EP1372362A2 - Circuit with a current control and a near-capacitive mode detection for operating a discharge lamp - Google Patents

Abstract

A lamp control circuit (12, 13) controls the load circuit to a set value (ILamp soll). When the detection circuit signals proximity to capacitive operation, the operating circuit is designed to reduce the set value.

Description

Technical field

The invention relates to an operating circuit for discharge lamps.

It relates to operating circuits that the discharge lamp with supply a high-frequency supply power, which via an oscillator circuit is obtained from a supply service. In particular, however not exclusively, the invention relates to the case that the utility power for the oscillator circuit to an AC power supply goes back, which is rectified. Such operational circuits are common, especially with low pressure discharge lamps, and therefore do not need to be explained in detail.

State of the art

The oscillator circuit supplies a so-called load circuit in the the discharge lamp is switched, and that of one through the oscillator circuit generated high-frequency lamp current is flowed through. The The load circuit defines a resonance frequency that is different electrical parameters of the load circuit is influenced and also from Operating state of the discharge lamp depends. One tries the load circuit during continuous operation of the discharge lamp relatively close to the resonance frequency to operate. This has the advantage of small phase shifts between current and voltage and thus low reactive currents. It benefits one particularly in the dimensioning of a lamp choke. Otherwise contains the high-frequency supply power generating Oscillator circuit regularly switching elements. With small phase shifts due to operation close to resonance, the switching losses are in the switching elements are relatively small. This has advantages in terms of efficiency 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 area, So with an increased compared to the resonance frequency of the load circuit Operating frequency of the oscillator circuit. But you have to avoid that the operating frequency of the oscillator circuit becomes lower than the resonance frequency, because in capacitive operation, i.e. with smaller ones Operating frequency than the resonance frequency, disturbing current peaks in the Switching elements and other difficulties may arise. In particular can result from an incorrect synchronization between the switching times and the lamp choke current in capacitive operation a pronounced positive Current peak at the beginning of a lamp current half-wave carried by a switching element result. So the overall aim is, if possible to work close to the resonance frequency, but falling short the same 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 others Phenomena of aging as well as due to the distribution of specimens between various individual discharge lamps fluctuations in Lamp impedance (based on continuous operation).

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 security reasons a relatively large distance from the nominal resonance frequency kept, which takes into account the listed fluctuations and tolerances. This results in higher component costs and increased space requirements due to the correspondingly larger dimensioning and loss of efficiency.

Therefore, attempts have already been made to operate the circuits shown Design with detection circuits to recognize the proximity to a capacitive Equip operation of the load circuit. For example, US 6,331 755 in its Figure 5 a resistor RCS for measuring a lamp inductor current and a comparator COMP for comparing this inductor current with a threshold. The comparison takes place on a switch-off edge of a switching transistor of a half-bridge oscillator circuit instead. ever closer to the operating frequency of the resonance frequency and thus the capacitive Operation comes, the smaller is not only a reversed sign Switch-on peak of the measuring voltage at the resistor RCS, but all the more the measuring voltage also drops more sharply at the end of the switch-on time of the aforementioned Switching transistor. A limit state can thus be established with the threshold value can be set at which the circuit is switched off altogether is (shown on the right in FIG. 6 there) if the operation is too resonant becomes.

Presentation of the invention

Based on the state of the art, the invention technical problem, an operating circuit for a discharge lamp with an oscillator circuit and a detection circuit for detection the proximity to a capacitive operation of the load circuit improve.

The invention relates to an operating circuit of the type shown, in which a control circuit for regulating the load circuit, in particular the lamp power or the lamp current is provided to a control setpoint and the operating circuit is configured in response to detection close to capacitive operation by the detection circuit reduce the control setpoint.

Preferred embodiments are specified in the dependent claims.

According to the invention it is provided that the operating circuit during the detection a certain proximity to the capacitive operation, as in the state technology, is switched off, but at least continues to operate as a rule becomes. The detection of the proximity to the capacitive operation should therefore lead to an influence on the mode of operation, so that this closeness at least is not further increased or even decreased to continue operations to be able to. For this purpose, the control setpoint, e.g. the power or Current setpoint, a control circuit reduced. The control circuit has in itself the sense and advantage of influencing lamp operation through Variations in specimens and temporal fluctuations such as temperature fluctuations or reduce the effects of aging. In the invention A control circuit also offers a particularly cheap and simple Possibility of preventing capacitive operation by influencing of the control setpoint. With the change in the control setpoint, a preferred embodiment of the control circuit also an indirect Influencing the operating frequency of the oscillator circuit connected be because the control circuit preferably influences the operating frequency takes to regulate the load circuit. The invention is clearly spoken Operating circuit so designed to operate continuously not too close to capacitive operation and too close to counteract a further approach, however, the lamp operation continue. From the point of view of the invention, it is rather tolerable that the discharge lamp will become slightly darker in such cases, than completely turning it off.

