The driving IC specification is an important key to determine the light output performance of LED bulbs. If the LED bulb is subjected to too high voltage or current, it will seriously affect its luminous performance and even cause damage. Therefore, in the LED drive IC, precise voltage rise and fall and current density control functions shall be set to maintain the stable luminous performance of the LED bulb.

The first news comes from Cree, a well-known manufacturer of LED lighting and semiconductor products. They have launched a series of LED bulbs that subvert the tradition, and believe that the retail price of this series will improve the product acceptance of their customers. The company claims that the new Cree bulb series has a design life of 25000 hours and a retail price of $12.97. It can be used to replace 60 watt (W) warm lighting incandescent bulbs, which can save 84% power compared with traditional incandescent bulbs. In addition, more importantly for consumers, Cree also claims that if the five most frequently used incandescent bulbs in the home are replaced with Cree’s LED bulbs, consumers can save up to US $61 (equivalent to about NT $1780) per year.

Moreover, Cree also further expanded its marketing channel and made the home depot chain store the exclusive supply channel of such LED bulbs, which can ensure that the general public can buy these LED bulbs close to home and get sufficient supply.

The second news is the suspension block of the famous San Francisco Bay Bridge. In the past 18 months, the lighting design company has suspended 25000 LED lights from the top of the bridge tower and suspension cable to the bridge deck. The project is promoted by a company called baylights, which creates a changing and alternative algorithm to make LED lighting full of vitality. The company controls each lamp individually so that the lighting pattern generated will be unique within the 2-year project period. The lighting system, developed by color dynamics, a subsidiary of Philips, is designed to reduce power consumption by 85% compared with traditional (non LED) solutions. The special LED provided by Philips color kinetics in this project is an ewflex SLX product with 4200k related color temperature.

These two events show the technological progress in the field of LED lighting output (measured in lumens per watt (LM / W)) and the commerciality brought by the annual power saving effect due to the long installation life of LED bulbs.

LED bulbs have multiple advantages and gradually replace CFL / incandescent bulbs

There is no doubt that led technology has made great progress in the past 10 years, with higher brightness, better efficiency, longer service time and lower cost. These are due to the progress of heat dissipation, packaging and technology. Unlike incandescent bulbs, LEDs have no filament, will not burn out, and emit colder light.

The output light of high-power and high brightness (HB) led has exceeded the key milestone of 100lm / W. some manufacturers have announced that they can produce output light greater than 230lm / W in the laboratory. Another advantage is the long service life of LED. The service life of white LED bulb is at least 50000 hours. However, the service life of incandescent lamp is about 1000? 2000 hours.

The main reason for the rapid growth of LED lighting market is that the power consumption of LED lighting is greatly reduced compared with traditional lighting. Compared with incandescent lighting, LED lighting can provide the same brightness (expressed in lumens) with less than 20% of power consumption. As shown in Table 1, LED lighting has many advantages, but there are also some disadvantages. The advantages of LED lighting include that its service life is several levels higher than that of incandescent lamps, which greatly reduces the cost of changing bulbs. LED bulbs can be dimmed by using the old triode for alternating current (triac) dimmer, which is also a great advantage of LED bulbs, especially in the application of replacement bulbs.

How to select the appropriate driver IC for the LED lamp

High power drive IC check voltage / current LED lighting system reliability improvement

Not only that, the LED lamp can be lit immediately. Unlike the energy-saving fluorescent lamp (CFL), which requires preheating time, and the LED lamp is not sensitive to the power cycle, which is also different from the energy-saving fluorescent lamp. Most importantly, LED lighting devices do not contain any harmful materials, so there is no need to deal with or dispose of these harmful substances, while energy-saving fluorescent lamps use harmful mercury gas to work. Finally, LED bulbs can achieve a very small package, which other bulbs cannot.

The above factors make LED bulbs have the advantages of being used in a variety of commercial applications, such as ceiling lighting in factories, outdoor building lighting in bridges and buildings, and lighting applications in stadiums and other large-scale event venues.

Select the appropriate drive icled to reduce the risk of damage

LED is an electrical equivalent diode. Strictly speaking, it is an element driven by current. If the LED is driven by an appropriate current, a certain luminous efficiency (LM / W) can be obtained. If the power supply current is too small, the LED bulb cannot provide all-optical output. For a single LED bulb, this does not seem to be a big problem, but if many LED bulbs are connected in series at the same time, the output light will change significantly without the support of uniform current.

