LED drive power, how much do you know?

　　1. What is the LED drive power

　　LED drive power to convert the power supply voltage for a specific current to drive the LED light voltage converter, under normal circumstances: LED drive power inputs including 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. The LED drive power output is mostly with the LED forward voltage value may change while changing the voltage constant current source. LED power supply core components include a switch controller, inductor, switching components (MOSfet), the feedback resistor, the input filter components, output filter components, and so on. Depending on the situation, we should also have input overvoltage protection, input under-voltage protection circuit, LED open circuit protection, overcurrent protection circuit.

　　2, LED drive power features

　　(1) High Reliability

　　Especially as LED street lamp drive power, packed in high altitude, maintenance inconvenience, cost of maintenance is also large.

　　(2) High efficiency

　　LED is energy-saving products, power supply efficiency is higher. For the power knot cooling installation in the fixture it is very important. High-efficiency power supply, its small power consumption, heat in the fixture is small, it reduces the temperature rise of the lamp. LED light failure advantageous to delay.

　　(3) High Power Factor

　　Power factor is the grid load requirements. Usually 70 watts of electrical appliances, there is no mandatory targets. Although it is not a single low-power electrical power factor has little impact on the grid, but at night use lighting capacity, similar load is too concentrated, the grid will produce more serious pollution. For 30 watts to 40 watts of LED drive power, it is said in the near future, perhaps there are certain factors have the power specs.

　　(4) Drive system

　　Now prevailing in two ways: one is a constant source for a plurality of constant current source, a constant current source for each channel individually to each LED power supply. In this way, the combination of flexible, all 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 is "in Section Hui Bao" change adopted drive mode, LED series or parallel. 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 exist for some time. Multi-channel constant current output power supply, in terms of cost and performance will be better. Perhaps the future direction of the mainstream.

　　(5) Surge Protection

　　LED anti-surge capacity is relatively poor, especially against reverse voltage capability. Strengthen the protection of this area is also important. Some LED lights installed in the outdoors, such as LED lights. Since the start grid load rejection and lightning induction, from the grid system will invade a variety of surge, some surge can cause LED damage. Therefore, the analysis "in Section Hui Bao" drive power surge protection should have a certain lack, as for the power supply and frequent replacement lamps, LED drive power surge suppression invasion have the ability to protect LED is not damaged.

　　(6) protection

　　In addition to conventional power protection, the best increase in the constant current output LED temperature negative feedback to prevent LED overheating; to meet safety and electromagnetic compatibility requirements.

　　3, by driving classification

　　(1) constant flow

　　Constant current drive circuit output is constant, and the output DC voltage but with the load resistance of different sizes vary within a certain range, the load resistance is small, the output voltage is low, the greater the load resistance, the output voltage higher;

　　Constant load short circuit is not afraid, but non-load completely open;

　　LED constant current drive circuit drives are ideal, but relatively higher prices;

　　It should be noted that to withstand the maximum current and voltage values to use, it limits the number of LED use.

　　(2) Regulators formula

　　When the voltage regulator circuit parameter determination, the output voltage is fixed, and the current output, but varies with the load increase or decrease;

　　Load open circuit voltage is not afraid, but totally non-load short-circuit;

　　In the regulator drive circuit drive LED, each string only need to add an appropriate resistor so that each string of LED display brightness uniformity;

　　Brightness will be affected by changes in the rectified voltage from the.

　　4, the overall constant current and constant work by road pros and cons

　　Compared with the overall constant current, constant despite shortcomings by road more, the cost is relatively high. But it can really protect and extend the life of the LED LED, so by way of constant current is the future trend.

