月份：2012年7月

Resolutions for LED tube drivers

Presently, more and more LED lighting products are used in our daily lives, from outdoor lighting applications to indoor lighting applications. LED is new developed technique which is more environmental friendly, more durable, more easily to control and of course can save more energy. LED tube is one of the most widely used indoor LED lighting products.

Presently, more and more LED lighting products are used in our daily lives, from outdoor lighting applications to indoor lighting applications.

LED tubes drivers can be divided into two types: isolated drivers and non-isolated drivers.

 

The input and the output of an isolated driver are “blocked” by transformers. These transformers can improve the security of the entire device but reduce the efficiency (around 85%) as well. Another disadvantage is that these transformers are too large to be put into the limited space within LED tubes.

 

Different from isolated drivers, the input and the output of non-isolated drivers are connected directly by circuits. Although these drivers are more efficient, they are somehow life-threatened. Currently, the most commonly used LED tube drivers are non-isolated BUCK drivers: convert AC to DC through rectifiers and meanwhile drop voltages through Buck block to maintain a constant current.

 

These improved drivers are much simpler in circuit design, smaller than isolated drivers. The normal efficiency of them is 80% ~ 90%. One can support more than 100 LEDs (in cascade or in parallel or both). However, these LED tube drivers have their own defects. Since non-isolated drivers will introduce the high energy from input to output, risking user’s live. To solve this problem, an insulated PCB will be inserted between AC input and aluminum cover. This design can block as high as 2000V voltage, even so it cannot pass the CE Certificate.

 

Comprehensively, both of isolated drivers and non-isolated drivers have their own advantages and disadvantages. Non-isolated drivers think more of efficiency, reducing energy consumption; while isolated drivers are safer to use. In fact, most people intend to use isolated drivers due to their small sizes, high efficiencies, low costs, more cost-effective.

 

LED tube drives can be divided into internal drivers and external drivers. LED tubes using internal drivers can directly replace the traditional fluorescent tubes without any changes. There are two different internal drivers: 1. Rectangular driver, installing within the aluminum shell. It is less practical due to the unevenly heat distribution. LED chips around the power supplier will continuously worked in high temperature. LED tubes using this kind of drivers usually have shorter service life. 2. Divide the entire power supplier into two parts and mounted separately on both sides of the tube. Because of the limited space of tube ends, design is more complicated.

 

How to choose right LED divers for your LED lighting applications

The load of LEDs impacts the reliability of drivers. The voltage-current characteristic of LEDs determines that within a certain region, with the increase of voltage, current will be exponentially raised. Therefore, many LED lamps driven by switching power suppliers worked in an unstable state. Since the output of switching power suppliers is a complex multiple harmonic waves rather than a constant value, leading to the inevitable fluctuation of the output signal.

 The load of LEDs impacts the reliability of drivers. The voltage-current characteristic of LEDs determines that within a certain region, with the increase of voltage, current will be exponentially raised.

As we all known, LED is very sensitive to voltage variations. Working under normal current, the voltage should be within the range of 0.3V-0.6V (different power consumptions may have slightly differences in voltages). Once working under unstable voltage, tiny changes can cause huge vibrations in current, and consequently the output power. Excess current may cause irrecoverable damages to LED Lights.

 

Facts proved that adding voltage-limited elements (like TVR, TVS) have little effect on reducing the current fluctuations. It seems that LED drivers were never as competent as other drivers. Reliability and efficiency are restricted with each other. Divers with higher efficiency must have a lower reliability, vise versa. The increase of reliability is at the cost of efficiency. In order to reduce the impact of input voltage to output voltage, energy must be converted through multiple levels.

 

In fact, all these problems were caused by LED loads. Recent years, experts put forward another proposal: to improve the internal connection of LED products to reduce the load. The simplest way is connecting resistors between LED chips. The larger the resistance, the more stable the load. Definitely, this method is still at the cost of efficiency. It is the simplest but not the best way. After this, some people introduced another resolution: adding linear barostats, using transistors as auto-adjusting elements. In this way, transistors are treated as changeable resistors. By doing so, the final circuit will be more stable and meanwhile efficiency can be guaranteed as well. This kind of LED drivers, to a large extent, is more advanced than that just simply insert resistors between LED chips. At present, the most commonly used method is constant-voltage sources combined with linear galvanostats. This method still provides very limited efficiency to the entire device.

 


Differences among LED, LED backlighting, OLED

At present, many companies played trick with concept of LED backlighting displays and LED displays. As a matter of fact, they are completely different applications. Another confusable LED technology is OLED. How to distinguish these three types of LED technologies?

 At present, many companies played trick with concept of LED backlighting displays and LED displays.

LED displays directly use LED Lights as pixel components. These LEDs will be arranged in matrix, sending out red/green/blue rays to comprise color screens. Thank to the “oversized” LED, the distance between pixels (known as dot pitches) of the same color is also oversized. In another word, LED displays are only suitable for large screens.

 

LED display is a combination of micro-electronic technology, computer technology and information processing, having many advantages: colorful, wide dynamic range, high light intensity, long service life, stable working condition, etc. Due to these advantages LED panel displays quickly became the most popular public media. Currently, LED displays have been widely used in many places: large plazas, commercial advertisements, stadiums, press releases, stock exchanges, etc.

