lcd panel led backlight voltage factory
As on charts, LED driving Voltage is always higher than controller"s supply voltage (usually 12V) , that way we need driver/inverter, whatever we call step-up converter, it make from regular Vcc (12V) voltage, voltage able to supply and light up LED strings.
Most problem become, because for newer bigger screens are not available any datasheets, so many times I have to test LED backlight parameter by myself.
That is the reason, we must tread LED backlit screens very carefully, normally we must to test each particular screen separately and be sure to NOT overCurrent or overVoltage LED strings, as LEDs will simply burn off.
Some of those universal drivers have on-board places(pads) for I-Sense resistors mount/remove, and/or short-pads to regulate Current for particular LED backlight, this made possible to match LED driver"s Current parameter with particular screen. However such work need some experience and knowledge about DC/DC step-up converters, to avoid any LED burning disaster.
So there is only one I-Sense Resistor for total current, so if we know the driver"s chip reference voltage and proper Current value for particular screen backlight, we can count Sense Resistor for particular current, as from Ohm"s law R=U/I , but we have to know exactly the Current of particular LCD display we use, and reference (feedback threshold) voltage of IC used in driver (usually 150mV to 500mV depend on IC type and manufacturer) .
This article goes over the inner circuitry and workings of an LED strip light. This information is for engineering discussion purposes and is not necessary for typical users interested in regular use of LED strips.
LEDs are typically 3-volt devices. What this means is that if a 3-volt differential is applied between the positive and negative ends of an LED, it will light up.
Therefore, 3 LEDs in series will require a forward voltage of 9 volts (3 volts x 3 LEDs), and 6 LEDs in series will require a forward voltage of 18 volts (3 volts x 6 LEDs).
In addition to the LEDs, one or more current limiting resistors is also necessary to ensure that the LED strip does not go into overcurrent mode. The resistor is also placed in series with the LEDs, and its resistance value is calculated such that it will draw approximately 3 volts as well.
These are the "building blocks" for each group of LEDs on an LED strip. The way it is laid out on an LED strip can be visualized in our graphic below:
What happens to LEDs in parallel? The voltage remains the same but the current is split equally among each of the parallel circuits. Therefore, if you have 3 parallel groups that each draws 50 mA at 24 volts, the total power draw is 150 mA, also at 24 volts.
These two examples of 3 LEDs and 6 LEDs show how a typical 12 and 24 volt LED strip is configured. Because the LED strips utilize 3 volt LED devices, and are configured to have multiple parallel strings of 3 or 6 LEDs.
You may be wondering if 12 volts means exactly 12.0 volts or if 11.9 volts would still work? The good news is that there is quite a bit of leeway in the power supplied to an LED strip.
You will see that at 3.0V, for example, this particular LED will draw about 120 mA. If we decrease the voltage to 2.9V, the LED will draw a bit less, only about 80 mA. If we increase the voltage to 3.1V, the LED will draw more, about 160 mA.
Because in a 12V LED strip there are 3 LEDs and a resistor in series, supplying 11V instead of 12V is a bit like reducing the voltage for each LED by 0.25V.
What if we supplied just 10V to a 12V LED strip? In this case, we are reducing the voltage per LED by 0.5V each. If we reference the chart, at 2.5V, the LEDs will barely draw any current.
All voltages less than the LED strip rating are safe, as you will always be drawing less current and therefore avoiding any possibility for damage or overheating. But what about voltage levels over 12V?
And keep in mind that each LED will have a different rating, and inherent variation in manufacturing can affect the actual voltage ranges that are acceptable for a particular LED strip.
While it is possible to supply a voltage that is slightly different from the rated voltage, you will have to be careful and precise to ensure that you do not cause any damage to the LEDs.
One way to dim an LED strip is to adjust the input voltage to below is rated level, as we saw above. In reality, however, power electronics are not very good at reducing the voltage output in this way.
The preferred method is to use what is called PWM (pulse width modulation) - where the LEDs are turned on an off at a rapid rate. By adjusting the ratio of time on vs time off (duty cycle), the apparent brightness of an LED strip"s light output can be adjusted.
Similarly, we also know that an LED will draw the same amount of current when it is in its "on" state, regardless of its duty cycle. This is an added advantage for LED strip lights whose color temperature consistence must remain constant even when its brightness is altered.