　　Less than 5, LED power supply

　　LED drive power existing shortage of reasons:

　　Production of LED lighting and related products company's technical staff is not enough understanding of the switching power supply, power supply can be made to work properly, but some of the key considerations evaluation and electromagnetic compatibility is not enough, there are still some risks too;

　　Most LED power production enterprises are transformed from ordinary switching power supply LED power supply do come, characteristics and use of the LED is not enough awareness;

　　Currently on a standard LED little, mostly reference switching power supplies and electronic ballasts standards;

　　Now most of the LED power supply is not unified, so most of them relatively small amount. Small purchases, the price is high, and component suppliers is not very complex;

　　LED power supply stability: wide voltage input, high temperature and low temperature, over-temperature, over-voltage protection problems are not solve them;

　　The first is the overall life of the drive circuit, especially in key components such as: capacitor life at high temperature directly affects the power of life;

　　LED drive should be challenging higher conversion efficiency, especially when driving high-power LED is especially true, because all is not as light output power is dissipated as heat, power conversion efficiency is too low, the impact of LED energy saving effect play;

　　Currently in smaller power (1-5W) applications, constant current drive power cost share is close to 1/3, close to the cost of the light source, to some extent affected the marketing.

　　LED drive two categories of power

　　LED drive power is the power supply voltage is converted to a specific current to drive the LED light voltage converter, under normal circumstances: LED drive power inputs including high frequency alternating current (ie, electricity), low-voltage direct current, high-voltage direct current, low-voltage high frequency AC (eg electronic transformer output) and the like. The LED drive power output is mostly with the LED forward voltage value may change while changing the voltage constant current source.

　　Due to a variety of different LED power supply performance and efficiency vary, so choose a suitable and efficient LED dedicated power supply, can be truly revealing the LED light source of high-performance features. Because of the low efficiency of the LED power supply itself consumes a lot of power, so the power supply to the LED process can not highlight the energy-saving features of the LED. In short, LED power supply LED work stability, energy efficiency, longevity, have an important role.

　　LED power supply which classified it?

　　A, LED power by driving can be divided into two categories:

　　A. Regulator type:

　　1, to determine the parameters of the regulator circuit, the output voltage is fixed, but the current output varies with the load increase or decrease

　　2, the voltage regulator circuit, although not afraid of open load, but completely non-load short-circuit

　　3, the rectified voltage change will affect LED brightness

　　4. For each string display brightness uniformity in the voltage regulator circuit drive LED, the need to add an appropriate resistor can

　　B. Constant formula:

　　1, LED constant current drive circuit drives are ideal, the disadvantage is that higher prices

　　2, the constant current load short circuit although afraid, but non-load completely open

　　3, the output of the constant current drive circuit is constant, but the DC output voltage as the load resistance of different sizes within a certain range

　　4, to limit the number of LED's used, as it has to withstand the maximum current and voltage values

　　Second, LED power supply according to the circuit configuration can be divided into six categories:

　　1, the conventional step-down transformer:

　　The advantage of this power supply is small, the weight of emphasis on the inadequacies of the power efficiency is very low, generally 45% to 60%, because reliability is not high, it is seldom used.

　　2, capacitor step-down:

　　In this way the LED power supply vulnerable to fluctuations in grid voltage, low power efficiency, should not be used when the flashing LED, since the circuit through a capacitor step-down, in the flash use, due to the charge and discharge of the role, the instantaneous current through the LED great, easy to damage the chip.

　　3, the electronic step-down transformer:

　　This lack of power structure at low conversion efficiency, narrow voltage range, generally 180 ~ 240V, moire large.

　　4, the resistance voltage:

　　This power supply is very low power efficiency, and reliability of the system are relatively low. Because the circuit through the resistance voltage, interference from the larger grid voltage changes, not easy to make the power supply, and the dropping resistor itself also consumes much of the energy.

　　5, RCC buck switching power supply:

　　LED power advantages of this approach is relatively wide voltage range, power efficiency is relatively high, generally 70% to 80%, a wider application. The main drawback is the switching frequency is not easy to control, load larger voltage ripple factor, abnormal load conditions and poor adaptability.

　　6, PWM control of switching power supply:

　　For now, PWM control of LED power supply design is ideal, because this output voltage or current switching power supply very stable. High power conversion efficiency, typically up to 80% to 90%, and the output voltage and current is very stable. In this way the LED power supply is mainly composed of four parts which are: input rectifier filter section, the output rectifier filter section, PWM voltage control section, switching energy conversion section. And this has improved the protection circuit, a high reliability of power supply.

