EP1377135A2 - Circuit with a near-capacitive mode detection for operating a discharge lamp - Google Patents

Abstract

The circuit has an oscillator circuit that generates radio frequency supply power for a load circuit containing a discharge lamp (1) from a variable supply power. A detection circuit identifies proximity to a capacitive operation of the load circuit. The detection circuit senses magnitude of fluctuations in the lamp current based on changes in the supply power.

Description

Technical field

The invention relates to an operating circuit for discharge lamps.

It relates to operating circuits which supply the discharge lamp with a high-frequency supply power which is obtained from a supply power via an oscillator circuit. In particular, but not exclusively, the invention relates to the case in which the supply power for the oscillator circuit is based on an AC supply power that is rectified. Such operating circuits are common, in particular in the case of low-pressure discharge lamps, and therefore do not have to be explained in detail.

State of the art

The oscillator circuit supplies a so-called load circuit, into which the discharge lamp is connected, and through which a high-frequency lamp current generated by the oscillator circuit flows. The load circuit defines a resonance frequency that is influenced by various electrical parameters of the load circuit and also depends, among other things, on the operating state of the discharge lamp. Efforts are made to operate the load circuit relatively close to the resonance frequency during continuous operation of the discharge lamp. This has the advantage of small phase shifts between current and voltage and thus low reactive currents. You benefit from this when dimensioning the components, especially a lamp choke. Otherwise, the oscillator circuit generating the high-frequency supply power regularly contains switching elements. With small phase shifts due to operation close to resonance, the switching losses in the switching elements are relatively small. This has advantages with regard to the efficiency of the operating circuit as well as the thermal load and the dimensioning of the switching elements.

As a rule, the aim is to work in the so-called inductive range, that is to say with an operating frequency of the oscillator circuit that is higher than the resonance frequency of the load circuit. However, one must avoid that the operating frequency of the oscillator circuit becomes lower than the resonance frequency, because in capacitive operation, that is to say at an operating frequency lower than the resonance frequency, disturbing current peaks in the switching elements and other difficulties can result. In particular, a faulty synchronization between the switching times and the lamp inductor current in capacitive operation can result in a pronounced positive current peak at the start of a lamp current half-wave carried by a switching element. Overall, the aim is to work as close to the resonance frequency as possible, but falling below it should not occur or should occur only to a limited extent.

However, as a result of temperature changes and aging processes such as electrode erosion, mercury diffusion in phosphors and other aging phenomena, and also as a result of specimen scatter between different individual discharge lamps, fluctuations in lamp impedance (based on continuous operation) occur.

As a result of these lamp impedance fluctuations and the usual component tolerances, the operating circuits cannot easily be relatively accurately set to a resonance-near operation. Rather, for safety reasons, a relatively large distance is kept from the nominal resonance frequency, which takes into account the fluctuations and tolerances listed. This results in higher component costs and increased space requirements due to the correspondingly larger dimensions and loss of efficiency.

For this reason, attempts have already been made to equip operating circuits of the type shown with detection circuits for recognizing the proximity to capacitive operation of the load circuit. For example, US Pat. No. 6,331,755 shows in FIG. 5 a resistor RCS for measuring a lamp inductor current and a comparator COMP for comparing this inductor current with a threshold value. The comparison takes place on a switching-off edge of a switching transistor of a half-bridge oscillator circuit. The closer the operating frequency comes to the resonance frequency and thus to the capacitive operation, the smaller is not only a reversed sign-on peak of the measurement voltage at the resistor RCS, but also the more the measurement voltage drops at the end of the on-time of the switching transistor mentioned. A threshold state can thus be set with the threshold value, in which the circuit as a whole is switched off (shown on the right in FIG. 6 there) if the operation becomes too close to resonance.

Presentation of the invention

Starting from the prior art mentioned, the invention is based on the technical problem of further improving an operating circuit for a discharge lamp with an oscillator circuit and a detection circuit for detecting proximity to a capacitive operation of the load circuit.

The invention relates to an operating circuit of the type shown, in which the detection circuit detects the magnitude of fluctuations in the lamp current or a manipulated variable of a lamp control circuit, which fluctuations in the supply power.

Preferred embodiments are specified in the dependent claims.

The invention is characterized by a particularly favorable form of detection of the proximity to the capacitive operation by the detection circuit. To this end, in a variant of the invention, the detection circuit detects the magnitude of fluctuations in the lamp current in accordance with the frequency of the supply power. If the oscillator circuit is supplied with a rectified AC supply power, the supply power of the oscillator circuit fluctuates with the fluctuations of the rectified supply voltage (so-called intermediate circuit voltage) given by the AC voltage frequency. The intermediate circuit voltage is therefore modulated at twice the frequency of the original AC voltage. The doubling of the frequency is a consequence of the rectification. It is theoretically also conceivable that no frequency doubling occurs here; in any case, the modulation of the intermediate circuit voltage is related to the frequency of the original AC voltage.

