leave the current cycle in this case. If the count Z2 was greater than zero, then the count Z2 is decreased by one and then checked again if it is still greater than zero. If the counter reading is now equal to zero, the status bit S2, which indicates the occurrence of the end of life of the lamp FL2, is deleted or reset and the counter reading Z2 is stored. Otherwise, only the counter Z2 is stored. Then in both cases the current cycle is left.

If the status bit S1 or the status bit S2 lingers in the set state for more than a predefined period of time, that is to say for a predetermined number of successive monitoring cycles, for example, then the operating device is switched off.

The invention is not limited to the embodiment explained in more detail above. For example, the lamps FL1, FL2 may be interrogated instead of alternately also in the same cycle. Furthermore, the counter readings Z1, Z2 can be increased or decreased by a value greater than 1 if the permissible limit value is greatly exceeded or fallen short of. In addition, other evaluation methods can be used. For example, instead of the conditions (6a, 6b), the difference P · | U _dc1 | - P _max · U _ac or P · | U _dc2 | - P _max · U _ac for both lamps FL1, FL2 are formed and evaluated. In particular, the values of the aforementioned Difference at different times during lamp operation are added by means of integrators to monitor the exceeding or falling below predetermined upper or lower limits. Instead of switching off the operating device or the lamps FL1, FL2 when the permissible maximum limit value is exceeded, it is also possible to operate the lamps FL1, FL2 with considerably reduced power until the permissible limit value is again permanently undershot.

Claims (12)

1. Method for operating at least one low-pressure discharge lamp using an inverter (T1, T2), the occurrence of a rectifier effect in the at least one low-pressure discharge lamp (FL1, FL2) being monitored during the operation of the at least one low-pressure discharge lamp (FL1, FL2) in order to determine the end of its life, characterized in that, for the purpose of monitoring the rectifier effect of the at least one low-pressure discharge lamp (FL1, FL2), the d.c. voltage drop (U_dc1, U_dc2) across the electric connections of the at least one low-pressure discharge lamp (FL1, FL2), the electric power (P) fed into the inverter (T1, T2), or a first variable which is proportional thereto, and a second variable (U_ac) correlated with the running voltage of the at least one low-pressure discharge lamp (FL1, FL2) are evaluated.
2. Method according to Claim 1, characterized in that the second variable (U_ac) correlated with the running voltage of the at least one low-pressure discharge lamp (FL1, FL2) is the r.m.s. value of the a.c. voltage component of the running voltage of the at least one low-pressure discharge lamp (FL1, FL2).
3. Method according to Claim 1, characterized in that the second variable (U_ac) correlated with the running voltage of the at least one low-pressure discharge lamp (FL1, FL2) is a constant value which corresponds to the average value of the running voltage which is characteristic of the lamp type of the at least one low-pressure discharge lamp (FL1, FL2).
4. Method according to one of Claims 1 to 3, characterized in that the product of the electric power (P) fed into the inverter (T1, T2) and the quotient of the d.c. voltage drop (U_dc1, U_dc2) across the electric connections of the at least one low-pressure discharge lamp (FL1, FL2) and the second variable (U_ac) correlated with the running voltage of the at least one low-pressure discharge lamp (FL1, FL2) is compared with a predetermined power value (P_max).
5. Method according to one of Claims 1 to 3, characterized in that the product of a predetermined power value (P_max) and the second variable (U_ac) correlated with the running voltage of the at least one low-pressure discharge lamp (FL1, FL2) is compared with the product of the electric power (P) fed into the inverter and the d.c. voltage drop (U_dc1, U_dc2) across the electric connections of the at least one low-pressure discharge lamp (FL1, FL2).
6. Method according to one or more of Claims 1 to 5, characterized in that the electric power (P) fed into the inverter (T1, T2), the d.c. voltage drop (U_dc1, U_dc2) across the electric connections of the at least one low-pressure discharge lamp (FL1, FL2) and the r.m.s. value (U_ac) of the a.c. voltage component of the running voltage of the at least one low-pressure discharge lamp (FL1, FL2) are determined from measured values which are fed to a microcontroller (MC), and a program-controlled evaluation is carried out by the microcontroller (MC).
7. Method according to Claim 4 or 5, characterized in that the comparison is cyclically repeated during the lamp operation.
8. Method according to Claim 7, characterized in that a counter operation (Z1, Z2) is performed as a function of the result of the comparison and a status bit (S1, S2) is set or reset in the event of the counter overflowing.
9. Method according to one of Claims 1 to 3, characterized in that the values, which are determined at different points in time in the lamp operation, for the difference between the product of the electric power (P) fed into the inverter and of the d.c. voltage drop (U_dc1, U_dc2) across the electric connections of the at least one low-pressure discharge lamp (FL1, FL2) and the product of a predetermined power value (P_max) and of the second variable (U_ac) correlated with the running voltage of the at least one low-pressure discharge lamp (FL1, FL2) are added up and evaluated.
10. Method according to Claim 1, characterized in that the electric power (P) fed into the inverter (T1, T2) is determined from the voltage drop (U2) across a voltage divider (R6, R7) which is arranged in parallel with the input of the inverter (T1, T2), and from the voltage drop (U1) across a resistor (R4) which is connected in series with an inverter transistor (T2) during a switching phase of the inverter (T1, T2) and which at the same time has all of the current of the inverter (T1, T2) flowing through it.
11. Method according to Claim 1, characterized in that the inverter (T1, T2) is supplied with an approximately constant d.c. voltage (U2), and the first variable is the voltage drop (U1) across a resistor (R4) which, during a switching phase of the inverter (T1, T2), has all of the current of the inverter (T1, T2) flowing through it.
12. Operating device for at least one low-pressure discharge lamp (FL1, FL2) having

    - a half-bridge inverter (T1, T2), to which is connected a load circuit (L1, C1) in which electric connections for at least one low-pressure discharge lamp (FL1, FL2) and at least one half-bridge capacitor (C2, C3) are arranged,

    - a first measuring apparatus (R4, R5, C4) for measuring a first voltage (U1) which is proportional to the electric power (P) injected into the half-bridge inverter (T1, T2),

    - a second measuring apparatus (R8, R9, C6; R10, R11, C7) for measuring a second voltage (U3, U4) which is proportional to the voltage drop across the at least one half-bridge capacitor (C3, C2),

    - a third measuring apparatus (R12, R13, C8, C9, D1, D2) for measuring a third voltage (U5) which is proportional to the r.m.s. value (U_ac) of the running voltage of the at least one low-pressure discharge lamp (FL1, FL2),

    - a fourth measuring apparatus (R6, R7, C5) for measuring a fourth voltage (U2) which is proportional to the supply voltage of the half-bridge inverter (T1, T2),

    - an evaluation unit (MC, TR) which is connected to the outputs of the measuring apparatuses and comprises a program-controlled microcontroller (MC) and which serves the purpose of evaluating the first to fourth voltage (U1, U2, U3, U4, U5) as well as of controlling the half-bridge inverter (T1, T2) as a function of the result of the evaluation.

Images