NO177520B - Discharge lamp drive circuit - Google Patents

Abstract

A ballast for a discharge lamp (L) contains, in the normal manner, an invertor (C) supplied from a DC voltage source (B), which invertor generates the alternating voltage for the discharge lamp (L). The discharge lamp (L) is located in parallel with the capacitor (C10) of a series-resonant circuit (L6/C10). A variable frequency AC voltage is supplied from a variable frequency generator (6, 7, 8, 9, 15, 16) to the invertor (C). The frequency of the variable frequency generator can be controlled by a control unit (10). Control signals are supplied to the control unit, which signals correspond to various parameters of the ballast. These parameters are the lamp voltage, the lamp current, the DC power, the DC voltage, a low-voltage DC voltage and the phase angle of the load circuit (E) for the invertor (C). The reaction of the control unit (10) to changes in the parameters mentioned is to change the frequency of the frequency generator or to switch off the frequency generator or the invertor (C). <IMAGE>

Description

The invention relates to a drive circuit for a discharge lamp which is arranged in parallel with the capacitor in a series resonance circuit in an inverter's load circuit, which inverter is connected to a direct voltage source and controlled by a frequency generator whose frequency can be changed by means of a control unit between an operating frequency that is close to the series resonance circuit's resonance frequency and an idle frequency that lies above the operating frequency, with a voltage measuring unit connected to the control unit which measures the lamp voltage and which produces a corresponding signal, as well as with a regulator unit.

According to DE-PS 30 14 419, it is known to measure lamp current. When it is too high, a bistable circuit breaker is activated which short-circuits the inverter's transformer.

According to DE-PS 31 52 951, it is known that when switching on the drive circuit, a monostable flip-flop is tilted to an unstable state, where it short-circuits an extra winding in the inverter's transformer, so that it works at a high frequency. At this high frequency, there is a reduced voltage in the lamp so that no ignition can occur. However, right up until the tilting back of the monostable rocker stage to the stable rocker state, the lamp's filament coils can be heated and emit. After tilting back the rocker step, the short circuit of the additional winding is canceled again, whereby the inverter works at a lower frequency that is close to the resonant frequency of a series resonant circuit. The lamp is connected in parallel with the capacitor of the series resonant circuit. At the lower frequency, an increased voltage across the capacitor and the lamp occurs due to the resonance, which is sufficient for ignition.

According to DE-OS 32 08 607, it is known to arrange two frequency generators for controlling the inverter, one of which produces approximately the operating frequency and the other a higher idle frequency. Instead of two generators, a tunable generator can also be arranged. First, the high idle frequency is engaged, where no ignition can occur. Thereby, the lamp electrodes are preheated. Due to the large positive temperature coefficient of the electrode resistance, one can judge whether the electrodes are already sufficiently preheated by means of a corresponding selection of the threshold value switch's call threshold. If this is the case, the threshold value switch alerts, and the generator with the lower frequency is switched on,

or - if a tunable generator is provided - the frequency is lowered towards the operating frequency, so that ignition can take place.

From WO-A 82/01276, a drive circuit is known which exhibits an overcurrent detector for the thyristors of the inverter. The detector must notify when both thyristors are in the conducting state at the same time. In such a case, the thyristors are switched off.

Furthermore, it is known from DE-A 3,432,266 to connect a measuring winding together with the lamp's choke, from which measuring winding a signal can be derived showing the ignition state and fed to a monitoring circuit. The monitoring circuit determines from this signal whether the series resonant circuit containing the choke oscillates at the ignition frequency through the lamp without noticeable damping, which is the case with an unlit lamp, or whether the series resonant circuit is damped by a lighted lamp. The corresponding message from the monitoring circuit is interrogated by a control circuit for an oscillator that drives the inverter. This control circuit controls the oscillator in such a way that it alternately oscillates for a certain length of time first with a high starting frequency, then with a lowered heating frequency, then with the further lowered resonant frequency for the series resonant circuit, and finally with the even lower operating frequency. When the polling of the monitoring circuit about the ignition frequency shows that the lamp has not lit, the control circuit sets the oscillator back to the start frequency, and the procedure is repeated.