　　Nine Features LED drive power requirements

　　According to the rules of the electricity grid and the characteristics of LED drive power requirements, the selection and design of LED drive power to take into account the performance characteristics of the following nine requirements:

　　1. High reliability especially as LED street lamp drive power, packed in altitude, convenient maintenance, cost of maintenance is also large.

　　2. High efficiency LED energy-saving products, power supply efficiency is higher. For structural power installed in the luminaire, it is particularly important. Because the LED's luminous efficiency increases as temperature decreases with LED, the LED's heat is very important. High-efficiency power supply, its small power consumption, heat in the fixture is small, it reduces the temperature rise of the lamp. LED light failure advantageous to delay.

　　3. High Power Factor Power factor is the grid load requirements. Usually 70 watts of electrical appliances, there is no mandatory targets. Although it is not a single low-power electrical power factor has little impact on the grid, but the night we lit, similar load is too concentrated, the grid will produce more serious pollution. For 30 watts to 40 watts of LED drive power, it is said in the near future, perhaps there are certain factors have the power specs.

　　4. Drive traffic now, there are two: one is a constant source for multiple constant current source, a constant current source for each channel individually to each LED power supply. In this way, the combination of flexible, all 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, LED in series or parallel. 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 exist for some time. Multi-channel constant current output power supply, in terms of cost and performance will be better. Perhaps the future direction of the mainstream.

　　5. The anti-surge protection LED surge capacity is relatively poor, especially against reverse voltage capability. Strengthen the protection of this area is also important. Some LED lights installed in the outdoors, such as LED lights. Since the start grid load rejection and lightning induction, from the grid system will invade a variety of surge, some surge can cause LED damage. Therefore, LED drive power surge suppression invasion have the ability to protect LED is not damaged.

　　6. protection in addition to conventional power protection, the best increase in the constant current output LED temperature negative feedback to prevent LED overheating.

　　7. protection aspects outside lighting installation type, the power structure to be waterproof, moisture, shells and Fast.

　　8. The drive power of life to be adapted and LED life.

　　9. To meet safety and electromagnetic compatibility requirements.

　　With the increasingly widespread application of LED, LED drive power performance will be more suitable for LED requirements.

　　Seven principles determine the quality of the LED drive power

　　1, giving up four or more outputs, development, one or two outputs, high current and large current to abandon the development of small current.

　　The more complex of outputs, current interference between different way to solve the high cost, if not solve the high failure rate. Further more the total output current of outputs is greater, and the heat is the main reason for the current, the voltage itself does not lead directly to heat, simply heat and made proportional to the square of the current, i.e. the current increases to the original 2 times, then the heat will increase fourfold, current is increased 3 times the original, the heat will increase to nine times the original. In summary, single or two-output LED lamp power supply failure rate will be much lower.

　　2, intelligent control is one of the advantages of LED lamps, and power is the key to intelligent control.

　　Intelligent Control LED lights and LED tunnel lighting application conditions most obvious effect of the most mature, intelligent control can be at different times, according to the road traffic density to achieve lighting power stepless control, both to meet the application requirements and achieve significant energy savings effect, you can save a lot of money for road authorities. Application of tunnel lighting can not only energy-saving, but also can automatically adjust the brightness according to the tunnel outside the tunnel entrance of brightness, to give the driver a visual transitional phase, in order to ensure safe driving.

　　3, to give up power, super power, select a higher stability of small power supply.

　　Because the greater the power, the greater the heat issue, which parts are more compact, is not conducive to heat, and the temperature is the culprit of a power failure. Furthermore, the development of small power relatively more mature, stability and cost has advantages. In fact, many large power programs have not verified and proven, are hastily implemented projects are experimental products, so failure after another elapsed time. By contrast medium power supply due to the older, more sophisticated technical solutions to.

　　4, cooling and protection are the main external factor of a power failure.

　　Not only the power supply itself will heat will heat lamps, two heat is distributed out how reasonable lighting design engineer must consider the issue, be sure to prevent excessive concentration of heat to form a heat island effect, affecting power life. The use of separate power program is a good choice.

　　5, to maintain viability.