One of the significant advantages of LED strip light products is simple yet versatile they are given compatibility with simple constant voltage power supply devices.
It can sometimes be useful to understand the inner workings of such devices as it can help us understand some of the more nuanced aspects of its performance, such as dimming and voltage input changes.
We are often asked how to drive a backlight in our displays. There are three main ways to do so: with an LED driver, a current generating resistor, or a current generating resistor and a transistor. We will explore the benefits and drawbacks of these choices.
It’s important to remember that display backlights are typically LED arrays and, like all diodes, LEDs are all about current. Every datasheet will list the necessary current for the backlight, typically called IF (forward current) or ILED.
Using an LED driver is the ideal way to drive an LED backlight. An LED driver is current controlled so the backlight brightness is consistent. If a constant voltage is used instead of a constant current, the backlight brightness can change with the LED temperature. The brightness can also be controlled via pulse width modulation (PWM), which allows designs to be usable in a wider variety of lighting situations.
It’s important to note that an LED driver will not work in all situations. Some display backlights include an internal diode with a forward voltage of 3.3v. If this is the case, the drive will be unable to turn off the backlight. Additionally, some backlights have a forward voltage below 3.3v. In this case, a different method will have to be used or a series resistor will be needed so the voltage is above the forward voltage.
This method is less efficient than an LED driver, but is flexible and uses common parts. Plus, any forward voltage on the LEDs can use this method. However, this method leaves the brightness open to change with LED temperature and does not allow for brightness adjustment..
To calculate the value of resistor to use, find the supply or forward voltage and current specifications for the backlight from the datasheet. Determine what your voltage source and voltage value will be. Calculate the voltage drop between the voltage source and the forward voltage spec of the LED. Then use Ohm’s Law to calculate the resistor value (V=IR so R=V/I).
The final method is to use a resistor in series with the backlight and a transistor with the emitter tied to ground. This method is similar to method 2, but with the added bonus of being able to control the backlight brightness. Connect a fairly large resistor between a PWM signal and the base of the transistor. This makes the voltage drop between the collector and emitter close to zero.
You should have basic electrical test equipment such as a Digital Multimeter and a basic working knowledge of electronics. Soldering is a must if you plan on doing more than testing a screen.
We will be gaining access under the dreaded “DO NOT TOUCH” plastic tape. What they really mean to say is.. “Do not touch the multiple ribbons along the top or bottom edge of the board.” These ribbons interface with the LCD screen and pass information to the pixel matrix. They are fragile!
This guide applies to Laptop LCD panels more so than your desktop"s 24" LCD screen. Some LED backlights are fed with up to 96 volts, but not usually… and not on laptops.
Finally, if you break something.. it"s on you! There is a lot of variance between LCD manufacturers.. they are all different! Hold only yourself responsible if something goes wrong... that"s an order, not a suggestion
Figure 5(a) shows a half-bridge DC-DC Series-Resonant Converter (SRC) topology for driving the RGB LEDs. The soft-switched DC-DC resonant converter includes power switches Q1 and Q2, resonant inductor Lr, resonant capacitor Cr, transformer T1, rectifier diodes Df1 and Df2, filter capacitor Cf, and the LED arrays represented by an equivalent resistance Ro. The characteristic impedance and the resonant frequency are respectively [18-21].
