Last year, the "City 000" demonstration projects, and as expected did not bring fruitful results, but this is the disadvantage of high power LED lighting is fully exposed: LED driver power supply reliability, poor, high fever, a lot of LED drive power to be burned, many of which are due to a small inconspicuous peripheral device broken in the entire drive circuit failure. Now, TI propose an innovative architecture can greatly improve the situation.
Last year, the "City 000" demonstration projects, and as expected did not bring fruitful results, but this is the disadvantage of high power LED lighting is fully exposed: LED driver power supply reliability, poor, high fever, a lot of LED drive power to be burned, many of which are due to a small inconspicuous peripheral device broken in the entire drive circuit failure. "Because of the efficiency of the LED driver circuit due to the low ambient temperature is high, many devices can not stand such high temperatures have been burned and I help our customers read the burned-out devices, and some only a small transistor, diode burned, the entire Power lost to collapse. "Texas Instruments High Performance Analog Business Development Manager Liu Chao on the" EE "Sunchang Xu explained," It looks like a small problem, but the LED throughout the ambient temperature is very high, 70% of the heat through the heat sink scattered, is the driving power behind the heat sink, the temperature is high, difficult to control. "Therefore, the industry is now the goal is to improve efficiency and reduce the degree of diffuse, improve integration, reduce component count, and a possible use of reliable meta- devices. "For example, the inductor is a very reliable thing, inductance is thousands of years we use things, rather than active devices." He added.
For this goal, TI launched this year a revolutionary and innovative LED driver architecture - multi-string LLC bipolar topology architecture, as opposed to the traditional high-power (power is 250W-300W) LED driver architecture thus eliminating the expensive multi-string high-voltage DC / DC buck, to more than one series of LLC resonant transformer circuit, a direct realization of constant efficiency increased dramatically to 92%, 4-5% than the traditional architecture upgrade, components numbers are greatly reduced, enhancing reliability, EMI design easier. (Pictured) "In simple terms, it is the original module with the constant need of synthesis as a constant current module module, saved the equivalent of a constant module." Liu Chao said.
High-power LED driver technology Raiders
Comparison of old and new architecture
TI UCC25710 image above is just for the production of a high-power LED lighting and LED backlight, based on more than one series of LLC resonant transformer circuit, has been with the LED lights and ultra-thin LED backlight on the TV. UCC25710 is based on TI's innovative, patented two-stage multi-string LLC topology architecture designed.
(Above) in the traditional high-power LED lighting can be seen on the chart, it has two Block, Block called the first constant voltage module, DC / DC came out to be a constant voltage output, the second is the constant modules, each module is a constant string of LED will require DC / DC step-up or step-down circuit, constant current for each LED string. This is typical of the traditional high-power LED driver topologies.
The efficiency of the distribution of such topologies are: the critical mode PFC is about 97% of maximum, LLC resonant half-bridge current efficiency in the industry we found is relatively high, its efficiency is about 96%, each string of constant-current buck Its efficiency is about 95%, multiply the total three efficiency (typical value) should be 88% or less. "Of course, some customers can be 90%, but at the expense of many costs, such as a diode rectifier diodes need to synchronize it with money and improve efficiency. In addition, a large buck on some interference with the work of the soft block to get an efficient, these are some of the methods, but it will not be too high overall efficiency is about 88% should be about. "Liu Chao explained.
He continued to analyze: the shortcomings of conventional topologies, the first of its cost is very high, because it comes with PFC, LLC circuit, there are many string high-voltage Buck (input voltage is 54V), high-voltage DC / DC cost is very high, each requires a string. Now the lamp power is actually 4-12 strings, strings that you need 4-12 Buck circuit, there are a lot of devices.
The second is the efficiency is very low, from our experience, its efficiency should be at 88%. The third is the reliability is very poor, because the practical application of each string requires a Buck, PFC, LLC circuit, the circuit is made up of many components, resulting in reliable change worse.
The last one, is also very important, is the traditional architecture of EMI problem is very serious. Buck switching frequency of each string because no synchronization between the string and the string of mutual interference. "I see where the client, when the output of each string plus a common mode inductor, common mode choke on the one hand increases the cost, the other will have an impact on the reliability of the above, so, EMI is a big problem, The reason is each string DC / DC is difficult to achieve synchronization, because the whole of the LED string with 4-12 in the above. "Liu Chao said.
In view of this situation, TI experts in late 2008 to propose a topological structure: Direct constant current, a transformer-driven multi-string LLC. Compared with traditional architecture, the efficiency increased to 93%. However, because of the use of more than three strings transformer structure, or requires three IC: a PFC, a BUCK, a PWM, the cost is not significantly reduced.
Now, Fengyun UCC25710 out to two-level multi-string structure of the transformer, can Buck buck this way to get rid of, no DC / DC step-down, that is, direct control of each LED string, followed by direct control of the LLC resonant half-bridge circuit, to achieve the resonance of each string of the current control. Premise only need a boost of PFC. This circuit is very simple: just need a boost of the PFC, coupled with a multi-string constant current transformer in series resonant half-bridge, you can get high-power LED drive current.
