DE102006035071A1 - Lamp e.g. gas-discharge lamp, brightness adjusting device for background lighting, has modulator applying voltage to lamp, such that voltage is sufficient and reduced during intervals, where transitions between intervals are decelerated - Google Patents

Abstract

The device (10) has a pulse width modulation modulator (14), modification circuit (16), pulse width modulation circuit (18), bridge circuit (20), and transformer (22) for applying pulsed effective voltage to a fluorescent lamp (12), such that the effective voltage is sufficient during pulse in intervals for igniting the fluorescent lamp and is reduced during pulse out intervals for igniting the fluorescent lamp. The transitions between the pulse in intervals and pulse out intervals are decelerated. An independent claim is also included for a method for adjusting brightness of a background lighting.

Description

The The present invention relates to a device for adjustment the brightness of a backlight according to the generic term of claim 1. Furthermore, the invention relates to a method according to the preamble of claim 15.

Backlights be among others for liquid crystal displays (LC displays) needed, for example for Computer screens are used. Similar backlighting can also be found in other types of advertisements in a wide range of applications, For example, in motor vehicles, illuminated billboards and the like. As a fluorescent lamp are at backlights in general Cold cathode tubes used. They have the advantage of producing low amounts of heat a relatively long life and high efficiency. Further For example, the electrode structures are simple, producing very small cold cathode tubes can be which can also be used in small liquid crystal displays are. However, increasingly hot cathode tubes are being used. Especially at larger display areas with high standards the light homogeneity a variety of lamps are used (e.g., 8, 16, or 32 single lamps) in a regular grid are arranged behind optical diffuser plates.

A Cold cathode tube comprises a tube with a high voltage connection a first end of the tube and a low voltage terminal at the second end of the tube. Of the High voltage connection is supplied with a high-frequency alternating voltage, whereby usual supply voltages a frequency of about 30 to 70 KHz and an amplitude of about 600 to 1500 V have. These voltages are sufficient for a lamp current in the cold cathode tube due to which the cold cathode tube lights up.

in principle Can the brightness of fluorescent lamps by the strength of the Current flowing through them (in the present document as "lamp current") Dimming the lamp is doing the lamp current through appropriate Lowering the lamp voltage lowered. A disadvantage of this method but is that the The lamp then works beyond its optimal operating point results in a reduced light output. Another serious Problem is that with this method no strong dimming possible is because then disturbing on the display Lichtinhomogenitäten become visible. Because with small lamp currents is especially the high voltage end Lamps due to parasitic capacitive currents considerably brighter than their low-voltage end.

Out For this reason, one usually gives way another method, namely on pulsed lamp operation off. This is due to the lamps for their optimum operating current necessary voltage for a few milliseconds. Then the voltage for turned off a few more milliseconds. Usually you work with a constant pulse rate of the order of 100-300 Hz. Lower frequencies would cause noticeable flicker, significantly higher frequencies burden the Lamps with unnecessary many power-ups and reinforce acoustic noise, because the sensitivity of human hearing in this frequency range quickly increases with the frequency.

in the pulsed lamp operation can use a so-called burst signal be used to modulate the lamp current. The burst signal is typically a PWM signal and modulates the lamp current such that during the Pulse duration of the burst signal a lamp current flows, and between the pulses, this means with a burst signal of 0 V, no lamp current flows.

The Burst signal typically has a frequency of 90 to 300 Hz and is thus slow compared to the AC voltage applied to the fluorescent lamp is applied. At the same time the frequency of the burst signal is like that high, that the individual light and dark phases with the human visual sense not can be differentiated, but a uniform brightness is perceived, which corresponds to the temporal mean of the light and dark phases. Thus, the duty cycle corresponds of the burst signal of the human perceived brightness of the Fluorescent lamp.

An apparatus for adjusting the brightness of a backlight having such a PWM burst signal is disclosed in U.S.Pat US 6,198,234 B1 described.

at The pulsed operation of backlights more often disturbing noises. The invention is based on the object, the occurrence of such disturbing Sounds to avoid or at least reduce.

These Task is done according to a First aspect of the invention by an apparatus and a method for adjusting the brightness of a backlight according to claims 1 or 15 solved. Advantageous developments are specified in the dependent claims.

