Design Article
LED driving techniques reduce power in LCD TVs
Peter Rust, Werner Schögler, Manfred Pauritsch, Herbert Truppe, ams
1/15/2013 2:19 PM EST
Multi-string with single DC-DC converter
A more radical approach to reducing BOM cost can be found in the multi-string with a single DC-DC converter topology (see Figure 4). The drawback of this approach is that the SMPS voltage must be regulated higher than the LED string with the highest forward voltage, which means that it operates at a higher voltage than is necessary for those strings with a lower forward voltage. This means that the ILED sink must dissipate the excess power from the strings with lower forward voltage, generating heat that must be conducted away from the circuit board, and resulting in reduced power efficiency.
Multi string mixed architecture
The architecture that provides the best balance between efficiency and BOM cost is one that combines elements of the multi-string and multi DC-DC converter architectures previously described. This mixed architecture (see Figure 5) has multiple DC-DC converters supplying groups of LED strings.
Figure 5: A mixed architecture optimizes balance between BOM cost & and power efficiency
Click on image to enlarge
Regulating current to match the characteristics of LEDs
The LED manufacturing process causes wide variations in brightness and color temperature from one LED to the next. As a guide to users, white LED manufacturers allocate each manufactured unit to groups or ‘bins’ of LEDs with comparable performance in terms of color, brightness and forward voltage. But the manufacturer’s specification for each brightness and color temperature bin is only valid under specific nominal operating conditions. This means that the LED current must be set to the nominal current stated in the datasheet in order to generate the specified brightness and color.
Consequently, dimming and brightness control can only be implemented by switching the current to any single LED either to ON (nominal current) or OFF (zero current) through a digital PWM control signal. In analog dimming, the LED would be operating outside its specified nominal current, leading to unacceptable changes in color temperature and poor LED-to-LED brightness matching (see Figure 6).
Figure 6: Brightness of LEDs from the same bin is guaranteed to match only at nominal current (in this case, 20 mA)
A more radical approach to reducing BOM cost can be found in the multi-string with a single DC-DC converter topology (see Figure 4). The drawback of this approach is that the SMPS voltage must be regulated higher than the LED string with the highest forward voltage, which means that it operates at a higher voltage than is necessary for those strings with a lower forward voltage. This means that the ILED sink must dissipate the excess power from the strings with lower forward voltage, generating heat that must be conducted away from the circuit board, and resulting in reduced power efficiency.
Figure 4: With one DC-DC converter serving multiple LED strings, SMPS voltage is not optimized
Multi string mixed architecture
The architecture that provides the best balance between efficiency and BOM cost is one that combines elements of the multi-string and multi DC-DC converter architectures previously described. This mixed architecture (see Figure 5) has multiple DC-DC converters supplying groups of LED strings.
Figure 5: A mixed architecture optimizes balance between BOM cost & and power efficiency
Click on image to enlarge
This solution offers the best overall power efficiency because it combines the advantage of local dimming in direct backlit systems with good DC-DC output voltage regulation. It also offers a substantial BOM saving over the efficient multi-string, multi DC-DC converter architecture.
Regulating current to match the characteristics of LEDs
The LED manufacturing process causes wide variations in brightness and color temperature from one LED to the next. As a guide to users, white LED manufacturers allocate each manufactured unit to groups or ‘bins’ of LEDs with comparable performance in terms of color, brightness and forward voltage. But the manufacturer’s specification for each brightness and color temperature bin is only valid under specific nominal operating conditions. This means that the LED current must be set to the nominal current stated in the datasheet in order to generate the specified brightness and color.
Consequently, dimming and brightness control can only be implemented by switching the current to any single LED either to ON (nominal current) or OFF (zero current) through a digital PWM control signal. In analog dimming, the LED would be operating outside its specified nominal current, leading to unacceptable changes in color temperature and poor LED-to-LED brightness matching (see Figure 6).
Figure 6: Brightness of LEDs from the same bin is guaranteed to match only at nominal current (in this case, 20 mA)
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docdivakar
1/16/2013 10:23 PM EST
Good article! The authors claim that the 'edge-lit LED backlighting TV's provide good optical uniformity in screen sizes up to 40"...' but there are many brands that go higher, up to 60" that display reasonably good pictures. Perhaps these larger ones employ more sophisticated light guides?
MP Divakar
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William Miller
4/1/2013 10:09 AM EDT
This technology is growing so fast) 15 years ago I had a lamp TV. 5 years ago I bought a plasma. A year ago I replaced it for a LED Tv. Now what? I will have to buy this new direct backlit system TV? I know it's more efficient. But at the moment I have doubts..
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William - http://www.carid.com/
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anne-francoise.pele
4/12/2013 8:55 AM EDT
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