EP3401897A1

EP3401897A1 - Light source driver circuit and method of driving a string of light sources

Info

Publication number: EP3401897A1
Application number: EP17170497.6A
Authority: EP
Inventors: Cengiz TARHAN; Cihan ERDÍNÇ
Original assignee: Vestel Elektronik Sanayi ve Ticaret AS
Current assignee: Vestel Elektronik Sanayi ve Ticaret AS
Priority date: 2017-05-10
Filing date: 2017-05-10
Publication date: 2018-11-14
Also published as: TR201707475A2

Abstract

A light source driver circuit (1) has a driver integrated circuit (4), which provides a drive voltage for driving a string (2) of light sources (3). One end of the string (2) is connected to a power supply. The light source driver circuit (1) has a transistor (6) having a drain, a source and a gate. The drain-source path of the transistor (6) is connected between the other end of the string (2) and the driver integrated circuit (4) and receives a drive voltage from the driver integrated circuit (4). A voltage regulator (10) controls the voltage applied as the gate-source voltage of the transistor (6) so that the transistor (6) operates in the linear region.

Description

Technical Field

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.

Background

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.

Summary

According to a first aspect disclosed herein, there is provided a light source driver circuit for driving a string of light sources, the circuit comprising:

a driver integrated circuit for providing a drive voltage for driving a string of light sources which is arranged in use such that one end of the string is connected to a power supply;
a transistor having a drain, a source and a gate,
the drain-source path of the transistor being for connection between the other end of the string and the driver integrated circuit for receiving a drive voltage from the driver integrated circuit; and
a voltage regulator for controlling the voltage applied as the gate-source voltage of the transistor so that the transistor operates in the linear region.

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.
In an example, the voltage regulator controls the voltage applied as the gate-source voltage of the transistor to be between 2.5V to 7V.
In an example, 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.
In an example, the transistor is a MOSFET.
In an example, the driver circuit is arranged to drive plural strings of light sources, the driver circuit comprising:

a respective transistor for each of the strings of light sources, and
a respective voltage regulator for each of the transistors for controlling the voltage applied as the gate-source voltage of the transistor so that the transistor operates in the linear region.

There may also be provided a backlight unit for a display device, the backlight unit comprising:

plural strings of light sources;
a light source driver circuit as described above for driving the plural strings of light sources.

In an example, the light sources are LEDs.
According to a second aspect disclosed herein, there is provided a method of driving a string of light sources in a backlight unit for a display device, the method comprising:

using a driver integrated circuit to drive a string of light sources, wherein one end of the string is connected to a power supply and wherein a drain-source path of a transistor having a drain, a source and a gate is connected between the other end of the string and the driver integrated circuit for receiving a drive voltage from the driver integrated circuit; and
for controlling the voltage applied as the gate-source voltage of the transistor so that the transistor operates in the linear region.

Brief Description of the Drawings

To assist understanding of the present disclosure and to show how embodiments may be put into effect, reference is made by way of example to the accompanying drawings in which:

Figure 1 shows schematically an example of a light source driver circuit;
Figure 2 shows a graph for drain-to-source resistance v. gate-to-source voltage for a MOSFET; and
Figure 3 shows schematically an example of a voltage regulator circuit.