The invention is based on the observation that the noise is related to the pulsed operation and on the recognition that the abrupt changes in the lamps are effective tion are responsible for the noise development.

The mentioned above be acoustic noise Of the acted upon with high magnetic field strengths Magnetic components, in particular emitted by the transformers. As the cause of the noise In particular, those components were identified in the Fourier spectrum of the pulses, the above magnetic force effects and the effect of magnetostriction mechanical Natural frequencies of the components stimulate. These are mainly Fourier components in the range of 0.5 kHz to 10 kHz. But not only the magnetic ones Components, but also the long thin fluorescent lamps are going through the pulsed high voltage field in the rhythm of the dimming frequency triggered and vibrate in their mechanical natural frequencies.

• 1. Magnetostriktion: Der Magnetkern in dem Transformator, der zum Erzeugen der Lampenspannung verwendet wird, wird in Abhängigkeit des ihn durchsetzenden magnetischen Felds deformiert und kann dadurch in Schwingung versetzt werden. Beim Transformator hängt die Magnetostriktion von der Transformatorspannung, nicht aber vom Transformatorstrom ab.
• 2. Magnetkräfte zwischen getrennten Teilen des Transformatorkerns: Viele Transformatoren haben Kerne, die aus zwei Teilen bestehen, beispielsweise zwei E-förmigen Teilen oder einem U- und einem I-förmigen, die mit einem geringen Abstand zwischen sich aneinander angrenzend angeordnet sind. Wenn das Magnetfeld im Transformatorkern eingeschaltet ist, ziehen die beiden Teile des Transformatorkerns sich mit einer erheblichen Kraft an. Durch das sich zeitlich ändernde Magnetfeld im Transformatorkern können daher die beiden Teile des Transformatorkerns gegeneinander in Schwingung versetzt werden, wodurch Geräusche entstehen können. Die Resonanzfrequenz für diese Schwingung hängt insbesondere von der Masse der Kernteile und dem Elastizitätsmodul eines Füllmaterials ab, mit dem der Zwischenraum zwischen den Kernteilen gefüllt ist.
• 3. Magnetkraft zwischen den Drähten in den Transformatorwicklungen: Diese Geräuschquelle hängt wesentlich von dem Transformatorstrom ab.
• 4. Elektrostatische Kräfte, die an der Leuchtstofflampe anliegen: An den Enden von Leuchtstofflampen liegen hohe Wechselspannungen an. Durch die Potentialunterschiede zwischen den Enden einer Lampe und deren Umgebung erfährt die Lampe eine elektrostatische Kraft, durch die die Lampe in Schwingung versetzt werden kann. Diese elektrostatischen Kräfte sind insbesondere deshalb wirksam, weil die Umgebung der Leuchtstofflampe bei einer Hintergrundbeleuchtung naturgemäß asymmetrisch ist, da sich ja auf der einen Seite der Beleuchtung die zu durchleuchtende Schicht, beispielsweise die LC-Schicht befindet und auf der anderen Seite ein Gehäuse mit den zum Betrieb der Leuchtstofflampen benötigten Schaltungen. Während sich die elektrostatischen Kräfte bei einer symmetrischen Umgebung gegenseitig aufheben könnten, ist dies bei der asymmetrischen Umgebung der Lampen in Hintergrundbeleuchtungen nicht der Fall, so daß durch die elektrostatische Anziehung beträchtliche Gesamtkräfte auf die Leuchtstofflampen ausgeübt werden, die die Lampen in Schwingung versetzen können.

In particular, the inventor identified the four sources as major sources of noise:

