WO2008084439A1

WO2008084439A1 - Power supply for led backlight

Info

Publication number: WO2008084439A1
Application number: PCT/IB2008/050048
Authority: WO
Inventors: Georg Sauerlaender; Gian Hoogzaad; Antonius M. G. Mobers
Original assignee: Nxp B.V.
Priority date: 2007-01-10
Filing date: 2008-01-08
Publication date: 2008-07-17

Abstract

A cost-efficient power source (110; 210) for driving a LED backlight system (100; 200) for an LCD panel. The backlight system comprises a plurality of red LEDs (102; 202), green LEDs (103; 203), and blue LEDs (104; 204). The power source comprises a first set of LED drivers for driving red LEDs, a second set of LED drivers for driving green LEDs, and a third set of LED drivers for driving blue LEDs. The power source further comprises a rectifier (112; 212) for connection to mains, and a mains filter (113; 213). In a first embodiment, the power source comprises one common one-stage power factor correction and DC/DC converter unit (114) for providing input voltage for all LED drivers. In a second embodiment, the power source comprises three one-stage power factor correction and DC/DC converter units (215, 216, 217) for separately providing input voltage for the sets of LED drivers.

Description

POWER SUPPLY FOR LED BACKLIGHT

FIELD OF THE INVENTION

The present invention relates in general to a system for driving a plurality of LEDs of different colors, more particularly multiple groups of LEDs. Specifically, the present invention relates to LCD backlights.

BACKGROUND OF THE INVENTION

It is known to use backlighting for LCD applications. A particular type of known backlighting comprises multiple thin fluorescent lamps arranged vertically above each other, each lamp arranged horizontally. More recently, RGB LEDs are used for backlighting. In such case, the LEDs are arranged in horizontal strips, with multiple such strips arranged above each other. In each strip, LEDs of different color, typically Red, Green, Blue, are arranged in spatial groups, such that the combined light output of such group generates white light. Electrically, LEDs of one color are connected in series and driven by one common driver. Typically, this applies to all LEDs of the same color in individual strips, but it may also apply to LEDs in neighboring strips or to only a subcollection of the LEDs of the same color in one strip. It is possible that all LEDs of all strips are switched ON simultaneously, but a scanning operation is also possible, where LED strips are switched sequentially in vertical direction.

SUMMARY OF THE INVENTION

A particular aspect of such LED backlight apparatus concerns the power supply. Conventionally, the power supply of a LED comprises a rectifier for connection to mains, a filter for filtering the rectified mains voltage, a power factor correction device assuring that the power factor as seen by the mains meets the regulations, a DC/DC up- converter for converting the filtered and rectified mains voltage up to a higher intermediate voltage, and a LED driver receiving the intermediate voltage and providing LED current. The LED driver behaves as a current source, and the output voltage of the LED driver corresponds to the forward voltage developing over the LED as caused by the current, which typically is a LED characteristic. In the case of multiple LEDs connected in series, the output voltage of the LED driver corresponds to the summation of all forward voltages of all LEDs.

In the case of a LED backlight apparatus, the LEDs of different color must have different drivers in order to be able to correctly adjust the relative light outputs of the LEDs of different color such that the overall light output is perceived as white light. A problem now is the design of a power supply for such case.

In one approach, it would be possible to provide separate assemblies of rectifier, filter, power factor correction, DC/DC converter, and LED driver for each color, but this would be costly. An object of the present invention is, therefore, to provide a cost-efficient power supply for LED backlights.

According to one aspect of the present invention, a power supply comprises an integral, one-stage power factor correction and DC/DC converter unit. Such unit performs DC/DC conversion and power factor correction in one stage in stead of two successive stages, requiring less components and causing less power loss.

According to another aspect of the present invention, the power supply comprises a common assembly of rectifier and filter, and separate LED drivers for each color.

In one advantageous embodiment, the one-stage power factor correction and DC/DC converter unit is common to all LED drivers.

In another advantageous embodiment, each LED driver is associated with a dedicated one-stage power factor correction and DC/DC converter unit.