This intermediate circuit voltage modulation can usually still be measured in the lamp current itself, even if the lamp current is determined by a current or power control circuit, which is a preferred embodiment of the invention. Depending on the technical complexity, control circuits are only able to weaken this modulation to a limited extent. If no control circuit is provided, the modulation of the intermediate circuit voltage is all the more recognizable in the lamp current.

Incidentally, this also applies to the case, which also represents a preferred embodiment of the invention, that the rectified AC supply power is converted to a largely constant DC voltage by a PFC circuit (Power Factor Correction, so-called power factor correction). The PFC circuit serves to limit the harmonic content of the power consumption from the AC network and generally charges a storage capacitor to the DC link voltage. The intermediate circuit voltage is then also modulated to a certain extent in accordance with the AC voltage frequency.

The magnitude of the lamp current fluctuations depends on the proximity to the resonance frequency and thus on the proximity to the capacitive operation. This follows from the increase in lamp current with increasing proximity to resonance on the one hand and the modulation of proximity to resonance by the intermediate circuit voltage modulation on the other hand.

The level of the fluctuations in the lamp current thus offers a particularly simple possibility for detecting the proximity to capacitive operation. In particular, this is a signal which can be varied, for example, at twice the mains frequency of the AC voltage network and which in this respect does not present any significant measurement difficulties. On the other hand, the conventional solutions for detecting the proximity to the capacitive operation are linked to the operating frequency of the oscillator circuit itself and must be related to these phases, which necessitates a considerably higher outlay in terms of circuitry. In many cases, the lamp current must be measured anyway for other reasons, for example in order not to exceed certain maximum values for safety reasons or to carry out the current regulation already mentioned. Then the invention is associated with all the less additional effort.

The general formulation of the invention in claims 1 and 2 speaks of a variable supply. As stated above, this can be a rectified AC supply power. However, the invention also includes the case in which the operating circuit is operated on a DC voltage source. Then the need for a rectifier is eliminated or an already provided rectifier is ineffective. In this case, however, it may also be desirable to use the invention. To this end, the DC voltage or DC link voltage can be modulated deliberately. In addition to the possibility of the detection according to the invention of the proximity to a capacitive load circuit operation, this also has the advantage that the modulation results in a broadening of the frequency spectrum of high-frequency interference transmitted by the operating circuit to the DC voltage source. The disturbances are therefore less problematic because they occur in a broader and therefore flatter interference spectrum. The variable supply services in the sense of the claims can therefore also be deliberately modulated direct voltage supply services. In particular, the invention also considers combination operating circuits which are provided both for operation on DC voltage sources and on AC voltage sources.

Furthermore, as an alternative to detecting the level of the fluctuations in the lamp current itself, the invention is also directed to the case where the lamp current is determined by a control circuit for regulating the load circuit, in particular the lamp current or the lamp power, So the changes in the control circuit in the effort of the control circuit to keep the controlled variable constant is detected. The manipulated variable could then be understood as an illustration of the lamp current fluctuations, even if the latter does not occur or only to a small extent.

The control circuit preferably has an I control element, that is to say an integrating element, in order to compensate for the comparatively slow parameter changes in the discharge lamp in the sense of the described impedance changes due to aging or other long-term fluctuations. In many cases, such an I control element will suffice. If necessary, it can be supplemented by a P control element (proportional element) or another additional device for better consideration of the DC link voltage modulation.

In particular, the control circuit and other control of the oscillator circuit can be carried out by an integrated digital circuit which only has to have a few additional functions. In addition, the digital circuit can be a programmable circuit or a so-called microcontroller, and the additional effort required for the invention can be limited to a pure software supplement.

In addition to controlling the oscillator circuit, such a digital control circuit or such a microcontroller can also take over the control of the aforementioned PFC circuit.

It is also preferably provided that the operating circuit is not switched off when a certain proximity to the capacitive operation is detected, as in the prior art, but is operated at least as a rule. The detection of the proximity to the capacitive operation is said to lead to an influencing of the operating mode, so that this proximity is at least not further increased or even reduced in order to be able to continue the operation. For example, the operating frequency of the oscillator circuit could be influenced directly. However, the preferred solution in the case of a control circuit is to reduce the current setpoint or power setpoint of the current control circuit, which can have an indirect influence on the frequency. That is clearly spoken The operating circuit according to the invention is therefore designed not to come too close to the capacitive operation in continuous operation and to counteract a further approach if it is too close, but to continue the lamp operation. For this purpose, it is particularly tolerated to change parameters that may be permanently fixed, such as the operating frequency or the lamp current, if necessary. From the point of view of the invention, it is rather tolerable that the discharge lamp becomes slightly darker in such cases than that it is switched off completely.