 

LED backlighting displays just replace traditional CCFL backlighting source of LCDs with LEDs. LCDs working principles can be simplified as: liquid crystal molecules deflect under external voltage as small gate for lighting sources, changing the transmittance of rays. These rays then project on the different color filters to form a complete picture.

 

Backlighting module’s transformation from CCFL to LED brought a lot merits to LCDs: more evenly distributed light, less power consumptions, more fashioned appearance. However, the majority of LED backlighting displays on the market used White LEDs. These are no other changes except for lighting sources. The display effect of this kind of devices is much worse than that of RBG-LED displays. The later failed to widely spread in the market due to its high cost. Thus these White LED backlighting displays and LED Street Lights are essentially different from normal LED displays and cannot be considered as one thing. Nowadays, there are more than 100 research institutions and enterprises has involved into this area, including SAMSUNG, LG, Philips, Sony, etc.

 

OLED is the abbreviation of Organic Light-Emitting Diode. As a matter of fact, the principle has been discovered in 1936. But until 1987, Kodak introduced a new OLED double-layer device. Since then, OLED has generally comes into our sight. OLED is a brand new flat panel display technology which can be commercialized.

 

LED exported to Japan needs circular PSE Certificate

Japan has become one of our most important overseas markets. However, Japanese market is not as good as our expected. LEDs(LED Lights) exported to Japan need circular PSE Certificate, which is the second doorsill after LED patents. According to the new policy proclaimed on 1st July 2011, every LED lighting product sold in Japanese market should have circular PSE Certificate. This article will give a small talk about the application scope of regulated LED lamps, test basis, main point of the standard and of course, coping strategies.

Japan has become one of our most important overseas markets. However, Japanese market is not as good as our expected.

PSE is the short term for Product Safety of Electrical Appliance & Materials Law (PSEAM Law), which is the compulsory standard for electrical appliances on the market. According to Electrical Appliance and Material Safety Law of Japan, electrical appliances can be dived into two parts: specified products and non-specified products. Specified products on Japanese market must go through an independent third-party certification body authorized by the Japanese government and mark diamond PSE labels. Non-specified products must provide legitimate self-testing reports and self-declarations and mark rounded PSE labels before promoted in the market.

 

There are totally 457 types of products been included in the scope of PSEAM Law. LED lamps have been classified as non-specified products and should have rounded PSE labels. LED Bulbs with one mental head, 100V-300V, 50Hz/60Hz, with a power consumption larger than or equal to 1W. Other LED lamps with one mental head, 100V-300V, 50Hz/60Hz, with a power consumption larger than or equal to 1W (except for LED Explosion Proof Lights).

 

In order to help the majority of LED lighting manufacturers and exporters to crack the technical barrier, a special PSE working group has been set up in 2011. After one year of study, this working group has basically digested the PSE standard and can provide professional tests for LED bulbs and LED lamps which are destined for the Japanese market. In accordance with the government regulation, products exported to Japan should pass the relevant technique standard of CQC and accept a series of tests prepared for PSE Certificate, even obtain the final certification before export. It made a large and active contribution to the Sino-Japanese trade.

 

ESD: a cannot-be-ignored factor for LED lighting manufacturers

ESD (Electro-Static discharge) is a common damage for electronic components. Although circuits for LED lighting technique is relatively simple than more advanced intelligent electrical apparatus, there are still many electronic components used in LED Lights circuit PCBs, which are rather vulnerable to ESD. Most of time, these damages are irretrievable, leading to the failure and worse scrap. ESD problem is gaining extensive attention from LED lighting manufacturers.

ESD (Electro-Static discharge) is a common damage for electronic components.

ESD leads to an instant current/voltage surge, which is the instant release of the pre-stored electronic energy by energy storing elements. There are two different energy releases: 1. Predictable energy release through controlled switching effect; 2. Unpredictable energy release through random export.

 

As we mentioned before, ESD can bring huge damages to electronic circuits. ESD is generally caused by friction between two different substances (charges accumulate on the surface). ESD larger than 15000V can be sensed by our bodies, normally in the form of pricking. Thousands of ESD happened in our daily lives, almost every of them can cause damages to our LED lighting products. How can LED lighting manufacturers block these external killers became a primary consideration.

 

Understanding the transmission of ESD can help us solve this problem. ESD can be transmitted though the air (induction discharge) and conduction. Technicians usually use shielding to prevent air induction discharge ESD and use absorbing to prevent conducting ESD.

 

The most effective method to keep LED lighting applications from ESD is restricting ESD trigger voltage, making it in the safe range of LED Panel working voltage. SPA can avoid the shock brought by ESD. Firstly, using diodes to absorb the instant current; secondly, current induction; in the last step, Zener diodes will be used to restrict the current level.

 

Intelligent LED lighting products need to realize sensitive human-machine interactions, including short-term control and information feedback. All these relays on chip set and interfaces, which are typically vulnerable to ESD and needs more effective ESD protective measurers.

 

The maximum capacitance and ESD level need special attention during the process. LED lighting manufacturers should provide anti-static workbenches for their workers, letting them accept specialized training in ESD aspect.