Electric lamp and discharge devices: systems – Current and/or voltage regulation
Reexamination Certificate
2002-12-23
2004-09-07
Clinger, James (Department: 2821)
Electric lamp and discharge devices: systems
Current and/or voltage regulation
C315S283000, C315S307000, C315S360000
Reexamination Certificate
active
06788010
ABSTRACT:
TECHNICAL FIELD
The invention is based on an operating appliance for high-pressure discharge lamps as claimed in the precharacterizing clause of claim
1
. The invention in this case relates in particular to a device and to a method for controlling an operating appliance for high-pressure discharge lamps, whose polarity is reversed at regular intervals. A particular feature in this case is the magnitude of overshoots which are initiated by the polarity reversal process. Since the literature also contains the expression very-high-pressure discharge lamp, it should expressly be mentioned that the expression high-pressure discharge lamps in the following text also covers very-high-pressure discharge lamps and the like.
BACKGROUND ART
The so-called square-wave operating mode is widely used for electronic operating appliances for high-pressure discharge lamps (also referred to as a lamp in the following text). In this case, a direct current is applied to the lamp, and its polarity is reversed at regular intervals. The polarity reversal is necessary in order to avoid electrophoresis effects and to load each electrode in the lamp uniformly.
The direct current is generally provided by a pulsed power supply. These pulsed power supplies are generally based on known topologies such as step-down converters (Buck), step-up converters (Boost), inverse converters (Buck-Boost), Cúk or Sepic converters. The voltage which is supplied to the pulsed power supply is generally higher than the voltage on the lamp, so that a step-down converter is preferably used. The power which the pulsed power supply can provide for a load can generally be adjusted by the duty ratio or the switching frequency of electronic switches which are contained in the pulsed power supply. For this purpose, the pulsed power supply has a control input at which a manipulated variable is fed in. If, by way of example, the pulsed power supply is a step-down converter, then a pulse width modulator (PWM) is generally used, which converts the manipulated variable to a drive signal for the electronic switch which is contained in the step-down converter.
The document EP 1 148 768 (Huber) describes a regulation for an operating appliance of this type.
The aim of regulation is to keep the lamp power constant. This is done by forming a set variable, in the form of a current set value, as a function of the lamp voltage. This current set value is compared with a control variable in a regulating device which essentially contains a control amplifier and an adder.
The control variable is provided by a measurement device and is a measure of the current emitted by the pulsed power supply. This current can generally be equated to an inductor current of an inductor which is contained in the pulsed power supply. The measurement device is preferably in the form of a current measurement resistance through which the current which the pulsed power supply emits flows. The regulating device provides a manipulated variable, which is supplied to the control input of the pulsed power supply. This results in regulation of the high-frequency inductor current (>20 kHz) of the inductor which is contained in the pulsed power supply. Two methods for doing this are generally known: presetting the peak current through the inductor (Peak Current Mode) or presetting the mean current through the inductor (Average Current Mode). Until now, the peak current mode has been preferred since it requires less circuitry complexity and inherently limits the inductor current. In order to make it possible to supply a direct current on which as little alternating current as possible is superimposed, the pulsed power supply is followed by a filter network, which generally comprises an LC low-pass filter which contains a filter inductor and a filter capacitor.
For polarity reversal, a polarity reversal network is connected between the pulsed power supply and the lamp. This has a polarity reversal signal input at which a polarity reversal signal is fed in. This allows a control device to reverse the polarity of the lamp voltage, by means of the polarity reversal signal. The polarity reversal network is generally in the form of a full bridge, which contains four semiconductor switches with an associated drive device.
In order to start the lamp, a starting transformer is connected between the pulsed power supply and the lamp and provides a starting voltage before operation of the lamp. A resonant network, comprising an LC resonant circuit, is frequently also connected between the pulsed power supply and the starting transformer, in order to increase the achievable starting voltage.
The described configuration of an operating appliance under discussion conceals the following problem: energy storage devices (starting transformer, resonant network, filter network) are connected between the pulsed power supply and the lamp and, together, they form a reactance network. Together with the lamp, the reactance network forms a load circuit which is caused to oscillate whenever a polarity reversal process takes place. A constant light flux is required, in particular, for use in projection appliances. The polarity reversal process must therefore on the one hand be carried out as quickly as possible, while on the other hand overshoots of the lamp current, and hence of the light flux, during polarity reversal must be kept as low as possible. The control structure described in the prior art produces overshoots which have a disturbing effect in projection applications.
DISCLOSURE OF THE INVENTION
The object of the present invention is to provide an operating appliance for operation of high-pressure discharge lamps as claimed in the precharacterizing clause of claim
1
, and a method for operation of high-pressure discharge lamps as claimed in the precharacterizing clause of claim
11
, which reverses the polarity of the lamp current with overshoots that are less than those in the prior art.
This object is achieved by an operating appliance and an operating method for operation of high-pressure discharge lamps having the features of the precharacterizing clause of claims
1
and
11
, respectively, by means of the features of the characterizing part of claims
1
and
11
, respectively. Particularly advantageous refinements can be found in the dependent claims.
The set variable is normally preset by the user of a system and does not change unless the user changes his requirements for the system. Assuming a constant lamp voltage, the set variable for the lamp current is equally predetermined and is constant in the prior art. The stated precondition is generally true since the lamp voltage varies only very slowly during operation, in comparison to the time intervals between polarity reversals.
According to the invention, the set variable is reduced by a reduction value with a time profile, in synchronism with the polarity reversal and hence in synchronism with the switching signal. The intention of this reduction is to stimulate oscillation in the energy storage devices in the abovementioned reactance network, which counteracts the overshoots caused by the polarity reversal. Ideally, the overshooting is compensated for exactly according to the invention. If the manipulated variable follows the set variable only very slowly, for example by means of a control amplifier which operates essentially as an integrator, for example in a time period which lasts for longer than one microsecond, then it is advantageous to reduce the manipulated variable directly, rather than reducing the set variable. The following statements therefore also apply in the same sense to a reduction in the manipulated variable.
Since at least one microsecond passes between the start of the reduction and any reaction in the reactance network, it is advantageous for the reduction to start even before the control device emits the polarity reversal signal. Practical experiments have shown that it is advantageous to start the reduction at least one microsecond before the output of the polarity reversal signal.
The optimum duration, for
Huber Andreas
Reiter Bernhard
Clinger James
Patent-Treuhand-Gesellschaft fur elektrische Gluhlampen mbH
Tran Thuy Vinh
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