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LED Driver
Generally it refers to synonyms LED drive LED drive
 
Constant voltage driver chips, constant current driver chip and a pulse driver chips: the current LED driver chips can be divided by type. Among them, the constant drive chip is usually our common DC / DC boost chip majority. The advantage of this program is not cheap cost of complex chip peripheral circuits. But only a constant voltage LED driver circuit output will cause the drive current is not controllable. LED brightness can not guarantee consistency.
 
Constant current driver chip, the drive current constant pressure can not be controlled until the problem is resolved. Relatively good precision constant current constant current chip can do about 1%, and there are simple peripheral control interface, the flexibility to set the size of the output current is so popular. But these chips than many constant chip and peripheral circuit complexity, more expensive. And due to constant output current to the battery as a power source in the discharge entire chip will be faster.
 
The current pulse driver chip is a high-frequency pulse generator output interface, power supply to the LED lights. Because it is a high-frequency pulse signal so that the human eye can not feel LED strobe to effectively save energy and vision required output in a manner consistent with another aspect. This type of chip and the operating frequency is usually through an external interface. But now this type of chip oscillation frequency is generally in the range of 500KHz to 100KHz ~ of. Thus, the current driving capability is only suitable for small power applications. But I believe that in the near future will increase the power LED driver applications.
 
 
 
 
 
Brief
 
LED driver (LED drivers), it is referring to the power-driven LED lights or LED module assembly work appropriately adjusting the electronic device. The decision to transfer characteristics LED PN junction, which can adapt to changes in the voltage range and power of electric current is very narrow, a slight deviation may not be severely reduced or luminous efficiency LED lights or shorten the life of even burned chip. Current power frequency and common battery power is not suitable for direct LED, LED driver can drive this LED electronic components in the optimum voltage or current status of the job.
 
Because LED application almost throughout the electronic application, luminous intensity, light color changes in all areas off control is almost impossible to estimate, so the LED drive has almost become a part of the server, so that the devices become diverse. The most simple LED drive (if you can call it whatever) may be one or several series and shunt voltage divider circuit parallel RC element, it will not be a separate product. As for the request for a more widespread commercial application of a constant current output steady, precise power regulation range of system solutions form. Implement these solutions usually require more complex circuit design, its core is the LED driver IC integrated applications. By setting different support circuits in the periphery of the LED driver IC, LED construct a different application solutions, small mobile phone display backlighting and keypad lighting drive, large high-power LED lights and a large outdoor LED display.
 
The more common high-power LED driver design and supply of the general by a professional company to play. After the end of its second package of plug-in module, these companies will provide LED application products manufacturer. The drive's design is not very flexible LED terminal application, you may need to do it yourself design. It has become an important part of the technical content of this unique LED end applications. Because, as packaging products, LED upstream, its technical performance has been cured of LED products and create a unique high-end LED application products, other places in addition to the efforts of hard drive functions LED light source aspect you can work that we have run out.
 
Because of the LED drive LED application products unique importance and wide range of user needs, so that when the heart of the LED driver LED driver IC element become a key element of the whole technology. Prompted many vendors, many of them listed companies as the main products LED drive, large supply LED driver IC downstream industries, such as Hangzhou Silan Microelectronics, Shenzhen-chuen core electronics, Shenzhen Guanghua Source Technology, Shenzhen State Microelectronics, China Resources Powtech , point crystal technology accumulated in Taiwan, technology, and wide Peng Technology, crystal Taiwan's technology, rainbow, ANPEC, comprehensive records. US manufacturers in this area, there are many industry leadership styles, such as National Semiconductor, Maxim, Texas Instruments, Semiconductor, Linear Technology, Fairchild, etc.
 
LED drive power supply voltage is converted to a specific current to drive the LED lamp voltage converter, under normal circumstances: LED drive power input includes a high frequency alternating current (ie, electricity), low-voltage direct current, high-voltage direct current, low-voltage high-frequency AC (such as electronic transformer output) and the like. LED drive power output and LED forward voltage drop is mostly change can alter the voltage value of the constant current source.
 