Therefore, high power LED lighting, new dual-class structure of the advantages of multi-string LLC can be summarized in the following four points:
First, the efficiency, the actual structure of this new topology the overall efficiency can reach 92%.
Second, the cost is very low, because not every string of the DC / DC step-down, TI with a bunch of transformers can drive two strings of LED, two strings of series transformer can drive four LED, used in transformers are the traditional transformer, the cost is far much lower than with constant voltage DC / DC to drive each string. "In fact, we left out a constant voltage module." Liu Chao said.
Third, it's components are very small, so it's very good reliability, the constant voltage and constant current on the whole in a single module, instead of two, making the reliability has been improved. "In addition, the capacitance of the traditional program with more, because each series DC / DC capacitors are required, PFC is also a capacitor, LLC is the capacitance of the capacitor damage to the LED lights also affect the reliability of an important factor in the framework of fewer capacitors No LLC, DC / DC capacitor, so to enhance reliability. "
Fourth, EMI problems reduce the design easier. Circuit no more strings DC / DC, just to make a resonant half-bridge driver, this can very easily get EMI design. "We have conducted this experiment on board, this board is basically designed after the test can be conducted through, I think, the original characteristics of the LLC circuit is very good EMI, a very good performance of the circuit, it is relatively traditional topologies for, EMI is very good. "he said.
In addition, the circuit can be compatible with dimming, but also with TI's Zigbee wireless dimming, because the IC is on the side or analog dimming, whole strings of LED dimming, to achieve the overall PWM or analog dimming.
"The architecture is a key technology in the transformer drive two strings of LED, two transformer drive four strings LED, this cost reduction, and improved reliability." Liu Chao said, why do transformers flow control for each string are then ? The problem is actually very simple, we border the two transformers in series, if too large enough to reverse the inductor, coupled to the side of IP1 is equivalent to IP2, IS1 ... if Dhabi same, it is a Dhabi relationship is Ip1/Np = Is1/Ns1, this is the first transformer. The second transformer, because of its border is the same converter, we put along the border to do a series, so Ip1/Ip2, Np1 = Is2/Ns, so that we can get Is1 = Is2. Is through such a transformer balanced way to get a good hold of the current balance, in theory, this is a very simple theory, but we used it in the LED drive current.
Also, Liu Chao said, there are no special requirements for transformers, general transformer inductance redundancy within plus or minus 10 percent, plus or minus the transformer station generally require less than 10%, "as long as you can be plus or minus 10%, redundant output current is less than plus or minus 3% on it. "In fact, in addition to street lighting, general lighting systems and it is for commercial lighting is appropriate, as long as the strings take the form of architecture, this topology can be used. "Customers see us after the new architectures are very happy, because it reduces the real costs and improve reliability." Liu Chao said.
LED high clear competitive advantage compared to low voltage LED
The same output power, high voltage LED current is much lower than required to drive low-voltage LED. As in the high-voltage blue Epistar 1WLED example, the forward voltage drop of up to 50V, but also that it can only 20mA drive current output power 1W, and the general forward voltage drop of 3V 1WLED, need to 350mA drive current to the output power 1W, so the same output power of high-voltage LED power dissipation at work far below the low-voltage LED, which means that the heat aluminum housing costs can be greatly reduced.
High-Voltage LED can significantly reduce the loss of AC-DC conversion efficiency. To 10W output power, for example, if the use of forward voltage drop of 50V, 1W LED, the output can take the 2 and 4 string configuration, four series of LED forward voltage drop of 200V, that is just from the city 220V AC power (AC) to use the bridge rectifier and drop 20V on it. But if we use the forward voltage drop of 3V 1W LED, even with 10 strings, but the forward voltage drop 30V, 220VAC mains that need to step-down to 30VDC. We know that the lower the pressure input and output, AC to DC conversion efficiency of the higher visibility, such as the use of high-voltage LED, the efficiency of the transformer can be greatly improved, thereby significantly reducing AC-DC power conversion losses, which a reduced heat consumption can be further reduced cooling housing costs.
Therefore, if the use of high pressure to develop LED LED general lighting products, the overall power consumption can be greatly reduced, thereby significantly reducing the design requirements of thermal shell, as we can be thinner and lighter aluminum housing can meet the cooling needs of LED lamps, As the cost of cooling aluminum case LED lighting is one of the major cost components, aluminum casing also means lower overall cost-effective LED lighting costs reduce. Thus, the high-voltage LED lighting LED lighting can bring the effective cost and weight reduction, but more important is the cooling system significantly reduces the design requirements, thus effectively clearing the LED lighting into the interior of the largest lighting market technical obstacles. Therefore, the high-voltage LED will dominate the future of LED general lighting market.