WO2007093215A1 - Backlight system - Google Patents

Backlight system Download PDF

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Publication number
WO2007093215A1
WO2007093215A1 PCT/EP2006/050926 EP2006050926W WO2007093215A1 WO 2007093215 A1 WO2007093215 A1 WO 2007093215A1 EP 2006050926 W EP2006050926 W EP 2006050926W WO 2007093215 A1 WO2007093215 A1 WO 2007093215A1
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WO
WIPO (PCT)
Prior art keywords
trl
lamp
lamps
transformer
voltage
Prior art date
Application number
PCT/EP2006/050926
Other languages
French (fr)
Inventor
Albert Runtze
Jean-Paul Louvel
Achim Elgert
Original Assignee
Tte Germany Gmbh
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Tte Germany Gmbh filed Critical Tte Germany Gmbh
Priority to PCT/EP2006/050926 priority Critical patent/WO2007093215A1/en
Publication of WO2007093215A1 publication Critical patent/WO2007093215A1/en

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Classifications

    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B41/00—Circuit arrangements or apparatus for igniting or operating discharge lamps
    • H05B41/14—Circuit arrangements
    • H05B41/26—Circuit arrangements in which the lamp is fed by power derived from dc by means of a converter, e.g. by high-voltage dc
    • H05B41/28—Circuit arrangements in which the lamp is fed by power derived from dc by means of a converter, e.g. by high-voltage dc using static converters
    • H05B41/282—Circuit arrangements in which the lamp is fed by power derived from dc by means of a converter, e.g. by high-voltage dc using static converters with semiconductor devices
    • H05B41/2825—Circuit arrangements in which the lamp is fed by power derived from dc by means of a converter, e.g. by high-voltage dc using static converters with semiconductor devices by means of a bridge converter in the final stage
    • H05B41/2827—Circuit arrangements in which the lamp is fed by power derived from dc by means of a converter, e.g. by high-voltage dc using static converters with semiconductor devices by means of a bridge converter in the final stage using specially adapted components in the load circuit, e.g. feed-back transformers, piezoelectric transformers; using specially adapted load circuit configurations
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B41/00—Circuit arrangements or apparatus for igniting or operating discharge lamps
    • H05B41/14—Circuit arrangements
    • H05B41/26—Circuit arrangements in which the lamp is fed by power derived from dc by means of a converter, e.g. by high-voltage dc
    • H05B41/28—Circuit arrangements in which the lamp is fed by power derived from dc by means of a converter, e.g. by high-voltage dc using static converters
    • H05B41/282—Circuit arrangements in which the lamp is fed by power derived from dc by means of a converter, e.g. by high-voltage dc using static converters with semiconductor devices
    • H05B41/2821—Circuit arrangements in which the lamp is fed by power derived from dc by means of a converter, e.g. by high-voltage dc using static converters with semiconductor devices by means of a single-switch converter or a parallel push-pull converter in the final stage
    • H05B41/2822—Circuit arrangements in which the lamp is fed by power derived from dc by means of a converter, e.g. by high-voltage dc using static converters with semiconductor devices by means of a single-switch converter or a parallel push-pull converter in the final stage using specially adapted components in the load circuit, e.g. feed-back transformers, piezoelectric transformers; using specially adapted load circuit configurations

