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LED Driver

Writer:www.turnmax-tech.comSource:东莞市腾茂电子有限公司 Date:2016-12-14
The LED driver
Generally refers to a synonym for the LED drive LED drive
Constant voltage driver chip, constant current driver chip and pulse driver chip, the LED driver chip can be divided by type. Among them, the constant voltage driver chip is usually our common DC/DC booster chip in the majority. The advantage of this solution is not cheap chip cost complex peripheral circuit. But only a constant voltage LED driver circuit output will lead to the drive current is uncontrollable. The brightness of the leds can guarantee consistency.
Constant current driver chip, drive current constant voltage before uncontrollable problem is resolved. A relatively good precision of constant current constant current chip can achieve about 1%, and has simple peripheral control interface, flexible set required the size of the output current is so popular. But these chips than many constant chips and peripheral circuit is complex, the price is more expensive. And as a result of constant output current, so that the battery as the power of the whole chip in the discharge 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, make the human feel LED stroboscope, visual need to effectively save energy output another aspect is consistent. With the working frequency of this type of chip is usually through an external interface. But now, this type of chip oscillation frequency is in commonly 100 KHZ to 100 KHZ range.
Therefore, the current driving ability is only suitable for low power applications. But I believe that in the near future will increase power LED driver applications.
A simple introduction
LED drive (LED driver), refers to the electric drive LED lamp or LED module assembly work properly adjust the electronic device. Because of conduction properties LED the decision of the PN junction, the range can be adapted to the change of the voltage and current of power supply is very narrow, slightly deviation may not be the indicator light is lit or severely reduced, luminous efficiency and shorten the life even burned chip. The frequency of the current power and common battery power is not suitable for direct LED, LED driver can drive it in the best voltage or operation of the current state of the LED electronic components.
Due to the LED application almost throughout the electronic application, luminous intensity, light color changes, a close control of all areas of almost impossible to estimate, so the LED driver has become almost a server part, make the series devices diversification. The simplest LED drive (if you can any calls it) can be one or a few series and shunt voltage divider circuit parallel RC element, it will not be a separate product. As required to provide a wider range of business applications of a stable and constant current output, a series of system solutions of the formation of accurate ability of power regulation. To implement these solutions often require more complex circuit design, its core is the application in LED driver IC integration. 
Through the outside of the LED driver IC set different support circuit, LED to build the different application solutions, drive small mobile phone keyboard and display backlighting lighting, large high power LED lights and a large outdoor LED display.
Is the more common form of high power LED driver design and supply of general to make by professional company. Its one end of the second package insert module, the company will provide the LED application products manufacturers. The drive of design is very flexible LED terminal application, you may need to your own design. It has become the unique LED terminal application products technology content is an important part of. Because, as the packaging products, LED upstream, its technical performance has cured LED products, and create a unique high-end LED application products, in addition to the LED light source drive function to other ways you can work hard where we are all used up.
Due to the unique importance of LED drive LED application products and a wide range of user needs, in this way, when the heart of the LED drive LED driver IC components is the key element of the whole technology. Prompted many vendors, including the LED driver as the main products of listed companies, large supply of LED driver IC of downstream industries, such as micro silan, shenzhen spring core electronic, light huayuan science and technology in shenzhen, shenzhen state microelectronics, China science and technology, China resources silicon crystal science and technology in Taiwan, the accumulation of technology, guang peng science and technology, Taiwan chip technology, rainbow, manufacturers, comprehensive notes, etc. The manufacturers in this aspect also has a lot of the leadership style of the industry, such as national semiconductor, beautiful letter, Texas instruments, semiconductor, LingLiEr, fairchild, etc.
LED drive power is the power supply voltage is converted to a specific current to drive the LED lamp voltage converter, under normal circumstances, the LED drive power input including high frequency alternating current (power), low voltage direct current (dc), high voltage direct current, low voltage high frequency ac (such as electronic transformer output). LED drive power supply output is mostly positive pressure drop can be changed with the LED constant current source voltage changes.