It is clearly seen from (13) that the switching frequency must be varied to regulate the output voltage. The highest switching frequency appears at the highest input voltage and the lightest load. On the other hand, the lowest switching frequency happens at the lowest input voltage and the heaviest load. For the SRC to operate in the zero-voltage-switching (ZVS) region, the lowest switching frequency must be higher than the resonant frequency as expressed in (5). Moreover, due to the switching speed limitations of the power devices, the highest switching frequency is below a specified value. In other words, the variations of the input DC voltage and the load variations must be confined to a small range. Usually a power factor corrector (PFC) is added in front of the DC-DC converter to raise the input power factor and reduce the input current harmonics. A phase-shift pulse width modulation (PSPWM) dimming control can effectively confine the load variation of the DC-DC SRC. Consequently, the output voltage variation of the PFC can be limited to a smaller extent. This results in a better operating condition for the SRC. For the PSPWM dimming strategy, the working durations of the shunt LED arrays are properly phase-shifted to confine the variation of the output current of the SRC. Figure 6 illustrates the circuit arrangement for N shunt single-colored LED arrays with PSPWM dimming method. It is almost the same as the conventional one, except that the dimming signals are applied with a specified phase difference. With the PSPWM dimming, there are always overlaps between the LED driving currents. The maximum duty cycle, or the overlap, is 100 %, corresponding to the highest brightness. To prevent the DC-DC SRC from operating at no load, the minimum duty cycle of the PSPWM dimming signal is 1/N, where N is the number of the shunt LED arrays. Under this circumstance, the overlap is zero, corresponding to the lowest brightness. Compared with the conventional dimming scheme, it is apparently recognized that the load variation of the SRC is less with the proposed PSPWM dimming function. To further investigate the operating principle of the PSPWM dimming, a more general case with N shunt LED arrays is discussed as follows. Figure 7 shows the waveforms of the N driving currents and the output current of the SRC. As stated earlier, the duty cycle range of the dimming signal is from 1/N to 100 %. In terms of the phase angle, if a complete period is 360º, the duty cycle range is from 360º/N to 360º. Assuming that the dimming signal for the LED array 1 starts at 0º, then the dimming signal for the k-th LED array would start at
It can be observed from Figure 7 that if the end of the dimming signal for LED array 1 is at d, where d is between k and k+1 and k 1, then the output current of the SRC in the range of k to k+1 is
A favored feature is that the load variation of the SRC is always within one step change of Ip, no matter what the load level is. Therefore, by carefully designing the duty cycle and the amplitude of the driving current for each LED array, the no load operation of the DC-DC SRC may be precluded. Moreover, the output transient of the SRC is improved due to the confined load change. The number of the LED array for one color, and the peak driving current of each LED array are first determined according to the specifications of the LED and the spectrum of the white color. Then a suitable duty cycle is chosen allowing a reasonable span of variation for dimming control.
LED backlighting is the most commonly used backlight for small, LCD panels. Light-emitting diodes, or LEDs, are practical components for a light source because of their small size. LED backlighting is popular due to its overall low cost, long life, variety of colors and high brightness.
LED backlights are housed in a light box that has a diffuser to evenly distribute the LED light. The light box is then mounted behind the LCD’s viewing area. The LED backlight comes in two configurations: array and edge lit. The array configuration has the LEDs mounted in a uniform, grid layout within the light box. This configuration gives off a very bright, even light. The disadvantage of an array configuration is that it requires a thick light box design to accommodate the number of LEDs required. The high number of LEDs in this configuration also means it consumes more power.
The other configuration for LED backlights is edge lit. An edge lit configuration is the most commonly used construction for LED backlights. This configuration mounts the LEDs along one edge of the light box. The layout results in a thin design. Edge lit also uses less LEDs overall and therefore consumes less power than an array configuration.
Another type of backlight options is the use of fiber optic technology. Fiber optic backlights use sheets of fiber optic woven cloth and are bundled by a ferrule (metal cap) to an LED or halogen light source. Advantages for the fiber optic technology includes low voltage, low power, and a very uniform brightness. This type of backlighting is ideal for custom display shapes or sizes however it is priced at a higher cost compared to other technologies available.
A third type of backlight option available uses an electroluminescent (EL) panel. The EL backlight is constructed of a series of different material layers that work together to create the light. The EL panel generates light when an electric current (AC power) is applied to its conductive surfaces. The advantage with EL backlighting is its low power consumption, no heat emission, and overall thin composition. EL backlighting is limiting in that it requires an invertor to generate the VAC needed to emit the light.
The last common backlight option available are cold cathode fluorescent lamps (CCFLs). CCFL backlights are a cost effective option typically found in graphic displays. The CCFL backlight for LCDs is usually configured with the lamp on the edge of a diffuser to distribute the light. An inverter is required to supply the voltage required by the fluorescent lamp. CCFLs offer a bright white light with low power consumption. This backlight option is not ideal for cold-temperature applications (less than 15°C) as the light output decreases with decreased ambient temperature.
There are many different backlight options available for your LCD. The most common types are LED, fiber optic, EL, and CCFL backlights. Cost and application of your product will have the highest influences on which backlight technology is best for your LCD.