Feature
 
According to the rules of the grid and the LED drive power required characteristics, selection and LED driver power supply design to take into account the following points: 1. High reliability especially as LED lights drive power, packed in high altitude, maintenance inconvenient, maintenance costs also large. 2. High efficiency LED energy-saving products, higher power efficiency. For electrical equipment, LED drive power structure inside the lamp, it becomes particularly important. Since the LED luminous efficiency of LED temperature with lower, so the heat LED it is very important. High power efficiency, its low power consumption, small heat in the fixture, it reduces the temperature rise of the lamp. LED light failure favorable delay. 3. The high power factor power factor load power requirements. Typically 70 watts of electrical power, no mandatory targets. While this is not a single low-power electrical power factor has little impact on the grid, but we lit the night, the same load is too concentrated, the grid will produce more pollution. 30 watts to 40 watts LED driver power supply, it is said that in the near future, maybe there are certain factors that have the power to regulate. 4. Drive way traffic in two ways: one is a constant voltage source, a plurality of constant current source, a constant current source for power separately to each LED. In this way, a flexible combination of the way LED failure does not affect other LED's work, but the cost will be slightly higher. Another is a direct constant current power supply, in series or parallel operation indicator. Its advantages are low cost point, but the flexibility is poor, but also to solve a LED failure, the problem does not affect other LED operation. Both forms coexist for some time. Multi-channel constant current output power, will be better in terms of cost and performance. It may be the future direction of the mainstream. 5. Surge Protection LED rapid deployment capability is relatively poor, particularly for reverse voltage capability. Strengthen the protection of this area is also very important. Some installed outdoors LED lights, such as LED lights. Since starting grid load shedding and lightning induction, from the grid system will invade a variety of surge, the surge will cause some damage to the LED. Thus, LED drive power surge suppression intrusion has the ability to protect the LED without being damaged. In addition to conventional protection function 6. power protection, the best increase in the constant current output LED temperature negative feedback to prevent LED overheating. 7. type of external lighting installation protection, the power structure to water, moisture, housing and fast. Life 8. The drive power adjustment and LED life. 9. To meet safety and electromagnetic compatibility requirements. With the increasingly wide range of LED applications, the performance of LED drive power would be more suitable for the needs of the LED.
 
Classification
 
Drive mode can be classified
 
 
 
(1) constant flow: an output current of the constant current drive circuit is constant, and the output DC voltage, but having different sizes within a certain range of load resistance changes, the load resistance is small, the output voltage is low, compared with large load resistance, higher output voltage; B, constant load short circuit is not afraid, but no load completely open. C, constant-current LED drive circuit drives are ideal, but the price is relatively high. R & D, to withstand the maximum current and voltage values ​​to note, thus limiting the number of LED's use; (2) adjusting the formula: First, when the voltage regulator circuit, in order to determine the parameters, the output voltage is fixed, but but with current output load reduce and change; b, the regulator circuit is not afraid of open load, but not load completely short-circuited. C, in order to adjust the drive circuit drive LED, each string only need to add an appropriate resistor, the LED display brightness per string; research and development, will be affected by changes in the brightness of the rectified voltage.
 
Circuit configuration mode classification
 
 
 