　　Problem Power can not be completely avoided, Chengdu Photovoltaic made towards ease of maintenance principles. Only the replacement of the power supply to do with the replacement of conventional lighting sources so convenient to the user with the happy, even if the power supply is broken, the mood is not too bad, and the mood of the user determines the quality of the LED light manufacturers fate.

　　6, protective properties.

　　Protection issues are also important, moisture penetration may cause a short circuit power, dust will affect the heat on the housing supply, exposure is likely to cause high temperature and aging wires and other components, from the actual use of the experience, the rotation Wiring plug failure rate is high, the majority of leaks cause a malfunction.

　　7, modular design.

　　The modular design has become a trend, we must find a way to power the module integration, that if power can plug a way to resolve maintenance issues, will be welcomed by users, is also required to interface standardized, so that different manufacturers LED lamp power supply can be common.

　　Summary: LED lamp power the best use of split design, while focusing on reliability and life power, even if a little more cost, only from the customer's point of view to design products, enterprises can obtain long-term development.

　　Five principles designed to simplify LED

　　Chip fever

　　This is mainly for the built-in power modulator high voltage driver chip. If the current consumption of the chip is 2mA, 300V voltage is applied on top of the chip, the chip's power consumption is 0.6W, of course, will cause fever chip. Maximum current driver chip from the drive power MOS consumption, a simple formula is I = cvf (consider resistance charging efficiency, the actual I = 2cvf, where c is the capacitance cgs power MOS tube, v is the power MOSFET gate voltage, so in order to reduce chip power consumption, must find ways to reduce the c, v and f. If c, v and f can not be changed, then think of ways to chip's power consumption assigned to off-chip devices, be careful not to introduction of additional power. then simply, is to consider better cooling it.

　　Power tube heat

　　On this issue, it was also seen in the Forum sent a paste. Power tube power is divided into two parts, switching losses and conduction losses. Note that the majority of occasions especially mains LED driver applications, switch damage much larger than the conduction losses. Switching losses and power tube cgd and cgs and chip drive capability and working frequency, so to solve the heating power tube may be resolved from the following aspects: A, not one-sided according to the on-resistance MOS power transistor size to choose, because The smaller the resistance, cgs and cgd capacitance greater. The cgs as 1N60 about 250pF, 2N60 of cgs about 350pF, 5N60 of cgs about 1200pF, the difference is too great, the choice of power tubes, enough on it. B, the rest is the frequency and chip drive capability, here talk about the frequency of. Frequency and conduction losses are proportional, so when the power tube heat, we must first think about is not the frequency of choice is a bit high. To find ways to reduce the frequency of it! Note, however, when the frequency is lower, in order to get the same load capacity, the peak current is bound to become larger or inductance becomes large, which may lead inductance into saturation region. If the inductor saturation current is large enough, you can consider the CCM (continuous current mode) is changed to DCM (discontinuous current mode), so you need to add a load capacitance of.

　　Operating Frequency down

　　This is also the user during commissioning relatively common phenomenon, mainly caused by reduced frequency of two aspects. Small input voltage and load voltage ratio, system interference large. For the former, be careful not to load voltage is set too high, although the load voltage is high, the efficiency will be high. For the latter, you can try the following aspects: a, will then set a minimum current dot; b, wiring clean point, especially sense this critical path; c, the inductor selection of small dots or closed circuit inductor selection ; d, plus RC low-pass filter it, the impact of a little bad, C of consistency is not good, deviation a little big, but it should be enough for lighting. Anyway down no good, only harm, it must be resolved.

　　Choose an inductor or transformer

　　Finally Turning the key, I have not started, only nonsense saturation point affected. Many users response, the same driver circuit, with a production of inductors is no problem with the production of b inductor current becomes small. In such cases, we should take a look at the inductor current waveform. Some engineers do not notice this phenomenon directly adjust the current sense resistor or achieve the desired operating frequency, doing so might seriously affect the life of LED. So, before designing, reasonable calculation is necessary, parameters and debugging parameters if the theoretical difference a bit far, to consider whether the down-and transformer is saturated. Transformer saturation, L becomes smaller, leading to a sharp rise in the peak current incremental transfer delay caused so LED peak current will also increase. Under the premise of constant average current, you can only watch the light fades.