LCD displays don’t emit light by themselves. They need a light source, and LED backlights are now dominating the market. In this article, Orient Display’s Bill Cheung provides a complete overview of LED backlight technology, discussing different types, driver technologies, color deviation, brightness options and more.
LCD (liquid crystal display) has long been the dominant technology in the display world. Certainly, there are some emerging competing display technologies—such as OLED (Organic Light Emitting Diode) [1] and micro-LED—that have the potential to threaten LCD’s position in the market. But both are currently only used for niche and high-end markets.
An LCD display can’t emit light by itself. In order to have an LCD display [2] used in a dim environment, a backlight has to be used as the light source. There are a few different technologies that are able to produce backlight ranging from EL (electroluminescent), CCFL (cold cathode fluorescent lamps) and LED (light emitting diode). However, a breakthrough in blue LED technology by Shuji Nakamura [3] led to LED backlights dominating the market.
One of the greatest benefits of LED backlighting is its long lifetime. Normally, LED lifetime can be measured with half-life when the original brightness decreases by 50%. With different LED chip manufacturing materials, technologies and environment used, the LED life can vary from 20,000 hours to well over 100,000 hours.
LED backlights have low power consumption and produce much less heat than other backlight technologies, which extends the durability and performance of the other display components. Furthermore, this reduces the risk of fire and explosion. LED backlights are also driven with DC (direct current) and low voltage (can be as low as 1.5V), which are good for battery drive and emit no interference to the circuitry. With the development of LED technology, the LED chips become small. So, it is possible to produce very thin backlight (0.5mm thick or thinner).
Although white LED is the most popular color, LED backlight can be made into different single colors, bi-colors and tri-colors [4] (Figure 1) (Figure 2). With RGB LED backlight color mixing, normal 8 color LED backlight can be produced (Figure 3).
LED backlight can be classified as bottom (array) lit and side (edge) lit backlights, and each have their plusses and minuses. The advantages of the bottom lit (array) backlight are that it is uniform and bright. Its disadvantage is high current draw, thickness, heat dissipation and cost. Meanwhile, the advantages of the side lit backlight are its thinness, flexibility in design, low current and lower cost. The main disadvantage of the side lit backlight is its non-uniformity—hot spots can be seen from most of the side lit backlight from certain angle. Figure 4 compares the bottom lit and side (edge) lit backlight LCD types.
Now let’s look at LED backlight structures. An LED backlight can be simplified into layers starting with a LED chip, light guide, diffusor and reflector (Figure 5). This is the lowest cost structure. Except for some very low current efficiency LCD displays—such as utility meters, battery-powered clock, watch, GPS and so on—most LCD displays need backlights to be visible in the dim lighting. Most often the backlight is actually at the back of the LCD. In rare cases, this light can be done as front light. The traditional LCD structure with LED backlight shown in Figure 6.
Direct current driving: This is the simple and low-cost way to drive a LED backlight, however, be mindful of the current limit otherwise the LED life can deteriorate quickly. The solution is simply to add a current limiting resistor in the circuit. Current limitation resistors value calculation formula: R = (V0– Vf)/If.Also be mindful of reverse drive, otherwise, the LED chip can break down easily.
LED driver with constant current: The advantage of constant current LED driver is that it will be the best option to use when building your own fixture or working with high powered LED because they avoid violating the maximum current specified for the LEDs, therefore avoiding burnout/thermal runaway. They are easier for designers to control applications, and help create a more consistent bright light.
LED driver with constant voltage: Using a constant voltage LED driver makes sense when using an LED or array that has been specified to take a certain voltage. This is helpful because constant voltage is a much more familiar technology for design and installation engineers. Moreover, the cost of these systems can be lower, especially in larger scale applications.
There are a variety of ways to connect a backlight and LCD module electrically. It can be done with wires that are soldered on the LCD or LCD module. It can be connected using pins, which can be soldered onto the LCD or LCD module. A third way is to use a FPC (flexible printed circuit), which can be soldered or plugged in a ZIF (zero insertion force) connector. And finally, there is the connector method. With this method you use connectors which can be plugged into mating connectors.
As the LED is manufactured via the semiconductor process, there are some color deviations that can be a quality control issue. One way to solve the issue is through a process of selection and sorting after manufacturing the LEDs. The LEDs are sorted into different categories or bins. How this sorting is done and what each bin actually contains is defined differently by each LED manufacturer. The backlight manufacturer can choose from which bin they take the LEDs for backlight color hue.