Further advantageous elaborations are mentioned in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features and advantages of the present invention will be further explained by the following description of one or more preferred embodiments with reference to the drawings, in which:

Fig. 1 schematically shows a block diagram illustrating a first embodiment of a power supply according to the present invention;

Fig. 2 schematically shows a block diagram illustrating a second embodiment of a power supply according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION Fig. 1 schematically shows a block diagram of a backlight system 100 for an LCD panel, for instance an LCD-TV, an LCD-monitor, etc. The backlight system 100 comprises a plurality of red LEDs 102, a plurality of green LEDs 103, and a plurality of blue LEDs 104, suitably arranged in horizontal rows. The red LEDs of the first row are indicated at 121; they may all be coupled in series, as shown. Likewise, the green LEDs 131 of the first row are all coupled in series in this embodiment. Likewise, the blue LEDs 141 of the first row are all coupled in series in this embodiment. Within the first row, the red, green and blue LEDs 121, 131, 141 are arranged at suitably distributed locations such as to be capable of generating homogenous white light. The same would apply to the red, green and blue LEDs of the second row 122, 132, 142, etcetera, until the red, green and blue LEDs of the last row 123, 133, 143.

The backlight system 100 further comprises a power supply 110, comprising an input terminal 111 for connection to mains, a rectifier 112 for rectifying the mains voltage, a filter 113 for filtering the rectified mains voltage as provided by the rectifier 112, and a one-stage power factor correction and DC/DC converter unit 114. Since mains rectifiers are known per se while known rectifiers can be used in implementing the present invention, an elaborate explanation of design and operation of the rectifier 112 is not needed here. Since filters for filtering the rectified mains voltage are known per se while known filters can be used in implementing the present invention, an elaborate explanation of design and operation of the filter 113 is not needed here. Since one-stage power factor correction and DC/DC converter units are known per se while known such units can be used in implementing the present invention, an elaborate explanation of design and operation of the one-stage power factor correction and DC/DC converter unit 114 is not needed here; it suffices to say that unit 114 behaves as a voltage source, providing an intermediate DC voltage. The power supply 110 further comprises LED drivers having their input coupled to receive the intermediate DC voltage provided by the one-stage power factor correction and DC/DC converter unit 114, and providing LED current for the LEDs. It is noted that each LED driver substantially behaves as a current source. All rows 121, 122, ... 123 of red LEDs may be driven simultaneously, but a scanning operation is also possible, for which purpose each string of LEDs is individually driven by an associated controllable string driver 124, 125, ... 126. Likewise, individual controllable string drivers 134, 135, ... 136 are shown for the green LED rows 131, 132, ... 134, and individual controllable string drivers 144, 145, ... 146 are shown for the blue LED rows 141, 142, ... 143. The string drivers are controlled by a controller 150. For sake of convenience, the string drivers 124, 125, ... 126 for driving the red LEDs will also be indicated as red drivers. Likewise, the string drivers 134, 135, ... 136 for driving the green LEDs will also be indicated as green drivers, and the string drivers 144, 145, ... 146 for driving the blue LEDs will also be indicated as blue drivers. The use of the one-stage power factor correction and DC/DC converter unit

114 has, as compared to an implementation with a separate power factor correction stage and a separate DC/DC converter stage, an advantage of reduced number of components and reduced costs.

In the power supply 110, the red, green and blue drivers have mutually different output voltages (as determined by the forward voltage of the LEDs 102, 103, 104, respectively), while their input voltages are equal. The output voltage of the unit 114 must be high enough to accommodate the highest driver output voltage; consequently, the driver with the lowest output voltage will experience a relatively high voltage drop. In the case of linear drivers, this will lead to relatively large power dissipation. For this reason, it is preferred that the red, green and blue drivers are implemented as switching drivers; this applies at least to the drivers with the lowest output voltage, which typically will be the red drivers 124, 125, ... 126.

It is noted that switching drivers are known per se, so that a more detailed explanation of design and operation of a switching driver is not necessary here. Typically, a one-stage power factor correction and DC/DC converter unit 114 has a relatively large voltage ripple component in its output voltage, at a frequency of the mains frequency, i.e. typically in the order of 100 Hz. Consequently, the string drivers will experience relatively large fluctuations of their input voltage. For this reason, it is preferred that the red, green and blue drivers are implemented as broadband switching drivers. It is noted that broadband switching drivers are known per se from WO-2004/100614 and that such known broadband switching drivers can be used when implementing the present invention; therefore, an elaborate description of design and functioning of a broadband switching driver is not needed here.