In particular, it can be provided that the detection circuit compares the level of the fluctuations with a predetermined threshold value and, as long as the threshold value is not exceeded, does not further influence the operation. If the threshold value is exceeded, the detection circuit can either continuously change the operating frequency, the control setpoint or another variable in accordance with a control context or can also change it by a predetermined fixed variable, as shown in the exemplary embodiment. In any case, the comparison with the threshold value preferably provides a function of the detection circuit which does not normally influence operation.

Description of the drawings

The invention is illustrated in more detail below with the aid of an exemplary embodiment, the features illustrated here also being essential to the invention in other combinations. In particular, it is pointed out that the above and the following description should also be understood with regard to the process category.

Figure 1 shows a schematic representation of an operating device according to the invention;

Figure 2a shows schematically the relationship between the intermediate circuit voltage, discharge lamp current and qualitative current form in switching elements of an oscillator circuit in an operating circuit according to the invention;

FIG. 2b corresponds to FIG. 2a, but relates to an operating state closer to resonance;

FIG. 3 shows a block diagram of a program sequence in a control circuit of the operating circuit from FIG. 1.

In FIG. 1, reference numeral 1 designates a low-pressure discharge lamp with two filament electrodes 2 and 3. Between a ground connection 4 and an intermediate circuit supply voltage 5 there is an oscillator half-bridge circuit known per se with two switching transistors 6 and 7 Switch center tap 8 back and forth between the DC link supply voltage and the ground potential. As a result, a high-frequency supply voltage for the discharge lamp 1 can be generated from the rectified intermediate circuit supply voltage present at the connection 5, which is obtained from a mains voltage via a rectifier bridge circuit known per se with a PFC circuit.

The PFC circuit not shown in FIG. 1 can be a so-called step-up converter, the structure of which is known per se and is not of particular interest for the invention. It can also be a different PFC circuit. Despite the PFC circuit remains a certain residual modulation of the intermediate circuit voltage with twice the mains frequency, usually with 100 Hz.

A so-called coupling capacitor 9, a lamp inductor 10 and the discharge lamp 1 are connected in series between the ground connection 4 and the center tap 8. The coupling capacitor 9 is used to decouple the discharge lamp 1 from DC components; the lamp choke 10 is used in particular to compensate for the negative derivation of the current-voltage characteristic of the discharge lamp 1 in places. Both circuit components are generally known in this function and need not be explained in more detail here.

The same applies to a resonance capacitor 11 which is parallel to the discharge lamp 1 and also in series with the coupling capacitor 9 and the lamp inductor 10 and which is used to generate resonance-excessive ignition voltage amplitudes for igniting the discharge lamp 1.

As far as described so far, the operating circuit is completely conventional. However, the control connections of the switching transistors 6 and 7, as indicated by dashed lines in FIG. 1, are controlled by control signals from a digital control circuit 12. The digital control circuit 12 is a programmable microcontroller and detects a signal indicating the level of the current through the lamp inductor 10 via a measuring resistor 13.

The control circuit 12 contains in particular a current regulating circuit which regulates the lamp current tapped via the resistor 13 to a largely constant value I _Lamp . The operation of the control circuit 12 is shown in more detail in Figure 3.

The control circuit 12 can therefore measure the lamp current I _lamp via the measuring resistor 13, and also regulates to a constant value via the operating frequency of the half-bridge oscillator with the switching transistors 6 and 7 Lamp current and, finally, by evaluating the remaining modulation of the lamp current amplitude as a result of the modulation of the intermediate circuit voltage, is able to recognize an operating mode which is too close to capacitive operation. For this purpose, as will be explained with reference to FIG. 3, a threshold value is used for the difference between the lamp current amplitude _maximum I _max and minimum I _{min shown in} FIGS. 2a and 2b.

FIGS. 2a and 2b schematically show the qualitative form of the fluctuations mentioned for a near-resonance but favorable operating state shown in FIG. 2a and an unfavorable operating state shown in FIG. 2b. One can see the change in the level of the fluctuations of the lamp current I _lamp tapped off at the resistor 13 and the corresponding changes in the intermediate circuit voltage U _between the point 5 and the ground connection 4. The lamp current is shown with its envelope, which illustrates the fluctuations in the amplitude with the intermediate _{circuit voltage} U _zw . The lamp current I _Lamp actually oscillates at the operating frequency of the half-bridge oscillator circuit, which is only indicated schematically in FIGS. 2a and 2b.