Some customers might request very fine binning by the LED manufacturer, which can be very expensive since only a very small percentage of the LEDs manufactured would meet the requirements for a specific bin. Figure 7 shows an example of the bin selection from Nichia, the most renowned LED manufacturer in the world. Figure 8 shows the 1931 CIE chromaticity diagram. And Figure 9 shows the color deviations (bin definition) by Cree for a qualified production lot.
In actual LED backlight production, most customers will accept the LED color for two big categories: white with yellowish (warm) and white with bluish (cold). Of course, the LED brightness will also need to be defined. For general application, most customers will accept a brightness tolerance of 70 percent.
It is extremely hard to estimate the LED backlight lifetime or MTBF (mean time between failures) because there are so many variable factors. However, the most important is the temperature on the LED chip. The factors that can affect the LED chip temperature include: surrounding temperature, humidity, driving current, voltage, backlight design (how many LED chips to be used, how close to each other, heatsink design), backlight manufacturing process (type and thickness of adhesive), quality of the LED chip and so forth.
To test the LED life is also very time consuming, requiring at least 1,000 hours. That’s the reason why no LED manufacturers can guarantee LED backlight life and most backlight manufacturers also are reluctant to provide lifespan data. As for LCD manufacturers, they need to discuss it with the customer to understand the applications and provide suggestions. It is normal that the LCD datasheet lists the typical life time and avoids providing a minimum lifetime. From Figure 10, we can see that over room temperature, the current needs to decrease as the temperature increases. At over 85°C, the LED is not usable.
To estimate LED backlight lifetime, you can use ballpark estimation or theoretical calculation. Let’s first examine the ballpark method. To take white LED as example, the nominal biasing current is 20mA. If we use a safe lifetime estimation, we can estimate using Table 1.
Now let’s use the theoretical calculation approach. As we previously mentioned, LED life is affected by a lot of factors: surrounding temperature, humidity, driving current, voltage, backlight design (how many LED chips to be used, how close to each other, heatsink designed), backlight manufacturing process (type and thickness of adhesive), quality of the LED chip and so on. LED chip manufacturers are not willing to give absolute values of LED chip lifetimes, but there is a theoretical calculation that we can use.
Temperature is the determination factor for LED chip life, while LED chip manufacturers use LED junction temperature to predict LED chip life more accurately. An example is:
Finally, let’s look at ways to increase LED backlight brightness. There are many ways to increase LED backlight brightness, but all these measures are balanced with performance and cost. Here are some of the methods:
For the LCD module side, using better aperture opening ratio, anti-reflection coating on surface, optical bonding. This results in higher cost. Actually, this measure is not to increase LED backlight brightness directly but to increase to the visibility to users.
Bill Cheung is an engineering lead and marketing manager at Orient Display, an LCD and display technology provider with over two decades of industry experience in delivering cutting edge display solutions. You can browse Orient Display"s knowledge base [7] to learn more about LCDs.
LED backlight driver ICs control the voltage and current for white LEDs used to illuminate LCD panels. Typical applications include portable appliances such as Smartphones, Tablets, Cameras, Notebook PCs as well as displays such as Computer Monitor and TV. Kinetic Technologies’ portfolio is one of the largest in the industry and features high efficiency operation with high accuracy at low dimming duty cycles. To enhance reliability, products are designed with several protection mechanisms including, but not limited to, LED short circuit, LED open circuit and over-current.
My TV recently turned off the backlight. Sound works and with the flashlight I can see the picture. I pulled out the power cord, after a few hours I tried again, worked fine, after about 5 minutes the backlight turned off again, the sound is fine and the flashlight test too. I noticed that if I let it be without power cord around 24 hours, the light will work for about 10 or 15 minutes before it turns off. When the light is working then the LED switch has 111.3-114.6 volts, but when it turns off, it is 144.6 volts. I also tried factory reset, not help.
Visually does not see the traces of burning on the power supply. And when the backlight is on, I don"t see dark spots on the screen, all the LEDs seem to work.
LCD panel:The driving voltage for most all LCD panels is above 3.3V. It is necessary to then add a “negative voltage” IC on the PCB of the module or to the customer’s motherboard to raise the voltage. A couple of NV generators is as follows: