The power factor of LED driver power supply will soon reach over 95%

With the advancement of technology, lighting tools and methods are constantly changing. Users have higher and higher standards for illumination, uniformity, color rendering and energy efficiency of lighting fixtures. At this stage, the most eye-catching rising star in lighting is LED lighting . Taking the general lighting market such as replacement of light bulbs and fluorescent tubes, recessed lights, street lights and parking lights, task lighting (desk lamps, cabinet lighting), landscape lighting, billboard text circuits, and building lighting as an example, it is expected that by 2012, the LED lighting market will grow to US$1.308 billion, with an annual compound growth rate of 17.4%. Therefore, LED general lighting has become a hot market.

However, there are still some problems in the application of LED in the lighting market, including LED consistency, life testing, and especially the power supply and drive system specifications. The requirements for LED drive power supply include high efficiency, high power factor, high current control accuracy, high reliability, safety isolation, compliance with EMI standards, small size, low cost, etc. It is worth noting that the latest Energy Star standard requires that the power factor index (PFC) must be greater than 0.7 for LED lighting products greater than 5W, so as to further achieve the goal of energy saving.

Power input system requirements vary

Analyzing the cost structure of LED lamps, the heat dissipation components account for about 30%, the power supply system accounts for 20%, the assembly part accounts for 5%, and the LED components account for the largest proportion, reaching 45%. Among the various parts of LED lamps, the power module shoulders the important task of energy saving. Currently, the LED lighting power input system on the market can be divided into two categories. One is the front-end AC power input system plus the back-end constant current control module. Such products include freezer light strips, indoor lamps, street lamps, table lamps, MR16, AR111, etc. The other is the AC power direct input system integrating AC/DC converters and constant current circuits. Such products include E27 and GU10 bulb-type LED lamps, PAR lamps, T5 and T8 LED tubes, etc.

According to He Shaojia, senior manager of Kaiyu Technology, in addition to selecting a constant voltage power supply with better front-end efficiency, the power module of the back-end constant power control should adopt a power design with better efficiency according to its product characteristics. The advantage is that the front-end AC/DC power supply, such as switching power supply, DC adapter and other constant voltage power supplies, has a high selectivity and many safety certification solutions, thus lowering the design threshold.

The second type of power supply design integrates AC/DC circuits and constant current circuits, so the main difficulty lies in the consideration of power factor, power efficiency and safety regulations, and the design is also difficult. In response to this, the industry said that the power supply topology suitable for AC-DC LED lighting applications of different powers is different. For example, in applications with power below 80 W, the flyback topology is the standard choice; while in applications that emphasize high efficiency, the resonant half-bridge dual inductor plus single capacitor (HB LLC) is the first choice.

High power conversion efficiency is the goal

It is understood that ultra-high-efficiency LED lighting topologies have been quite popular in recent years. The industry mainly hopes to provide an efficiency of more than 90% at a relatively low power level (<50W). Such high efficiency needs to be achieved with a new topology, such as replacing the flyback topology with a resonant half-bridge topology, so that the advantages of zero voltage switching (ZVS) can be fully utilized.

From the perspective of application power level, AC-DC powered LED general lighting applications include low power, medium power and high power. The power range of low power applications is usually between 1 and 12W; medium power covers the range of 8 to 40W; and the power of high power applications is often higher than 40W. Overall, the main challenge facing LED lighting Driver power is that LED drivers should challenge higher conversion efficiency, especially when driving high-power LEDs, because all power that is not output as light is dissipated as heat. Too low power conversion efficiency will affect the energy-saving effect of LEDs.

First, in the 1W to 8W low-power LED general lighting, typical applications include G13, GU10, PAR16, PAR20 and recessed lights. The input voltage range of such applications is between 90 and 264V AC, and the constant current output current includes 350mA and 700mA. The efficiency requirement is 80%, and short-circuit protection and overvoltage protection are required. In terms of 8W to 25W LED general lighting application requirements and solutions, they are divided into non-PFC and PFC.

For the case without PFC, typical applications of 8W to 25W AC-DC LED lighting are PAR30, PAR38 and recessed lights. In such applications, the input voltage requirement is 85~135 Vac or 185~264Vac (or universal input), the efficiency requirement is greater than 80%, short-circuit protection and open-circuit protection are provided, and the constant current output current is 350mA, 700mA and 1A. In terms of 8W to 25W AC-DC LED lighting that requires PFC, the typical applications are also PAR30, PAR38 and recessed lights. The input voltage specification of such applications is 90 to 264 Vac, the efficiency requirement is 80%, and it supports 350mA, 700 mA and 1A constant current output, short-circuit and overvoltage protection are provided, and the power factor is required to be higher than 0.9. As for 50W to 200W LED general lighting, which is mostly used for street lighting and high-power area lighting, different LED solutions for the 50W-150W or 100~200W power range can be used.

