The LED lighting industry has been paying more and more attention in recent years, and its core device LED driver chip is an important guarantee for achieving efficient LED lighting. However, how to reduce the cost of the device while improving the energy efficiency and reliability of the LED driver has been a major problem for many design engineers. Driven by this demand, Texas Instruments (TI) has introduced the new multi-string half-bridge resonant high-efficiency LED driver solution UCC25710. The product reduces costs by greatly simplifying design and greatly increases efficiency and reliability.
For high-power LED lighting drivers, traditional topologies face many issues such as high cost, low efficiency, low reliability, and EMI interference. The typical high-power LED lighting driver topology on the market usually includes a constant voltage module and a constant current module, the total efficiency can only reach about 88%, and the higher efficiency requires sacrifice of cost; multi-string Buck circuit + PFC + LLC, chip The large number leads to high cost; since each series requires a Buck in practical applications, and the PFC and LLC circuits are very complicated, the reliability is poor; in addition, each of the Buck switching frequencies is not synchronized, and EMI interference is severe.
So, at the end of 2008, TI proposed a serial input, multi-parallel equivalent LED driver (SIMPLDrive). Dr. Liu Xuechao, Business Development Manager of High Performance Analog Products, Texas Instruments Semiconductor Division, said that this architecture still requires three ICs: the first level is the traditional PFC, the second level is the critical mode Buck stepping; the third level is Constant current of multiple strings of transformers. The overall efficiency of this three-stage multi-string transformer solution is very high, exceeding 93%, but it does not significantly improve the cost.
Figure 1: Dr. Liu Xuechao, Business Development Manager, High Performance Analog Products, Texas Instruments Semiconductor Division.
TI's innovative IC UCC25710, which was mass-produced last quarter, removed the Buck step-down section and solved the previous IC cost problem. This is the industry's first single-chip LLC controller that directly drives multiple LED strings through a PFC output for high-efficiency LED driver power supplies. Liu Xuechao said, “It is now possible to directly control each string of LEDs and then directly control the LLC resonant half-bridge circuit. This makes the circuit very simple, requiring only one boosted PFC, plus a multi-string transformer in series with constant current. The resonant half-bridge provides a new topology for high-power LED drive currents. The advantages of the new topology include: high efficiency: up to 92%; low cost: no need for CC DC/DC drivers; high reliability: few components, constant voltage Constant current is integrated in one module; it can be compatible with PWM or analog dimming; EMI design is simple: only need to boost PFC and multi-string transformer series resonant half-bridge, so it is easy to design.
Liu Xuechao pointed out that this is the only resonant circuit based on multiple transformers in series to achieve high efficiency and low cost LED-driven lighting. The current sharing control by transformer magnetic balance is the core of the architecture. However, the cost of driving a string of LEDs with a single voltage converter is still high. "TI's patented multi-transformer architecture uses a transformer to drive two LED strings. A transformer that drives a current reduces the cost."
The switching frequency of the UCC25710 can be freely set from the minimum to the maximum switching frequency with an accuracy of 3%-6%. It can support a dimming range of 1%-100%, and a linear dimming curve can be obtained by delaying the duty cycle. The chip integrates overvoltage, undervoltage, overcurrent protection and overtemperature protection. It features a PWM dimming interface that can be driven with ZigBee and dimming control.
TI's PMT4302 universal LED lighting multi-string LLC AC/DC driver reference design uses the UCC28810 as the critical mode PFC and the UCC25710 as the multi-string LLC controller. The input voltage is 90V-260V wide voltage range input, the output is 500mA, supports four strings of LEDs, can achieve 92% efficiency.
Figure 2: PMT4302 multi-string LLC AC/DC driver Demo board with UCC25710.
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