The purpose of the present invention is to provide a drive circuit whose operational reliability is increased by means of monitoring the lamp voltage, where the requirement for increased operational reliability is particularly satisfied by the realization of most building elements within the framework of an integrated semiconductor chip.

The task is solved using the characterizing features

in patent claim 1.

Fig. 1 shows a block current core for the drive circuit and

Fig. 2 shows the time course of the drive voltage

for the inverter compared to the lamp's total current.

The block circuit shown in Figure 1 consists of several modules which will be explained in more detail in order.

Module A is connected to the AC mains and serves as an HF overtone filter to reduce the harmonic overtones of the mains frequency as well as radio frequency interference. It is constructed in the usual way and contains chokes LI and L2 as well as capacitors Cl, C2, C3 and C4.

Connected to module A is module B. This consists of a rectifier bridge which is only schematically indicated, with a smoothing network. On connection 16 for module B there is a +DC voltage of approx. 250V available.

Connected to module B is module K, which serves to generate a voltage UEG corresponding to a direct current drawn by module B. Module K consists of a resistor RI with which a series connection of a choke L2 and a capacitor C5 is connected in parallel. The voltage UI6 can be taken from the connection point between L2 and C5.

A further module M serves to produce an actual value voltage UG which corresponds to the direct voltage produced by Module B. Module M is connected to the connection 16 in module B and consists of a resistor-voltage divider R2/R3, whereby it is connected in parallel with the resistor R3 a capacitor C6. The voltage UG that can be taken from the voltage dividing point is proportional to the direct voltage emitted from module B at connection point 16, but is lower than this.

Module C is an inverter that produces a variable alternating voltage from the direct voltage produced from module B. In addition, an alternating voltage is supplied to module C from module D, which will be explained in more detail later, on the winding L3 of a counter-phase transformer Tlf. Counter-phase -transformer a Tl has two windings L4 and L5 on the secondary side. The voltage that occurs on the winding L4 is fed across a resistor R4 to the gate connection of a field effect transistor FETI, where a diode Dl is connected in parallel with this resistor. Between the port connection and the source connection for FETI, two zener diodes Zl and Z2 are connected in series, but opposite. The voltage that occurs on the counter-phase transformer T1 winding L5 is supplied via a resistor R5 to the gate connection of a field-effect transistor FET2, where a diode D2 is connected in parallel with the resistor. Between the port connection for FET2 and the source connection, two zener diodes Z3 and Z4 are connected in series, but opposite. Resistors R4 and R5 are high-resistance and serve for the switch-on delay of the FETs. The diodes Dl and D2 must ensure a quick switch-off of the FETs. In this way, it will be achieved that both FETs are never completely conducting at the same time, as this would result in a short circuit. The zener diodes Zl, Z2, Z3 and Z4 must protect the FET's port connections against voltages. Otherwise, the inverter according to module C is of normal structure and its function is known, so that it does not need to be explained in more detail here.

At the output of module C which forms the inverter, a load circuit designated module E is connected, which contains a gas discharge lamp L. This contains, in addition to the gas discharge lamp L, a series resonant circuit formed by a choke L6 and a capacitor CIO. The capacitor C9 serves for disconnection. The oscillating circuit capacitor CIO lies in parallel with the gas discharge lamp L. The use of a series resonant circuit has several known advantages. Before lighting the gas discharge lamp L, the alternating current flows through the lamp's filament coils over the parallel capacitor CIO. Thereby, the glow spirals are heated and stimulated to emission. When the frequency of the voltage supplied from module C to module E is close to the resonance frequency of the series resonant circuit L6/C10, a voltage increase occurs on the parallel capacitor CIO, which is used to light the lamp L. After lighting, the lamp L dampens the quality of the series resonant circuit -factor, and the voltage across the lamp L drops, because the lamp L is then conductive. Now the choke L6 acts as a 1 amp current limiter. In order to extend the life of the lamp L, efforts are made to preheat the lamp's L filaments sufficiently long before ignition. To achieve this, module E is first supplied with a voltage with a frequency that is sufficiently high above the resonance frequency of the series resonant circuit L6/C10 so that the filament coils of the lamp L are sufficiently heated, but that lighting of the lamp L still does not occur, because the voltage that falls across the parallel capacitor CIO because of its high frequency is too low. The frequency is then lowered downwards in the direction of the series resonance circuit's L6/C10 resonance frequency, until said voltage increase on the parallel capacitor CIO occurs and ignition can occur. How the aforementioned change in the frequency supplied to module E is effected will be explained in more detail later.