• 1. Magnetostriction: The magnetic core in the transformer used for generating the lamp voltage is deformed in response to the magnetic field passing through it, and can thereby be vibrated. For the transformer, the magnetostriction depends on the transformer voltage, but not on the transformer current.
• 2. Magnetic forces between separate parts of the transformer core: Many transformers have cores which consist of two parts, for example two E-shaped parts or one U-shaped and one I-shaped, which are arranged adjacent to each other with a slight distance between them. When the magnetic field in the transformer core is turned on, the two parts of the transformer core attract with considerable force. Due to the time-varying magnetic field in the transformer core, therefore, the two parts of the transformer core can be vibrated against each other, which can cause noise. The resonant frequency for this vibration depends in particular on the mass of the core parts and the modulus of elasticity of a filling material, with which the gap between the core parts is filled.
• 3. Magnetic force between the wires in the transformer windings: This noise source depends substantially on the transformer current.
• 4. Electrostatic forces applied to the fluorescent lamp: high alternating voltages are present at the ends of fluorescent lamps. Due to the potential differences between the ends of a lamp and its surroundings, the lamp undergoes an electrostatic force by which the lamp can be vibrated. These electrostatic forces are particularly effective because the environment of the fluorescent lamp is naturally asymmetric in a backlight, since yes on one side of the illumination to be screened layer, for example, the LC layer and on the other side a housing with the Operation of the fluorescent lamps required circuits. While the electrostatic forces could cancel each other out in a symmetrical environment, this is not the case with the asymmetric environment of the backlight lamps, so that the electrostatic attraction exerts considerable overall forces on the fluorescent lamps which can cause the lamps to vibrate.

In most cases the first two sources are predominantly for noise development causally.

As previously mentioned, the inventor has observed that it a connection between the pulsed operation and the noise gives. The above-identified sources of noise have after observation However, the inventor's resonance frequencies in the range of 0.5-10 kHz, thus well above the frequency of the voltage pulses of typically approximately 100-300 Hz are. Apparently it is not the low frequency Fourier components of the voltage pulses on the fluorescent lamp, the for the noise are responsible. But since the voltage pulses conventionally a rectangular shape with approximate vertical flanks, they also contain a significant proportion high-frequency Fourier components. The invention is the realization based on that straight these high-frequency Fourier components of the usual voltage pulses for noise responsible in the sources of resonance identified above.

The inventive solution to the problem of noise development is based on the reduction of said Fourier components in the voltage pulses at the lamp. Two physical methods are used for this: First, the edge steepness of the pulses is reduced. The voltage is not turned on and off abruptly, as in the prior art, but adequate hardware and / or software in the lamp control circuit ensures a sufficiently slow voltage drop. Slow is called in a period of typically 1 millisecond or more. If the transition time is chosen to be less than 0.5 milliseconds, the effect of the measure is gradually lost again, since the proportion of higher frequency Fourier components in the kilohertz range increases accordingly and increasingly mechanical oscillations of the Magnetic components are excited.

According to the second Method, the lamp voltage in the dark phases does not become zero withdrawn but merely reduced to a value at which a stationary Lam penbetrieb not possible anymore is, the fluorescent lamp goes out. By raising the Voltage levels in the dark phases become the amplitudes of all AC Fourier components reduced, and thus the acoustic noise.

While at the first solution the jump in the magnetic flux and voltage amplitude between the light and dark phases of the Fluorescent lamp stretched in time and thus made less sharp will be, according to the second solution the jump height reduces what re the noise a similar one Has effect. Preferably, however, both solutions are combined, as below based on an embodiment is explained in more detail.

Preferably a burst generator is used which generates a burst signal, with which the effective lamp current or the effective lamp voltage is modulated.

In an advantageous development of the burst generator comprises a PWM modulator for generating a square wave signal and a modification circuit for modifying the square wave signal, wherein the modification circuit comprises a low-pass filter, in particular an RC element and / or a voltage window circuit, the incoming signals, which under a lower threshold V _ref1 , than outputs V _ref1 , and outputs the incoming signals, which are above an upper threshold V _ref2 , as V _ref2 . By means of this burst generator, a burst signal modified in the sense of the invention can be generated with simple means.

Preferably comprises the device further comprises a PWM circuit having an input for the burst signal has and outputs a PWM switching signal whose duty cycle is the current signal value of the burst signal corresponds. This PWM switching signal can in a bridge circuit a transformer, which has an AC voltage across the primary of the transformer generated. The frequency of the PWM switching signal is preferably 30 to 70 KHz.

at This development is therefore the duty cycle of the PWM switching signal predetermined by the current signal value of the burst signal. By the duty cycle of the PWM switching signal In turn, the output power of the transformer and thus the Lamp current set. In this way, the lamp current can be on simple way using standard components according to the Modulate burst signal.