Definitions

  • the present invention is related to a backlight system with a power supply arrangement comprising a first circuit supplying a DC voltage for a power supply which has a transformer with a mains isolation and a power stage controlled by a backlight controller (BC) , and several lamp transformers for the operation of cold cathode fluorescent lamps.
  • Backlight systems of this kind are used in particular to illuminate LCD screens for the operation of LCD computer monitors and LCD television sets.
  • a backlight system with lamps and with a power supply arrangement for the lamps, known by the public at large, is schematically shown in figure 1.
  • the power supply arrangement comprises a mains connection and a power factor controller PFC supplying a high DC voltage, as well as a switched mode power supply and a transformer TRl with a mains isolation MI.
  • the power supply arrangement comprises further a backlight controller BC, a multitude of power stages PS controlled by the backlight controller BC and a multitude of lamp transformers TRL. Each of the lamp transformers TRL is coupled to one lamp L.
  • This system first converts the supplied high DC voltage into a low DC voltage with the switched mode power supply, and then converts the low DC voltage into high AC voltages by the power stages PS and the lamp transformers.
  • the high AC voltages are needed to operate the lamps L, which are cold cathode fluorescent lamps (CCFL) .
  • the characteristics of the lamps L are a high ignition voltage and different discharge voltages.
  • the high ignition voltage is generated due to a resonant circuit with a leakage inductance of transformer TRL and a capacitor CL.
  • Each of the lamps L is driven by one of the lamp transformers TRL.
  • each of the lamp transformers TRL can deliver the individual discharge voltage as necessary for the operation of each of the lamps L.
  • the conversion of a supplied high DC voltage, first into a low DC voltage and then into high AC voltages needs some intermediate components, such as the switched mode power supply or a multitude of power stages PS.
  • the intermediate components increase the number of the components of the power supply arrangement and as a result, the costs of the backlight system.
  • a further disadvantage of this system is the energy loss due to the efficiency value of each converter.
  • the efficiency of a power factor controller PFC is about 90%, while that of a switched mode power supply about 75% and that of a power stage PS is about 85%. This ends up in an overall efficiency of about 57% only.
  • a specific multi-lamp backlight system is described in US 2004/0066151 Al.
  • This system comprises a special lamp transformer with a first coil set, to which a first AC voltage is applied. It comprises a second and a third coil set, on which a second and a third AC voltage for driving a first and a second lamp are induced.
  • This kind of lamp transformers needs a lot of copper material.
  • a bigger transformer core is necessary for the second and third coil set.
  • the power supply is built up by the power stage and the power stage works as a DC-AC converter, e.g. it converts the supplied high DC voltage into a high AC voltage at the secondary side of a first transformer.
  • the first transformer includes a mains isolation. Several lamp transformers are connected with a respective primary winding to a secondary- winding of the first transformer, which provides the AC voltage for the lamp transformers. No further intermediate components are necessary.
  • the backlight system of the invention avoids converting the voltage into a low DC voltage first.
  • the system succeeds to convert the DC voltage into a high AC voltage, for example 150 Vrms, in two steps.
  • This system with one converter less, has an increased efficiency per se.
  • the efficiency of the DC-AC converter is high due to the first transformer, which can be chosen with a turns ratio near 1 and with a good coupling behaviour.
  • the low turns ratio of the first transformer causes low copper losses.
  • An overall efficiency of 70 to 80% can be achieved.
  • the turns ratio of the lamp transformers can be chosen lower than that of the prior art. This enables the use of lamp transformers with an easier design.
  • the lamp transformers in particular have a low coupling ratio.
  • the coupling ratio should be lower than 0,75.
  • the low coupling ratio of the lamp transformers diminishes the influence of the intermediate high AC voltage and assures that at least two lamps with their individual discharge voltages can be operated with one lamp transformer.
  • a backlight system of the invention can be easily adapted to an appliance with a different number of lamps .
  • the first transformer In order to deliver the correct power, only the first transformer has to be adapted to the new number of lamps .
