Author: Yen Kuang China Resources Powtech (Shanghai) Co., Ltd.
Science and technology are advancing with each passing day, and science and technology create a new life for mankind. In recent years, the global human consensus: the future of the industry related to green environmental protection and energy conservation is bright. The LED lighting industry is the first to bear the brunt of the infinite vitality and infinite mass market. In 2009, the LED lighting industry was not affected by the global economic crisis. Under the strong promotion of the Chinese government, the LED lighting industry in China is rapidly gaining momentum. In 2009, it became the first year of LED lighting. The Chinese LED lighting industry chain has grown rapidly during this economic crisis. Perfect, LED next-generation lighting technology is developing rapidly, and millions of companies that drive the LED lighting industry chain are thriving.
LED light source is a dazzling star, based on LED can do shape and color, ever-changing lighting and lighting fixtures, LED advertising large screen, LED information screen, mobile phone and MP3/MP4 consumer electronics display backlight, notebook computer and LCD- The backlight of the TV display.
The normal working illumination of the LED light source is inseparable from the driving IC. This article is about the development trend of LED lighting fixture driving technology in the recent stage.
The basic structure of LED lighting
The basic architecture of LED lighting fixtures is: LED light source + drive power + luminaire metal / plastic structural parts. The LED light source relates to the brightness, color and life of the LED lamp product; the driving power source relates to the electrical performance and stability, reliability and life of the LED lamp product; the metal/plastic structural component relates to the heat dissipation performance and the novel fashion of the LED lamp product, and different Application.
Figure 1 The basic structure of LED lighting
LED light source is energy efficient and has no carbon emissions
The LED light source is a new generation of light source for solid-state illumination. As a fourth-generation lighting fixture that is green, energy-saving, energy-saving, and long-lived, the LED light source has sprung up, attracts attention, and is rapidly developing. As the fourth generation of new energy-saving light source, LED light source was used as the light source of various lamps. The incandescent lamp as a light source has a luminous efficiency of only 5%, and the luminous efficiency of the LED light source is almost 90%. LED lighting has become the focus of everyone's attention because of its high energy saving, long life and environmental protection. In recent years, high-brightness LED light sources have experienced rapid advances in their manufacturing technology, and their production costs have been declining. Now almost all lamps can use LED light sources as high-brightness, high-efficiency, energy-saving, carbon-free lighting sources. .
LED light source needs to drive IC
The current DC LED light source is a semiconductor device that operates at low voltage (VF=2→3.6V) and large current (IF=200→1500mA), and must provide a suitable DC constant current to emit light normally. The technology of direct current (DC) driving DC LED light source has become more and more mature. Since the power supply used in our daily lighting is high voltage alternating current (AC100-220V), it is necessary to use the buck technology to obtain lower voltage. The switching power supply is stepped down, and then the alternating current (AC) is converted into direct current (DC), and then converted into a direct current constant current source by the driving IC to cause the LED light source to emit light. Therefore, the system application scheme of the DC drive LED light source must be: switching power supply + constant current source. LED lamps must have a certain amount of space to house this power module, but for E27 standard screw luminaires, the space is very limited and difficult to place. Whether it is step-down through switching power supply or other methods, the system will have a certain amount of loss. When DC LED lamps are switched between AC and DC, about 8 to 25% of the power is lost, and the system efficiency is difficult to achieve more than 90%. Therefore, improving conversion efficiency has always been the goal pursued by power chip and power supply manufacturers.
Common LED light source driver IC
At present, common LED light source driver ICs are borrowed from general-purpose power ICs. There are still very few driver ICs designed for LED light sources. Many companies are in design. It is expected that there will be many marketable cities by 2010. At present, the low-cost and unsafe solution of LED light source driving is realized by the method of resisting and reducing voltage, such as the previous Christmas lighting; the low-cost driving can also use CCR and LDO, their driving current is not large, CCR Can only drive LEDs below 0.5W, such as 0.06W straw hat lamp, LDO can drive 1W LED with voltage regulation method, but it is not good for LED light source life; DC/DC→ constant current source may be the most known One of the ideal solutions.
Common LED light source driver IC and its function table:
DC/DC constant current source driver IC
The DC/DC constant current source driver IC provides a constant DC power supply directly to the LED source. The most used one is to use DC/DC-Buck as the buck constant current source. The buck constant current source is like taking water from the large reservoir, so the constant current characteristics are stable and the supply capacity is strong. DC/DC-Boost is used to make the boost type constant current source. It is like lifting water from the water channel. The output voltage can be higher than the input voltage. If the output current is required to be large, the current needs to be expanded.
LED light sources are often used in series and parallel applications to expand the brightness of the lamps. Since the LED light sources have a certain degree of dispersion, multiple series applications are more reasonable than parallel applications, which can reduce the consistency requirements for LED light sources. DC/DC-Buck is used as the step-down constant current source. The total voltage required after connecting multiple LED light sources in series cannot be higher than the input voltage of DC/DC-Buck. If the low voltage is 2V, it is ideal. The efficiency of the source can be greater than 90%. For this reason, some of the main indicators of the new generation DC/DC-Buck constant current source IC will be improved qualitatively, Vin=5-70V, even Vin=5-100V, such a chip can reach the basic conditions of automotive electronic applications, output The voltage can also meet the string requirements of 20-30 LED sources. The key to increasing the voltage of Vin is the manufacturing process of integrated circuits.