Detailed Description

As mentioned, 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.
One type of display device that generates displayed images in this way is a liquid crystal display (LCD) device. 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. In the LCD display device, 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). However, problems can arise because of manufacturing tolerances of the light sources, which can lead to heating of the driver IC because of current returning to the IC.
As a particular example in the case of the lights sources being LEDs (light emitting diodes), 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.) So, if for example 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. the pin that sets the drive voltage for string 1) at around 0.4V for the 99V LED string, which means that the voltage of the LED2 pin (i.e. the pin that sets the drive voltage for string 2) is around 99-93V = 6V. This approximate 6V difference at the LED2 pin in this example causes a problem for the driver IC because the LED string current ILED for string 2 return to the LED2 pin and there is therefore a power loss of 6V x ILED. This causes heating of the IC, which can exceed the IC's maximum power dissipation values and possibly damage the IC.
Separately, in known backlight units, particularly where the light sources are LEDs, 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. It is known to use pulse-width modulation (PWM) for the drive voltage that drives a string of light sources for this purpose. In some cases, transistors, such as MOSFETs (metal-oxide-semiconductor field-effect transistor), are used in the drive circuit as part of the PWM dimming control. (As is known, a MOSFET is a type of IGFET or insulated gate field effect transistor.)
In particular, in an example, for a backlight unit that has n LED strings in parallel, each string typically having plural LEDs, n external MOSFETs are necessary for PWM dimming, with one MOSFET being provided per string of LEDs. In PWM dimming, as mentioned, the LEDs are open and closed in one cycle. When the LEDs of a string are closed, 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.
In outline, 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. In some examples, the transistors are controlled so as to operate in their linear region and not in their saturated region.
Referring to Figure 1, an example of a light source driver circuit 1 in accordance with the present disclosure is shown schematically. The driver circuit 1 drives plural strings 2 of light sources 3. In the example shown, there are four strings 2 of light sources 3, though there may be fewer or more strings 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. In this example, there are four light source drive pins 5, one for each of the strings 2 of light sources 3, referred to generically as LEDX and marked LED1, LED2, LED3 and LED4 respectively in the figure. 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. at the lower end of each string 2 in the figure.) The other end of 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. For each transistor 6, 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. For each transistor 6, 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 6 to control the operation of the transistor 6.
For a string 2, when the light sources 3 are off (e.g. when no light is required to be emitted by the light sources 3 or during the off portion of a dimming mode cycle, such as a PWM mode as described above), the voltage rating for the drive pin 5 is effectively increased by the drain-source voltage VDS of the transistor 6. When the light sources 3 are on (e.g. during for example continuous operation of the light sources 3 or during the on portion of a dimming mode cycle, such as a PWM mode as described above), VSINK is higher than the sum of the VGS (gate-source voltage) threshold and the driver pin voltage for that string VLEDX. Current flows through the light sources 3, returning to the driver pins 5 of the driver IC 4. However, as noted above, 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.
To address this, in accordance with an example of the present disclosure, 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. As can be seen in 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. In an example, 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. When the transistor 6, such as a MOSFET, is operating in the linear region, the transistor 6 is turned on and a channel has been created which allows current to flow between the drain and the source. In effect, 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. This corresponds to the region to the left in the graph of Figure 2, where at lower values of the gate-to-source voltage (say below about 7V or so) the value of the drain-to-source on-resistance changes significantly as the gate-to-source voltage is changed. On the other hand, in 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. In general, 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. 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.) Moreover, 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. In the case of a MOSFET transistor 6, 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.
Other 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.
It will be understood that the specific example described above and shown in the drawings is just one way of driving light sources in a backlight unit for a display device. Many other ways of connecting a driver IC to drive light sources in strings are known and possible, with for example different arrangements for generating the drive voltages, sink voltages, gate-source voltages, etc.
It will further be understood that the 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. In this regard, 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).
The examples described herein are to be understood as illustrative examples of embodiments of the invention. Further embodiments and examples are envisaged. Any feature described in relation to any one example or embodiment may be used alone or in combination with other features. In addition, any feature described in relation to any one example or embodiment may also be used in combination with one or more features of any other of the examples or embodiments, or any combination of any other of the examples or embodiments. Furthermore, equivalents and modifications not described herein may also be employed within the scope of the invention, which is defined in the claims.

Claims

A light source driver circuit for driving a string of light sources, the circuit comprising:
a driver integrated circuit for providing a drive voltage for driving a string of light sources which is arranged in use such that one end of the string is connected to a power supply;

a transistor having a drain, a source and a gate,

the drain-source path of the transistor being for connection between the other end of the string and the driver integrated circuit for receiving a drive voltage from the driver integrated circuit; and

a voltage regulator for controlling the voltage applied as the gate-source voltage of the transistor so that the transistor operates in the linear region.
A light source driver circuit according to claim 1, wherein the voltage regulator is arranged to control the voltage applied as the gate-source voltage of the transistor to be between 2.5V to 7V.
A light source driver circuit according to claim 1 or claim 2, wherein the driver integrated circuit is arranged to control the transistor to achieve dimming of the light sources.
A light source driver circuit according to claim 3, wherein the driver integrated circuit is arranged to control the transistor to achieve dimming of the light sources using pulse width modulation.
A light source driver circuit according to any of claims 1 to 4, wherein the transistor is a MOSFET.
A light source driver circuit according to any of claims 1 to 5, the driver circuit being arranged to drive plural strings of light sources, the driver circuit comprising:
a respective transistor for each of the strings of light sources, and

a respective voltage regulator for each of the transistors for controlling the voltage applied as the gate-source voltage of the transistor so that the transistor operates in the linear region.
A backlight unit for a display device, the backlight unit comprising:
plural strings of light sources;

a light source driver circuit according to claim 6 for driving the plural strings of light sources.
A backlight unit according to claim 7, wherein the light sources are LEDs.
A method of driving a string of light sources in a backlight unit for a display device, the method comprising:
using a driver integrated circuit to drive a string of light sources, wherein one end of the string is connected to a power supply and wherein a drain-source path of a transistor having a drain, a source and a gate is connected between the other end of the string and the driver integrated circuit for receiving a drive voltage from the driver integrated circuit; and

for controlling the voltage applied as the gate-source voltage of the transistor so that the transistor operates in the linear region.
A method according to claim 9, comprising the voltage regulator controlling the voltage applied as the gate-source voltage of the transistor to be between 2.5V to 7V.
A method according to claim 9 or claim 10, wherein the driver integrated circuit is arranged to control the transistor to achieve dimming of the light sources.
A method according to claim 11, wherein the driver integrated circuit is arranged to control the transistor to achieve dimming of the light sources using pulse width modulation.
A method according to any of claims 9 to 12, wherein the transistor is a MOSFET.
A method according to any of claims 9 to 13, wherein the light sources are LEDs.