  • the lamp transformers can have a low coupling ratio of 0,4 to 0,7.
  • the first transformer can have a high coupling ratio near 1, e.g. greater or equal to 0,9.
  • the high coupling ratio increases the efficiency of the voltage conversion.
  • the first transformer can further have a turns ratio in the range of 2:1 to 1:2, and in particular a turns ratio of 0.8:1 to 1:1,5. Due to the turns ratio near 1:1 a good coupling behaviour can be easily achieved.
  • the lamp transformers can have a turns ratio of 1:7 to 1:20. This enables a design of the lamp transformers, which is easy to manufacture.
  • Each lamp can be connected in series with a capacitor.
  • the capacitor forms a resonant circuit with a leakage inductance of the lamp transformers, which produces the needed high ignition voltage.
  • a lamp transformer can be connected with a terminal of the secondary winding to at least two lamps, which are coupled in parallel to one another, and between each two lamps a coil can be connected.
  • each lamp transformer is connected to two lamps, which are coupled in parallel, and the two lamps are connected to one coil.
  • the coil which forms a resonant circuit with the capacitors of a lamp pair of one or two lamp transformers insures the ignition of the second lamp of each lamp pair. In addition, the coil equalizes the currents of the two lamps in a lamp pair.
  • a high DC voltage of 400 Vdc is converted into a maximum AC voltage of 250 Vrms at the secondary side of the first transformer by the power stage. This AC voltage is delivered to the lamp transformers.
  • the backlight system lowers therefore the number of components and as a result, lowers the costs and increases the reliability due to a reduced number of components .
  • the power efficiency of the backlight system is increased and the material costs of the lamp transformers are low.
  • figure 1 a simplified circuit diagram of a backlight system according to prior art
  • figure 2 a simplified circuit diagram of a backlight system of a first embodiment of the invention
  • figure 3 a simplified circuit diagram of a backlight system of a second embodiment of the invention.
  • a backlight system of a prior art with lamps L and with a power supply arrangement for the lamps L is schematically shown in figure 1.
  • the power supply arrangement comprises a mains connection and a power factor controller PFC supplying a high DC voltage, as well as a switched mode power supply with a transformer TRl having a mains isolation MI.
  • the switched mode power supply has a controller SC und a switching transistor Tl, which is driven by the controller SC and connected to the primary winding of the transformer TRl.
  • the switched mode power supply converts the high DC input voltage into a low stabilised DC output voltage during operation, as known.
  • the switched mode power supply comprises further a feedback circuit for the stabilisation of the DC output voltage, not shown.
  • the DC output voltage is provided by the transformer TRl via a rectifier diode Dl and a smoothing capacitor Cl .
  • the power supply arrangement further comprises a backlight controller BC, a multitude of power stages PS controlled by the backlight controller BC and a multitude of lamp transformers TRL.
  • Each of the power stage PS drives some, in particular three, lamp transformers TRL to convert the low DC voltage into a high AC voltage.
  • Each of the lamp transformers TRL is coupled to one of the lamps L, and each of the lamps L is connected in series to a lamp capacitor CL.
  • a backlight system of an embodiment of the invention with lamps L, and with a power supply arrangement for the lamps L, is schematically shown in figure 2.
  • the power supply arrangement comprises in this embodiment a mains connection and a first circuit supplying a DC voltage, which is in particular a power factor controller PFC, which are well- known components of a power supply arrangement.
  • the first circuit delivers a rectified mains, in particular a high DC voltage with e.g. an amount of 400 Vdc .
  • the power supply arrangement comprises further a power supply with a backlight controller BC, a power stage PS, and a first transformer TRl with a mains isolation.
  • the power stage PS is connected to the output of the first circuit PFC and to the primary winding of the transformer TRl for converting the applied DC voltage into an AC voltage by means of the transformer TRl.
  • the backlight controller BC is connected to the power stage PS.
  • the power stage PS comprises for example a half bridge configuration with power MOS transistors.
  • the power stage PS is controlled by the backlight controller BC for an operation as a DC-AC converter, which converts the DC voltage, supplied by the power factor controller PFC, into an AC voltage at the secondary side of the transformer TRl.
