The LED drive power source converts the power supply into a specific voltage and current to drive the LED light-emitting voltage converter. Usually, the input of the LED drive power source includes high-voltage power frequency AC (ie, mains), low-voltage DC, high-voltage DC, low-voltage high-frequency. AC (such as the output of an electronic transformer). The output of the LED driver is mostly a constant current source that changes the voltage as the LED's forward voltage drop changes. The core components of the LED power supply include a switch controller, an inductor, a switching component (MOSfet), a feedback resistor, an input filter component, an output filter component, and the like. According to the requirements of different occasions, there are also input overvoltage protection circuit, input undervoltage protection circuit, LED open circuit protection, overcurrent protection and other circuits.
LED drive power features(1) High reliability
Especially like the driving power supply of LED street lamps, it is installed at high altitude, the maintenance is inconvenient, and the maintenance cost is also large.
(2) High efficiency
LEDs are energy-saving products, and the efficiency of driving power supplies is high. It is very important to dissipate heat from the power supply installed in the fixture. The power supply has high efficiency, its power consumption is small, and the heat generated in the lamp is small, which reduces the temperature rise of the lamp. It is beneficial to delay the light decay of LEDs.
(3) High power factor
The power factor is the grid's load requirements. Generally, there are no mandatory indicators for electrical appliances below 70 watts. Although the power factor of a single power consumer with low power has little effect on the power grid, the amount of lighting used at night is large, and the similar load is too concentrated, which will cause serious pollution to the power grid. For 30 watts to 40 watts of LED driver power, it is said that in the near future, there may be certain indicators for power factors.
(4) There are two types of driving methods: one is a constant voltage source for multiple constant current sources, and each constant current source supplies power to each LED separately. In this way, the combination is flexible, and all the LED faults do not affect the work of other LEDs, but the cost will be slightly higher. The other is direct constant current power supply, which is the driving mode adopted by “Zhongke Huibaoâ€. The LEDs operate in series or in parallel. Its advantage is that the cost is lower, but the flexibility is poor, and it is necessary to solve a certain LED failure without affecting the operation of other LEDs. These two forms coexist for a while. Multi-channel constant current output power supply mode will be better in terms of cost and performance. Perhaps it is the mainstream direction in the future.
(5) Surge protection
The ability of LEDs to withstand surges is relatively poor, especially against reverse voltage capability. It is also important to strengthen protection in this area. Some LED lights are installed outdoors, such as LED street lights. Due to the start of the grid load and the induction of lightning strikes, various surges will be invaded from the grid system, and some surges will cause LED damage. Therefore, the analysis of the "Zhongke Huibao" drive power supply should have a certain lack of surge protection, and as the power supply and lamps are frequently replaced, the LED drive power supply should have the ability to suppress the intrusion of surges and protect the LED from damage.
(6) Protection function
In addition to the conventional protection function, the power supply preferably increases the LED temperature negative feedback in the constant current output to prevent the LED temperature from being too high; it must meet the requirements of safety regulations and electromagnetic compatibility.
(1) Constant current type
The output current of the constant current drive circuit is constant, and the output DC voltage varies within a certain range according to the magnitude of the load resistance. The load resistance is small, the output voltage is low, the load resistance is larger, and the output voltage is also The higher the constant current circuit is not afraid of the load short circuit, but the load is completely open; the constant current drive circuit drives the LED is ideal, but relatively high price; should pay attention to the maximum withstand current and voltage value used, it limits The number of LEDs used.
(2) Voltage regulation
When the parameters in the voltage regulator circuit are determined, the output voltage is fixed, and the output current changes with the increase or decrease of the load; the voltage regulator circuit is not afraid of the load open circuit, but the load is completely short-circuited; The circuit drives the LEDs, and each string needs to be coupled with a suitable resistor to make each string of LEDs display brightness average; the brightness is affected by the rectified voltage changes.
Seven principles for determining the quality of LED drive power1. Give up 4 or more outputs, develop single or two outputs, abandon large currents and large currents, and develop small currents.
The more the number of output channels is more complicated, the current interference between different outlets is solved, and the cost is high. If not solved, the failure rate is high. In addition, the more the number of output channels, the larger the total output current, and the current is the main cause of heat generation. The voltage itself does not directly cause heat. Simply speaking, the heat is proportional to the square of the current, that is, the current increases to the original. 2 times, the heat will increase to 4 times, the current will increase to 3 times, and the heat will increase to 9 times. In summary, the single- or two-output LED lamp power 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 is the most mature condition in LED street light and LED tunnel light lighting application. Intelligent control can realize stepless control of lamp power according to road traffic density in different time periods, which not only meets application requirements, but also realizes huge energy saving. The effect can save a lot of money for the highway supervisory unit. The application in tunnel lighting not only saves energy, but also automatically adjusts the brightness of the tunnel entrance and exit according to the brightness outside the tunnel, providing the driver with a visual over-stage to ensure driving safety.