In the respective lower area of the figures, qualitative current forms of the half-period currents flowing through the respectively closed switching transistor 6 or 7 are shown. The limited negative deflection that can initially be seen in the respective left-hand current form is typical for inductive operation and means that the current follows the voltage. As long as the negative peak is not too pronounced, this can be seen as a favorable operating condition. In the right-hand current form in FIG. 2a, it can be seen that in the region of the small amplitudes of the lamp current, that is to say the minimum intermediate _{circuit voltages} U _ZW , the negative deflection indicating inductive operation has almost disappeared. The proximity to capacitive operation thus fluctuates with the intermediate _{circuit voltage} U _between . Accordingly, the right-hand current form in FIG. 2b shows a pronounced positive peak at the beginning of the current form, which symbolizes the beginning of capacitive operation. This tip leads to thermal loads and possibly damage to the switching transistors 6 and 7 and should be avoided.

FIG. 3 shows in the form of a block diagram the mode of operation of the operating circuit from FIG. 1. The sequence shown runs as software stored in the microcontroller 12. According to the upper end of the block diagram, a measured intermediate circuit voltage (between points 4 and 5 in FIG. 1) Uzw is subtracted from a desired intermediate value voltage U _ZW-Soll . The difference is integrated via an integration element symbolized by I, multiplied by a normalization constant denoted by k ₃ and used to regulate the PFC circuit (not shown in FIG. 1) to a constant output voltage. For this purpose, the switching processes of a switching transistor of the PFC circuit, for example a step-up converter, are clocked accordingly, ie ultimately the operating frequency of the switching transistor is changed so that the output voltage and thus the intermediate circuit voltage U _{zw is} as constant as possible. The intermediate circuit voltage is output by the PFC circuit via points 4 and 5 in FIG. 1 to the half-bridge oscillator formed by the switching transistors 6 and 7 and the load circuit containing the lamp 1.

The half-bridge oscillator with the switching transistors 6 and 7 supplies the lamp current I _Lamp flowing through the lamp 1, which is measured by the microcontroller 12 via the measuring resistor 13. This is symbolized by the arrow emerging from the half-bridge oscillator in FIG. 3 to the right. In the microcontroller, the lamp current is rectified and amplified by the elements labeled with the corresponding electrical switch symbols, then in one labeled PT ₁ Low-pass filter filtered in the sense of averaging and finally AD converted.

A branch follows, which leads on the one hand to a block designated as a detection circuit. This detection circuit calculates the fluctuations in the lamp current amplitude over a period of 10 ms, i.e. the difference between the maximum and the minimum of the lamp current amplitude or the envelope within the specified period. If this difference exceeds a value of, for example, 50 mA, the detection circuit increases its output signal, otherwise it decreases it. The detection circuit therefore assumes that in the normal case no output signal is necessary and in this normal case has the output signal 0 (which is also not further reduced). If the threshold value of 50 mA is exceeded, the output signal is increased by a certain fixed value and increased again after the 1Oms period by this fixed amount as long as the 50 mA threshold value is exceeded.

As soon as the threshold value is no longer exceeded, the output signal is gradually reduced, preferably using smaller step sizes than for the increase. This happens up to an output signal of 0 if the threshold value for the lamp current fluctuations is not exceeded again beforehand. The detection circuit therefore uses the threshold value to recognize that it is too close to capacitive operation, responds to this detection with an output signal and slowly moves the output signal back as soon as this detection no longer applies.

The output signal described is limited with regard to conceivable measurement errors and then subtracted from a lamp current setpoint I _{Lamp Soll} in the case of the differential element symbolized with a minus sign. The corrected lamp current setpoint is in turn subtracted from the actual value of the lamp current I _Lamp averaged by the digital mean value element.

The difference between them is integrated and multiplied by the normalization constant symbolized with k ₁ . The integrated and standardized difference between the lamp current target value corrected by the detection circuit and the lamp current actual value is then added to a value in the link symbolized by a circle according to the arrow described with offset in order to carry out an operating point setting. This value stands for a period, which in turn is limited with regard to conceivable measurement errors and is used to control the switching transistors 6 and 7 of the half-bridge oscillator.