(1) resistors, capacitors buck ways: through capacitor step-down, while taking advantage of charging and discharging due to flicker effect, the instantaneous current through the LED large, easy to damage the chip. Susceptible grid voltage, high power efficiency, low reliability fluctuations. (2) Resistance Buck ways: through the resistor voltage, interference from the power grid voltage fluctuation is large, it is not likely to cause power dropping resistor consumes most of the energy, so this is a very low power efficiency power supply and system reliability sex is low. (3) conventional step-down transformer modes: power, small size and weight to emphasize the power efficiency is very low, generally only 45% to 60%, and therefore rarely used, reliability is not high. (4) Electronic transformer buck mode: Low power efficiency, voltage range is not wide, general 180~240V, large ripple interference. (5) The RCC buck mode switching power supply: wide voltage range, power efficiency is relatively high, generally can achieve 70 to 80 percent, applications are wide. Since this type of oscillation frequency control is not continuous, the switching frequency is not easy to control, load voltage ripple coefficient is relatively large, abnormal loads and poor adaptability. (6) PWM control mode switching power supply: mainly composed of four parts, the input rectifier filter section, the output rectifier filter section, PWM voltage control section, switching energy conversion section. The basic working principle is that when the input voltage, the internal parameters and external load changes, the control circuit by a differential signal with a reference signal for closed-loop feedback to control, adjust the main circuit switching element PWM switching regulator pulse width, so that the switching power supply output voltage or current stabilization (corresponding to the power supply or constant current power supply). High power efficiency, it can usually do 80-90%, the output voltage, current stability. This circuit improves the general protection measures, is a high-reliability power supply. 3. The power supply mounting position classification driving force installation location can be divided into external power and internal power supply. (1) As the name suggests an external power supply, an external power supply is installed outside. Typically the voltage is relatively high, there is a security risk, we need an external power supply. The difference is that with the built-in power supply and a shell, a common street. (2) The internal power supply is installed in the lamp power, it is generally relatively low voltage, 12V to 24V, who, without security risks. This common light bulb.
 
Application
 
The main application markets high-power applications ---- the largest market share in LED landscape lighting LED, power LED market analysis, landscape lighting market is about 43%. This is a new round of rapid growth will drive the rapid development of LED lighting industry as a whole. Landscape Lighting Shanghai, Xiamen, Beijing, Dalian, Nanchang, and some of the cities with large, complete LED lighting demonstration project a considerable part of the successful completion of these cities, as well as demonstration projects successfully used in Olympic venues, indicating LED lighting landscape lighting technology has mature. Whether domestic or foreign, urban landscape is a city landmark product, but as a landscape lighting neon lights all over, because the world's energy, energy conservation, environmental protection and other issues are gradually being LED landscape lights, around the world there are about 700 000 5000 Calculation city to city, about 1,000 yuan a lamp, the light of these huge economic benefits generated is unpredictable.
 
Drive is a core component of LED lighting, with LED chip technology matures, the quality has been very reliable LED light source, in many cases, LED lighting is the result of a hard disk failure. High-power LED drive power is an emerging industry, key industry gathering has not yet formed, therefore, regional obvious. The number of firms to enter the industry is not much, and therefore, less competitive level.
 
Not enough
 
LED driver power supply shortcomings reasons: (1) the production of LED lighting and related products, the company's technical staff, understand the switching power supply is not enough, in the normal power supply, can work, but some of the key considerations evaluation and electromagnetic compatibility not enough there are still some risks too; (2) most of the LED power supply manufacturers to come up with common LED power supply switching power supply make the transition, LED characteristics and uses of understanding is not enough; (3) with respect to the standard LED almost no major components are standard reference switching power supplies and electronic ballasts; and (4) Most of LED power supply is not uniform, so most of a relatively small amount. Small purchases, the price is high, and component suppliers is not very complex; stability (5) LED Power Supply: wide input voltage, high temperature, low temperature, over-temperature, over-voltage protection problems are not resolved these issues; the first is a driver circuit as a whole life, especially in key parts, such as: lifespan of the capacitor at high temperature directly affects the power of life; LED drive higher conversion efficiency to be followed by the challenge, especially when driving high-power LED is especially true, because all not as light output power is dissipated as heat, power conversion efficiency is too low, the impact of the play, LED energy saving effect; application in smaller power (1-5W), the proportion of constant current drive power costs accounted for nearly one-third It is close to the source of the cost, to a certain extent affected the marketing.
 