3, give up high power, super high power, choose a medium and small power supply with higher stability.
Because the greater the power, the greater the heat, the more compact the components inside, which is not conducive to heat dissipation, and the temperature is the culprit of power failure. Moreover, the low-power power supply is relatively mature in development, and has advantages in terms of stability and cost. In fact, many high-power power solutions have not been time-tested and proven in practice. They are all rushed projects, all of which are experimental products, so the faults are endless. In contrast, small and medium-sized power supplies have matured due to their earlier development.
4. Heat dissipation and protection are the main external factors of power failure.
Not only the power supply itself will heat up, but also the lamps will heat up. How to properly dissipate these two heat sources is a problem that the lighting design engineer must consider. It is necessary to prevent excessive heat concentration, form a heat island effect, and affect the life of the power supply. A split power solution is a good choice.
5. Feasibility of maintenance.
The problem of power failure cannot be completely avoided. Chengdu Chaoyue Optoelectronics proposed the principle of maintenance simplicity. Only when the replacement of the power supply is made as simple as the replacement of the light source of the conventional illumination can the user be happy, even if the power supply is broken, the mood will not be too bad, and the user's mood determines the LED lamp manufacturer. fate.
6, protection performance.
The protection problem is also very important. The penetration of moisture may cause short circuit of the power supply. The dust on the outer casing will affect the heat dissipation of the power supply. Exposure will easily cause high temperature and aging of wires and other components. From the experience in actual use, the rotation The failure rate of the wiring plug is high, and most of them are faults caused by water leakage.
7, modular design.
Modular design has become the trend of today. It is necessary to find a way to integrate the module power supply. If the power supply can solve the maintenance problem by plugging and unplugging, it will be welcomed by users. At the same time, it is necessary to establish interface standardization and let LEDs of different manufacturers. The lamp power supply can be used universally.
1, LED drive circuit directly affects LED life
The LED driver we refer to includes digital drive and analog drive. Digital drive refers to digital circuit drive, including digital dimming control, RGB full color change. Analog driving refers to analog circuit driving, including AC constant current switching power supply, DC constant current control circuit. The driving circuit is composed of electronic components, including semiconductor components, resistors, capacitors, inductors, etc. These components have a service life, and any failure of the device may cause the failure of the entire circuit or some functions may be invalid. The service life of LEDs is 50,000-100,000 hours. According to 50,000 hours, they are lit up successively and have a life span of nearly 6 years. The life of the switching power supply is difficult to reach for 6 years. The warranty period of the switching power supply sold in the market is usually 2-3 years. The power supply that reaches the 6-year warranty is military-grade. The price is 4-6 times that of the normal power supply. Usually, the price is usually 4-6 times. The lighting factory is hard to bear. Therefore, the faults of LED lamps are mostly driven circuit faults.
2, LED drive power cooling problem
The LED is a cold light source, and the operating junction temperature cannot exceed the limit, and a certain margin must be left when drawing. The whole luminaire should be considered in terms of beautiful appearance, convenient installation, light distribution, heat dissipation and many other problems. It is necessary to find a balance among many elements, so that all the lamps are the best. LED lamps are not developed for a long time. There are not many experiences, and many depictions are constantly improving. Some LED luminaire manufacturers use the power supply for the outsourcing or outsourcing. The luminaire painter knows little about the power supply, and the heat dissipation space for the LED is large, and the heat dissipation space for the power supply is small. It is usually necessary to draw a suitable power supply to match the lighting, so that it will bring certain difficulties to the power supply. It is often encountered because the internal space of the luminaire is small or the internal temperature is high, and the cost control is low, and it is not suitable for a suitable power source. Some LED luminaire factories have the ability to develop power. At the beginning, the luminaires are evaluated in the early stage, and the power supply is drawn simultaneously, which can handle the above problems. In the depiction, it is necessary to summarize the heat dissipation of the LED and the heat dissipation of the power supply, and the overall temperature rise of the control lamp can be used to describe a better lamp.