The characteristics of
According to the rules of power grid and the characteristics of LED drive power requirements, selection and design of LED drive power supply should consider the following: 1. The high reliability, especially as the LED street lamp driver power supply, in a high altitude, maintenance inconvenient, maintenance cost is also big. 2. LED energy saving products with high efficiency, power efficiency is higher. For electric power equipment, LED drive power structure within the lamps and lanterns, is particularly important. Due to LED temperature in LED luminous efficiency is lower, so it is very important to heat the LED. High power efficiency, its power consumption is small, small heat in jig, it reduces the temperature of the lamp. Better delay LED light failure. 3. High power factor power factor of load power supply requirements. General is 70 watts of electrical appliances, there is no mandatory indicators. Though this is not a single low power electric power factor, the impact on the grid is not big, but we lit the night, the same load is too concentrated, the grid will result in more serious pollution. Of 30 to 40 watts LED drive power supply, it is said that in the near future, there may be some factors has the authority to regulate. 4. Drive way passage way has two kinds: one kind is the constant voltage source, for multiple constant current source, a constant current source, used to separately to each LED power supply. In this way, the flexibility, the combination of LED failure will not affect other LED all the way to work, but the cost will be slightly higher. The other is a constant current power supply directly, series or parallel operation indicator light. It has the advantage of low cost, but poor flexibility, but also solve a LED failure occurs, the problem does not affect the other LED to run. These two forms coexist for a period of time. Multichannel constant current output power, in terms of cost and performance will be better. May be the mainstream direction in the future. 5. Surge protection LED rapid deployment ability is poor, especially for reverse voltage capability. To strengthen the protection of this field is also very important. Installed in outdoor LED lights, such as LED lights. Since the launch power grid load rejection and lightning induction, into the various surge from the grid system and surge can cause some damage to the LED. Therefore, LED drive power surge suppression invasion have the ability to protect the LED was damaged. In addition to the conventional protection function 6. Power protection, in the constant current output increase LED temperature feedback best LED to prevent overheating. Outside lighting installation type 7. Protection, power supply structure to waterproof, moistureproof, shell and fast. Life 8. To adjust the driving power and the service life of the LED. 9. To meet the requirements of safety and electromagnetic compatibility. As LED application is becoming more and more widely, the performance of the LED drive power supply will be more suitable for the LED needs.
Drive mode can be classified
Constant flow: (1), a constant current drive circuit of the output current is constant, and the output dc voltage, but with different load resistance within a certain range of change, the size of the load resistance is small, the output voltage is low, bigger load resistance, high output voltage; B, a constant load short circuit will not be afraid, but no load completely open. C, LED constant current drive circuit drive is ideal, but the price is relatively high. Research and development, the attention should be paid to the maximum withstand current and voltage value, thereby limiting the use of the LED quantity; (2) to adjust type: a, when the voltage regulator circuit, to determine the parameters of the output voltage is fixed, but with current output load decrease and change; B, not afraid of open circuit load regulator circuit, but not completely short-circuit load. C, in order to adjust the drive LED drive circuit, each string only need to add an appropriate resistance, make the brightness of the LED display on average every string; The rectifier voltage to research and development, the brightness will be susceptible to change.
Circuit structure classification
(1) resistance, capacitance step-down by capacitance step-down, use for flashing charging and discharging effect at the same time, the instantaneous electric current through the LED is big, easy to damage the chip. Vulnerable to grid voltage, high power efficiency, reliability and low volatility. 
(2) resistance step-down method: through the resistance voltage, by the interference of grid voltage change big, it is not easy to cause the power supply, step-down resistance consume much energy, so it is very low in efficiency of the power supply, and the reliability of the system is low. 
(3) the conventional step-down transformer model: power, small volume, weight stressed power efficiency is very low, generally only 45% to 60%, so rarely used, the reliability is not high. 
(4) the electronic transformer step-down modes: low power efficiency, voltage range is not wide, general 180 ~ 240 v, the larger ripple interference. 
(5) of the RCC step-down mode switching power supply: wide voltage range and power efficiency is relatively high, generally can reach 70% to 80%, the application is wide. Because of this type of control oscillation frequency is discontinuous, switching frequency is not easy to control, load voltage ripple coefficient is larger, abnormal load and poor adaptability. PWM control mode switching power supply 
(6) : mainly consists of four parts, the input rectifier filter part of the output rectifier filter parts, PWM voltage control part, part of energy conversion switch. Basic principle is that when the input voltage, internal parameters and external load change, control circuit and used for reference signal of the closed-loop feedback error signal to control, adjust the main circuit switching element conduction PWM switching regulator of pulse width, makes the stability of the switch power supply output voltage or current (i.e., the corresponding power supply or constant current power supply). High power efficiency, and can usually be between 80% and 90%, the output voltage and current is stable. This circuit improves the general protection measures, is a kind of high reliability of power supply. 3. According to according to the installation position of the power driver installation position classification can be divided into the external power supply and internal power supply. 
(1) as the name implies the external power supply, it is the external power supply is installed in the outside. Usually relatively high voltage, there is a safety risk, we need an external power. The difference is that with the built-in power supply, and a shell, a common street lamp. 
(2) the built-in power supply is installed in the lamp power, are generally low voltage, 12 v and 24 v, people live without a safe hidden trouble. The common light bulb.