  • the transformer TRl contains the mains isolation MI and has a turns ratio from 2:1 to 1:2, in particular from 0,8:1 to 1:1,5. As a result, the generated AC voltage has an amount of up to 250 Vrms .
  • the transformer TRl has advantageously a high coupling ratio of 0,9 to 1, which can be easily achieved when using the above mentioned low turns ratio of the transformer TRl.
  • the power supply arrangement comprises further several lamp transformers TRL, which are arranged in parallel and which are each connected with a primary winding to the secondary winding of the first transformer TRl.
  • the lamp transformers TRL have a low coupling ratio in order to make possible individual discharge voltages together with similar or equal lamp currents.
  • the coupling ratio has advantageously a low value in the range of 0,4 to 0,7.
  • the lamp transformers TRL have a turns ratio of 1:7 to 1:20.
  • Each lamp transformer TRL is coupled with one terminal of the secondary winding to two lamps L, which are connected in parallel to one another, and with the other terminal to ground.
  • Each lamp L is connected in series to a capacitor CAL and the two lamps L of a lamp pair are connected with a coil COL.
  • Each terminal of the coils COL is in particular connected to a tab between a capacitor CAL and lamp of a lamp pair for providing a resonance circuit.
  • the turns ratio of the transformer TRl is 1:1, its coupling ratio is near 1, while the coupling ratio of the lamp transformers TRL is about 0,5 and its turns ratio 1:17.
  • the output voltage of the first circuit is 400 Vdc and the output voltage as provided by the transformer TRl is max. 150 Vrms in this embodiment.
  • the AC voltage generated by a lamp transformer TRL has an amount of up to 2500 Vrms, which is needed for a lamp ignition.
  • the first circuit is a step-up circuit for providing power factor correction
  • the backlight controller BC includes an integrated power supply controller circuit.
  • the power stage PS can be built up by a full bridge configuration also with power MOS transistors.
  • a step-down circuit can be used and coupled to the power stage PS built up by a half bridge, or the backlight controller BC can drive the step-down circuit as well as the power stage PS.
  • each of the lamp transformers TRL causes a specific leakage inductance, which forms a resonant circuit with the capacitor CAL, which is connected in series with the corresponding lamp L.
  • the switching frequency of the power supply is close to this resonant circuit, a high secondary voltage will be generated and will ignite the lamp L. Once the lamp L is ignited the voltage will drop to a level, at which the lamp L is operating.
  • the backlight controller BC will regulate the transferred energy in order to keep the lamp current at its nominal value.
  • the capacitor CAL also works as a ballast to compensate the lamp equivalent negative impedance.
  • a typical switching frequency of the power supply is for example 50 kHz, which can be adjusted at the backlight controller BC.
  • the coil COL which is connected to each lamp L of one lamp transformer TRL, forms a resonant circuit with the two capacitors CAL.
  • This resonant circuit will generate a high voltage, when only one of the lamps L is ignited. A high voltage present at the second lamp L will ignite therefore the second lamp L as well.
  • the coil COL also equalizes the currents in the two lamps L, when the two lamps L are in operation.
  • a backlight system of a second embodiment of the invention corresponds to a backlight system of the first embodiment in all ways, except for the following features:
  • the backlight system comprises second lamp transformers TRL- 2, which are connected in parallel to one another to the transformer TRl.
  • the number and the characteristics of the second lamp transformers TRL-2 correspond to that of the lamp transformers TRL.
  • Each of the second lamp transformers TRL-2 is connected to the two lamps L of a lamp pair at the end of the lamp pair, which is opposite to connection of lamp transformer TRL.
  • the second embodiment is schematically shown in figure 3.
  • the lamp transformers together with the respective lamps can be arranged also in groups, as shown in figure 4:
  • a first group comprises lamp transformers TRLl and TRL2, which are connected in parallel with regard to each other.
  • the respective lamps L are coupled to ground, as shown and explained already in figure 2 .
  • the last group comprises two lamp transformers TRLn-I and TRLn, which are also coupled in parallel.
  • the transformers TRL3-TRLN-2, not shown, are connected respectively.
  • the groups of the arrangement as shown in figure 4 are connected in series to the secondary winding of the transformer TRl, in correspondence with the lamp transformers as shown and explained with regard to figure 2.

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Circuit Arrangements For Discharge Lamps (AREA)

Abstract

A backlight system with lamps (L) and with a power supply arrangement, which comprises a mains connection, a circuit supplying a high DC voltage, a power supply and a transformer (TR1) with a mains isolation. The power supply arrangement further comprises a backlight controller (BC), a power stage (PS), which is controlled by the backlight controller (BC), and multiple lamp transformers (TRL). Each of the lamp transformers (TRL) is coupled to at least one of the lamps (L). The power supply is build up by the power stage (PS), which converts a supplied, high DC voltage into a high AC voltage at the secondary side of the transformer (TR1). The lamp transformers (TRL) are connected to the transformer (TR1). The lamp transformers (TRL) have a low coupling ratio.

Description

Backlight System
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention is related to a backlight system with a power supply arrangement comprising a first circuit supplying a DC voltage for a power supply which has a transformer with a mains isolation and a power stage controlled by a backlight controller (BC) , and several lamp transformers for the operation of cold cathode fluorescent lamps. Backlight systems of this kind are used in particular to illuminate LCD screens for the operation of LCD computer monitors and LCD television sets.
DESCRIPTION OF THE PRIOR ART
A backlight system with lamps and with a power supply arrangement for the lamps, known by the public at large, is schematically shown in figure 1. The power supply arrangement comprises a mains connection and a power factor controller PFC supplying a high DC voltage, as well as a switched mode power supply and a transformer TRl with a mains isolation MI. The power supply arrangement comprises further a backlight controller BC, a multitude of power stages PS controlled by the backlight controller BC and a multitude of lamp transformers TRL. Each of the lamp transformers TRL is coupled to one lamp L. This system first converts the supplied high DC voltage into a low DC voltage with the switched mode power supply, and then converts the low DC voltage into high AC voltages by the power stages PS and the lamp transformers. The high AC voltages are needed to operate the lamps L, which are cold cathode fluorescent lamps (CCFL) . The characteristics of the lamps L are a high ignition voltage and different discharge voltages. The high ignition voltage is generated due to a resonant circuit with a leakage inductance of transformer TRL and a capacitor CL. Each of the lamps L is driven by one of the lamp transformers TRL. As a result, each of the lamp transformers TRL can deliver the individual discharge voltage as necessary for the operation of each of the lamps L.
The conversion of a supplied high DC voltage, first into a low DC voltage and then into high AC voltages needs some intermediate components, such as the switched mode power supply or a multitude of power stages PS. The intermediate components increase the number of the components of the power supply arrangement and as a result, the costs of the backlight system. A further disadvantage of this system is the energy loss due to the efficiency value of each converter. The efficiency of a power factor controller PFC is about 90%, while that of a switched mode power supply about 75% and that of a power stage PS is about 85%. This ends up in an overall efficiency of about 57% only.
A specific multi-lamp backlight system is described in US 2004/0066151 Al. This system comprises a special lamp transformer with a first coil set, to which a first AC voltage is applied. It comprises a second and a third coil set, on which a second and a third AC voltage for driving a first and a second lamp are induced. This kind of lamp transformers needs a lot of copper material. A bigger transformer core is necessary for the second and third coil set. SUMMARY OF THE INVENTION
It is therefore desirable to develop a backlight system, which lowers the number of components and as a result, lowers the costs .
According to the invention this can be achieved by the features of claim 1. Advantageous developments of the invention are specified in the dependent claims.
The power supply is built up by the power stage and the power stage works as a DC-AC converter, e.g. it converts the supplied high DC voltage into a high AC voltage at the secondary side of a first transformer. The first transformer includes a mains isolation. Several lamp transformers are connected with a respective primary winding to a secondary- winding of the first transformer, which provides the AC voltage for the lamp transformers. No further intermediate components are necessary.
Only one power stage is used to drive the lamp transformers. All components of the DC-DC converter, such as the supplementary switched mode power supply, are no longer needed. The number of components is significantly decreased and as a result, the costs of the backlight system are decreased and also the reliability is increased.
The backlight system of the invention avoids converting the voltage into a low DC voltage first. The system succeeds to convert the DC voltage into a high AC voltage, for example 150 Vrms, in two steps. This system, with one converter less, has an increased efficiency per se. In addition, the efficiency of the DC-AC converter is high due to the first transformer, which can be chosen with a turns ratio near 1 and with a good coupling behaviour. The low turns ratio of the first transformer causes low copper losses. As a result, the efficiency of the system is significantly increased. An overall efficiency of 70 to 80% can be achieved. Further, the turns ratio of the lamp transformers can be chosen lower than that of the prior art. This enables the use of lamp transformers with an easier design.
