EP3401897A1 - Lichtquellentreiberschaltung und verfahren zur ansteuerung einer reihe von lichtquellen - Google Patents

Lichtquellentreiberschaltung und verfahren zur ansteuerung einer reihe von lichtquellen Download PDF

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Publication number
EP3401897A1
EP3401897A1 EP17170497.6A EP17170497A EP3401897A1 EP 3401897 A1 EP3401897 A1 EP 3401897A1 EP 17170497 A EP17170497 A EP 17170497A EP 3401897 A1 EP3401897 A1 EP 3401897A1
Authority
EP
European Patent Office
Prior art keywords
transistor
light sources
voltage
string
source
Prior art date
Legal status (The legal status 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 status listed.)
Withdrawn
Application number
EP17170497.6A
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English (en)
French (fr)
Inventor
Cengiz TARHAN
Cihan ERDÍNÇ
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Vestel Elektronik Sanayi ve Ticaret AS
Original Assignee
Vestel Elektronik Sanayi ve Ticaret AS
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 Vestel Elektronik Sanayi ve Ticaret AS filed Critical Vestel Elektronik Sanayi ve Ticaret AS
Priority to EP17170497.6A priority Critical patent/EP3401897A1/de
Priority to TR2017/07475A priority patent/TR201707475A2/tr
Publication of EP3401897A1 publication Critical patent/EP3401897A1/de
Withdrawn legal-status Critical Current

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    • 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
    • H05B45/00—Circuit arrangements for operating light-emitting diodes [LED]
    • H05B45/40—Details of LED load circuits
    • H05B45/44—Details of LED load circuits with an active control inside an LED matrix
    • H05B45/46—Details of LED load circuits with an active control inside an LED matrix having LEDs disposed in parallel lines
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2330/00—Aspects of power supply; Aspects of display protection and defect management
    • G09G2330/04—Display protection
    • G09G2330/045—Protection against panel overheating
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/34—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source
    • G09G3/3406—Control of illumination source
    • G09G3/342—Control of illumination source using several illumination sources separately controlled corresponding to different display panel areas, e.g. along one dimension such as lines
    • G09G3/3426—Control of illumination source using several illumination sources separately controlled corresponding to different display panel areas, e.g. along one dimension such as lines the different display panel areas being distributed in two dimensions, e.g. matrix

