Breakthrough design of high-power LED driver technology

Last year, the " 10 Cities, 10,000 Lights " demonstration project did not bring fruitful results as expected. Instead, the shortcomings of this high-power LED lighting were fully exposed: the reliability of LED driver power supply was extremely poor, the heat was high, and many LED driver power supplies were burned out, many of which were caused by the failure of inconspicuous peripheral small devices, resulting in the failure of the entire driver circuit. "Because the LED driver circuit is inefficient and the ambient temperature is high, many devices cannot withstand such high temperatures and are burned out. I have also helped our customers to see the burned out devices. Some of them are only small triodes and diodes that are burned out, and the entire power supply collapses." Liu Xuechao, business development manager of high-performance analog products at Texas Instruments , explained, "It seems to be a small problem, but the entire ambient temperature of the LED is very high. 70% of the heat is dissipated through the heat sink. Behind the heat sink is the driver power supply, and the temperature is very high and difficult to control." Therefore, the industry's current goal is to improve efficiency, reduce diffusion, increase integration, reduce the number of components, and try to use reliable components. "For example, inductors are very reliable. Inductors have been used for thousands of years, not active devices," he added. To achieve this goal, TI officially launched a new LED driver architecture this year - a bipolar multi-string LLC topology architecture. Compared with the traditional high- power (power is 250W-300W) LED driver architecture, this architecture eliminates the expensive multi-string high-voltage DC/DC buck and replaces it with an LLC resonant circuit with multiple transformers in series, which directly achieves constant current. The efficiency is greatly improved to 92%, which is 4-5% higher than the traditional architecture. The number of components is also greatly reduced, the reliability is improved, and the EMI design is simpler. (As shown in the figure) "In simple terms, it combines the original constant voltage module and constant current module into one module, which is equivalent to saving a constant voltage module." Liu Xuechao said.

Comparison of old architectures

The UCC25710 in the above figure is a LLC resonant circuit based on multiple transformers in series for high-power LED lighting and LED backlighting that TI has just mass-produced. It has been used in LED street lights and ultra-thin LED TV backlighting. UCC25710 is specially designed based on TI's innovative and patented dual-stage multi-string LLC topology architecture.

(As shown above) In the traditional high-power LED lighting architecture diagram, it can be seen that it has two blocks. The first block is called the constant voltage module. After the DC/DC comes out, a constant voltage output is obtained. The second is the constant current module. The constant current module is a circuit that requires DC/DC boost or buck for each string of LEDs to provide constant current for each string of LEDs. This is a traditional typical high-power LED drive topology.

The efficiency distribution of this topology is: the maximum value of the critical mode PFC is about 97%, the LLC resonant half-bridge efficiency is currently recognized by the industry as being relatively high, and its efficiency is about 96%. The efficiency of each string of constant current buck is about 95%, and the total efficiency (typical value) of the three multiplied should be within 88%. "Of course, some customers can achieve 90%, but they will sacrifice a lot of costs, such as the diodes that need synchronous rectification diodes, which is to use money to improve efficiency. In addition, some soft block interference work is done on the large buck to get a high efficiency, which are some methods. But overall, its efficiency will not be too high, and it should be around 88%." Liu Xuechao explained.

He continued to analyze: The disadvantages of the traditional topology architecture are that first, its cost is very high, because it has PFC, LLC circuits, and multiple strings of high-voltage Buck (input voltage is 54V). The cost of high-voltage DC/DC is very high, and each string requires one. The current street light power supply is actually 4-12 strings, which means you need 4-12 strings of Buck circuits, with a lot of components.