Active power factor adjustment reaches over 95%

In response to the demand for LED lighting power drive, in terms of constant current drivers, Kaiyu Technology mainly promotes T6316, which is a constant current LED driver with 4 channels, and the output current can be determined according to the external resistor. Kaiyu Technology's T6322 is a step-down constant current LED driver, whose current is determined by the external resistor, and can support up to 1.5A output current, providing ±5% current accuracy and high power efficiency (low energy consumption) and high line adjustment capabilities. T6316 has ±6% current accuracy and ±3% matching accuracy between channels, and can be used for lighting equipment such as street lamps and lamp tubes.

He Shaojia further stated that in terms of power factor, currently passive power factor adjustment can reach 0.8. It has a simple structure, low price, and the same operating frequency as the power supply, but the passive components are large and heavy, and need to be designed for specific input voltage and load conditions. In response to this, Kaiyu can provide corresponding passive valley-filling filters to correct the overall power factor, so that the power factor of the LED driver can reach more than 85%. It is understood that the driver that Kaiyu Technology is about to develop will adopt active power factor adjustment, and its overall power factor will reach more than 95%, which will further achieve the energy-saving goal.

Richtek has also launched the RT84 series of LED lighting driver ICs. Richtek said that unlike the buck-type LED driver ICs on the market that mostly use hysteresis control, Richtek's entire series of products use fixed-frequency control to keep the reduction in inductance value to one-third of that of hysteresis control. Its fixed-frequency control method is also less likely to cause poor accuracy due to changes in input voltage and current than hysteresis control. Richtek also proposes different buck, boost, and buck-boost LED solutions for different LED lighting applications.

Richtek said that currently, most of the products on the market are designed with buck LED driver ICs. The advantage of this method is that the design concept is simple, but the number of LED drivers is limited by the limitation of the buck circuit, so a large number of buck ICs are required. The use of Richtek's boost driver IC can significantly reduce the use and cost of LED buck driver ICs by more than half. Richtek's current LED driver power products can meet customers' buck, boost and buck-boost applications, and provide corresponding products from 1W to 400W. It also provides solutions for standard lamps such as MR16, AR111, GU10, E27, PAR30, PAR38 and other specifications.

Supports both isolated and non-isolated applications

When it comes to LED lighting driver power supplies, NXP Semiconductors must be mentioned. The company launched a new LED controller SSL2103 in the first half of this year. NXP said that this product further expands NXP's market-proven AC/DC LED driver product series (SSL2101/SSL2102), making it suitable for a variety of lighting applications. Jacques Le Berre, marketing director of NXP's semiconductor lighting department, said that SSL2101 not only meets the market demand for LEDs, but also leads the entire industry to provide more energy-efficient lighting solutions. In fact, SSL2101 has sold more than 10 million units since it was launched in the spring of 2009. SSL2101 and SSL2102 products are already quite mature and can be applied to various dimmable LED drivers. They are very suitable for high-integration, low-power applications, such as independent rectifiers below 25 watts or LED bulbs below 15 watts.

The SSL2103 has the functions and features of its similar product SSL2101 and can be used for higher power solutions. As a multi-purpose AC/DC LED lighting controller, the SSL2103 has a wider power range and better dimming compatibility, making it suitable for dimmable and non-dimmable LED lighting. In addition, the SSL210x series products integrate controllers with the same functions to improve the versatility of this series of products in various LED lighting applications. The SSL2103 is mainly designed for higher power LED lighting fixtures (PAR20, PAR30 and PAR38), which can provide greater flexibility in power, size and dimming control for LED lighting design.

It is understood that in addition to the functions and performance of SSL2101 and SSL2102, SSL2103 can also meet higher power applications. This function is achieved through an external power switch and can support the application of dimmable or non-dimmable lighting drivers at the same time. Similar to the design of previous products, the life of SSL2103 can exceed 75,000 hours at normal operating temperature. NXP's AC/DC LED driver product series can provide a wide variety of technical solutions and can support the application of isolated or non-isolated lamps. In addition, this series of products can also be applied to 100V, 110V or 230V AC power networks, and the circuit board form factor can be applied to E27, GU10 or PARx lamps. This product series also includes products that integrate power switches and bleeder circuitry, which can make the size of the PCB board smaller.