Fig. 2 shows a block diagram of a second embodiment of a backlight system 200 for an LCD panel. Components which are comparable to components of the system 100 of figure 1 have the same reference numeral yet increased by 100. An explanation of these components will not be repeated here.

In stead of one common one-stage power factor correction and DC/DC converter unit 114 for all LED drivers, power supply 210 has separate one-stage power factor correction and DC/DC converter units 215, 216, 217 for corresponding red drivers 224, 225, ... 226, green drivers 234, 235, ... 236, and blue drivers 244, 245, ... 246, respectively. The one-stage power factor correction and DC/DC converter units 215, 216, 217 have their inputs coupled to the output of the filter 213, and provide respective intermediate DC voltages for the corresponding red drivers 224, 225, ... 226, green drivers 234, 235, ... 236, and blue drivers 244, 245, ... 246, respectively.

An advantage of this second embodiment is that the different units 215, 216, 217 can be adapted to the corresponding string drivers, such that their respective intermediate DC voltages match the input voltage requirements of the corresponding string drivers. As a consequence, the power dissipation in the string drivers will be lower, and it is possible to use linear string drivers, which will further reduce the costs.

The one-stage power factor correction and DC/DC converter units 215, 216, 217 are preferably implemented as flyback converter. It is noted that flyback converters are known per se so a more detailed explanation of design and operation of a flyback converter is omitted here.

A flyback converter is a switching converter, comprising a switch that is opened and closed at a certain operating frequency, which frequency in practice is selected to be well above the audible range and which in practice may be in the range of about 100 kHz. When the switch is conductive, the converter draws current from mains; when the switch is non-conductive, the converter draws no current from mains. The filter 113, 213 needs to filter this high-frequency current load such as to avoid noise pollution towards the mains. In a further preferred embodiment, also illustrated in figure 2, the power supply 210 further comprises a common converter controller 218 controlling the operation of the converter units 215, 216, 217 such that their switching frequencies are synchronized and such that their switching phases are equally distributed over a full 2π phase. In the illustrated embodiment with three converter units 215, 216, 217, the ideal relative phases would be 0, π/3, 2π/3. This prevents all converter units 215, 216, 217 from drawing mains current at the same time: the current load towards mains is distributed more evenly in time, and the mains effectively "sees" a load with a higher frequency, so that the mains filter 213 can have a reduced size, further reducing the costs of the power supply.

Summarizing, the present invention provides a cost-efficient power source 110; 210 for driving a LED backlight system 100; 200 for an LCD panel. The backlight system comprises a plurality of red LEDs 102; 202, a plurality of green LEDs 103; 203, and a plurality of blue LEDs 104; 204. The power source comprises a first set of LED drivers 124, 125, ... 126; 224, 225, ... 226 for driving the red LEDs 102; 202, a second set of LED drivers 134, 135, ... 136; 234, 235, ... 236 for driving the green LEDs 103; 203, and a third set of LED drivers 144, 145, ... 146; 244, 245, ... 246 for driving the blue LEDs 104; 204. The power source further comprises a rectifier 112; 212 for connection to mains, and a mains filter 113; 213.

In a first embodiment, the power source comprises one common one-stage power factor correction and DC/DC converter unit 114 for providing input voltage for all LED drivers.

In a second embodiment, the power source comprises three one-stage power factor correction and DC/DC converter units 215, 216, 217 for separately providing input voltage for the three sets of LED drivers.

According to the invention, total power loss and system costs of the power source have been reduced.

While the invention has been illustrated and described in detail in the drawings and foregoing description, it should be clear to a person skilled in the art that such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments; rather, several variations and modifications are possible within the protective scope of the invention as defined in the appending claims. For instance, instead of only three different colors the backlight system may comprise LEDs of four or more different colors; by way of example, a fourth type of LED may be a white-light LED. Further, it is possible that the power supply comprises two, or more than three drivers. Further, it is possible that all LEDs have the same color. Further, it is possible that a driver supplies only a subcollection of LEDs of one string, another subcollection of LEDs of the same string being driven by another driver. Further, it is possible that all strings of the same color are driven by the same driver. Further, it is possible that a driver drives LEDs arranged in parallel. Further, it is possible that a driver drives only one LED. Further, an embodiment is possible comprising multiple one-stage power factor correction and DC/DC converter units (like embodiment 200), wherein at least one of these converter units provides intermediate voltage to two or more LED drivers (like embodiment 100).