The lamp transformers in particular have a low coupling ratio. The coupling ratio should be lower than 0,75. The low coupling ratio of the lamp transformers diminishes the influence of the intermediate high AC voltage and assures that at least two lamps with their individual discharge voltages can be operated with one lamp transformer.
A backlight system of the invention can be easily adapted to an appliance with a different number of lamps . In order to deliver the correct power, only the first transformer has to be adapted to the new number of lamps .
The lamp transformers can have a low coupling ratio of 0,4 to 0,7. The first transformer can have a high coupling ratio near 1, e.g. greater or equal to 0,9. The high coupling ratio increases the efficiency of the voltage conversion. The first transformer can further have a turns ratio in the range of 2:1 to 1:2, and in particular a turns ratio of 0.8:1 to 1:1,5. Due to the turns ratio near 1:1 a good coupling behaviour can be easily achieved. The lamp transformers can have a turns ratio of 1:7 to 1:20. This enables a design of the lamp transformers, which is easy to manufacture. Each lamp can be connected in series with a capacitor. The capacitor forms a resonant circuit with a leakage inductance of the lamp transformers, which produces the needed high ignition voltage.
A lamp transformer can be connected with a terminal of the secondary winding to at least two lamps, which are coupled in parallel to one another, and between each two lamps a coil can be connected. Preferably, each lamp transformer is connected to two lamps, which are coupled in parallel, and the two lamps are connected to one coil.
The coil, which forms a resonant circuit with the capacitors of a lamp pair of one or two lamp transformers insures the ignition of the second lamp of each lamp pair. In addition, the coil equalizes the currents of the two lamps in a lamp pair.
In a preferred embodiment, during operation a high DC voltage of 400 Vdc is converted into a maximum AC voltage of 250 Vrms at the secondary side of the first transformer by the power stage. This AC voltage is delivered to the lamp transformers.
The backlight system lowers therefore the number of components and as a result, lowers the costs and increases the reliability due to a reduced number of components .
Further, the power efficiency of the backlight system is increased and the material costs of the lamp transformers are low.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be explained in more detail using two embodiments, which are illustrated in the figure 2 and 3. It shows : figure 1, a simplified circuit diagram of a backlight system according to prior art; figure 2, a simplified circuit diagram of a backlight system of a first embodiment of the invention; and figure 3, a simplified circuit diagram of a backlight system of a second embodiment of the invention.
DETAILED DESCRIPTION OF A PRIOR ART
A backlight system of a prior art with lamps L and with a power supply arrangement for the lamps L is schematically shown in figure 1. The power supply arrangement comprises a mains connection and a power factor controller PFC supplying a high DC voltage, as well as a switched mode power supply with a transformer TRl having a mains isolation MI. The switched mode power supply has a controller SC und a switching transistor Tl, which is driven by the controller SC and connected to the primary winding of the transformer TRl. The switched mode power supply converts the high DC input voltage into a low stabilised DC output voltage during operation, as known. The switched mode power supply comprises further a feedback circuit for the stabilisation of the DC output voltage, not shown. The DC output voltage is provided by the transformer TRl via a rectifier diode Dl and a smoothing capacitor Cl .
The power supply arrangement further comprises a backlight controller BC, a multitude of power stages PS controlled by the backlight controller BC and a multitude of lamp transformers TRL. Each of the power stage PS drives some, in particular three, lamp transformers TRL to convert the low DC voltage into a high AC voltage. Each of the lamp transformers TRL is coupled to one of the lamps L, and each of the lamps L is connected in series to a lamp capacitor CL.
DETAILED DESCRIPTION OF A FIRST EMBODIMENT OF THE INVENTION
A backlight system of an embodiment of the invention with lamps L, and with a power supply arrangement for the lamps L, is schematically shown in figure 2. The power supply arrangement comprises in this embodiment a mains connection and a first circuit supplying a DC voltage, which is in particular a power factor controller PFC, which are well- known components of a power supply arrangement. The first circuit delivers a rectified mains, in particular a high DC voltage with e.g. an amount of 400 Vdc .
The power supply arrangement comprises further a power supply with a backlight controller BC, a power stage PS, and a first transformer TRl with a mains isolation. The power stage PS is connected to the output of the first circuit PFC and to the primary winding of the transformer TRl for converting the applied DC voltage into an AC voltage by means of the transformer TRl. The backlight controller BC is connected to the power stage PS.