Module G serves to produce a voltage UIL, which is a measure of the lamp L's total current. Module G consists of a resistor R8 which is connected to the lamp's one electrode, through which the total lamp current flows.

Module H serves to produce a low supply direct voltage U^ of around 10...12V for the building elements of module D, which module is preferably designed as a semiconductor chip, and which will be explained in more detail later. Module H is connected to module G's output, which, as mentioned, produces a voltage UIL, which corresponds to the total lamp current. This voltage is thus derived from the total lamp current. It is supplied to module H on the diode D3, which rectifies the voltage UIL, which is an alternating voltage. The resistor R9 and both the zener diodes Z5 and Z6 which are connected in series with this ensure voltage stabilization. The resistor RIO and the capacitor Cll, which is connected in series with this, as well as the capacitor C12, ensure smoothing of the rectified voltage Ujjy. The voltage U^ could probably also be obtained by voltage division from the equal voltage (250V) which is available from connection 16 on module B; but then the amount corresponding to the voltage difference between 250V and 10...12V would have to be destroyed, which would be associated with corresponding unwanted heat loss and unnecessary energy consumption.

The drive circuit's core is module D. This essentially serves to produce a variable frequency for module C which forms the inverter, and in dependence on several of the drive circuit's parameters. With the variable frequency, it is possible to change the voltage across the lamp's L (module E) connection terminals. At an idle frequency that is significantly higher than the series resonant circuit's L6/C10 resonant frequency, module E, which forms the load circuit, is inductive, and the voltage across the lamp's L connection terminals effectively amounts to no more than 250V. This relatively low voltage is prescribed by associated standards (for example Deutsche VDE 0712), to enable a relatively harmless replacement of the lamp L. At the ignition or operating frequency which is close to the resonance frequency of the series resonant circuit L6/C10, on the other hand, the alternating voltage supplied the lamp's L connections, approx. 500V.

Module D contains a quartz-equipped fixed-frequency generator 6, which e.g. producing a frequency of 8 MHz. This frequency is supplied to a frequency control unit 10, which can for example contain a correspondingly programmed microprocessor (CPU). The frequency control unit leads the frequency of 8 MHz emitted from the fixed frequency generator 6 to a block 7, which supplies corresponding counting pulses (clock) to an 8-bit counter 9 and at the same time causes the 8-bit counter 9 to count up or down ( up/down). The latter depends on the state of the parameters for the drive circuit that are supplied to the frequency control unit 10. The counter value of the 8-bit counter 9 is supplied to a 7-bit comparator 16. This is also via a 7-bit bus line with series-connected flip-flops' is FF connected to a counter logic 8. The flip-flop chain is supplied with the 8MHz frequency from the fixed frequency generator 6 for counting, whereby each flip-flop FF in the chain divides the frequency by a factor of 2. When the state of the flip-flop chain is equal to 8- the counter value of the bit counter 9, then the logic part of the counter logic 8, which is connected to the 7-bit comparator 16, resets the flip-flop chain via line R. At the same time, the logic part of the counter logic 8 transmits the corresponding assigned frequency to a drive circuit 15, whose output is connected to the winding L3 of the counter-phase transformer Tl in the module C which forms the inverter.

The control unit 10 (CPU) which contains the programmed microprocessor, processes the signals supplied to it, corresponding to the individual measured parameters, in parallel and/or in series. The microprocessor is programmed in such a way that it works according to a specific program when processing these signals. This will be described in the following with regard to the way it works.

A first parameter is the lamp voltage UL produced by block F and which is an alternating voltage. This is supplied to the input of a comparator/regulator unit 12. The comparator/regulator unit 12 is also connected to a shaker-digitizer 11 which supplies it with three target value voltages UK, UH and Uz. These three setpoint voltages are DC voltages. The comparator/regulator unit 12 constantly compares the UL peak value of the lamp AC voltage with the current target value voltage.