Second, the efficiency is very low. From our experience, its efficiency should be around 88%. Third, the reliability is very poor, because in actual applications, each string requires a Buck, PFC, LLC circuit, and the entire circuit is composed of many components, resulting in poor reliability. The last and most important one is that the EMI problem of this traditional architecture is also very serious. Because the switching

frequency of each string of Buck is not synchronized, there is mutual interference between strings. "I saw from the customer that a common-mode inductor was added to each string of outputs. The common-mode inductor not only increases the cost, but also affects the reliability. Therefore, EMI is a big problem because it is difficult to synchronize each string of DC/DC at present, because there are 4-12 strings of LEDs on it." Liu Xuechao said. In response to this situation, TI experts proposed a topology architecture at the end of 2008: direct constant current, one transformer drives multiple strings of LLC. Compared with the traditional architecture, the efficiency is improved to 93%. However, since the three-stage multi-string transformer architecture was adopted at that time, three ICs were still required : a PFC, a BUCK, and a PWM , and the cost was not significantly reduced. Now, this UCC25710 has come out and changed to a two-stage multi-string transformer architecture, which can remove the Buck buck path without DC/DC buck, that is, directly control each string of LEDs, and then directly control the LLC resonant half-bridge circuit to achieve current resonance control of each string. The premise is that only a boost PFC is required. This circuit is very simple: it only needs a boost PFC and a resonant half-bridge with multiple transformers in series to generate constant current to obtain a high-power LED drive current.

Therefore, the advantages of the new dual-stage multi-string LLC architecture for high-power LED lighting can be summarized into the following four points:

The first is efficiency improvement. The actual overall efficiency of this new topology can reach 92%.

Second, the cost is very low, because there is no need to step down the voltage of each string of DC/DC. TI can drive two strings of LEDs with one string of transformers, and two strings of transformers can drive four strings of LEDs. The transformers used are all traditional transformers, and the cost is much lower than using a constant voltage DC/DC to drive each string. "In fact, we have saved a constant voltage module." Liu Xuechao said.

Third, it has very few components, so its reliability is very good. The constant voltage and constant current are integrated in one module instead of two, which improves reliability. "In addition, the current traditional solution uses more capacitors, because each string of DC/DC requires capacitors, PFC is also a capacitor, and LLC is also a capacitor. Capacitor damage is also an important factor affecting the reliability of LED street lights. This architecture has a small number of capacitors, and there are no LLC and DC/DC capacitors, so the reliability is improved."

Fourth, EMI problems are reduced and the design is simpler. The circuit does not have multiple strings of DC/DC, and only needs a resonant half-bridge to drive, so it is very easy to get EMI design. "We have a conduction experiment on this board. After the board is designed, it can basically pass the conduction test . I personally think that the LLC circuit has very good EMI and very good performance. Compared with the traditional topology architecture, the EMI is very good." He said.

In addition, the circuit is compatible with dimming and can also be dimmed with TI's wireless Zigbee, because the IC is placed on the side or analog dimming, dimming the entire LED string to achieve overall PWM or analog dimming.

"The technical key in this architecture is that one transformer drives two strings of LEDs, and two transformers drive four strings of LEDs, so the cost is reduced and the reliability is improved." Liu Xuechao said, why can the transformer control the current sharing of each string? This problem is actually very simple. We connect the edges of the two transformers in series. If the reverse inductance is large enough, the coupling to the side IP1 is equal to IP2, IS1... If the Zhabi is the same, it is a Zhabi relationship, that is, Ip1/Np = Is1/Ns1, this is the first transformer. For the second transformer, since its edges are the same current transformer, we connect the edges in series, so that Ip1/Ip2, Np1=Is2/Ns, so we can get Is1=Is2. It is through this transformer that a good current balance is obtained. Theoretically, this is a very simple theory, but we use it for the driving current of LEDs.

In addition, Liu Xuechao said that there are no special requirements for transformers here. The general transformer inductance redundancy is within plus or minus 10%, and the transformer station generally requires within plus or minus 10%. "As long as you can achieve within plus or minus 10%, the output current redundancy is less than plus or minus 3%. "In fact, in addition to street lamp applications, it is also suitable for general lighting systems and commercial lighting. As long as a multi-string architecture is required, this topology can be used. "Customers are very happy to see our innovative architecture because it really reduces costs and improves reliability." Liu Xuechao said.