The invention is preferably distinguished by a particularly favorable one Form of detection of the proximity to the capacitive operation by the detection circuit out. For this purpose, the detection circuit detects the height of Fluctuations in lamp current according to the frequency of the supply power. If the oscillator circuit with a rectified AC supply power is supplied, the supply power fluctuates the oscillator circuit with the by the AC voltage frequency given fluctuations in the rectified supply voltage (so-called intermediate circuit voltage). The DC link voltage is therefore twice the frequency of the original AC voltage modulated. The doubling of the frequency is a consequence of Rectification. It is theoretically also conceivable that there is no frequency doubling here occurs; in any case there is modulation of the intermediate circuit voltage in relation to the frequency of the original AC voltage.

This intermediate circuit voltage modulation is usually still in the lamp current itself measurable, even when the lamp current is through a current or power control circuit is regulated. control circuits are only able to do this modulation to a limited extent, depending on the technical complexity mitigate.

Incidentally, this also applies to the case of a preferred embodiment The invention represents the rectified AC supply power through a PFC circuit (Power Factor Correction, so-called power factor correction) to a largely constant DC voltage is converted. The PFC circuit is used for limitation the harmonic content of the power consumption from the AC network and usually charges a storage capacitor to the DC link voltage on. The intermediate circuit voltage is then also in a certain way Scope modulated according to the AC voltage frequency.

The magnitude of the lamp current fluctuations depends on the proximity to the resonance frequency and thus from 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 the resonance proximity by the intermediate circuit voltage modulation on the other hand.

The level of the fluctuations in the lamp current thus offers a special one easy way to measure proximity to capacitive operation. In particular it is a double, for example Mains frequency of the AC network variable signal, so far offers no significant measurement difficulties. on the other hand are the conventional solutions for detecting the proximity to the capacitive Operation at the operating frequency of the oscillator circuit itself linked and must be related to these phases, which is significant higher circuit complexity. The lamp current must be measured anyway in the invention to the already mentioned Current control. Therefore, the invention as a whole with one all the less additional effort involved.

The term variable supply performance is generally used here. As stated above, this can be rectified on the one hand AC supply power. However, the invention encompasses also the case that the operating circuit on a DC voltage source is operated. Then there is no need for a rectifier an already provided rectifier ineffective. In this case, too however, it may be desirable to use the invention. For this, the DC voltage or DC link voltage are deliberately modulated. In addition to the possibility of the detection of proximity to one according to the invention capacitive load circuit operation, this also has the advantage that due to the modulation a broadening of the frequency spectrum of transmitted to the DC voltage source by the operating circuit results in high-frequency interference. The disturbances are less problematic, because they occur in a broader and therefore flatter interference spectrum. The changing benefits in the sense of claims can also deliberately modulate DC power supplies his. In particular, the invention also draws combination operating circuits considered for both operation on DC voltage as well are provided at AC voltage sources.

Furthermore, the invention is directed alternatively to a detection of the height of the Fluctuations in the lamp current itself even in the event that the lamp current by a control circuit for regulating the load circuit, in particular the lamp current or the lamp power is determined, where then a manipulated variable of the control circuit, i.e. the changes in the control circuit in an effort to keep the control circuit constant the controlled variable is recorded. The manipulated variable could then be used as an illustration of the lamp current fluctuations, even if the latter do not occur or only to a small extent.

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

In particular, it can be provided that the detection circuit detects the height which compares fluctuations with a predetermined threshold value, and as long as the threshold is not exceeded, the operation will not continue affected. If the threshold is exceeded, the detection circuit can the control setpoint either according to a control context change continuously or by a predetermined fixed Change size, as shown in the embodiment. In any case is preferably a function by comparison with the threshold given the detection circuit that does not normally affect operation.

In particular, the control circuit and other control of the oscillator circuit through an integrated digital circuit, only a few Must have additional functions. In addition, the Digital circuit around a programmable circuit or a so-called Act microcontrollers, the necessary for the invention Can limit additional effort to a pure software addition.

Such a digital control circuit or such a microcontroller can in particular, in addition to the control of the oscillator circuit, the control take over the mentioned PFC circuit.

Description of the drawings

Figur 1 zeigt eine schematisierte Darstellung eines erfindungsgemäßen Betriebsgerätes;

Figur 2a zeigt schematisiert den Zusammenhang zwischen Zwischenkreisspannung, Entladungslampenstrom und qualitativer Stromform in Schaltelementen einer Oszillatorschaltung bei einer erfindungsgemäßen Betriebsschaltung;

Figur 2b entspricht Figur 2a, bezieht sich jedoch auf einen resonanznäheren Betriebszustand;

Figur 3 zeigt ein Blockdiagramm eines Programmablaufs in einer Steuerschaltung der Betriebsschaltung aus Figur 1.

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 Figure 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 is a known one Oscillator half-bridge circuit with two switching transistors 6 and 7. By an alternating switching operation of the two switching transistors 6 and 7 there is a center tap 8 between the intermediate circuit supply voltage and switch the ground potential back and forth. This can result in the rectified DC link supply voltage applied to terminal 5, via a known rectifier bridge circuit is obtained from a mains voltage using a PFC circuit, generates a high-frequency supply voltage for the discharge lamp 1 become.