　　LED current size

　　We all know LEDripple too large, then, LED life will be affected, the extent of impact, have not seen what the experts said. LED plant previously asked this data, they say less than 30% are acceptable, but later has not been verified. Recommendations or try to control the small dot. If the thermal solution is not good, then, LED must be derated. Some experts also hope to give specific indicators, otherwise affect LED promotion.

　　LED drive power to common problems

　　For energy saving, LED has been greatly implemented, but there is a power LED are demand-driven, it will directly affect the life expectancy of the LED, and thus how to do a LED power supply LED drive power is portrayed by the weight of the weight. This article analyzes some LED driver power supply problem, engineers expect to be able to supply a little assistance.

　　1, the driving circuit directly affect the LED life

　　He said LED driver includes a digital drive and so on drive types, the digital drive means drive digital circuits, including digital dimmer control, RGB full color changing and so on. Driving means driving analogy analogy circuit, including AC constant current switching power supply, DC constant current control circuit. A drive circuit by the electronic components, including semiconductor elements, resistors, capacitors, inductors, etc. These components are the use life expectancy, any equipment failures would cause failure of the entire circuit may be some function failure. LED use life expectancy is 5-10 million hours, according to 50,000 hours count, successive lights have life expectancy nearly six years. Life expectancy of switching power supply is difficult to reach six years, switching power supply warranty available in the market are usually 2--3 years, to reach 6-year warranty is military grade power supply, the price is 4-6 times the normal power supply, usually lighting plant is difficult to bear. LED lighting problems so much trouble for the driver circuit.

　　2, the heat problem

　　LED is cold light source, the job can not exceed the junction temperature limits must also remain balance when depicted. To depict the entire fixture Thinking beautiful appearance, convenient device, with light, heat, and many other aspects of the problem, to find a balance point in many elements, so all lighting is the best time to carry out .LED lamps not long to learn little experience, many depicting all continuous improvement. Some LED lighting manufacturers used the power of outsourcing outsourcing perhaps, lamps depicting the division of power do not have much to LED heat larger space, the smaller supply of heat to space. Good lighting is often portrayed and then find a suitable power source facilities, which gives the power to bring a certain degree of difficulty matching. Often encountered inside the lamp due to a smaller space and perhaps internal temperature is high and the cost of control is low, can not be assigned to the appropriate power supply. Some LED lamps factory have the power to develop talent, at the beginning of the pre-painted lamps were evaluated simultaneously depicting power, will be able to deal with the above problems. Consideration should be summarized in portraying the heat LED heat dissipation and power supply, all the lamps temperature control, such competence depict better lighting.

　　3, power portrayed the problem

　　a, power portrayed. Although the LED luminous efficiency, but also 80-85% of the heat loss, resulting in the lamp internal temperature 20-30 degrees, if the room temperature of 25 degrees status, there are 45-55 degrees inside the lamp power long time in a high temperature environment Under the job, to ensure that life we must increase the power margin, generally left to the 1.5-2-fold margin.

　　b, component selection. The internal temperature at 45-55 degrees lighting conditions, as well as the internal power supply temperature around 20 degrees, the temperature of the element adjacent to reach 65-75 degrees. Some elements in the high temperature condition parameter drift, and some life will be shortened, so equipment can be selected at a higher temperature long time use, with particular attention electrolytic capacitors and wires.

　　C, is depicted electrical function. Switching power supply LED of the alignment parameters depicted, the resolution is mainly constant size parameters, the LED current brightness, if the bulk current error is large, unevenness in brightness of the entire batch of lamps. And changes in temperature can cause the output current offset. Batch error handling is generally within +/- 5%, ability to ensure the brightness of light together .LED forward voltage drop error, constant current power supply voltage scale drawn to include the size of the LED voltage. Use time division multiple LED series, the smallest drop in series multiplied by the number of lower voltage, maximum voltage drop is multiplied by the number of series as the upper limit voltage, constant current power supply voltage range slightly wider than the size, usually on the lower limits 1-2V left margin.