14W high power LED driver power supply design
 
 
 
First, the design features
 
1, for the high ambient temperature (75 degrees)
 
2, energy efficient
 
3, combined with the EU Assembly / CEC requirements 2008 / Energy Star 2.0, high-load mode efficiency (up to 86%, requiring 79.6%); when you enter the 265 VAC <250 mW, 300 mW no-load input power requirement
 
4, hysteretic thermal shutdown protection
 
5, load disconnect protection
 
6, foot EN55015B conducted EMI limits, EMI margin> 8 db millivolt
 
Second, the principle
 
The figure below shows a typical 20-volt, 14-watt constant voltage (CV), constant current (CV) output of the power supply circuit. The amount of light output and flow through the proportion of LED array. Thus, LED drive should have a constant current output, rather than a constant voltage output. In this design, DC output is not isolated from the AC input, the LED array and the housing in response to user and security isolation.
 
 
 
 
 
From BR 1, C1 and C2 AC input is rectified and filtered. Inductor L1 and C1 and C2 together form a π-type filter, and provides EMI filtering. Fuse F1 provides protection against catastrophic failure. In order to make the power supply to the load without damaging the work, using a constant voltage of the zener diode VR2 and voltage regulation is maintained at about 21 V.
 
By detecting the current sense resistor R7 voltage drop, constant current characteristic. Shunt regulator integrated circuit U3 and R9, R8 and R8A together to generate at the inverting input of the operational amplifier U2 accurate 0.07 V reference voltage. Reaches the set current, voltage R7 will exceed the reference voltage, it will increase the output of the operational amplifier. The positive bias D4, driving the base of Q1, and then pulled out of the EN / UV pin of U1 current. Capacitor C7 and resistor R11 provide loop compensation. Using the current sampling voltage is minimized, thereby reducing wear of the operational amplifier by way of limitation, the maximum efficiency.
 
As long as the EN / UV pin is pulled out of the current exceeds 115μA, U1 will be recycled in the MOSFET mode is disabled (ON / OFF control). Enabling disabled by adjusting the ratio of the switching cycle, the feedback loop may adjust the output voltage or current. ON / OFF control, while optimizing converter efficiency under different load conditions, in order to meet energy efficiency standards.
 
Due to high ambient temperature, U1 will reduce the current limit mode. This can improve the overall efficiency of the power and improve its thermal performance. Primary clamp (D1, VR1, C3 and R3) internal maximum peak drain voltage controlled
 
700 V under the BVDSS MOSFET breakdown voltage. Resistor R23 reduce high frequency leakage inductance ringing, thereby reducing EMI. Output by the secondary side of the diode D2, D3 and C6 rectified and filtered.
 
Third, design elements
 
1, to select fast diode can not choose ultrafast diodes to improve efficiency by recovering some of the leakage inductance energy.
 
2, let C3 to improve EMI performance.
 
3, the optional resistor R10, for the minimum output voltage 6 V to the U3 provides supply current to 1 mA.
 
4, U1 selectable current limit allows the current limit to optimize the size and equipment options to suit the ambient temperature. For example, in order to reduce power consumption, it can be changed from 1μFC3 TNY280GN 0.1μF to use equipment in the same design. In addition, the high thermal performance environment, you can change from C31μF to 10μF to use TNY278GN devices.
 
5, the LED string voltage source is between 6 to 20V can also work. However, since the output current is constant, low light string voltage, lower output power.
 
LED-based lighting driver buck converter design
 
 
 
The output voltage may be lower than the input voltage than the peak current mode control of non-continuous buck converter LED driver to a higher good choice. However, when using this step-down converter designed for driving, LED voltage variation will change the LED current, LED open circuit will result, to the detriment of the high voltage output of the converter. This article will discuss the design for LED converter, and given a variety of ways to overcome their inherent disadvantages.
 