Further Advantages and features of the invention will become apparent from the following Description in which the invention with reference to an embodiment of Reference to the attached Drawings closer explained becomes. Show:

1 a block diagram of the device according to an embodiment of the invention,

2 a circuit diagram of a modification circuit according to an embodiment of the invention,

3A a diagram in which the timing of a burst signal and the associated PWM switching signal is shown schematically,

3B a diagram in which the time course of a modified according to the first aspect of the invention burst signal is shown schematically, and

3C a diagram in which the time course of a modified according to the first and the second aspect of the invention burst signal is shown schematically, and

4 a block diagram of a device according to another embodiment of the invention.

1 shows a block diagram of a device 10 to adjust the brightness of a lamp 12 a backlight. The device 10 comprises a burst generator consisting of a PWM modulator 14 and a modification circuit 16 consists. The PWM modulator 14 is used to generate a square wave signal, which in the modification circuit 16 is inputted and modified therein to give the burst signal. The modification circuit 16 is with an input of a PWM circuit 18 connected. In the PWM circuit 18 a PWM switching signal is generated whose duty cycle corresponds to the signal value of the burst signal present at the input of the PWM circuit 18 is applied.

With the PWM circuit 18 is a bridge circuit 20 connected. The bridge circuit 20 includes transistors which are the PWM switching signal from the PWM circuit 18 be switched accordingly to generate an AC voltage across a primary winding 24 a transformer 22 is applied.

The secondary winding 26 of the transformer 22 is over a capacitor 28 with the fluorescent lamp 12 connected, which is typically a cold cathode tube.

In 2 is the modification circuit 16 shown in detail. The Modifcationsschaltung 16 has an entrance 30 for the square wave signal previously used in the PWM modulator 14 (please refer 1 ) was generated. At the entrance 30 includes a low pass filter 32 on, in the embodiment shown by an RC element of a resistor 32 and a capacitor 36 is formed.

To the RC member 32 closes a voltage window circuit 38 at. The voltage window circuit 38 includes a first potentiometer 40 connected to the non-inverting input of a first operational amplifier 42 connected is. The output of the first operational amplifier 42 is over a first diode 44 with the inverting input of the first operational amplifier 42 connected. The first potentiometer 40 is set so that the non-inverting input of the first operational amplifier 42 a voltage V _ref2 is applied. The first operational amplifier 42 causes in connection with the forward direction of the first diode 44 in that a tension in the node 46 can not rise above a value of V _ref1 .

The voltage window circuit 38 further includes a second potentiometer 48 comprising the non-inverting input of a second operational amplifier 50 at a voltage V _ref1 which is lower than V _ref2 . The output of the second operational amplifier 50 is via a second diode 52 with the inverting input of the first operational amplifier 50 connected. Note that the reverse direction of the second diode 52 is the reverse of that of the first diode 44 , Thus, the second operational amplifier causes 50 that one at the node 47 voltage applied can not fall below the value V _ref1 . (For simplicity, the internal resistances of the diodes 44 . 52 neglected here). To the voltage window circuit 38 closes a voltage follower 54 at.

The following is the function of the device of 1 and 2 with reference to 3A to 3C explained. First, the function of the device without the modification circuit 16 described. For this we first imagine that the modification circuit 16 in 1 is missing and the PWM modulator 14 directly with the PWM circuit 18 connected is. Such an arrangement without the modification circuit 16 is known from the prior art. The PWM modulator 14 gives a pulse width modulated square wave signal 56 in the diagram of 3A is plotted over time t. This pulse width modulated square wave signal 56 is hereinafter referred to as "unmodified burst signal" because it is without the modification circuit 16 to be entered into the PWM circuit. The unmodified burst signal 56 consists of pulses that are in 3A hatched and during which the unmodified burst signal 56 assumes a first signal value. Between pulses the unmodified burst signal decreases 56 a signal value of zero. The unmodified burst signal 56 has a low frequency of about 100 Hz compared with the frequency of the lamp AC voltage.

The unmodified burst signal 56 gets into the PWM circuit 18 entered. In the PWM circuit 18 becomes a high-frequency PWM switching signal 58 generated, which is also in the diagram of 3A is shown. The switching signal 58 has a frequency of 30 to 70 KHz, and its duty cycle corresponds to the current signal value of the unmodified burst signal 56 which is in the PWM circuit 18 is input as an analog input signal. As in 3A can be seen, is the duty cycle during the pulse of the unmodified burst signal 56 nearly 100% and between pulses 0%.

