Innovative bipolar multi-string LLC topology for high-power LED drive technology

Disadvantages of high-power LED lighting: LED driver power supply has poor reliability and high heat generation. Many LED driver power supplies are burned out, many of which are caused by the failure of inconspicuous peripheral small devices, which leads to 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. I have also helped our customers to see the burned devices. Some are just small transistors and diodes that are burned, and the entire power supply collapses." Liu Xuechao, business development manager of Texas Instruments' high-performance analog products, 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 a very reliable thing. Inductors have been used by everyone 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 saves the expensive multi-string high-voltage DC/DC buck and replaces it with an LLC resonant circuit with multiple transformers in series, directly achieving constant current, greatly improving efficiency to 92%, 4-5% higher than the traditional architecture, greatly reducing the number of components, improving reliability, and making EMI design 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 with the old architecture

The UCC25710 in the above picture is a newly mass-produced LLC resonant circuit based on multiple transformers in series for high-power LED lighting and LED backlighting. It has been used in LED street lights and ultra-thin LED TV backlighting. The 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, which gets a constant voltage output after the DC/DC comes out, and the second is the constant current module. The constant current module is a circuit that requires a DC/DC boost or buck circuit for each string of LEDs to provide a constant current for each string of LEDs. This is a traditional typical high-power LED driver topology architecture.

The efficiency distribution of this topology is: the maximum value of critical mode PFC is about 97%, the LLC resonant half-bridge efficiency is currently considered to be relatively high in the industry, and its efficiency is about 96%, and the efficiency of each string of constant current buck is about 95%. 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. For example, the diode needs a synchronous rectifier diode, 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. These 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 are that it is very expensive 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, which have a lot of components.

The second is that the efficiency is very low. From our experience, its efficiency should be around 88%. The third is that 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, which leads to 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 Buck string is not synchronized, there is mutual interference between strings. "I saw from the customer that a common-mode inductor was added to each string output. The common-mode inductor not only increases the cost, but also affects the reliability. Therefore, EMI is a big problem because it is currently difficult to synchronize each string of DC/DC because there are 4-12 strings of LEDs on it." Liu Xuechao said.

In response to this situation, TI experts proposed a topology at the end of 2008: direct constant current, one transformer driving multiple LLC strings. Compared with the traditional architecture, the efficiency was improved to 93%. However, due to the three-level multi-string transformer architecture adopted at that time, three ICs were still required: one PFC, one BUCK, and one PWM, and the cost was not significantly reduced.

Now, the UCC25710 is out, and it is changed to a two-stage multi-string transformer architecture, which can remove the Buck step-down path, and no DC/DC step-down is required, that is, each string of LEDs is directly controlled, and then the LLC resonant half-bridge circuit is directly controlled to achieve current resonance control of each string. The premise is that only a boost PFC is required. This circuit is very simple: only a boost PFC is required, plus a multi-string transformer series constant current resonant half-bridge, 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 in the following four points:

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

The second is that the cost is very low, because there is no need to step down the voltage of each DC/DC string. TI can drive two strings of LEDs with one transformer, and two 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. 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 DC/DC string requires capacitors, PFC also requires capacitors, and LLC also requires capacitors. Capacitor damage is also an important factor affecting the reliability of LED street lights. This architecture has fewer capacitors and no LLC or DC/DC capacitors, so 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 for driving, 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 used with TI's wireless Zigbee for dimming, because the IC is placed on the side or analog dimming, dimming the entire LED string to achieve overall PWM or analog dimming.

"The key technology in this architecture is that one transformer drives two strings of LEDs, and two transformers drive four strings of LEDs, which reduces costs and improves reliability." Liu Xuechao said, why can transformers control the current balance of each string? This problem is actually very simple. We connect the edges of two transformers in series. If the reverse inductance is large enough, the IP1 coupled to the side 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 transformer, we connect the edges in series, so Ip1/Ip2, Np1=Is2/Ns, so we can get Is1=Is2. It is through this transformer balancing method that a good current balance is obtained. In theory, this is a very simple theory, but we use it on the driving current of LEDs.