　　d, PCB layout depicting .LED lamp left power smaller scale (except external power supply), so a higher demand on PCB painted, too much to think about the elements. Security interval to leave enough demand for the input and output power barrier, the primary circuit and the secondary side circuit demand pressure 1500-2000VAC, at least enough to stay on the PCB 3MM interval. If it is metal lamps, the entire power distribution board also ponder some high and safe separation of the housing. If no space is necessary to ensure the safe use of interval condition other ways to ensure that the insulation, such as punch holes in the PCB, add insulation paper, potting insulating plastic. Layout also ponder other heat balance, the heating element is distributed evenly, can not be set to place, to prevent part of the temperature rise. Electrolytic capacitors away from heat, slow down aging, extend the use of life expectancy.

　　e, certification problems. Currently no alignment of LED lighting specifications, the state is studying the development of the relevant parts of the domestic sale of lighting fixtures certified reference standard, the export is to do CE or UL certification, and some reference to an overseas specification LED lamps to do . Therefore, the alignment of this situation, the switching power supply depict these specifications are to be satisfied together more difficult match, we can only demand the alignment is not the same satisfaction is not the same certification.

　　4, the use of parameters.

　　Purchased power in the selection mainly depends on the size of constant voltage and constant current. Current value chosen as the lower side LED current norms. Scale voltage selection should be moderate, and try not to pick a larger scale, to prevent power waste.

　　LED driver power supply circuit of three kinds of protection methods

　　1. LED Circuit protection fuse (tube)

　　Since the fuse is a one-off, and the reaction is slow, ineffective, cumbersome to use, so the fuse is not suitable for LED lamps in the finished product, because LED lights are now mainly in the city of glamor, and lighting project. It requires LED protection circuit to be very harsh: in normal use beyond current protection can be activated immediately, allowing LED power supply path is disconnected, the LED and power supply can be protected in the whole lamp can then automatically restore the normal power supply, It does not affect the LED work. Circuit can not be too complicated volume is not too large, the cost is even lower. So it is difficult to achieve using fuse way up.

　　2. Use transient voltage suppression diode (referred to TVS)

　　Transient voltage suppression diode is a diode in the form of high-performance protection devices. When it is subjected to reverse transient bipolar high-energy shock, can be a negative 12 power of 10 seconds, a very short time the speed, so that high impedance between the poles themselves immediately reduced to low resistance, absorbing up to several kilowatts of power surge , the voltage clamping element between the poles in a predetermined voltage value, the effective protection of electronic circuits, precision components. Transient voltage suppression diodes have fast response times, transient power, low leakage current, breakdown voltage deviation consistency, with clamp voltage is easier to control, no damage to the limit, the volume is small.

　　However, in actual use was found to meet the requirements to look for TVS device voltage value is not easy. LED light bead damage mainly because current Ambassador chip overheating. TVS can detect over-voltage can not detect overcurrent. To select the appropriate voltage protection point difficult to grasp, can not produce such a device it would be difficult to use in practice.

　　3. Select PPTC

　　Since the resumption of fuses, also known as polymer positive temperature thermistor PTC, it is composed of a polymer and conductive particles and the like. After a special processing, conductive particles constituting the conductive path in the polymer chain. When the normal operating current through (or elements in a normal ambient temperature), PTC resettable fuse low resistance state; when the circuit is abnormal over-current through (or ambient temperature), the large current (or ambient temperature) heat generated by the rapid expansion of the polymer, thus cutting off the conductive paths composed of conductive particles, PTC resettable fuse high-impedance state; when the circuit overcurrent (over-temperature condition) disappeared, the polymer cools LED drive power , volume returned to normal, in which conductive particles constitute a conductive path again, PTC resettable fuse and was the initial low resistance state. In normal operation since the resumption of heating a small fuse in abnormal operating conditions of its high heat resistance is enormous, also limits the current through it, which play a protective role. In a specific circuit, you can select:

　　① shunt protection. LED lights are generally divided into many series branch. We can add a PTC element were protected in front of each branch. The advantage of this approach is high accuracy, good reliability of protection.

　　② overall protection. In front of all light beads affixed a PTC element, to protect the whole lamp. The advantage of this approach is simple and does not account for the volume. For consumer products, this results in actual use, the protection is satisfactory.