3, the problem of LED drive power portrayal
a, power depiction. Despite the high luminous efficiency of the LED, there is still 80-85% of the thermal energy loss, resulting in a temperature rise of 20-30 degrees inside the lamp. If the room temperature is 25 degrees, the inside of the lamp has 45-55 degrees, and the power supply is always in a high temperature environment. In order to ensure the longevity, it is necessary to increase the power headroom, usually leaving a margin of 1.5-2 times.
b, component selection. Under the condition that the internal temperature of the lamp is 45-55 degrees, the internal temperature rise of the power supply is about 20 degrees, and the temperature adjacent to the component should reach 65-75 degrees. Some components will drift in the high temperature condition, and some life will be shortened, so the equipment should be selected for use at higher temperatures for a long time, paying special attention to electrolytic capacitors and wires.
c, electrical function depiction. The switching power supply is aligned with the parameters of the LED, mainly the constant current parameter. The current is determined by the brightness of the LED. If the batch current error is large, the brightness of the whole batch of lamps is not uniform. And temperature changes can also cause the power supply output current to shift.
e, certification problems. At present, there is no specification for LED lamps in China. The relevant parts of the country are being researched and formulated. The lamps and lanterns sold in China are based on the specifications of lighting fixtures. The export is done with CE or UL certification, and some are based on foreign LED lamp specifications. . Therefore, in order to align this situation, it is more difficult to describe the switching power supply together with the above specifications. We can only align with different requirements and satisfy different certifications.
4, LED drive power supply parameters.
The selection of the externally driven power supply mainly depends on the voltage scale of the constant current and the constant current. The constant current value is chosen to be the standard current of the LED. The selection of the voltage scale should be moderate, try not to choose a larger scale to prevent the dross of power.
1. Protect the LED driver power supply with a fuse (tube)
Since the fuse is disposable and the reaction speed is slow, the effect is poor, and the use is troublesome, the fuse is not suitable for use in the finished LED lamp, because the LED lamp is now mainly in the glory engineering and lighting engineering of the city. It requires the LED protection circuit to be very demanding: the protection can be started immediately when the current is exceeded, the LED power supply path is disconnected, the LED and the power supply can be protected, and the power can be automatically restored after the entire lamp is normal. Does not affect the LED work. The circuit should not be too complicated, the volume should not be too large, and the cost should be low. Therefore, it is very difficult to implement the fuse.
2, LED drive power supply uses transient voltage suppression diode
The transient voltage suppression diode is a high efficiency protection device in the form of a diode. When its two poles are subjected to reverse transient high energy impact, the high resistance between the two poles can be immediately reduced to low resistance at a very short time of 10 minus 12 power seconds, absorbing up to several kilowatts of surge power. The voltage clamp between the two poles is at a predetermined voltage value, which effectively protects the precision components in the electronic circuit. The transient voltage suppression diode has the advantages of fast response time, large transient power, low leakage current, good uniformity of breakdown voltage, easy control of clamp voltage, no damage limit, and small volume.
However, in actual use, it has been found that it is not easy to find a TVS device that satisfies the required voltage value. The damage of the LED beads is mainly caused by overheating inside the chip due to excessive current. TVS can only detect overvoltages and cannot detect overcurrents. It is difficult to master the proper voltage protection point, and the device cannot be produced and is difficult to use in practice.
3, LED drive power supply selects self-recovery fuse
The self-recovering fuse is also called a polymer polymer positive temperature thermistor PTC, and is composed of a polymer and conductive particles. When the normal operating current passes (or the component is at normal ambient temperature), the PTC resettable fuse is in a low-resistance state; when there is an abnormal overcurrent in the circuit (or the ambient temperature rises), the large current (or the ambient temperature rises) The generated heat causes the polymer to expand rapidly, which cuts off the conductive path formed by the conductive particles. The PTC self-recovery fuse is in a high-resistance state; in normal working state, the self-recovering fuse has little heat, and it is in an abnormal working state. The high heat resistance is very large, which limits the current through it, thus protecting the LED drive power.
In order to solve the problem of high failure rate of LED driving power supply and difficult maintenance, through the analysis of LED lighting principle and power demand, combined with the current practical application, we try to adopt low-voltage DC power supply mode in LED road lighting. The DC power supply not only reduces the LED drive power failure rate, but also reduces the safety risks of road lighting and facilitates the charging of electric vehicles in the future.
With the continuous development of light-emitting diode (LED) technology, LED lighting gradually expands from indoor to outdoor. The reason for the slow promotion of LED in the field of road lighting is the high power of road lighting and the harsh operating environment. After a period of tracking and detection of high-power LED street lamps, some LED lamps have failed one after another. Through the analysis of the fault, we found that the proportion of LED drive power damage is as high as 90%. Although the theoretical life of LED street lamps is up to 50,000 hours (13.7 years), the life of the drive circuit is short, about 12,000 hours (3 years). The drive power becomes a short board that limits the service life of the LED street light. At the same time, because the LED driving power supply needs to match the LED particles, there is no uniform standard. The output interfaces of the driving power supplies produced by various suppliers are not uniform, the quality is not uniform, which brings inconvenience to the maintenance of the LED street lamps, and the cost of replacing the driving power supply is high.