The output constant current accuracy is also an important indicator worthy of attention. It is related to the consistency of the constant current drive power supply in large-scale industrial production, such as constant current due to the constant current accuracy of the DC/DC-Buck constant current source IC output. The inconsistency of the output current of the driving power supply will affect the brightness difference of the LED lamps produced in batches. Therefore, the constant current output accuracy of the new generation DC/DC-Buck constant current source IC is 2%-1%. Overcurrent, overvoltage, overtemperature, and open circuit protection are essential for such ICs. Switching frequency dithering technology can effectively reduce the EMI of the application circuit and will therefore be used more and more. The built-in MOSFET is still not too big, one is to take into account the cost of the chip, the second is to consider the heat dissipation of the chip itself during operation, generally the output current of a single chip is below 2A.
In addition, since the current flows from 750mA to 1500mA, as in a silicon die similar to 1.4X1.1mm, it takes a long time to pass these currents, which inevitably consumes power, which inevitably generates heat. The physical heat dissipation structure of the chip itself is also Critically, the package of the driver chip should facilitate the rapid dissipation of the driver chip die. For example, the die is directly bonded to the copper plate, and a pin extends directly to the outside of the package, so that it can be directly soldered on the copper foil of the PCB to quickly radiate heat.
The high level of integration of the DC/DC constant current source driver IC is the key to simplifying the application circuit. For example, the PT4115 requires only four components in the periphery, which greatly saves the application cost.
Figure 2 PT4115 with simple application circuit
AC/DC constant current source driver IC
Lighting fixtures that use AC power directly on the lamp head require an AC/DC constant current source driver IC with a simple application circuit and low cost of use. Most of the currently used driver ICs are flyback constant current driving schemes that are switched from switching power supplies, using the isolation buck principle. Divided into primary side feedback control and secondary side feedback control two types of applications, secondary side feedback control system accuracy of up to 2%, but the entire application circuit requires more than 20 components; primary side feedback control system accuracy is generally up to 5 -3%, the minimum number of components required for the entire circuit is only 7-8. LED lamp manufacturers are very cost-conscious.
At present, the AC/DC constant current source driver IC is moving from a flyback topology to a high efficiency resonant half bridge (LLC) + PFC topology to take full advantage of the zero voltage switching topology (ZVS) and to meet the LED fixtures for PFC. (The power factor is corrected) Increasing requirements and require an efficiency of >90% at lower power levels (eg <50 W). Wide voltage input, short circuit and over power protection, open circuit protection, and lower total harmonic distortion (THD) are basic requirements.
Figure 3 AC/DC constant current source driver IC is moving from flyback topology to high efficiency resonant half bridge LLC
Another AC/DC constant current source driver IC development idea is composed of power factor correction (PFC) and pulse width modulation (PWM) two average current mode controllers to form a new driver IC, which uses the switch charging type of PFC stage. Multiplier technology can achieve higher power factor and lower total harmonic distortion (THD); PWM can choose to use current mode control or voltage mode control. PFC regulates rising edge modulation, while PWM uses falling edge. Modulation, because modulation with different triggers can reduce the ripple voltage on the PFC output capacitor. Adding a programmable two-stage PFC output function can improve system efficiency at low voltage input and light load. Add various protection functions, including PWM Soft start with PFC, PFC overvoltage/undervoltage, cycle-by-cycle current limit, PFC input undervoltage, etc., to ensure that the LED light source of the power supply is not damaged. The PFC part works in continuous current mode, which can help reduce the boost inductor current. The rate of change is suitable for larger power applications. The gain regulator can provide a higher power factor and lower total harmonic distortion for the power supply. It is the core of the PFC stage and can be tailored to different The input voltage, frequency, rms voltage and PFC output voltage respond to the current loop. The function of the gain regulator is to generate a control signal to the PFC stage, controlling its duty cycle to maintain the output voltage stable; the reciprocal of the VRMS square can be High pressure and low pressure provide constant power.
AC/DC constant current source driver IC application and thyristor dimmer are also compatible with existing applications. There is a considerable demand for guest industry and home lighting, and it is also a branch that is developing. New development should be adapted to isolation. AC/DC off-line 100VAC / 230VAC applications for front-cut phase dimmers, rear-cut phase dimmers, from 2% to 100% wide dimming range, with resonant control for 85% High efficiency, meeting harmonic requirements, power factor up to 0.9 without dimmer.
For AC/DC constant current drive sources, conduction and radiation are important parameters related to whether LED luminaire products can pass CE and UL certification. Therefore, EMI results must be tested according to EN55015 B under 230 VAC input conditions. .
IC prices follow the pyramid's market rules
Since the day of its birth, integrated circuit products have entered the natural law of constant price reduction. It is precisely because of the price reduction that the space for survival will continue to expand. LED light source and driver IC are also integrated circuits, and naturally follow this natural law, which is the fundamental reason why LED lighting can be rapidly expanded in just a few years. Every time an integrated circuit product is lowered, its market is expanded several times. LED lighting is a massive product that enters the human space. It is inevitable that the sales price will go lower and lower. The main raw material of the integrated circuit, silicon, is everywhere in nature, and it is huge and low.
Reference materials:
"Market Analysis of LED Driver IC Technology for Backlighting and Lighting" Yan Zhongguang 2008-03
"LED Lamp Low Voltage Drive Technology" Yan Zhongguang 2008-09
"Developing AC Direct Drive LED Light Source Technology" Yan Zhongguang 2008-12
"Thinking about the Design of Using LED Table Lamps" Yan Zhongguang 2009-08
Fairchild, Onsemi, PI technical information
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