Appliance Design - November 2007 - (Page 25)

DISPLAYS & INDICATORS mA, the total brightness diminishes, but the difference in their brightness levels increases greatly. Medium-size LCD backlights must simultaneously illuminate dozens, even hundreds of high-brightness LEDs. If the forward currents deviate too much from the LEDs’ nominal value, the LEDs exhibit unequal brightness levels, even if those currents are all identical. To solve this problem, the drive current is often pulse-width modulated. Compared with direct analog control of the drive current, a PWM dimmer maintains LED currents at their nominal value, and adjusts the brightness via duty-cycle control of the PWM signal. As a result, all LEDs maintain a uniform appearance as their brightness changes. Multiple LED strings 20 mA/string An external BJT can be used to drive up to 100 strings (2 A). Fig. 2. Use of an LDO regulator for driving the LED-backlight group in a medium-size LCD. Essential requirements To devise an effective power-driving circuit for LEDs, one must understand their essential photoelectric characteristics. LEDs are semiconductor devices. It is possible to fabricate about 30,000 5 mm LED die (chips) on a 3-inch epitaxial wafer, but testing, classifying, and packaging contribute to a reduction in the final number of usable LED products. The chips on a 3-inch epitaxial wafer can be sorted into various bins. (See Fig. 1.) Chips on the dark blue field are the best, and chips on the middle fields are less stable, with the quality of chips on the outer field the worst on the wafer. Thus, the forward conducting voltage, wavelength, and brightness of LEDs from the same epitaxial wafer differ somewhat, providing a basis for sorting into various bins. Moreover, the fabrication processes of different manufacturers impose different environments for creating and cutting wafers and packaging the LEDs, thereby producing LED products with a range of characteristics. LEDs are typically sorted and classified according to wavelength, brightness, and other parameters. When an LED conducts, its forward-current variation is much greater than the rate of variation for forward conducting voltage. The testing and classification for optical characteristics, therefore, is based mostly on a consistent nominal current value such as 20 mA or 350 mA. The range of allowable variation in forward conducting voltage is then noted for the brightness level specified. To guarantee equal brightness and chroma for similar LEDs, they must all have the same driving current. To guarantee reliability, however, the drive current for LEDs must be lower than the nominal value. Moreover, this permitted nominal value decreases as the ambient temperature increases. An LED used for backlighting, therefore, must be driven with a known constant current. Otherwise, current that exceeds the maximum nominal value can affect the LED’s reliability. Factory testing and classifying of LEDs is based on equal nominal current values. As a consequence, the LEDs purchased have similar optical characteristics around the nominal current value. LEDs are often dimmed by simply reducing their drive current. But, if those LEDs operate at a current far from their nominal value (for the purpose of dimming, perhaps), their optical characteristics will differ considerably, even if all have the same driving current. If two LEDs with nominal drive currents of 20 mA are driven at 5 Drive scheme Medium-size LCDs (5 in. to 17 in.) are the displays most often found in portable DVDs, notebook computers, and GPS receivers. The LED driving strategy in these applications has developed according to the strategy for small-size LED backlighting, and is compatible with the current-driving circuits for CCFL backlights, as far as possible. One simply replaces the CCFL with an aluminum-based LED lighting bar, and replaces the complicated high-voltage AC driver with a simple low-voltage DC driver. The resulting cost is very close to that of a traditional CCFL backlighting module. To extend the LED-backlighting drive scheme from small-size to medium-size LCD panels, one simply adds LEDs as required, and considers the series-parallel block of LEDs as an integral load. One then adjusts Fig. 3. Use of switched power for driving the LED-backlight group in a medium-size LCD. Each LED branch can support up to 10 white LEDs connected in series. In other words, one LED group supports up to 80 LEDs. The current through each branch can be set up to 55 mA. www.applianceDESIGN.com applianceDESIGN November 2007 25 http://www.appliancedesign.com

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Appliance Design - November 2007 Contents Editorial Shipments/Forecasts News Watch Today’s LEDs are Brighter and More Colorful than Previous Versions, Allowing Designers More Opportunities to Improve and Differentiate Their Products. When Converting from CCFL to LED Backlights in LCD Panels, New Driving Methods are Needed. Electrowetting Technology Allows Displays to be Designed in an Extensive Range of Colors. Noise Problems are Not Always Easy to Find or Fix, but Acoustic Labs Can Help Designers Ferret Out the “Good” Noise from the “Bad.” A New Sanitization Protocol for Clothes Washers Combines the Best of Three Well-Established Provisions to Ensure Efficiency, Quality and Safety. Stricter EMC Requirements Raise the Bar for Appliances with Electronic Controls and Require Testing to New Levels. Despite Added Cost and Complexity, Gas-Modulating Systems have Migrated from Large-Scale Boiler Applications to Smaller Residential and Commercial Furnaces and Appliances. A New Treatment Technology Hardens the Surface of Light Alloys such as Aluminum and Magnesium and Expands Their Potential Applications. DesignMart Advertiser’s Index Association Report: AHAM

Appliance Design - November 2007

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