EP1148768A2 - Stabilization of gas dicharge lamps operation - Google Patents

Abstract

Flickering phenomena in gas discharge lamps are undesired in projection technology, in particular. According to the invention, the problem is solved by setting a lamp operation which does not form a focal spot. A specific control structure which includes a cascade structure and feedforward control is proposed for implementing this operation.

Description

Technical field

The invention relates to a method for operating gas discharge lamps according to the preamble of claim 1. The invention also relates a ballast for operating gas discharge lamps according to the preamble of claim 6.

State of the art

When operating a gas discharge lamp (hereinafter also referred to as a lamp) the type of approach to discharge on the electrode depends on whether the electrode emits (cathode) or captures (anode) electrons. In the Anode applies the discharge over a large area over the electrode, while a so-called focal spot (hotspot) usually forms at the cathode, whereby the discharge begins rather punctiform. The point at which the focal spot is dependent on the electrode geometry, the Electrode material and the temperature distribution on the electrode. This Parameters are subject to changes during operation, so the starting point the focal spot can change its position, which is characterized by a Expressed instability of gas discharge (arc restlessness) or flickering. In particular this flickering occurs when the lamp is operated with alternating current, since an electrode alternately forms cathode and anode and therefore forms reform the focal spot with every change from anode to cathode got to.

To reduce flickering, e.g. the so-called rectangular operation from US 4,484,434 the lamp known. It has been found to be for stability the AC operation of high pressure gas discharge lamps advantageous is a rectangular one instead of a sinusoidal lamp current choose. Usual values for the frequencies of the square wave lie at 50Hz to 200Hz. Especially for applications in image acquisition and projection technology, where the constancy of the luminous flux is important rectangular operation prevailed. So that the period in which the Luminous flux does not correspond to the rectangular amplitude, is as short as possible, commutation is sought as quickly as possible.

Despite the rectangular operation, especially with short-arc high-pressure discharge lamps, which are preferably used in projection technology, the stability of the discharge is not yet satisfactory. About the restlessness of the bow To improve, is a pulse-shaped in PCT application WO 95/35645 Excessive increase in lamp current at the end of a rectangular period suggested. With the current increase, a temperature increase occurs which has a stabilizing influence on the position of the focal spot. The duration and level of the pulses and the operating frequency only rough information given. Also the mode of operation of the procedure is only hinted at. Thus the application of the method to a Lamp with a different design (e.g. with a different electrode geometry or something else Filling pressure) than the lamp mentioned in the exemplary embodiment possible after extensive experimentation.

However, not only is it a problem to determine an appropriate current waveform, but, as will be explained below, it is also a problem to create a desired curve shape. The load circuit of an arrangement for operating a discharge lamp contains, inter alia, energy stores, which can also be parasitic and the lamp which is a non-linear Represents load.

The network of energy stores forms resonance frequencies that can be excited by the nonlinear load. Especially during operation short-arc high-pressure lamps lead to long-lasting settling processes after commutation of the lamp current in rectangular operation. These vibrations can of course also be observed in the luminous flux. For applications that require a high level of luminous flux (e.g. video projection), care must therefore be taken that the time period in which transients occur is small compared to that Period of the square wave. Significant influence on the duration the transient has the one used in the operating device in question Regulator. In conventional control gear for the applications mentioned a quantity is generated which represents a measure of the lamp power and compared to a reference measure. The result of this comparison is the Control variable for the power section of the control gear. The settling time for a light source with rectangular operation can be defined by the time which elapses from commutation to the time the Luminous flux leveled off in a band of +/- 5% around the setpoint Has. For conventional controllers described above, this settling time is 250µs-300µs. Since the settling time is at most 10% of a half period the square wave should be with conventional ones Regulator frequencies for the square wave of at most 200 Hz can be realized.

Presentation of the invention

The object of the present invention is divided according to the statements to the state of the art in two parts: On the one hand, the invention is intended Provide method according to the preamble of claim 1, the with clearly defined parameters an almost flicker-free operation of a gas discharge lamp allowed. On the other hand, the invention means according to the Provide the preamble of claim 6 with which the above method is implemented can be.

The first part of the task is followed by a procedure with the characteristic Features of claim 1 solved. Particularly advantageous configurations can be found in claims 2 to 5, which are dependent on claim 1.

