Innovative architecture of high-power LED driver significantly reduces costs

The " 10,000 LED Street Lights in 10 Cities " demonstration project vigorously promoted by the Chinese government was ultimately proven to be just a policy promotion behavior. LED street lights are far from commercialization, because after a period of time, the brightness of LED street lights will gradually decrease or completely stop lighting. The reason is not that the life of high-power LEDs is insufficient, but that the heat dissipation technology of LED driver power supply is not up to standard. Therefore, either the brightness of LEDs is accelerated by heat decay, or the LED driver power supply fails due to the heat of inconspicuous peripheral small components.

"The low efficiency of high-power LED drivers , limited working space and poor thermal design will cause the working environment temperature of LED driver power supplies to be very high. Many devices cannot withstand such high temperatures and are burned. I have also helped our customers see the burned devices. Many of them are just small transistors and diodes, but once they burn out, the entire power supply collapses." Liu Xuechao, business development manager of TI's high-performance analog products, explained, "This seems to be a small problem, but the temperature generated by LEDs when working 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 temperature, 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 revolutionary high-power LED (>100W) driver architecture this year: a two-stage multi-string LLC topology architecture. Compared with the traditional LLC half-bridge driver architecture, this innovative architecture eliminates the need for multiple strings of constant current DC/DC buck circuits (thus eliminating the need for expensive high-voltage MOSFETs, inductors and electrolytic capacitors for each string), and replaces it with a two-stage LLC resonant circuit with multiple transformers in series, which directly achieves constant current, greatly improving efficiency to 92%, 4-5% higher than the traditional architecture, significantly reducing the number of components, improving reliability, and making EMI design simpler. (As shown in the figure below)

"In simple terms, it combines the original required constant voltage stage and constant current stage into one stage, which is equivalent to eliminating one constant current stage." Liu Xuechao said.

TI's innovative dual-stage multi-string LLC topology for high-power LED lighting (Image source: TI)

Comparison of old and new 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.

Liu Xuechao pointed out: "UCC25710 is the industry's first single-chip LLC controller that drives multiple LED strings directly through PFC output . LLC and series LED switch control support PWM dimming, and programmable dimming LLC switch ramp can eliminate noise."

He explained that the disadvantages of the traditional LLC half-bridge topology are that it is very expensive, because each string of DC-DC buck circuits at its output requires a high-voltage MOSFET, an inductor and an electrolytic capacitor, and the cost of these components is very high. The current LED street light driver actually has 4 to 12 strings, which means you need 4 to 12 strings of DC-DC buck circuits, which makes the architecture very expensive.

The second problem is the low efficiency, which is generally less than 88%. The third problem is the low reliability, because it requires many components. The last and most important one is that the EMI problem of this traditional architecture is very serious. Because the MOSFET switching frequency of each string of buck circuits is difficult to synchronize, there is mutual interference between strings.

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 is improved to more than 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.

TI's new 100W multi-string half-bridge resonant high-efficiency LED driver solution

Now, the UCC25710 is out, and it is changed to a two-stage multi-string transformer architecture, which can remove the Buck step-down circuit, 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 resonant 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 as follows: First, the efficiency is improved. 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 transformer, 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.

Third, it has very few components, so its reliability is very good. The constant voltage and constant current are integrated in one level instead of two, which improves reliability. In addition, the traditional driving scheme uses a lot of capacitors, because each string of DC/DC buck 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, 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 to achieve the drive, which makes EMI design very easy.

In addition, the architecture supports PWM dimming and can also be used with TI's wireless Zigbee for remote 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.

In addition, Liu Xuechao said: "This architecture has no special requirements for the transformer, as long as you can keep it within plus or minus 10%, and the output current redundancy is less than plus or minus 3%." The general transformer inductance redundancy on the market can be within plus or minus 10%.

"In addition to LED 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 can significantly reduce costs and improve reliability." Liu Xuechao said.