The application form
The main application of high power market, LED landscape, analysis of the market power LED, LED lighting market share on the biggest application of 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 in Shanghai, xiamen, Beijing, dalian, nanchang, and some large and medium cities with large, full LED lighting demonstration project smoothly has quite part of these cities and demonstration project in Olympic venues successfully used, explain LED lighting landscape lighting technology has been mature. Whether domestic or foreign, urban landscape is the city's iconic products, but as a landscape illumination neon lights all over, because of energy, energy saving, environmental protection problem of the world are being LED landscape lamp, and about 700000 cities across the city in the world 5000 calculation, a lamp 1000 yuan or so, the enormous economic benefits generated by the light is unpredictable.
Drive is the core part of LED lighting with LED chip technology matures, the quality is very reliable LED light source, in many cases, the LED lighting is due to hard disk failure. High power LED drive power supply is an emerging industry, has not yet formed the focus of the industry gathered, therefore, regional obvious. And the number of enterprises to enter the industry is not much, as a result, less competitive levels.
Not enough
LED drive power supply: (1) the reason of insufficient production of LED lighting and related products, the company's technical staff, understand the switch power supply is not enough, the power supply is normal, can work, but some of the key factor assessment and electromagnetic compatibility is not enough there is a certain risk is too big; (2) most of the LED power production enterprises to come and transformation of normal LED power supply switching power supply, LED features and USES of knowledge is not enough; (3) relative to the standard LED almost no large components are standard reference switch power supply and electronic ballast; And (4) most of the LED power supply is not unified, so much smaller amounts. A small amount of purchase, the price is high, and the parts supplier is not very complex; Stability (5) the LED power supply: wide input voltage, high temperature, low temperature, over temperature, over voltage protection problem does not solve these problems; First is driving circuit of whole life, especially in the key components, such as: the life of the capacitor in high temperature directly affects the service life of the power; Should follow the LED drive higher conversion efficiency of the challenge, especially when driving high power LED, in particular, because all is not as light output power as heat dissipation, power conversion efficiency is too low, the influence of the play, the LED energy-saving effect; In smaller power (1-5 w) the application of constant current drive power cost accounts for the proportion of close to a third, it is close to the cost of the light source, a certain extent, affected the market promotion.
14 w high power LED drive power supply design
First of all, the design characteristics
1, for high temperature (75 degrees)
2, high efficiency and energy saving
3, combining with the European Union general assembly / / energy star 2.0 2008 request CEC, high efficiency of load model (as high as 86%, so we need 79.6%); At 265 VAC input < 265 milliwatts, 250 milliwatts no-load input power
4, hysteresis heat off protection
5, load disconnect protection
6, sufficient EN55015B conduction EMI limits, EMI margin > 8 decibels millivolts
Working principle of the two,
Figure shows a typical 20 v, 14 w constant voltage (CV), constant current (CV) of the output power of the circuit. The quantity of LED array and through the light output ratio. Therefore, LED drive should have a constant current output, rather than a constant voltage output. In this design, the DC output is not isolated from the input communication, so the LED array and the shell should be from the security of the user.
From BR 1, C1 and C2 AC input rectifier and filter. Inductors L1 and C1 and C2 together to form a PI type filter, and provided the EMI filter. The fuse F1 offer serious fault protection. In order to make the work under the power supply to the load without damage, zener diode VR2 using constant voltage and the voltage regulation are kept in about 21 V.
By testing in current detection of the voltage drop on resistor R7, realize constant current characteristics. Shunt regulator IC U3 and R9, the R8 and R8A together to generate the U2 in operational amplifier inverting input accurate reference voltage of 0.07 V. To set the current, voltage reference voltage R7 will exceed stated, it will increase the output of the operational amplifier. At this point will be offset D4, driving the base of Q1, and then from the current EN/UV pin U1 is pulled out. Provide R11 loop compensation capacitor C7 and the resistance. Using the current sampling voltage is minimized, thus reduce wear operational amplifier restrictive way and make the maximum efficiency.
As long as the EN/UV pull pin of the current more than 115 mu, the MOSFET will be circulation patterns in the U1 is prohibited (ON/OFF control). Through the adjustment made to disable switch cycle, the ratio of feedback loops can adjust the output voltage or current. ON/OFF control, optimization of converter efficiency under different load conditions at the same time, in order to conform to the energy-saving standards.
Owing to the high ambient temperature, U1 cut in current limit flow pattern. It can improve the overall efficiency of power supply and improve the heat dissipation performance. Primary clamp (D1, VR1, C3 and R3) drain voltage maximal peak value of insider control
The breakdown voltage of 700 V BVDSS MOSFET. Resistance R23 reduce the leakage inductance of the high frequency oscillation, thus reducing EMI. Secondary side by the output of the diode D2, D3, and C6 rectifier and filter.
Third, the design elements
1, choose fast diode cannot choose super fast diode, the leakage inductance of the part by restoring the energy to improve efficiency.
2, let the C3 to improve EMI performance.
3, optional resistor R10, used in 6 V supplies power to the smallest output voltage U3 current of 1 ma.