As explained in the explanations on the prior art, the cause lies for the flickering of a lamp is due to the fact that the focal spot that the Approach of gas discharge on the cathode represents its position constantly changes. A closer analysis shows that immediately after an electrode commutated to the cathode, no focal spot is formed. Rather finds first of all a flat discharge approach. Only after one results in thermal inhomogeneity on the cathode, the discharge is laced and forms a focal spot. According to the invention, flickering can occur the lamp can be greatly reduced by commutating the Lamp current is carried out before discharging a focal spot trains. For the quickest possible change of an electrode from the Steep current edges are necessary for the cathode to the anode, which is why Realize the method very well with a rectangular current profile leaves. Because of a flicker-free operation especially for applications in projection technology is important, the process has special meaning for lamps, which are used in such applications. These are first Line high and high pressure discharge lamps and because of the optical Image qualities, especially those with a short discharge arc. For Such lamps must have the frequency of the rectangular lamp current are at least 300 Hz around the teaching of the method according to the invention to suffice.

Is the procedure used for the first time on a lamp copy or the lamp has been operated in the meantime using another method, so it can despite the application of the inventive method for flickering appears a short time after the lamp has been put into operation. The reason for this is an electrode structure that is quick to train favored by focal spots in different positions. The application of the method according to the invention, however, causes the Electrodes that have a stabilizing influence on the discharge arc. Thus, an almost flicker-free operation comes through the inventive Procedure after a short time.

As described above, the method according to the invention is implemented for ultra-high pressure short-arc lamps, a frequency for the rectangular one Lamp current of at least 300 Hz is required while using control gear, which contain a conventional controller structure, a frequency of a maximum of 200 Hz can be realized. Closing this gap is the second part the object of the present invention. It is controlled by an operating device solved with the characterizing features of claim 6. Especially advantageous configurations can be found in the dependent on claim 6 Claims 7 to 10.

In an operating device for gas discharge lamps, an output voltage UA is usually generated from a constant, so-called intermediate circuit voltage U0 with the aid of a clocked DC / DC converter. This is a DC voltage that can be set using a manipulated variable Us . The DC / DC converter can be of different types, such as step-up, step-down or inverse converters. The manipulated variable Us causes a variation in the duty cycle of the circuit breakers contained in the converters in these converters. The rectangular operation of the lamp is usually realized in that the output voltage UA is reversed by means of a full bridge circuit with the desired frequency for the square wave.

The control variable of the control gear is the power of the lamp (pist). In cases where the lamp power is difficult to determine and the power loss of the control gear is known with sufficient accuracy, the input power of the DC / DC converter can also be used as a controlled variable. In conventional operation devices Pist is compared with a target value thereof Psoll and without the aid of further measurable variables directly or after weighting by a control characteristic (P, PI, I, PID) of the manipulated value U determined. However, this structure does not allow a short settling time after commutation of the lamp current.

According to the invention, the problem is solved by two measures: cascade control and feedforward control. The cascade control, as it is also used in principle in the so-called current mode in switched-mode power supplies, is implemented in the operating device according to the invention in that the evaluated control difference from Pist and Psoll does not determine the control value Us , but rather defines a setpoint for the lamp current Isoll . Isoll is compared with the List value , which is a measure of the lamp current , and it is only this comparison result that determines the manipulated variable Us directly or after weighting using a control characteristic (P, PI, PID). The feedforward control is implemented as follows in the operating device according to the invention: The output voltage UA, which is to be measured at the lamp terminals, is one of the factors determining the lamp power. Auxiliary circuits (eg ignition circuits) and supply lines can lead to fluctuations in the output voltage UA . Fluctuations in UA interfere with the control process, especially during the transient process after lamp current commutation. Therefore Isoll according to the invention not only determined by the control difference of Pist and Psoll, but also brought into a function of the output voltage UA. This can also be done by means of a weighting using a control characteristic, a differentiating characteristic preferably being selected in order to emphasize the fluctuations in UA .

The invention is illustrated by the following figures.

Description of the drawings

Figur 1

eine flackernde Entladung

Figur 2

eine flackerfreie Entladung

Figur 3

ein Blockschaltbild der Reglerstruktur

Figur 4

einen Schaltplan eines bevorzugten Ausführungsbeispiels

A preferred embodiment of the controller structure according to the invention and the results which can be achieved thereby when operating a gas discharge lamp are explained in more detail below with reference to the accompanying drawings. Show it:

Figure 1

a flickering discharge

Figure 2

a flicker-free discharge

Figure 3

a block diagram of the controller structure

Figure 4

a circuit diagram of a preferred embodiment

1 shows the discharge of a short-arc high-pressure lamp immediately before the commutation of the lamp current. You can see the trained one Focal spot. Such a discharge does not correspond to the teaching of the present Invention and therefore tends to flicker.