After switching on the drive circuit, the lamp voltage UL usually increases. Only if there is a short-circuit in the connection, it continues to be below the setpoint short-circuit voltage UK. The control unit 10 now tests for a specific time, for example over a mains AC voltage period, whether the lamp voltage UL remains below the setpoint short-circuit voltage UK. If this is the case, the control unit 10 disconnects the drive circuit 15. Furthermore, the control unit 10 switches the entire module D to a high-resistance state. This means that module D's blocks and building elements are probably ready for operation, but still only occupy a very insignificant effect. As module E, and thus also modules G and H, when the drive circuit 15 is switched off, is no longer supplied with any alternating voltage, the blocks and building elements in module D are supplied with voltage from module B. As already mentioned earlier, at module B's connection only a high DC voltage available; The power loss that the building elements, respectively the blocks, of module D, which is designed as a semiconductor chip, are exposed to due to the voltage difference to the supply voltage, is nevertheless negligible, because the power absorption of module D is also small due to the high-resistive state. When module D is again switched from the high-resistance state to the operating state, the voltage supply is delivered through the low direct voltage U^ which is available at the output of module H. The switching between the two possibilities for voltage supply takes place via the control unit 10 on block 17 for the module's D voltage supply.

Since the lamp voltage UL drops sharply immediately after lighting the lamp L, and even below the short-circuit setpoint voltage UK, it is necessary immediately after the start (lighting) of the lamp L to interrupt the testing of the lamp voltage UL by means of the comparator/regulator unit 12. The interruption is programmed into the control unit's 10 microprocessor.

Disconnection of the drive circuit 15, as well as setting module D to a high-resistance state by means of a detected short circuit, can only occur by disconnecting and reconnecting the mains AC voltage. If there is no short circuit in the connection, then the lamp voltage UL exceeds the setpoint short-circuit voltage UK and rises until it reaches the setpoint preheating voltage UH. This amounts to approx. 1.4V. This corresponds to a voltage of approx. 200V on the lamp's L connections. When the comparator/regulator unit reports to the control unit 10 that the lamp voltage UL is equal to the setpoint preheating voltage UH, the control unit 10 regulates the frequency supplied to module C, so that the lamp voltage UL remains equal to the setpoint preheating voltage UH for a certain preheating time. During this time, the filaments of the lamp L in module E heat up and begin to emit. The idle frequency supplied to module C (inverter) in this case reaches around 123kHz. The preheated id is contained in the control unit's (CPU) microprocessor.

When the preheating time is over, the control unit causes, according to the program entered into it, that the frequency supplied to module C is lowered in the direction of the operating frequency, which amounts to approximately 62kHz. The lowering of the frequency of the inverter (module C) results in the AC voltage on Lamp L (module E) rising to approximately 500V. When comparing the lamp voltage UL with the third setpoint voltage emitted from the setpoint transmitter 11, the setpoint ignition voltage Uz in the regulator 12, the control unit 10 now ensures, while evaluating the output signals of the regulator 12, that the lamp voltage UL is kept equal to the setpoint ignition voltage Uz. The rated ignition voltage Uz is approx. 3.5V. The lamp L should now light up. When ignition occurs, the lamp voltage UL drops sharply. This is detected by the control unit 10 above the comparator/regulator unit 12. The drive circuit with the lamp L is thus in operation.

When the lamp voltage UL does not drop below the set value ignition voltage Uz over a certain period of time, this means that ignition has not occurred. The reason for the non-ignition could be, for example, a gas defect in the lamp L, or the emission capability in the lamp's filaments could be exhausted. If, for this or other reasons, the lamp L does not start even if the starting voltage is reached, there is a risk that the connection may be destroyed if the relatively high starting voltage is maintained for a long time. For this reason, the control unit (CPU) 10 is programmed so that a futile start attempt is canceled after a certain time. This happens by the control unit 10 increasing the frequency that is supplied to the inverter (module C) in the direction of the idle frequency, with the result that the voltage across the lamp's L connection terminals drops again. The control unit 10 is further programmed in such a way that in the event of a first unsuccessful start attempt, a specific number of such start attempts can be added. If, after this determined number of starting attempts, the lamp L also does not light up, then the frequency of the inverter is set to the highest possible value, i.e. the idle frequency and remains there either until the lamp L is replaced, or until the AC mains is disconnected and reconnected.