Further, although the present invention has been explained in the context of driving backlight LEDs for an LCD screen, the present invention can also be applied in other applications where it is desirable to drive a plurality of LEDs with multiple drivers. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.

In the above, the present invention has been explained with reference to block diagrams, which illustrate functional blocks of the device according to the present invention. It is to be understood that one or more of these functional blocks may be implemented in hardware, where the function of such functional block is performed by individual hardware components, but it is also possible that one or more of these functional blocks are implemented in software, so that the function of such functional block is performed by one or more program lines of a computer program or a programmable device such as a microprocessor, microcontroller, digital signal processor, etc.

Claims

CLAIMS:

1. Power source (110; 210) for driving a plurality of LEDs (102, 103, 104; 202, 203, 204), the power source comprising multiple LED drivers (124, 125, ... 126, 134, 135, ... 136, 144, 145, ... 146; 224, 225, ... 226, 234, 235, ... 236, 244, 245, ... 246), each LED driver for driving at least one LED; wherein the power source comprises at least one one-stage power factor correction and

DC/DC converter unit (114; 215, 216, 217) for providing input voltage for the multiple LED drivers.

2. Power source according to claim 1, wherein multiple LED drivers (124, 125, ... 126, 134, 135, ... 136, 144, 145, ... 146) have their input coupled to the output of one common one-stage power factor correction and DC/DC converter unit (114).

3. Power source according to claim 2, wherein at least one of said LED drivers (124, 125, ... 126, 134, 135, ... 136, 144, 145, ... 146) is implemented as a switching driver.

4. Power source according to claim 2, wherein all of said LED drivers (124, 125, ... 126, 134, 135, ... 136, 144, 145, ... 146) are implemented as switching drivers.

5. Power source according to claim 2, wherein all of said LED drivers (124, 125, ... 126, 134, 135, ... 136, 144, 145, ... 146) are implemented as broadband switching drivers.

6. Power source according to claim 1, wherein the power source (210) comprises multiple one-stage power factor correction and DC/DC converter units (215, 216, 217), each converter unit providing input voltage to at least one of said multiple LED drivers (224, 225, ... 226, 234, 235, ... 236, 244, 245, ... 246).

7. Power source according to claim 6, wherein each one-stage power factor correction and DC/DC converter unit (215, 216, 217) provides input voltage to all LED drivers (224, 225, ... 226, 234, 235, ... 236, 244, 245, ... 246) associated with the same color of LEDs (202, 203, 204).

8. Power source according to claim 6, wherein each one-stage power factor correction and DC/DC converter unit (215, 216, 217) is implemented as flyback converter.

9. Power source according to claim 8, further comprising a common converter controller (218) controlling the operation of the converter units (215, 216, 217) such that their switching frequencies are synchronized and such that their switching phases are distributed over a full 2π phase.

10. LED backlight system (100; 200) for an LCD panel, comprising a plurality of red LEDs (102), a plurality of green LEDs (103), and a plurality of blue LEDs (104), further comprising a power source according to any of the previous claims.

11. LCD panel, provided with a backlight system (100; 200) according to claim 10.

12. LED backlight system (100) for an LCD panel, comprising: - a plurality of red LEDs (102), a plurality of green LEDs (103), and a plurality of blue LEDs (104); a power source (110), comprising at least one LED driver (124, 125, ... 126) for driving the red LEDs (102), at least one LED driver (134, 135, ... 136) for driving the green LEDs (103), and at least one LED driver (144, 145, ... 146) for driving the blue LEDs (104); the power source further comprising a rectifier (112) for connection to mains, and a mains filter (113); wherein the power source comprises one common one-stage power factor correction and DC/DC converter unit (114) for providing input voltage for all LED drivers.

13. LED backlight system (200) for an LCD panel, comprising: a plurality of red LEDs (202), a plurality of green LEDs (203), and a plurality of blue LEDs (204); a power source (210), comprising at least one red LED driver (224, 225, ... 226) for driving the red LEDs (202), at least one green LED driver (234, 235, ... 236) for driving the green LEDs (203), and at least one blue LED driver (244, 245, ... 246) for driving the blue LEDs (204); the power source further comprising a rectifier (212) for connection to mains, and a mains filter (213); wherein the power source comprises separate one-stage power factor correction and DC/DC converter units (215, 216, 217) for separately providing input voltage for the red, green and blue LED drivers.