Fig. 2 also shows the discharge of a short-arc high-pressure lamp directly before commutating the lamp current. However, the frequency is now of the rectangular lamp current so high that there is no focal spot trains. This corresponds to the teaching of the present invention, which is why this discharge shows only negligible flickering.

3 shows a block diagram of a controller structure according to the invention. Since the lamp power is to be regulated in a higher-level control circuit, the control difference from pist and Psoll is first formed in a first subtraction point S1 and evaluated with a control characteristic RC1. The control characteristic RC1 can be a P, PI, I, or PID characteristic. The weighted signal is fed to a second subtraction point S2. The output voltage UA evaluated with the control characteristic RC2 is subtracted. The control characteristic RC2 is implemented in FIG. 3 in a preferred differential characteristic (DT1), but can in principle also have a different characteristic (e.g. P, PI, I, or PID). In the second subtraction point S2, the feedforward control implemented in the description section is implemented.

The output of the second subtraction point S2 represents the setpoint Isoll of the inner control loop of the cascade control carried out in the description part. Isoll is compared in the third subtraction point S3 with a variable which corresponds to the value of the lamp current . The result of this comparison becomes, after weighting with a control characteristic RC3, the manipulated variable Us . The control characteristic RC3 can be a P, PI, or PID characteristic.

FIG. 4 shows a circuit in which the control structure shown in FIG. 3 is implemented. In the following, components denoted by an R followed by a number are resistors, components denoted by a C followed by a number are capacitors and components denoted by a T followed by a number are to transistors. The central component is the UCC3800 Current Mode Controller available from Unitrode . This IC contains the first (S1) and the third (S3) subtraction point, options for determining the control characteristic RC3, as well as a circuit that generates the manipulated variable Us as a clocked signal for controlling the circuit breaker of the DC / DC converter mentioned in the descriptive part. This circuit breaker is typically a MOSFET, the time during which it is switched on is varied by its signal at the gate. This signal is available on the UCC3800 at pin 6 (OUT). An internal oscillator is required to generate the signal. The frequency of the oscillator can be set by R108 and C103 if it runs freely. In this case, the DC / DC converter works in the so-called continuous mode. R108 and C103 are connected in series. The connection point is connected to PIN 8 (REF) and a reference voltage of 5V. The other end of R108 is connected to PIN 4 (RC), the other end of C103 is connected to ground.

Under certain operating conditions which are not directly related to the invention, the DC / DC converter is switched on by means of a circuit part which contains the components C6, R1, R2, R107, T100, R106, C101, R105, D102, R104 and C102 transferred to discontinuous mode . This circuit part is controlled by the voltage at the drain of the above-mentioned MOSFET. The series connection of C6, R1, R2 and R107 lies between the drain and the operating voltage of 10.5V. The resistor R107 is connected to the supply voltage and the emitter of T100 at the same time. The other connector is connected to the base of T100. R106 and C101 are located on the collector of T100. The other terminal of R106 is grounded, the other terminal of C101 is connected to R105 and to the anode of D102. The other connection of R105 is grounded, the cathode of D102 is connected to R104 and C102. The other connection of R104 is grounded, the other connection of C102 is connected to pin 4 (RC) of the UCC3800 .

The UCC3800 is connected to an operating voltage (10.5V) and ground at pin 7 (VCC) and pin 5 (GND). Psoll is fed in via pin 8 (REF); in this case a reference voltage of 5V.

The circuit part which contains the components R11, R28, R29, R31, R117, R24, R25, IC11-B, R101 C13, C12, R20, R22 and IC11-A serves to provide Pist. IC11-A and IC11-B are operational amplifiers. The circuit section supplies a voltage at the output of IC11-A (Pin1) that is proportional to the input power of the DC / DC converter. For this purpose, the intermediate circuit voltage U0 via the terminal UA1 is an inverting amplifier, the components R11, R28, R25, R24 and IC11-B contains fed. R11 and R28 form a voltage divider between UA1 and ground. The signal at the junction of R11 and R28 is fed to the inverting input of IC11-B (Pin6). The non-inverting input of IC11-B (Pin5) is connected to a reference voltage of 2.5V. The feedback resistor R25 is located between the output of IC11-B (Pin4) and the inverting input of IC11-B. The output of IC11-B is connected to the inverting input of IC11-A (Pin2) via the series connection of R24 and R101.