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

Between the ground connection 4 and the center tap 8 are a so-called in series Coupling capacitor 9, a lamp inductor 10 and the discharge lamp 1 switched. The coupling capacitor 9 is used for decoupling the discharge lamp 1 of 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. Both circuit components are generally known in this function and do not need to be explained here become.

The same applies to one parallel to the discharge lamp 1 and also in Series to the coupling capacitor 9 and the lamp choke 10 lying Resonance capacitor 11, which is used to generate resonance-inflated Ignition voltage amplitudes used to ignite the discharge lamp 1.

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

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 mode of operation of the control circuit 12 is shown in more detail in FIG. 3.

The control circuit 12 can therefore measure the lamp current I _lamp via the measuring resistor 13, further regulates the operating frequency of the half-bridge oscillator with the switching transistors 6 and 7 to a constant lamp current and is finally capable of evaluating the remaining modulation of the lamp current amplitude as a result of the modulation of the intermediate circuit voltage operating mode 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 therefore 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) U _{zw 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 operations of the switching transistor 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 switching symbols, then filtered in a low-pass element labeled PT ₁ in the sense of averaging and finally converted to AD.

There follows a branch, on the one hand to a detection circuit designated block leads. This detection circuit calculates one Period of 10ms the fluctuations of the lamp current amplitude, 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 50 mA, for example, increases the detection circuit its output signal, otherwise it lowers it. The detection circuit therefore assumes that there is normally no output signal is necessary and has the output signal in this normal case 0 (which is also not further lowered). If the threshold of 50 mA is exceeded, the output signal is fixed by a certain Value increased and after the 10ms period again by this fixed Amount increased as long as the 50 mA threshold is exceeded.

As soon as the threshold is no longer exceeded, the output signal gradually decreased, preferably smaller increments than used for the increase. This happens up to one Output signal of 0 if the threshold for the Lamp current fluctuations is exceeded. The detection circuit uses the threshold to recognize that the capacitive is too close Operation, responds to this detection with an output signal and drives it Output signal slowly returns 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 in turn limited with regard to conceivable measurement errors and is used to control the switching transistors 6 and 7 of the half-bridge oscillator.

Overall, it can be seen 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 I 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 the invention is in addition to that already provided Lamp current control just a slow additional control loop in the Has the meaning of an additional software branch for which no other Measurement determination is necessary. Rather, the one that is measured anyway and digitized lamp current used.

If necessary, the regulation shown can be controlled by a further control element the lamp current control loop can be supplemented 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 doesn't change anything the fact that, albeit smaller, lamp current modulations remain. Self if the lamp current modulations were completely adjusted, in this respect they could be close for the detection according to the invention capacitive operation are used as the control signal of the lamp current control circuit representative of the fluctuations in lamp current is used. The fluctuations in the lamp current would then be to a certain extent only existing in terms of control technology and no longer physically available. The invention also relates to this variant. in the The current would also be capacitive in the case of perfect lamp current control Break in area.

In addition, it has already been found that the DC link voltage Uzw in Figure 2 or between the terminal 5 and ground 4 in Figure 1 also be a deliberately modulated voltage from a DC voltage source could. This would not change the principle of this embodiment. In this case, however, the PFC circuit would be superfluous.

The invention thus enables a with little additional effort despite component tolerances and lamp aging processes, very precise coordination the operating circuit to an average resonance-like continuous operation. If difficulties arise, contrary to the status of Technology of lamp operation continued and as a result of the change in Current setpoint only made a certain reduction in performance. From the perspective of the user is one with hardly noticeably reduced Luminous lamp compared to a non-functional one Lamp to see the far cheaper solution.

Claims (8)

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 a lamp control circuit (12, 13) is provided for controlling the load circuit (1, 8 -11) to a control setpoint (I _{Lamp soll} )
and that the operating circuit 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 Claim 1, in which the detection circuit (12, 13) detects the level of fluctuations in the lamp current (I _lamp ) corresponding to changes in the supply power (5). Operating circuit according to claim 1, wherein the detection circuit (12.13) the amount of changes in utility performance (5) corresponding fluctuations in a manipulated variable of Lamp control circuit (12, 13) detected. Operating circuit according to one of the preceding claims, in which the control circuit (12, 13) has an I control element. Operating circuit according to Claim 2, 3 or 4, in which the detection circuit (12, 13) compares the magnitude of the fluctuations (I _Lamp ) with a predetermined threshold value and only reduces the control setpoint (I _{Lamp should} ) if the threshold value is exceeded , Operating circuit according to one of the preceding claims with a the oscillator circuit (6, 7) with a DC voltage power (5) supplying and connected to a rectifier PFC circuit, which is regulated to the DC voltage (5). Operating circuit according to one of the preceding claims with a the oscillator circuit (6, 7) with a DC voltage power (5) supplying and connected to a rectifier PFC circuit, which is regulated to the DC voltage (5). Operating circuit according to Claim 7, in which a microcontroller (12) a positive control circuit for the oscillator circuit (6,7) and for the PFC circuit contains.

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