A further parameter that is monitored is the lamp current. The actual value voltage UIL corresponds to the lamp current, which is emitted from module G. This is supplied to a comparator 13

in module D which also receives a reference voltage UILM from a reference voltage transmitter 1, which voltage corresponds to a minimum lamp current. The comparator 13 compares the voltage UIL

with the voltage UILM and reports the result to the control unit 10. When the voltage UIL, which corresponds to the lamp current, falls below the reference voltage, the reason for this could be, for example, a broken filament coil in the lamp L or the fact that the lamp L is not seated correctly in its socket or has been removed. In that case, the control unit 10 controls the frequency that is supplied to the inverter (module C) again to the highest possible value, i.e. to the idle frequency. Thereby, there is a relatively low and therefore harmless voltage on the lamp's L connections (sockets), which is particularly important if the lamp L is removed.

A further monitored parameter is the low DC voltage produced by module H to supply the blocks and elements in module D. This voltage U^ is supplied to a comparator 2, which is also supplied with a reference voltage from the encoder 1 and which is equal to the minimum DC voltage value to be is produced by module H. The comparison result of the comparator 2 is again supplied to the control unit 10. If the low direct voltage XJm produced by module H goes below the reference voltage UNVM, then the control unit 10 controls the frequency of the inverter back to the idle frequency and'couples module D to the high-resistance state.

A further monitored parameter is the DC voltage produced by module B. A corresponding and divided voltage to this DC voltage is the voltage UG which can be extracted from module M. It is supplied to a comparator 3. In addition, the comparator 3 is also supplied with a reference voltage UGM from the encoder 1, which reference voltage corresponds to the minimum equivalent voltage. The comparison result of the comparator 3 is again communicated to the control unit 10. When the voltage UG falls below the reference voltage UGM, this means, for example, that the direct voltage produced by module B has fallen below 150V. At this direct voltage, the inverter can no longer generate energy to light the lamp. Attempts to start could then possibly lead to damage to the lamp. The control unit now tests for at least half a grid period whether the direct voltage UG, which is emitted by module M, at least once falls below the minimum direct voltage V^^. If this is the case, the drive device 15 is disengaged. This means that no more start attempts are made.

A further parameter to be monitored is the direct current power. The actual value direct current effect is found by multiplying the direct voltage UG that is emitted from module M, and the direct voltage UIG that is emitted from module K. The latter direct voltage UIG corresponds to the direct current that is taken up from the coupling. A voltage UPG which is produced by the multiplier 5 and which corresponds to the direct current power is supplied to a regulator 4. The regulator 4 is also supplied with a reference voltage Ups which corresponds to the setpoint power. The output signal from the regulator 4, which works as a comparator, is supplied to the control unit 10. This now regulates the frequency of the inverter (module C) so that the absorbed direct current power remains constant. In this way, the L light output of the lamp is also constant. It should also be mentioned in this context that one can foresee the possibility of making the reference voltage Ups on the reference voltage generator 1 adjustable from the outside, in order to be able to compensate building element tolerances, and to be able to adapt the power absorbed by the lamp L to these tolerances by trimming a ) resistance parts.

When the frequency generator connection is to be driven up from operating frequency to idling frequency, the entire frequency range must not be driven through continuously, but this can be done by the control unit 10 conveying a reset impulse via the wire R to the 8-bit counter 9.