Of course, the LED bulb also needs to overcome the inherent forward voltage drop, which is related to the LED type and the configuration of the terminal device. For white LEDs, the forward voltage drop is about 3.5 volts (V), but it will be larger at high temperature. The results show that the forward voltage drop of LED is related to the input power supply. Under different LED chain configurations, for example, series (led forward voltage drop accumulation), parallel (LED current accumulation), or multiple parallel chains connected in series (led forward voltage and current accumulation), the led forward voltage requires a variety of conversion architectures, but this will make the overall circuit more complex.

The above briefly describes what conditions are required for the normal operation of LED bulb. What will happen if there are factors that adversely affect its operation or the current operation of driving integrated circuit (IC)?

The answer is that it will adversely affect the output brightness of LED lamps, and even lead to serious consequences. These events include overvoltage (which occurs in the event of an open LED) and overcurrent (which occurs in the event of a short circuit or re plugging of the LED chain). Of course, poor heat dissipation environment will also have an adverse impact on the service life of LED lamps. Therefore, good heat dissipation design is also very key.

This shows what factors will damage the LED and even lead to the failure of the LED and its driving circuit. Based on these knowledge, lighting system designers should carefully consider the LED bulb design technology, especially the component selection standard of LED driving circuit, so as to protect the LED from damage. The key factors are detailed below.

Provide stable current for LED

LED driving IC is very critical because the component has to face a variety of input power supplies. With different application fields, the design of LED driving IC is also different, so as to convert the input power supply into the required voltage and current and achieve the best LED performance. Overvoltage or overcurrent will adversely affect the service life or optical output of LED. Therefore, the more stable the voltage and current, the more reliable the system is. Therefore, the voltage and current stabilization within ± 5% is conducive to prolong the service life of LED and reduce service faults.

The LED drive circuit must cope with abnormal transient voltage

In order to protect the LED from overvoltage, the LED drive IC must first be required to be able to handle greater transient voltage than normal operation. Automotive application environment is a good example. When the load drops suddenly, it will reach 42 volts or even higher.

Protect LEDs from overheating

LED lighting devices are generally small, and there is no space for heat sink, or even no fan for air convection heat dissipation. Therefore, most heat dissipation needs to be handled through conduction. The industry needs to consider good heat dissipation function in the design stage. Here, the use of LED Driver IC with very high conversion efficiency will also be of great help. The higher the conversion efficiency of the drive IC, the smaller the energy loss and the less heat generated. Therefore, the LED drive IC with an efficiency of more than 90% will help to achieve a good heat dissipation design.

Another way to help heat dissipation is to use an on-chip temperature sensor in the LED. If the system controller monitors the temperature signal, the controller can throttle the total current of the system, so as to reduce the generation of heat. Of course, the cost is to reduce the optical output, but it will be much better than the failure of the whole system. Normal operation can be resumed after the fault state disappears.

Finally, these heat dissipation methods adopted by LED can also be extended to LED driven IC. It is also a good idea to adopt these methods at the whole system level.

Taking the industry’s high-power LED driving IC (Figure 1) as an example, it has a very precise step-down and step-up LED controller. The driving IC can achieve a fixed output voltage no matter whether the input voltage is higher, lower or equal to the output voltage. In addition, different from the complex and inefficient single ended primary inductor converter (SEPIC), this four-way switch synchronous single inductor architecture not only simplifies the converter design, but also improves the work efficiency and reduces the heat consumption.

How to select the appropriate driver IC for the LED lamp

Figure 1 100 watt LED driver circuit using high conversion efficiency LED driver, with peak efficiency up to 98.5%

It can be seen from the schematic diagram of 100 watt lighting drive IC in Figure 1 that the conversion efficiency is as high as 96% when the output voltage / current reaches 33.3 volts / 3 amps (a) in the whole input voltage range.

The key features of the LED Driver IC include a wide input and output operating voltage range of up to 60 volts. In order to achieve the best and uniform LED output light on the lamp chain, the drive IC also provides very strict ± 2% output voltage accuracy, ± 6% LED current accuracy and ± 6% input current accuracy when the line, load and temperature change.

For ease of use, the user of LED driven IC can adopt pulse width modulation (PWM) or analog dimming. The four-way switch voltage rise and fall architecture not only realizes the seamless conversion between modes, but also brings very low noise to led in all modes, including the direct mode in which Vout is close to VIN, which is difficult to achieve, especially during this mode, load transient events will occur. In addition, in order to improve the system performance and reliability, it also provides input and output current regulation functions. This element will be used in the environment with large ambient temperature changes. Therefore, the high-order LED driving IC provides – 40 ℃+ Working interface temperature range between 150 ℃.

The combination of LED efficiency and its commercial price advantage makes it provide characteristics beyond many lighting applications. The two examples presented at the beginning of this paper are only a small part of a large number of practical applications in the world. Now the era of LED lighting has come.

Responsible editor; zl

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