The power stage PS comprises for example a half bridge configuration with power MOS transistors. The power stage PS is controlled by the backlight controller BC for an operation as a DC-AC converter, which converts the DC voltage, supplied by the power factor controller PFC, into an AC voltage at the secondary side of the transformer TRl.
The transformer TRl contains the mains isolation MI and has a turns ratio from 2:1 to 1:2, in particular from 0,8:1 to 1:1,5. As a result, the generated AC voltage has an amount of up to 250 Vrms . The transformer TRl has advantageously a high coupling ratio of 0,9 to 1, which can be easily achieved when using the above mentioned low turns ratio of the transformer TRl.
The power supply arrangement comprises further several lamp transformers TRL, which are arranged in parallel and which are each connected with a primary winding to the secondary winding of the first transformer TRl. The lamp transformers TRL have a low coupling ratio in order to make possible individual discharge voltages together with similar or equal lamp currents. The coupling ratio has advantageously a low value in the range of 0,4 to 0,7. The lamp transformers TRL have a turns ratio of 1:7 to 1:20.
Each lamp transformer TRL is coupled with one terminal of the secondary winding to two lamps L, which are connected in parallel to one another, and with the other terminal to ground. Each lamp L is connected in series to a capacitor CAL and the two lamps L of a lamp pair are connected with a coil COL. Each terminal of the coils COL is in particular connected to a tab between a capacitor CAL and lamp of a lamp pair for providing a resonance circuit.
In this embodiment, the turns ratio of the transformer TRl is 1:1, its coupling ratio is near 1, while the coupling ratio of the lamp transformers TRL is about 0,5 and its turns ratio 1:17. The output voltage of the first circuit is 400 Vdc and the output voltage as provided by the transformer TRl is max. 150 Vrms in this embodiment. As a result, the AC voltage generated by a lamp transformer TRL has an amount of up to 2500 Vrms, which is needed for a lamp ignition. In a preferred embodiment, the first circuit is a step-up circuit for providing power factor correction, and the backlight controller BC includes an integrated power supply controller circuit. Alternatively, the power stage PS can be built up by a full bridge configuration also with power MOS transistors. As a further alternative, a step-down circuit can be used and coupled to the power stage PS built up by a half bridge, or the backlight controller BC can drive the step-down circuit as well as the power stage PS.
The low coupling ratio of each of the lamp transformers TRL causes a specific leakage inductance, which forms a resonant circuit with the capacitor CAL, which is connected in series with the corresponding lamp L. When the switching frequency of the power supply is close to this resonant circuit, a high secondary voltage will be generated and will ignite the lamp L. Once the lamp L is ignited the voltage will drop to a level, at which the lamp L is operating. The backlight controller BC will regulate the transferred energy in order to keep the lamp current at its nominal value. The capacitor CAL also works as a ballast to compensate the lamp equivalent negative impedance. A typical switching frequency of the power supply is for example 50 kHz, which can be adjusted at the backlight controller BC.
In addition the coil COL, which is connected to each lamp L of one lamp transformer TRL, forms a resonant circuit with the two capacitors CAL. This resonant circuit will generate a high voltage, when only one of the lamps L is ignited. A high voltage present at the second lamp L will ignite therefore the second lamp L as well. The coil COL also equalizes the currents in the two lamps L, when the two lamps L are in operation.
DETAILED DESCRIPTION OF A SECOND EMBODIMENT OF THE INVENTION
A backlight system of a second embodiment of the invention corresponds to a backlight system of the first embodiment in all ways, except for the following features:
The backlight system comprises second lamp transformers TRL- 2, which are connected in parallel to one another to the transformer TRl. The number and the characteristics of the second lamp transformers TRL-2 correspond to that of the lamp transformers TRL. Each of the second lamp transformers TRL-2 is connected to the two lamps L of a lamp pair at the end of the lamp pair, which is opposite to connection of lamp transformer TRL. The second embodiment is schematically shown in figure 3.
THIRD EMBODIMENT OF THE INVENTION
The lamp transformers together with the respective lamps can be arranged also in groups, as shown in figure 4: A first group comprises lamp transformers TRLl and TRL2, which are connected in parallel with regard to each other. The respective lamps L are coupled to ground, as shown and explained already in figure 2 . The last group comprises two lamp transformers TRLn-I and TRLn, which are also coupled in parallel. The transformers TRL3-TRLN-2, not shown, are connected respectively. The groups of the arrangement as shown in figure 4 are connected in series to the secondary winding of the transformer TRl, in correspondence with the lamp transformers as shown and explained with regard to figure 2.