With the junction of R24 and R101, the resistors are R31, R29 and R117 connected. The other connection of R29 is grounded, the other Connection of R117 is connected to the reference voltage of 5V and the other connection of R31 leads to the connection Poti. On the Connection potentiometer, a potentiometer can be connected to ground and thus the lamp power can be adjusted.

The components R101, R22, C13, R20, C12 and IC11-A form an adder in which the amplified voltage signal UA1 and that via the connection source supplied signal, which is a measure of the input current, is added.

The signal from the source connection is the non-inverting via R22 Input from IC11-A (Pin3) fed. C13 lies between the non-inverting Input from IC11-A and ground. Between the inverting input IC11-A and the output of IC11-A are connected in series of C12 and R20.

The addition represents an approximation of the multiplication at the operating point, whereby a signal is present at pin 1 of IC11-A, the voltage value of which is a measure of the input power of the DC / DC converter. With the help of C12, the adder simultaneously generates the control characteristic RC1, in this case a PI characteristic. A weighted pist signal is thus available at pin 1 of IC11-A.

With a constantly regulated input power and constant DC link voltage U0 , the input current, for which the signal supplied via the source connection is a measure, is at the same time a measure of the lamp current List. To implement the inner control loop of the cascade control, the signal of the source connection is fed via R114 to pin 3 (CS) and thus to the third subtraction point S3 integrated in the UCC3800 .

The outer control loop of the cascade control is closed via R112, which connects the output of IC11-A and pin 2 (FB) of the UCC3800 . Pin 2 (FB) of the UCC3800 simultaneously represents the signal Isoll and the second subtraction point S2. The output voltage UA of the DC / DC converter is present at connection UA. Pin 2 (FB) of the UCC3800 is fed via the series connection of C100 and R111, thus realizing the described disturbance variable connection. C100 and R111 form the control characteristic RC2; in this case a DT1 characteristic.

The control characteristic RC3 can be determined by the components C104 and R109 connected in parallel, which are connected between pin 1 (COMP) and pin 2 (FB) of the UCC3800 ; in this case a PI characteristic.

The pin designations of the UCC3800 given in brackets refer to the manufacturer's data sheet, the company UNITRODE, Merrimack, USA.

Claims (11)

A method for AC operation of one or more parallel-connected gas discharge lamps having the lamp current carrying electrodes, due to the alternating current operation, alternately cathode and represent anode, the gas discharge on the cathode can constrict by a flat approach to a so-called. Focal spot, characterized characterized in that the polarity of the lamp current is switched over before the discharge on the electrode currently representing the cathode constricts from a flat attachment to a focal spot. Method for AC operation of one or more gas discharge lamps connected in parallel according to claim 1, characterized in that the lamp current is rectangular. A method for AC operation of one or more gas discharge lamps connected in parallel according to claim 1, characterized in that the lamp is a high or ultra-high pressure discharge lamp. Method for AC operation of one or more gas discharge lamps connected in parallel according to claim 3, characterized in that the lamp is a short-arc lamp. Method for AC operation of one or more gas discharge lamps connected in parallel according to claim 4, characterized in that the value of the frequency of the lamp current is greater than 300 Hz and the lamp current is rectangular. Ballast for operating one or more gas discharge lamps connected in parallel, which has the following features: Device for providing a DC voltage (output voltage UA ), Device for providing an electrical quantity which is a measure of the lamp power ( Pist ), Device for providing an electrical variable which is a measure of the setpoint value for the lamp power ( Psoll ) Device for providing an electrical quantity that is a measure of the lamp current ( Iact ), Device for regulating electrical quantities characterized in that, depending on the quantities Pist, Psoll and UA , the control device defines a quantity which is a measure of the setpoint of the lamp current ( Isoll ) and sets the lamp current by comparison with List. Ballast for operating one or more gas discharge lamps connected in parallel according to claim 6, characterized in that the quantities Pist and Psoll are evaluated with a proportional and / or integral and / or differential control characteristic. Ballast for operating one or more gas discharge lamps connected in parallel according to claim 6, characterized in that the variable UA is evaluated with a proportional and / or integral and / or differential control characteristic. Ballast for operating one or more gas discharge lamps connected in parallel according to claim 6, characterized in that the variables Iact and Isoll are evaluated with a proportional and / or integral and / or differential control characteristic. Ballast for operating one or more gas discharge lamps connected in parallel according to Claim 6, characterized in that lamp operation according to the teaching of one of Claims 1 to 5 is thereby realized. Method for operating one or more gas discharge lamps connected in parallel according to Claim 1, characterized in that the method is implemented by a ballast in accordance with the teaching of one of Claims 6 to 9.

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