Definitions

  • the present disclosure relates to a light source driver circuit and method of driving a string of light sources in a backlight unit for a display device.
  • a display device is used to display an image, which may be a static and/or moving image.
  • Some display devices have plural light sources provided in a backlight unit for illuminating pixels of a display screen that is in front of the backlight unit.
  • a problem with some backlight units is overheating of the driver integrated circuit which drives the light sources.
  • a light source driver circuit for driving a string of light sources, the circuit comprising:
  • Examples of the present disclosure effectively force the transistor (such as a MOSFET or other IGFET) in the driver circuit for the string of light sources to bear the power loss, or at least a portion of the power loss, rather than the light source driver IC having to bear the full power loss and suffer the associated serious heating problems.
  • the transistor such as a MOSFET or other IGFET
  • the voltage regulator controls the voltage applied as the gate-source voltage of the transistor to be between 2.5V to 7V.
  • the driver integrated circuit is arranged to control the transistor to achieve dimming of the light sources. In an example, the driver integrated circuit is arranged to control the transistor to achieve dimming of the light sources using pulse width modulation.
  • the transistor is a MOSFET.
  • the driver circuit is arranged to drive plural strings of light sources, the driver circuit comprising:
  • a backlight unit for a display device comprising:
  • the light sources are LEDs.
  • a method of driving a string of light sources in a backlight unit for a display device comprising:
  • a display device is used to display an image, which may be a static and/or moving image.
  • Some display devices have plural light sources provided in a backlight unit for illuminating pixels of a display screen that is in front of the backlight unit.
  • LCD liquid crystal display
  • the display screen of the LCD display device has a number of individually controllable elements for generating an image. These individually controllable elements correspond to the pixels of the image. (It is also common to refer to the individual controllable elements of an LCD display device as pixels. The term “pixels" will be used herein to refer to the individually controllable elements of the LCD display device, unless otherwise stated.)
  • the pixels of the display screen receive the light that is emitted by the light sources of the backlight unit to generate the displayed image.
  • each pixel includes a liquid crystal cell. The alignment of the liquid crystals in the various liquid crystal cells is controllable such that either light can pass through or light is blocked.
  • the light sources of the backlight unit are also controlled, in general either to emit light or not to emit light as required for illuminating or not the corresponding pixels in the display screen.
  • Light sources that are being controlled to emit a greater or maximum amount light are commonly referred to as "open”.
  • Light sources that are being controlled so as to emit no light or less than a maximum amount of light are commonly referred to as “closed”.
  • the light sources of the backlight unit are often arranged in parallel strings of light sources, in which each string has plural light sources. This is typically the case in so-called edge-lit display devices, in which the light sources are provided only at the edges of the display screen, but also in some direct-lit display devices in which the light sources are arrayed across substantially the whole area of the display screen.
  • An integrated power supply drives the strings of light sources, typically using a driver IC (integrated circuit).
  • driver IC integrated circuit
  • typical white LEDs have a forward voltage Vf of 3.3V with a 20% variation at its rated current.
  • the forward voltage Vf of an LED is the voltage that must be applied across the leads of the LED, from anode to cathode, in order for the LED to turn on.
  • the strings of LED light sources each have 30 LEDs arranged in series, it is possible that one string may require say 93V (30 x 3.1V) to drive it, while a second string may require say 99V (30 x 3.3V) at the same current to drive it. If the two strings are used in parallel, the LED driver IC may set the regulation voltage of the LED1 pin (i.e.
  • dimming of the light sources is achieved by rapidly switching the light sources on and off.
  • the amount of dimming is determined by the duty cycle of the switching on and off of the light sources.
  • PWM pulse-width modulation
  • transistors such as MOSFETs (metal-oxide-semiconductor field-effect transistor), are used in the drive circuit as part of the PWM dimming control.
  • MOSFET metal-oxide-semiconductor field-effect transistor
  • n external MOSFETs are necessary for PWM dimming, with one MOSFET being provided per string of LEDs.
  • PWM dimming as mentioned, the LEDs are open and closed in one cycle.
  • all the LEDs of that string have a voltage lower than say 3.1V in the example above, and the total voltage difference of the LEDs of the string arising from manufacturing tolerances returns to the IC's LED driver pin. Again, this voltage can damage and break the IC.
  • examples of the present disclosure address this problem in essence by forcing transistors (such as MOSFETs or other IGFETs) in the driver circuit for the strings of light sources to bear the power loss, or to bear at least a portion of the power loss, rather than the light source driver IC having to bear the full power loss and suffer the associated serious heating problems.
  • This can be achieved in some examples by controlling the gate-source voltage of the transistors in the driver circuit for the strings of light sources to be relatively lower, which in turn increases the source on resistance whereby the transistors carry the power loss, or at least a portion of the power loss, rather than the light source driver IC having to bear the full power loss.
  • the transistors are controlled so as to operate in their linear region and not in their saturated region.
  • the driver circuit 1 drives plural strings 2 of light sources 3.
  • the driver circuit 1 drives plural strings 2 of light sources 3.
  • Each string 2 may have one or more light sources 3.
  • the light sources 3 may each be for example LEDs.
  • the driver circuit 1 and the strings 2 of light sources 3 may be part of a backlight unit for illuminating a display screen of a display device (not shown).
  • the strings 2 may be provided only at the edges of the display screen as in a so-called edge-lit display device, or may be arrayed across substantially the whole area of the display screen as in a so-called direct-lit display device.
  • the driver circuit 1 has a driver chip or IC (integrated circuit) 4.
  • the driver IC 4 generates voltages to drive the plural strings 2 of light sources 3, in particular to control the turning on and off of the light sources 3 within the strings 2.
  • the driver IC 4 has various input and output pins.
  • the main pins of relevance for the present disclosure are the light source drive pins 5.