Claims

Claims
1. Backlight system with lamps (L) and a power supply arrangement comprising a first circuit (PFC) , connectable to a mains voltage, a power supply having a first transformer (TRl) with a primary winding, a secondary winding and a mains isolation (MI) , and a power stage (PS) controlled by a backlight controller (BC) , the first circuit supplying a DC voltage for the power supply, and several lamp transformers (TRL, TRL-2), which are each coupled with a primary winding to the secondary- winding of the first transformer (TRl) , and which are each coupled with a secondary winding to at least two lamps
(L) , and the power supply providing an AC voltage for the lamp transformers (TRL, TRL-2) .
2. The backlight system according to claim 1, characterized in that the lamp transformers (TRL, TRL-2) have a low coupling ratio, in particular between 0,4 to 0,7.
3. The backlight system according to claim 1 or 2, characterized in that the transformer (TRl) has a coupling ratio of 0,9 to 1.
4. The backlight system according to one of the claims 1 to
3, characterized in that the transformer (TRl) has a turns ratio in the range of 2:1 to 1:2.
5. The backlight system according to one of the claims 1 to
4, characterized in that the lamp transformers (TRL, TRL- 2) have a turns ratio in the range of 1:7 to 1:20.
6. The backlight system according to one of the claims 1 to
5, characterized in that at least several of the lamps (L) are connected in series with a capacitor (CAL) .
7. The backlight system according to one of the claims 1 to
6, characterized in that two lamp transformers (TRL, TRL- 2) are connected each to at least two lamps (L) , which are coupled in parallel and which have each a capacitor in series (CAL) , and each two lamps (L) are connected by a coil (COL) .
8. The backlight system according to claim 7, characterized in that each lamp transformer (TRL) is connected to two lamps (L) , which are coupled in parallel to one another, and the two lamps (L) are connected with a coil (COL) , which is connected with each capacitor and each lamp.
9. The backlight system according to claim 7, characterized in that two lamp transformers (TRL, TRL-2) are connected to two lamps (L) , which are coupled in parallel to one another, and the two lamps (L) are connected with a coil (COL) .
10. The backlight system according to one of the claims 1 to 9, characterized in that the lamp transformers (TRLl, ..., TRLn) are arranged in groups, the groups being connected in series to the secondary winding of the first transformer (TRl) , and the lamp transformers within each group being connected in parallel with regard to each other.
PCT/EP2006/050926 2006-02-14 2006-02-14 Backlight system WO2007093215A1 (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
PCT/EP2006/050926 WO2007093215A1 (en) 2006-02-14 2006-02-14 Backlight system

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
PCT/EP2006/050926 WO2007093215A1 (en) 2006-02-14 2006-02-14 Backlight system

Publications (1)

Publication Number Publication Date
WO2007093215A1 true WO2007093215A1 (en) 2007-08-23

Family

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Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/EP2006/050926 WO2007093215A1 (en) 2006-02-14 2006-02-14 Backlight system

Country Status (1)

Country Link
WO (1) WO2007093215A1 (en)

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20030001524A1 (en) * 2001-06-29 2003-01-02 Ambit Microsystems Corp. Multi-lamp driving system
US6534934B1 (en) * 2001-03-07 2003-03-18 Ambit Microsystems Corp. Multi-lamp driving system
US20030141829A1 (en) * 2002-01-31 2003-07-31 Shan-Ho Yu Current equalizer assembly for LCD backlight panel
US20040000879A1 (en) * 2002-04-12 2004-01-01 Lee Sheng Tai Circuit structure for driving a plurality of cold cathode fluorescent lamps
US20040066151A1 (en) * 2002-10-02 2004-04-08 Darfon Electronics Corp. Multi-lamp backlight system

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6534934B1 (en) * 2001-03-07 2003-03-18 Ambit Microsystems Corp. Multi-lamp driving system
US20030001524A1 (en) * 2001-06-29 2003-01-02 Ambit Microsystems Corp. Multi-lamp driving system
US20030141829A1 (en) * 2002-01-31 2003-07-31 Shan-Ho Yu Current equalizer assembly for LCD backlight panel
US20040000879A1 (en) * 2002-04-12 2004-01-01 Lee Sheng Tai Circuit structure for driving a plurality of cold cathode fluorescent lamps
US20040066151A1 (en) * 2002-10-02 2004-04-08 Darfon Electronics Corp. Multi-lamp backlight system

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