4, U1 optional current limit to current limit, to optimize the size and the equipment selection, to adapt to the environment temperature. For example, in order to reduce power consumption, can be from 1 mu FC3 into 0.1 u F to use TNY280GN devices in the same design. In addition, the high heat dissipation performance of environment, you can change to 10 from C31 microcomputer u F u F to use TNY278GN device.
5, in LED series voltage source is between 6 to 20 v can work normally. However, due to the output current is constant, low lamp voltage, low power output.
LED lighting driver based on buck converter design
The output voltage may also be lower than the input voltage is higher than peak current mode control of continuous step-down converter a good choice for the LED driver. However, when using the lifting pressure converter design, LED voltage change is going to change the LED current, the LED will lead to open, thus damaging the output of the high voltage converter. This article will be discussed in detail for the LED converter design, and gives the ways to overcome its inherent shortcomings.
Light-emitting diode (LED) applications for many years, with the latest technological progress, it is becoming a strong candidate for the lighting market. New high brightness leds have a very long life (100000 hours), high efficiency (about 30 lumens/watt). In the past 30 years, every 18 of the LED light output brightness will double ~ 24 months, and that growth will continue, this trend known as Haitz's law, the LED equivalent of Moore's law.
Electricity, LED diode is similar, they are also a unidirectional conductivity (although they are not very good reverse blocking ability, a high reverse voltage can easily damage (LED), and has a low dynamic similar to traditional diode VI characteristic impedance. In addition, the rated current of the LED usually has a safe when conducting (high brightness LED the rated current of 350 ma and 700 ma) generally, when the rated current, LED the difference of forward voltage drop can be relatively large, usually 350 ma white LED voltage drop between 3 ~ 4 v.
Driving leds needed to control the dc current. In order to make some long life LED, LED to low current ripple, because high ripple current, LED, produce large amounts of the resistance of the power consumption, reduce the service life of the LED. LED driver circuit needs higher efficiency, because the overall efficiency depends not only on the LED itself, but also relates to the driving circuit. Work in the current control mode switch of the converter, it is an ideal application driven solutions, to meet the requirements of high power LED and high efficiency.
Drive multiple leds also needs careful consideration. Figure 1 is a series - parallel connection circuit of LED. Desmond tutu 1 (a) is a parallel connection circuit of LED. Figure 1 (h) for the LED circuit in series connection. Because of the dynamic impedance of each LED and positive pressure drop is different, so, if there is no external current sharing circuit such as current mirror, it could not guarantee the same current through the LED; In addition, because the LED failure will LED string broken, leading to a series of all the rest of the LED LED current distribution, this will lead to increase the current in LED string, which may damage the LED. Therefore, for the above two reasons, the design is usually not as 1 (one), be shows for the LED circuit in parallel.
Therefore, better way is the LED series. The downside of this approach is that if an LED fails, the whole LED string will stop working. (2) in the rest of the LED string to continue to work, a simple method is to a zener diode (LED) rated voltage is greater than the highest voltage, and parallel LED each (or group), is shown in figure 1. Therefore, in the LED the failure of any of the items stated, current will flow to the corresponding zener diode, referred to the rest of the LED string can still work normally.
Basic single-stage switch converter can be divided into three categories: a buck converter, boost converter and step-down converter. When the LED string voltage than the low input voltage, in figure 2 (a) the step-down converter is an ideal choice; When the input voltage is always more than series of low output voltage, using the appropriate booster converter in figure 2 (b); Can when the output voltage is lower than the input voltage ratio (caused by the change of the output or input) is higher, and then use the buck converter (c) in figure 2 is more appropriate. Boost converter on the downside, any instantaneous input voltage (by raising the input voltage is more than the output voltage) can lead to large current flows through the LED (because of the low dynamic impedance of load), and damage of the LED. Converter can also boost converter by replacement, due to the input voltage does not affect the LED current transients.
Buck converter
For low voltage application, LED drive, buck converter is a good choice. The reason is that they are above and below the voltage are available, and to drive the LED string (step-up and step-down), high efficiency of the input voltage (it is easy to reach more than 85%), the discontinuous operation mode inhibiting the change of the input voltage, which offers excellent line voltage adjusting peak current control mode allows the converter to regulate the LED current, without the need for a complex compensation (simplified design), it is easy to realize linear and LED dimming PWM, switching transistor failure no damage to the LED and so on. Figure 2 shows the buck, boost and buck - boost converter and LED series connection circuit.
However, this method still has some disadvantages: first, the peak current control problem, because the use of continuous current mode buck converter is a constant power converter. Therefore, the voltage of any changes in LED string will lead to corresponding changes in LED current; Another problem is that the LED will open damage of high voltage converter circuit; In addition, the need for additional circuit with constant power converter into constant current converter, and the need to protect the converter under the condition of no load.

TypeInfo: Corporate News

Keywords for the information:LED driver