Light-emitting diode (LED) applications for many years, and with the latest technological advances, it is becoming a strong contender for the lighting market. New high-brightness LED have a very long life (about 100,000 hours), high efficiency (about 30 lumens / watt). In the past three decades, every 18 of the LED light output brightness ~ 24 months will be doubled, and this growth will continue, this trend is called Haitz Law, LED equivalent to Moore's Law.
 
Electrically speaking, LED diodes are similar, they are also one-way conductivity (although they are not very good reverse blocking capability, high reverse voltage can be easily damaged (LED), and has a low dynamic similar to a conventional diode VI characteristic impedance. In addition, LED usually has security rated current is turned on (high-brightness LED rated current is generally 350 mA or 700 mA), when the rated current, LED forward voltage difference may be relatively large drop between usually 350 mm Anbai Guang LED voltage drop 3~4V.
 
Driven by the need to control LED DC current. In order to make some long-life LED, LED current ripple to be low, because high ripple current, the LED produces a large amount of resistance of the power consumption, reduce LED life. LED driver circuit requires a higher efficiency, because the overall efficiency depends not only on the LED itself, but also related to the driving circuit. Current control mode switching converter work, is an ideal application-driven solutions to meet the high-power LED and high efficiency requirements.
 
To drive multiple LED also need careful consideration. Figure 1 is a series - parallel LED connection circuit. Wherein Figure 1 (a) is a parallel LED connection circuit. Figure 1 (h) of the series LED circuit. Due to the dynamic impedance and each LED's forward voltage drop is not the same, so if there is no external current sharing circuitry (such as current mirrors), it is impossible to guarantee the same current flows through the LED on; In addition, because the fault LED LED string is disconnected, thereby causing the LED current assignment of all the remaining LED string, which will lead to an increase of the current in the LED string, which may damage between the LED. Thus, for these two reasons, this design is not generally 1 (a) as a parallel LED circuit is shown.
 
 
 
Therefore, a better approach is to LED series. The disadvantage of this method is that if one LED fails, the entire LED string will stop working. (B) A simple method is provided in the LED string to continue the rest of the work is a Zener diode (rated voltage is greater than the highest voltage LED), and each (or group) parallel LED, shown in Figure 1. Thus, in case of failure of any one of the LED, the current will flow to the respective Zener diode, the rest of the LED string can still work.
 
The basic single-stage switching converter can be divided into three categories: a buck converter, boost converter and buck converter. When the LED string voltage lower than the input voltage, the buck converter in Figure 2 (a) is the ideal choice; when the input voltage is always lower than the output voltage of the string, use the appropriate boost converter in Figure 2 (b ); changes in output or input when the output voltage may be lower than the input voltage ratio (caused by) higher, and then use Figure 2 buck converter (c) is more appropriate. The disadvantage of the boost converter is that any instantaneous input voltage (the input voltage rises and exceeds the output voltage) can cause a large current to flow through the LED (due to the low dynamic impedance of the load), causing damage to the LED. Down converter may also be replaced by a boost converter, since the input voltage does not affect the LED current transients.
 
 
 
Buck converter operates
 
For low voltage applications, LED drivers, buck converter is a good choice. The reason is that they are above and below the available voltage to drive LED strings (boost and buck), high efficiency input voltage (it is easy to reach more than 85%), non-continuous mode of operation inhibition input voltage changes (provides superior line voltage adjustment), peak current control mode allows the converter to regulate the LED current, without the need for complex compensation (simplified design), it is easy to achieve linear and PWM LED dimming, switching transistor failure does not damage LED Wait. Figure 2 shows a buck, boost and buck - boost converter and LED string connected to the circuit.
 
However, this method still has some drawbacks: First, the problem of peak current control, because the use of non-continuous current mode buck converter is a constant power converter. Therefore, any change in the LED string voltage will result in a corresponding change in the LED current; Another problem is that, LED open circuit will damage the high voltage of the converter circuit; in addition, the need for additional circuitry to constant power converter to convert Constant current converter, and the need to protect the converter is under no load conditions.
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