It can thus be seen overall that the PFC circuit is first regulated to a constant _{DC link voltage} with a setpoint U _ZW-Soll . The modulation of the intermediate circuit voltage let through by the PFC circuit influences the lamp current via the half-bridge oscillator, which is regulated by a second control loop to a lamp current setpoint I _{Lamp Soll} . A simple, slow 1-control loop is used because only long-term drift effects need to be taken into account. This lamp current setpoint is in turn corrected by a third control circuit, into which the detection circuit is connected, in such a way that the threshold value of 50 mA for the lamp current amplitude modulations is not permanently exceeded.

It can also be seen that in addition to the lamp current control provided in any case, the invention only has a slow, further control loop in the sense of an additional software branch, for which no further measurement value determination is necessary. Rather, the already measured and digitized lamp current is used.

If necessary, the control shown can be supplemented by a further control element in the lamp current control circuit, with which the 100 Hz modulation of the lamp current is damped. For example, instead of a PI controller can be used for a simple I controller. This does not change the fact that, although smaller, lamp current modulations remain. Even if the lamp current modulations were completely compensated, they could be used for the detection according to the invention of the proximity to the capacitive operation, in that the control signal of the lamp current control loop is used as a representative of the fluctuations in the lamp current. The fluctuations in the lamp current would then to a certain extent only exist in terms of control technology and no longer be physically present. The invention also relates to this variant. Incidentally, even with perfect lamp current control, the current would drop in the capacitive range.

In addition, it has already been established that the intermediate circuit voltage U _zw in FIG. 2 or between the connection 5 and ground 4 in FIG. 1 could also be a deliberately modulated voltage from a DC voltage source. This would not change the principle of this embodiment. In this case, however, the PFC circuit would be superfluous.

The invention thus enables a very precise adjustment of the operating circuit to a continuous operation that is close to resonance on average, despite component tolerances and lamp aging processes. If difficulties arise, in contrast to the prior art, lamp operation is continued and, as a result of the change in the current setpoint, only a certain reduction in output is carried out. From the perspective of the user, the far cheaper solution is to be seen in a lamp that shines with hardly any noticeably reduced brightness compared to a lamp that is not functioning.

Claims (11)

Operating circuit for a discharge lamp (1) with
an oscillator circuit (6, 7) for generating a high-frequency supply power for a load circuit (1, 8-11) containing the discharge lamp (1) from a variable supply power (5)
and a detection circuit (12, 13) for recognizing the proximity to a capacitive operation of the load circuit (1, 8 -11),
characterized in that the detection circuit (12, 13) detects the level of fluctuations in the lamp current (I _lamp ) corresponding to the changes in the supply power (5). Operating circuit for a discharge lamp (1) with
an oscillator circuit (6, 7) for generating a high-frequency supply power for a load circuit (1, 8-11) containing the discharge lamp (1) from a variable supply power (5),
a detection circuit (12, 13) for recognizing the proximity to a capacitive operation of the load circuit (1, 8 -11)
and a lamp control circuit (12, 13) for controlling the load circuit (1, 8-11) to a lamp _setpoint (I _{lamp setpoint} ),
characterized in that the detection circuit (12, 13) corresponds to the amount of changes in the supply power (5) Fluctuations in a manipulated variable of the lamp control circuit (12, 13) are detected. Operating circuit according to claim 1 or 2, which is designed in response to detection of the proximity to a capacitive operation by the detection circuit (12, 13) to adapt the operation of the oscillator circuit (6, 7) in such a way that the proximity to the capacitive Operation is not increased further and the operation can be continued. Operating circuit according to one of the preceding claims with a current regulating circuit (12, 13) for regulating the lamp current (I _lamp ) to a current setpoint (I _{lamp setpoint} ). Operating circuit according to one of the preceding claims with a power control circuit for regulating the lamp power to a power setpoint. Operating circuit according to Claims 4 and 5, which is designed to reduce the control setpoint (I- _{lamp setpoint} ) in response to detection of the proximity to capacitive operation by the detection circuit (12, 13). Operating circuit according to one of Claims 4 to 6, in which the control circuit (12, 13) has an I control element. Operating circuit according to one of the preceding claims, in which the detection circuit (12, 13) compares the magnitude of the fluctuations with a predetermined threshold value. Operating circuit according to one of the preceding claims with a PFC circuit which supplies the oscillator circuit (6, 7) with a DC voltage power (5) and is connected to a rectifier and is regulated to the DC voltage (5). Operating circuit according to one of the preceding claims, which is designed for an AC voltage supply power and has a rectifier for generating a DC voltage power (5). Operating circuit according to Claim 9, in which a microcontroller (12) contains a positive control circuit for the oscillator circuit (6, 7) and for the PFC circuit.

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