Such a PWM circuit 18 is known from the prior art. For example, it may include a comparator containing the inputted unmodified burst signal 56 with a high-frequency carrier signal compares and so a high-frequency switching signal 58 whose duty cycle corresponds to the signal value of the unmodified burst signal 56 equivalent.

The switching signal 18 gets into the bridge circuit 20 of the transformer 22 entered. By the switching signal 58 are actuated switches, typically transistors, which in turn generate a high-frequency AC voltage at the primary winding 24 of the transformer 22 is applied. This will be the power of the transformer 22 by the duty cycle of the switching signal 58 certainly. As 3A can be seen, has the switching signal 58 during the pulses of the unmodified sausage signal 56 a high duty cycle, which corresponds to a high transformer output voltage and thus a high lamp current. Between the pulses of the unmodified burst signal 56 (that is at signal value equal to zero) is also the duty cycle of the switching signal 58 equal to zero, so that the output voltage of the transformer 22 also equal to zero. Thus, the pulses of the unmodified burst signal correspond to the luminous phases of the fluorescent lamp 12 and the portions between the pulses the dark phases of the fluorescent lamp 12 , This modulation of the lamp current is too fast at a burst frequency of 100 Hz for it to be perceived with the human sense of sight. The viewer perceives only a uniform, non-flickering de illumination whose brightness is the duty cycle of the unmodified burst signal 56 equivalent. Thus, the fluorescent lamp can be 12 by setting the duty cycle of the unmodified burst signal 56 dim.

As described in the beginning, the magnetic field in the transformer can be used 22 , by the current in the transformer (and thus the lamp current) and by the high voltage applied to the fluorescent lamp 12 is applied, mechanical vibrations are generated, which can cause a disturbing noise. However, these electrodynamic quantities oscillate at a frequency of 30 KHz to 70 KHz, ie with a frequency which is above the audible spectrum. Therefore, the "normal" oscillation of these quantities does not produce any mechanical resonances that would be audible.

In burst mode, however, the amplitudes or rms values of all of these quantities become the burst signal 56 modulated. Because the unmodified burst signal 56 a sharp square wave signal containing many high-frequency Fourier components, this modulation of the amplitudes can produce mechanical vibrations with resonance frequencies of a few kilohertz (eg 0.5 kHz to 10 kHz) and produce an unpleasant hissing noise. The disturbing effect on the user is exacerbated by the fact that human hearing is much more sensitive at higher frequencies than at low frequencies.

The noise can be significantly reduced or avoided altogether if the abovementioned variables (magnetic flux, lamp current, lamp voltage) are less abruptly modulated than with the square-wave signal 56 from 3A the case is. In 3B is a first modified burst signal 56 ' shown, whose flanks are slightly bevelled. At the first modified burst signal 56 ' the transition between the first and the second signal value is slowed down, so that the transition takes about 1 ms in total. This slowdown of the transition is made with the RC link 32 the modification circuit 16 from 2 achieved in a simple manner.

Experiments have shown that such a deceleration of the transition, or, graphically speaking, flattening of the edges of the burst signal noise can be effectively suppressed. The reason for this is apparently that the less "sharp", somewhat flattened burst signal contains less of the high frequency Fourier components responsible for exciting the interfering resonances 3B is the change in the duty cycle of the switching signal 58 shown schematically during the right flank of the first pulse. The duty cycle does not change immediately, as in the case of 3A the case was, but gradually changed over a period of several cycles.

In 3C is another modified burst signal 56 '' shown. This modified burst signal 56 '' has as well as the signal 56 ' from 3B Beveled flanks, with the same positive effect on noise as described above. In addition, however, the modified burst signal takes 56 '' not a value of zero between the pulses, but a value V _ref1 which lies between the first signal value and zero. Accordingly, there is between the pulses of the modified burst signal 56 '' a duty cycle of the switching signal 58 (in 3C not shown), which is different from zero.