A final parameter to be monitored is the load impedance of the field effect transistors FETI and FET2 of the inverter (module C). When the direct voltage that module B has produced for the inverter drops, for example in the event of a strong interruption of the mains alternating voltage or - when the supply direct voltage for the inverter is taken from a central battery system - when the voltage is lowered, then the load on the field-effect transistors as a result of the parallel capacitance CIO which lies in parallel with the lamp L, capacitive. A capacitive load of the field-effect transistors can then occur, especially with the aforementioned lowering of the supply DC voltage for the inverter, if the control frequency supplied to the inverter approaches its lower limit value. With capacitive loading, there is a risk of destroying the field-effect transistors. To avoid capacitive loading, the nature of the load impedance is therefore communicated to the field effect transistors. This happens so that the voltage UT is taken from the output of the drive circuit 15 and compared in a phase comparator 14 with the voltage UIL which corresponds to the lamp current. The ridge edge of the voltage UT, which is a rectangular voltage, thereby defines the comparison time tv. When the load for the field-effect transistors becomes inductive, the voltage UIL, which can consist of sine half-waves, is shifted to the right in Figure 2, as the current in the inductive case follows the voltage. In the capacitive case, it is the other way around. The output signal of the phase comparator 14 is supplied to the control unit 10. When the load of the field-effect transistors approaches the capacitive case, the control unit 10 causes an increase in the frequency that was supplied to the inverter, with the result that the load again becomes inductive.

Numerous variations of the drive circuit described above are conceivable. Thus, for example, it is not necessary that all parameters are measured and taken into account. The parameters can be measured continuously or periodically. The control unit must not be formed by a microprocessor (CPU), but can be formed by a majority of individual control block bearings etc. The measurement times, measurement intervals and other times such as e.g. the preheating time, can be determined by the teller logic (flip-flop chain) or by a separate timer circuit. The above design can of course also be used with a drive circuit with more than one lamp. Thus, for example, two lamps can be connected in a normal tandem connection.

Claims (4)

1. Drive circuit for a discharge lamp arranged in parallel with the capacitor in a series resonant circuit in an inverter load circuit, which inverter is connected to a direct voltage source and controlled by a frequency generator, the frequency of which can be changed by means of a control unit between an operating frequency that is close to the resonant frequency of the series resonant circuit and an idle frequency that lies above the operating frequency, with a voltage measuring unit connected to the control unit which measures the lamp voltage, and which produces a corresponding signal, as well as with a regulator unit, characterized in that the measurement signal (UL) corresponding to the lamp voltage is supplied to the regulator unit (12) which compares this (UL) with a stored reference signal (UH) which corresponds to a preheating voltage and communicates the comparison result to the control unit (10), that the control unit (10) sets the frequency generator's (6,7,8,9,15,16) frequency close to the idle frequency and then regulates so that the lamp voltage in a certain preheat etid is kept equal to the preheating voltage, and that the control unit (10) after the end of the preheating time regulates the frequency generator's (6,7,8,9,15,16) frequency so that it is lowered in the direction of the operating frequency. 2. Drive circuit according to claim 1, characterized in that the measurement signal (UL) which corresponds to the lamp voltage, in a comparator included in the regulator unit (12) is compared with a reference signal (Uz) which corresponds to the lamp ignition voltage and the comparison result is supplied to the control unit (10), that the control unit (10) regulates the frequency generator's (6, 7,8,9,15,16) frequency depending on the comparison result of the comparator in a closed control circuit so that the lamp voltage during a certain ignition time does not exceed the reference voltage, and that the control unit (10) after the end of the ignition time increases the frequency generator's (6,7,8, 9,15,16) frequency in the direction of the idle frequency, if the lamp voltage during the ignition time has not dropped significantly due to ignition. 3. Drive circuit according to claim 2, characterized in that the control unit (10) again lowers the frequency generator's (6,7,8,9,15,16) frequency down in the direction of the operating frequency, that the control unit (10) repeats the ignition attempts a predetermined number of times if no ignition is achieved in the meantime, and that the control unit (10) sets the frequency generator's (6,7,8,9,15,16) frequency permanently to the idling frequency, if no ignition occurs after the set number of ignition attempts. 4. Drive circuit according to one of the preceding claims, characterized in that the regulator unit (12) or the comparator compares the measurement signal (UL) which corresponds to the lamp voltage, with a stored reference signal (UK) which corresponds to a short circuit across the lamp, and notifies the comparison result to the control unit (10), and that the control unit (10) disconnects the frequency generator (6, 7, 8, 9, 15, 16) or the inverter (C) when the signal comparison shows that the lamp voltage does not exceed the short-circuit voltage within a certain period of time.