  • One end of each string 2 is connected to a respective one of the light source drive pins 5. (This is not shown in the schematic drawing of Figure 1 , but is indicated by the designation "LED1", etc.
  • each string 2 is connected to a source of an input voltage VIN.
  • the driver IC 4 controls the voltages at the drive pins 5 to turn the light sources 3 of the various strings 2 on and off as needed to cause an image to be displayed.
  • the light source drive pins 5 have an absolute maximum voltage rating, which is fixed and written into the specification of the driver IC 4. To effectively increase that voltage rating, a respective transistor 6 is provided for each string 2.
  • the transistors 6 may be for example IGFETs, such as MOSFETs, including in particular enhancement-mode, n-channel MOSFETs.
  • IGFETs such as MOSFETs, including in particular enhancement-mode, n-channel MOSFETs.
  • MOSFETs MOSFETs, including in particular enhancement-mode, n-channel MOSFETs.
  • the drain-source path of the transistor 6 is located between the end of the string 2 and the drive pin 5, with the drain of the transistor 6 being connected to the end of the string 2 and the source being connected to the drive pin 5.
  • a voltage VSINK 7 is applied via a resistor 8 to the gate of the transistor 6.
  • the resistor 8 is also connected by another resistor 9 to the drive pin 5. Accordingly, VSINK 7 is applied as the gate-source voltage for the transistor
  • the voltage rating for the drive pin 5 is effectively increased by the drain-source voltage VDS of the transistor 6.
  • VGS gate-source voltage
  • VSINK is higher than the sum of the VGS (gate-source voltage) threshold and the driver pin voltage for that string VLEDX.
  • this can cause a power loss and heating of the driver IC 4 because of the tolerances in the manufacture of the light sources 3, particularly in the case of the light sources 3 being LEDs.
  • the voltages that are respectively applied as the gate-source voltage of the various transistors 6 in the drive lines for the strings 2 are controlled so as to be lowered. That is, the gate-to-source voltage is lowered.
  • the graph of Figure 2 which shows how the drain-to-source on-resistance of a MOSFET varies with gate-to-source voltage, a lower gate-to-source voltage corresponds to a higher drain-to-source on-resistance.
  • the voltages that are respectively applied to the various transistors 6 in the drive lines for the strings 2 are controlled to lower the gate-to-source voltage of the transistors 6 such that the transistors 6 are operating in the linear region and not in the saturated region.
  • the transistor 6 such as a MOSFET
  • the transistor 6 is turned on and a channel has been created which allows current to flow between the drain and the source.
  • the MOSFET operates like a resistor, controlled by the gate voltage relative to both the source and drain voltages.
  • the gate-to-source voltage of the transistors 6 may in an example be in the range of approximately 2.5V to 7V, or in an example more specifically in the range of approximately 3.5V to 5V, in order for a transistor 6 such as a MOSFET to be operating in the linear region.
  • the "saturated" region which is at higher voltages to the right in the graph of Figure 2 , the value of the drain-to-source on-resistance hardly changes as the gate-to-source voltage is changed.
  • the effect of the higher drain-to-source on-resistance is that the transistor 6 itself bears the power loss, and therefore heating, or at least the major part of the power loss and therefore heating, caused by manufacturing tolerances of the light sources 3 (particularly manufacturing tolerances in the forward or drive voltages Vf of the light sources 3): the driver IC 4 is not subject to the power losses and therefore heating, or at least the effect on the driver IC 4 is significantly reduced.
  • transistors are much better able to withstand heating than an IC.
  • Figure 3 shows schematically an example of a voltage regulator circuit 10 for controlling the voltage applied as the gate-source voltage of the transistors 6, specifically for generating a low voltage VSINK which is applied as the gate-source voltage of the transistors 6.
  • a similar voltage regulator circuit 10 may be provided for each transistor 6 for each string 2 of light sources 3.
  • the voltage regulator circuit 10 has a bipolar junction transistor (BJT) 11, in this example an NPN BJT 11.
  • BJT bipolar junction transistor
  • the voltage VSINK, which is applied via the resistor 8 to the gate of the transistor 6, is presented at the emitter of the BJT 11.
  • the base of the BJT 11 is connected to earth via a Zener diode 12.
  • the collector of the BJT 11 is connected via a resistor 13 to the voltage VCC_LED of a power supply. (VCC_LED may in example be a minimum of Vsink+1V and a maximum of 24V.)
  • VCC_LED may in example be a minimum of Vsink+1V and a maximum of 24V.
  • the collector and emitter of the BJT 11 are connected by a resistor 14, and the collector and the base of the BJT 11 are connected by a further resistor 15.
  • the emitter of the BJT 11 is also connected to earth via a capacitor 16.
  • the Zener diode 12 of the voltage regulator circuit 10 effectively operates to provide a low VSINK (particularly compared to known similar circuits) and therefore a low voltage is applied to the gate of the transistor 6 of the LED driver circuit so that the transistor 6 operates in the linear region.
  • the gate-to-source voltage of the transistors 6 may be in the range of approximately 2.5V to 7V, or more specifically in the range of approximately 3.5V to 5V, in order for the MOSFET transistor 6 to be operating in the linear region, and VSINK may be controlled by the voltage regulator circuit 10 such that the gate-to-source voltage of the transistors 6 is in this range.
  • voltage regulator circuits may be used. Examples include the known LM317 voltage regulator and the 7805 voltage regulator IC (or more generally the 78XX voltage regulator IC where XX is the maximum voltage).
  • the voltage regulator may be fixed or adjustable.
  • the principal aim is to lower the voltage (VSINK above) that is applied to the gate of the (MOSFET) transistor 6 of the LED driver circuit so that the transistor 6 operates in the linear region. The effect of this is that the transistor 6 bears the major part or substantially all of the power loss and heating arising from manufacturing tolerances in the LEDs 3, thereby protecting the driver IC 4 from that heat.
  • processor or processing system or circuitry referred to herein may in practice be provided by a single chip or integrated circuit or plural chips or integrated circuits, optionally provided as a chipset, an application-specific integrated circuit (ASIC), field-programmable gate array (FPGA), digital signal processor (DSP), graphics processing units (GPUs), etc.
  • the chip or chips may comprise circuitry (as well as possibly firmware) for embodying at least one or more of a data processor or processors and a digital signal processor or processors, which are configurable so as to operate in accordance with the exemplary embodiments.
  • the exemplary embodiments may be implemented at least in part by computer software stored in (non-transitory) memory and executable by the processor, or by hardware, or by a combination of tangibly stored software and hardware (and tangibly stored firmware).