Due to this reduced duty cycle between the pulses, the bridge circuit becomes 20 between the pulses is not simply turned off, but it is a certain magnetic flux in the transformer and thus maintain a certain lamp voltage. As a result, the change in the amplitude of the magnetic flux and the lamp voltage is not only less abrupt but also smaller overall, which in turn contributes to the avoidance of noise. The second signal _value , V _ref1 , however, is selected so that the corresponding lamp voltage for igniting the fluorescent lamp 12 is too low. That is, between the pulses of the modified burst signal 56 '' is the fluorescent lamp 12 nevertheless dark, so that the adjustment of the brightness functions exactly as in the case of the diagrams 3A and 3B ,

The modified burst signal 56 '' from 3C is obtained by the unmodified burst signal 56 from 3A in the modification circuit 16 from 2 is entered. First, the vertical edges of the incoming square wave signal through the RC element 32 "Flattened." The voltage window circuit 38 causes the signal at the node 46 does not fall below the value of the lower threshold V _ref1 and does not exceed the upper threshold V _ref2 , whereby the waveform of 3C is obtained.

Both the slowing of the transition between the signal values (see 3B ) as well as the increased second signal _value (V _ref1 instead of V = 0) are each a solution to the problem of reducing or completely avoiding the noise and can also be used individually if this is advantageous for practical reasons. A particularly effective solution, however, is the combination of the two measures as described in 2 and 3C is shown.

In the embodiment described above, an analog burst signal was used, which was made of special hardware (burst generator 14 , Modification circuit 16 ) was generated. The described modification of the burst signal but is only one embodiment by which the abrupt modulation of the lamp effective voltage can be accomplished, and the invention is not limited to this embodiment. For example, the bridge transistors could also be controlled directly by a microcontroller, so that the method can also be programmed without any specific hardware. It is only essential that the transitions between the pulse-on intervals and pulse-off intervals of the lamp effective voltage be extended in time and / or reduced in their jump height, that a noise is suppressed. Note that the timing of the modified burst signal 56 ' . 56 '' from 3B and 3C qualitatively corresponds to the course of the lamp effective voltage.

In 4 is an alternative embodiment of a device 60 for setting the brightness of a backlight is shown schematically. The device 60 also includes a fluorescent lamp 12 and a transformer 22 , The AC operating voltage for the fluorescent lamp 12 is from a bridge circuit 62 generated, which is a half or full bridge with field effect transistors, and of the transformer 22 transformed to the required high voltage level of typically 600 to 1500 volts. The actual value of the lamp current is measured and via a line 64 in a bridge control circuit 66 entered. In the bridge control circuit 66 it is compared with a setpoint, and the effective value of the lamp voltage is increased or decreased accordingly. This creates a loop for the secondary flow.

Also with the device 60 becomes the effective value of the generated AC voltage of the bridge circuit 62 influenced by their pulse width. The lamp voltage is thus pulse width modulated, which is equal to a change in amplitude of the sine wave in the square wave signal. In the embodiment shown, the lamp effective voltage is set by the duty cycle of the bridge output voltage.

The bridge circuit 62 is through the bridge control 66 controlled such that the pulse width of the bridge voltage and thus the effective value of the lamp voltage in a selectable time interval of typically one millisecond between the two voltage ratings for the on and the off state of the lamp changes. The nominal value for the off-state of the lamp does not correspond to a voltage of zero volts, but a voltage that is different from zero, but to ignite the fluorescent lamp 12 is too low.

The in the foregoing description, claims and figures Features can both individually and in any combination for the realization the invention in its various embodiments of importance.

10

contraption for adjusting the brightness of a backlight

12

Fluorescent Lamp

14

PWM modulator

16

modification circuit

18

PWM circuit

20

bridge circuit

22

transformer

24

primary

26

secondary winding

28

capacitor

30

Input to the modification circuit 16

32

low pass filter

34

resistance

36

capacitor

38

Power window circuit

40

first potentiometer

42

first operational amplifiers

44

First diode

46 47

junction

48

second potentiometer

50

second operational amplifiers

52

Second diode

54

voltage follower

56

Not modified burst signal

56 ', 56' '

modified burst signals

58

circuitry

60

contraption for adjusting the brightness of a backlight

62

bridge circuit

64

management

66

bridge control

Claims (28)