LED driver power is included in the national 3C compulsory certification category. Power problems have become an important factor affecting the promotion and application of LED lamps. Only by solving the LED power problem can the LED lighting be used in road lighting.
First, the requirements of LED particles for power supply
In order to solve the LED power problem, we need to understand the basic working principle of LED particles and their power supply requirements.
At present, the LED lamps used in road lighting are an overall lighting structure, including an LED light source and a power supply. The LED light source is mixed by a certain number of high-power LED particles (first stringed and then combined) into an integral light-emitting chip. A single LED is actually a diode. When a certain forward voltage is applied across the diode to excite the PN junction to conduct current, the LED can emit light. The nominal voltage of a single LED is 3.4V ± 0.2V (the actual operating voltage is about 2.8 ~ 3.8V). The operating current is related to power and brightness. LEDs of different powers have different currents. Generally speaking, the higher the power, the higher the current, the more light is emitted. The high-power 1W LED particles used in road lighting have a nominal current of 350mA.
Through the structural analysis of the actual application of LED luminaires, we can clearly see that the LED luminaires are connected in series by a certain number of LED granules to obtain a LED string with an operating voltage of 40.8V±2.4V, and then parallelize these LED strings to obtain one. LED lamps with a working current of 3.5A. To calculate the loss, the luminaire requires a power supply of 48V/3.5A.
Second, LED drive power
The existing street lamp power supply line is 220V AC power, and must be stepped down, rectified, and stabilized in three steps to provide a stable low-voltage DC power supply for the LED lamp. Firstly, the 220V AC is stepped down to 48V low-voltage AC, and then the low-voltage AC is converted into a low-voltage DC by bridge rectification, and then converted into a constant current source through a high-efficiency switching regulator to provide constant LED particles. Current.
In order to reduce the chip failure rate, most manufacturers choose a combination of fewer strings and more. The voltage requirements of existing LED lamps are mostly 48V. The voltage and current requirements of each LED lamp may be slightly different. In practical applications, it should be based on the whole. Select the appropriate drive power for the voltage and current of the lamp.
Third, power failure analysis
Since the LED particles and the power source are distributed together in a small space of the lamp, in order to achieve the moisture-proof and waterproof requirements, the power source must be sealed, but the problem of heat dissipation is also caused. In summer, the temperature of the LED light source can reach more than 70 degrees. LED power supplies operate in this high temperature environment and their lifetime is greatly reduced. At the same time, due to the overall sealing, the damaged parts cannot be replaced on site, and can only be replaced as a whole, which is not only difficult to repair but also expensive. This is one of the reasons why the use of units is not enthusiastic about LED street lights.
Fourth, DC power supply mode
Since the problem of heat dissipation of the drive power source scattered in each street lamp fixture cannot be solved, why can't the dispersed drive power source be transferred from the lamp to the ground and concentrated in the transformer or switch box on the ground for uniform processing. The DC transformer concentrated on the ground not only has high conversion efficiency, but also has controllable temperature and is also easy to maintain. Once the LED street lamp realizes the low-voltage DC power supply mode, it will also provide safety for the street lamp, and it can also provide conditions for charging the electric vehicle in the future.
Five, technical approach
Existing equipment can be used to a large extent for existing LED street lights. At the end of the original transformer (or at the switch box), the transformer and rectification equipment are added, and 48V low-voltage DC power is output through voltage transformation, rectification, and filtering, and the LED street lamp is powered by the original cable (Example 4 & TImes; 25mm2 1 & TImes; 16mm2).
For the new and rebuilt LED street lights, the design of the distribution control lines can be further optimized. The high-power rectifier is used to centrally rectify 220V/380V AC to obtain a standard DC power supply, and directly provide low-voltage DC power for the LED street light. Therefore, the driving power supply of the traditional LED street lamp can be removed, and the DC power supply can be directly provided by the street lamp power distribution control. The structural design of the LED street lamp can be simplified and the reliability is correspondingly increased.
Sixth, the advantages of DC power supply
1. Solve the problem of short service life of current LED street lamp lighting power supply;
2. Improve the overall lighting efficiency of LED lamps (currently, domestic and European countries will contain light sources and power supplies, and the whole lighting effect will be used as an energy-saving detection indicator for LED lamps), and at the same time concentrate on step-down rectification to reduce system energy consumption;
3. Reduce the safety risks in the operation of street lighting facilities (in China, the AC power below 50V is a safe voltage, and the DC voltage is safer than AC in actual use);
4, LED street light drive power is removed, greatly reducing the point of failure, not only easy to maintain and the cost is greatly reduced;
5. The street lamp DC power supply on both sides of the road can facilitate the charging of electric vehicles and battery cars.
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