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  • Electronic Switches (AREA)
  • Circuit Arrangement For Electric Light Sources In General (AREA)
  • Semiconductor Lasers (AREA)
EP17170497.6A 2017-05-10 2017-05-10 Lichtquellentreiberschaltung und verfahren zur ansteuerung einer reihe von lichtquellen Withdrawn EP3401897A1 (de)

Priority Applications (2)

Application Number Priority Date Filing Date Title
EP17170497.6A EP3401897A1 (de) 2017-05-10 2017-05-10 Lichtquellentreiberschaltung und verfahren zur ansteuerung einer reihe von lichtquellen
TR2017/07475A TR201707475A2 (tr) 2017-05-10 2017-05-23 Işik kaynaği sürücü devresi̇ ve bi̇r işik kaynaği di̇zi̇si̇ni̇n sürülmesi̇ne i̇li̇şki̇n yöntem

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
EP17170497.6A EP3401897A1 (de) 2017-05-10 2017-05-10 Lichtquellentreiberschaltung und verfahren zur ansteuerung einer reihe von lichtquellen

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EP3401897A1 true EP3401897A1 (de) 2018-11-14

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EP17170497.6A Withdrawn EP3401897A1 (de) 2017-05-10 2017-05-10 Lichtquellentreiberschaltung und verfahren zur ansteuerung einer reihe von lichtquellen

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EP (1) EP3401897A1 (de)
TR (1) TR201707475A2 (de)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN113223444A (zh) * 2020-01-17 2021-08-06 厦门凌阳华芯科技有限公司 一种单像素led驱动芯片及led显示屏

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20100045194A1 (en) * 2008-08-19 2010-02-25 Microsemi Corp.-Analog Mixed Signal Group Ltd. Powering and controlling light emitting diodes via thermally separated arrays of dissipative active elements
WO2011021850A2 (en) * 2009-08-18 2011-02-24 Lg Innotek Co., Ltd. Led driving circuit
US20110089854A1 (en) * 2009-10-15 2011-04-21 Richtek Technology Corporation, R.O.C. Circuit And Method for Controlling Light Emitting Device, And Integrated Circuit Therefor
US20130026933A1 (en) * 2011-07-25 2013-01-31 Shenzhen China Star Optoelectronics Technology Co., Ltd. Led backlight drive circuit

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20100045194A1 (en) * 2008-08-19 2010-02-25 Microsemi Corp.-Analog Mixed Signal Group Ltd. Powering and controlling light emitting diodes via thermally separated arrays of dissipative active elements
WO2011021850A2 (en) * 2009-08-18 2011-02-24 Lg Innotek Co., Ltd. Led driving circuit
US20110089854A1 (en) * 2009-10-15 2011-04-21 Richtek Technology Corporation, R.O.C. Circuit And Method for Controlling Light Emitting Device, And Integrated Circuit Therefor
US20130026933A1 (en) * 2011-07-25 2013-01-31 Shenzhen China Star Optoelectronics Technology Co., Ltd. Led backlight drive circuit

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN113223444A (zh) * 2020-01-17 2021-08-06 厦门凌阳华芯科技有限公司 一种单像素led驱动芯片及led显示屏
CN113223444B (zh) * 2020-01-17 2022-03-11 厦门凌阳华芯科技有限公司 一种单像素led驱动芯片及led显示屏

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