Dimmable solution with ultra-high efficiency

Furthermore, Infineon's dimmable LED driver solution ICL8001G is mainly used for 10W-60W, supports dimming function, and is suitable for high-efficiency LED bulbs for residential lighting, which are enough to replace 40W/60W/100W incandescent bulbs and support various typical consumer lighting applications. Infineon said that the driver circuit board using ICL8001G uses innovative master control technology to manage LED power consumption. 25 components are enough to meet the needs of a 10-watt LED lamp (efficiency equivalent to 60 watts), which can save half of the power and reduce costs by about 30% compared to other alternative products. The reference design printed circuit board area is only 20mm x 70mm, which can be easily embedded in a general screw-in bulb. All components are assembled on a single-sided circuit board, which helps to further reduce costs. It is understood that the energy saving effect of ICL8001G can reach 90%, support a variety of installed wall dimmers, and is also the only master-controlled offline LED driver solution with integrated power factor correction (PFC) function, with a power factor of more than 98%.

Compared with many current dimmable LED dimmable solutions, dimming will cause a 30% drop in energy efficiency. Infineon's driver solution uses many phase-cut dimmers, so it can maintain ultra-high efficiency (>80%) throughout the entire dimming range. Other features of ICL8001G include: independent driver output, which can effectively prevent overheating problems; digital soft-start function, built-in startup unit, and light up immediately when turned on; cycle-by-cycle current limiting; short-circuit protection, overvoltage protection and overheating detection.

Electrolytic capacitor-free LED driver power solution

Furthermore, in the current market, LED lighting drive power solutions that do not require electrolytic capacitors are also worth mentioning. For example, Murata Manufacturing of Japan demonstrated a digital power circuit for LED lighting at TECHNO FRONTIER 2010 in July this year. Its input and output capacitors are replaced by the company's multilayer ceramic capacitors (MLCC) instead of electrolytic capacitors, which can be built into the tube of straight-tube LED lighting fixtures. Compared with the use of aluminum electrolytic capacitors, this solution can not only reduce the product size, but also extend the product life. In order to obtain a constant current, this solution uses a DSP microcontroller with a switching frequency of about 200kHz. The output capacitor uses two MLCCs of about 5μF. The input voltage supports AC100V and 200V. It is understood that under normal circumstances, MLCCs of this capacity cannot completely absorb pulsating currents, but by improving the control on the DSP side, the flicker of the lighting fixture will not be felt. The main circuit adopts a non-insulated buck-boost type, and no PFC (power factor improvement) circuit is set. The overall dimensions are 180mm×19.4mm×6.5mm.

Basically, the reason for replacing electrolytic capacitors is that the working life of ordinary LED lighting driver power supplies depends on the electrolytic capacitors that must be used in the smoothing circuit when converting AC to DC. The working life of LEDs is as high as 40,000 hours, while the life of electrolytic capacitors is only a few thousand hours. Since the life of the system is determined by the life of the electrolytic capacitors used in the power supply components, if the electrolytic capacitors are not removed, the life of the LED lighting driver power supply will not match the life of the LED, and the long working life of LED lighting will inevitably be affected. Therefore, the industry has recently been very active in developing LED lighting driver power supply solutions without electrolytic capacitors.

Directly driven by AC power can bring a wider range of applications

In order to solve the problem that the life of AC/DC converter is shorter than that of LED, in addition to the above solutions, the development of AC LED has also attracted much attention. For example, Acriche developed by Seoul Semiconductor does not require AC/DC converter and can be directly connected to AC power supply. In addition, the on-chip AC LED lighting technology developed by Taiwan Industrial Technology Research Institute also uses AC to directly drive LED chips to emit light. Zhu Mudao, head of the Optoelectronics Department, Optoelectronic Components and System Application Group of ITRI, pointed out that this technology does not require a rectifier transformer, which saves 15~30% of electricity compared to traditional LEDs. With three-dimensional heat conduction, pluggable packaging technology, and small size, it is easier to carry out a variety of lighting designs. ITRI said that this technology not only saves energy and reduces carbon emissions, but also improves the overall LED luminous efficiency, and brings broader space and market opportunities for the overall LED lighting application. It is believed that this technology will become a lighting technology with great potential in the future within 5 years.