Contraption ( 10 ) for adjusting the brightness of a backlight, the at least one fluorescent lamp ( 12 ), with means ( 14 . 16 . 18 . 20 . 22 . 62 . 64 . 66 ) for applying a pulsed rms voltage to the at least one fluorescent lamp ( 12 ), such that the effective voltage during pulse-on intervals for igniting the at least one fluorescent lamp ( 12 ) is sufficient and during pulse-off intervals for igniting the at least one fluorescent lamp is too low, characterized in that the transitions between the pulse-on intervals and pulse-off intervals are slowed down so that one by an abrupt modulation of the Lampeneffektivspannung inducible noise is reduced. Contraption ( 10 ) according to claim 1, wherein the rise time and fall time of the lamps are effective voltage at the transition between pulse on and pulse off at least 0.3 ms, preferably at least 0.5 ms takes. Contraption ( 10 ) for adjusting the brightness of a backlight, the at least one fluorescent lamp ( 12 ), with means ( 14 . 16 . 18 . 20 . 22 . 62 . 64 . 66 ) for applying a pulsed rms voltage to the at least one fluorescent lamp ( 12 ), such that the effective voltage during pulse-on intervals for igniting the at least one fluorescent lamp ( 12 ) and is too low during pulse-off intervals for igniting the at least one fluorescent lamp, characterized in that the lamp effective voltage is not reduced to zero during the pulse-off interval. Contraption ( 10 ) for adjusting the brightness of a backlight, with the features of claim 1 or 2 in combination with the features of claim 3. Contraption ( 10 ) according to one of the preceding claims, with a burst generator ( 14 . 16 ) for generating a burst signal ( 56 ' . 56 '' ) which alternately assumes a first or a second signal value and with which the current passing through the at least one fluorescent lamp ( 12 ), is modulated, with means ( 32 . 34 . 36 ) to slow down the transition of the burst signal ( 56 ' . 56 '' ) between the first and second signal values. Contraption ( 10 ) according to one of the preceding claims, with a burst generator ( 14 . 16 ) for generating a burst signal ( 56 '' ), which alternately assumes a first or a second signal _value (V _ref2 , V _ref1 ) and with which the current flowing through the at least one fluorescent lamp ( 12 ) is modulated, wherein the first signal value corresponds to a lamp effective voltage, which is used to ignite the fluorescent lamp ( 12 ) and the second signal _value V _ref1 corresponds to a lamp effective voltage, which is different from zero but to ignite the fluorescent lamp ( 12 ) is too low. Contraption ( 10 ) according to one of the preceding claims, in which the burst signal ( 56 ' . 56 '' ) is a PWM signal. Contraption ( 10 ) according to one of the preceding claims, in which the burst signal ( 56 ' . 56 '' ) has a frequency of 80 to 300 Hz. Contraption ( 10 ) according to one of the preceding claims, in which the burst generator ( 14 . 16 ) a PWM modulator ( 14 ) for generating a square wave signal ( 56 ) and a modification circuit ( 16 ) for modifying the rectangular signal ( 56 ), wherein the modification circuit ( 16 ) a low-pass filter ( 32 ), in particular an RC element ( 34 . 36 ) and / or a voltage window circuit ( 38 ) which outputs incoming signals which are below a lower threshold V _ref1 as V _ref1 , and outputs the incoming signals which are above an upper threshold V _ref2 as V _ref2 . Contraption ( 10 ) according to claim 9, wherein the first signal value of the burst signal ( 56 '' ) V _ref2 and the second signal _{value of} the burst signal ( 56 '' ) V _ref1 corresponds. Contraption ( 10 ) according to one of the preceding claims, with a PWM circuit ( 18 ), which has an input for the burst signal ( 56 ' . 56 '' ) and a PWM switching signal ( 58 ) whose duty cycle corresponds to the current signal value of the burst signal ( 56 ' . 56 '' ) corresponds. Contraption ( 10 ) according to claim 11, wherein the switching signal ( 58 ) has a frequency of 30 to 70 KHz. Contraption ( 10 ) according to claim 11 or 12, wherein the switching signal ( 58 ) in a bridge circuit ( 20 ) of a transformer ( 22 ) which generates an AC voltage which is applied to the primary winding of the transformer ( 22 ) is present. Contraption ( 10 ) according to one of claims 11 to 13, in which the second signal _value V _{ref1 of} the burst signal ( 56 '' ) corresponding duty cycle of the switching signal ( 58 ) 30% to 70%, preferably 40% to 60% of the duty cycle of the switching signal ( 58 ), which corresponds to the first signal _value V _{ref2 of} the burst signal ( 56 '' ) corresponds. Method for adjusting the brightness of a backlight, the at least one fluorescent lamp ( 12 ), in which a pulsed rms voltage is applied to the at least one fluorescent lamp ( 12 ), which during pulse-on intervals for igniting the at least one fluorescent lamp ( 12 ) is sufficient and during pulse-off intervals for igniting the at least one fluorescent lamp is too low, characterized in that the transitions between pulse-on intervals and pulse-off intervals are slowed down so that inducible by an abrupt modulation of the lamp effective voltage Noise is reduced. The method of claim 15, wherein the rise time and the fall time at the transition between pulse on and pulse off at least 0.3 ms, preferably at least Takes 0.5 ms. Method for adjusting the brightness of a backlight, the at least one fluorescent lamp ( 12 ), in which a pulsed rms voltage is applied to the at least one fluorescent lamp ( 12 ), which during pulse-on intervals for igniting the at least one fluorescent lamp ( 12 ) and is too low during pulse-off intervals for igniting the at least one fluorescent lamp, characterized in that the lamp effective voltage is not reduced to zero during the pulse-off interval. Method for adjusting the brightness of a backlight with the features of claim 15 or 16 in combination with the Features of claim 17. Method according to one of claims 15 to 18, b egg the a burst signal ( 56 ' . 56 '' ) which alternately assumes a first and a second signal value and with which the current flowing through the at least one fluorescent lamp ( 12 ) is modulated, wherein the transition of the sausage signal ( 56 ' . 56 '' ) between the first and second signal values is slowed down so that an inducible by an abrupt modulation of the lamp effective voltage noise is reduced. Method according to one of Claims 15 to 19, in which a burst signal ( 56 ' . 56 '' ) which alternately receives a first and a second signal _value (V _ref2 , V _ref1 ) and with which the current flowing through the at least one fluorescent lamp ( 12 ), wherein the first signal _value V ref2 corresponds to a lamp effective voltage whose amplitude for igniting the fluorescent lamp ( 12 ), and the second signal _value V _ref1 corresponds to a lamp effective voltage whose amplitude is different from zero but for igniting the fluorescent lamp ( 12 ) is too low. Method according to one of Claims 19 or 20, in which the burst signal ( 56 ' . 56 '' ) is a PWM signal. Method according to one of Claims 19 to 21, in which the burst signal ( 56 ' . 56 '' ) has a frequency of 80 to 300 Hz. Method according to one of Claims 19 to 22, in which the generation of the burst signal ( 56 ' . 56 '' ) comprises a first step in which a PWM square wave signal ( 56 ), and a second step in which the edges of the square wave signal ( 56 ) with the aid of a low-pass filter ( 32 ), in particular an RC element ( 34 . 36 ) and / or the upper value of the PWM signal, in case _it exceeds an upper threshold V _ref1 , is set to the upper threshold V _ref2 and the lower value of the PWM signal in case it is below a lower threshold V _ref1 is set to the lower threshold V _ref1 . Method according to Claim 23, in which the first signal value of the burst signal ( 56 '' ) V _ref2 and the second signal _{value of} the burst signal ( 56 '' ) V _ref1 corresponds. Method according to one of Claims 19 to 24, in which a PWM switching signal ( 58 ) whose duty cycle is equal to the current signal value of the burst signal ( 56 ' . 56 '' ) corresponds. Method according to Claim 25, in which the switching signal ( 58 ) has a frequency of 30 to 70 KHz. Method according to Claim 25 or 26, in which the switching signal ( 58 ) in a bridge circuit ( 20 ) of a transformer ( 22 ) which generates an AC voltage which is applied to the primary winding of the transformer ( 22 ) is present. Method according to one of Claims 25 to 27, in which the second signal value of the burst signal ( 56 '' ) corresponding duty cycle of the switching signal ( 58 ) 30% to 70%, preferably 40% to 60% of the duty cycle of the switching signal ( 58 ) corresponding to the first signal value of the burst signal ( 56 '' ) corresponds.