Chip fever

This is mainly for built-in power modulator high-voltage driver chip. If the chip current consumption of 2mA, 300V voltage added to the chip above, the chip power consumption of 0.6W, of course, will cause the chip heat. The maximum current of the driving chip is derived from the consumption of the driving power MOS tube. The simple calculation formula is I = cvf (consider the charge efficiency of the charge, the actual I = 2cvf, where c is the cgs capacitance of the power MOS tube, v is the power tube conduction When the gate voltage, so in order to reduce the power consumption of the chip, you must think of ways to reduce c, v and f. If c, v and f can not change, then think of the chip power to the chip outside the device, The introduction of additional power. Then a little easier, is to consider a better cooling it.

Power tube heat

On this issue, also seen someone posted in the forum. Power tube power consumption is divided into two parts, switching losses and conduction losses. It should be noted that, in most cases, especially LED mains drive applications, the switch damage is much greater than the conduction loss. Switching losses and power cgd and cgs and the chip drive capacity and operating frequency, so to solve the power tube heat can be resolved from the following aspects: A, can not be based on the size of the on-chip resistance to select the MOS power tube, because The smaller the internal resistance, the larger the cgs and cgd capacitors. Such as 1N60 cgs is about 250pF, 2N60 cgs is about 350pF, 5N60 cgs is about 1200pF, the difference is too big, select the power tube, enough to be on it. B, the rest is the frequency and chip-driven ability, and here only talk about the frequency of the impact. Frequency and conduction loss is also proportional to, so the power tube fever, the first thing to think about is not a bit high frequency selection. Think of ways to reduce the frequency of it! It should be noted, however, that when the frequency is reduced, the peak current must become large or the inductance becomes large in order to obtain the same load capacity, which may cause the inductance to enter the saturation region. If the inductor saturation current is large enough, consider changing the CCM (continuous current mode) to DCM (discontinuous current mode), which requires adding a load capacitor.

Operating frequency down frequency

This is also the user in the debugging process is more common phenomenon, the frequency is mainly caused by two aspects. The input voltage and the load voltage ratio is small, the system interference is big. For the former, be careful not to set the load voltage is too high, although the load voltage is high, the efficiency will be high. For the latter, you can try the following aspects: a, the minimum current set again small points; b, wiring clean point, especially the sense of this critical path; c, the inductance of the choice of small points or use closed magnetic circuit inductance ; d, plus RC low-pass filter it, the effect is a bit bad, C consistency is not good, the deviation is a bit big, but for lighting should be enough. No matter how the frequency is not good, only bad, so be sure to solve.

Inductance or transformer selection

Finally talked about the focus, I have not started, only blindly said the impact of saturation. Many users react, the same drive circuit, with a production of the inductor is no problem, with b production of the inductor current becomes smaller. In case of this, look at the inductor current waveform. Some engineers did not notice this phenomenon, directly adjust the sense of resistance or operating frequency to achieve the required current, this may seriously affect the LED life. So, before the design, a reasonable calculation is necessary, if the theoretical calculation of the parameters and debugging parameters are a bit far worse, to consider whether the frequency down and the transformer is saturated. When the transformer is saturated, L becomes smaller, resulting in a sudden increase in the peak current increment caused by the transmission delay, and the peak current of the LED is also increased. In the average current unchanged under the premise, can only look at the light failure.

LED current strength

We all know LED ripple too large, LED life will be affected, the impact of how much, have not seen which experts said. Previously asked the LED factory this data, they said that 30% can be accepted, but later did not verify. Suggestions or try to control small points. If the heat to solve the bad, then the LED must be used in derating. Also hope that experts can give a specific indicator, or else affect the promotion of LED.

In order to save energy and energy, LED has been a great implementation, but the LED are required to have a power supply drive, its good or bad will directly affect the LED life, and thus how to do a LED driver power is the focus of the LED power source weight. This paper analyzes some of the LED drive power problems, hoping to provide some assistance to the engineers.

1, the drive circuit directly affects LED life

We said that the LED driver, including digital-driven and analog push-driven two, digital- driven index word circuit driver, including digital dimming control, RGB full color changes and so on. Analog push refers to the analog circuit driver, including AC constant current switching power supply, DC constant current control circuit. The drive circuit consists of electronic components, including semiconductor components, resistors, capacitors, inductors, etc., these components have the use of life, any failure of a device will cause the failure of the entire circuit may be some function failure. LED use of life is 5-10 million hours, according to 50,000 hours count, a series of light, nearly 6 years of life. Switching power supply life is difficult to reach 6 years, the market to sell the switching power supply warranty period is usually 2-3 years to reach 6 years warranty power supply is military grade, the price is usually 4-6 times the power supply, usually The lighting factory is very difficult to bear. So LED lighting problems for the drive circuit problems.

2, heat problems

LED for the cold light source, operating junction temperature can not exceed the limit, the description will have to leave the remaining margin. The design of the entire lamp to consider the appearance of beautiful, easy to install, with light, heat and many other problems, to find a balance in many elements, so that all the lamps is the best.LED lamps are not long to develop The experience is not much, many depictions are constantly perfect. Some LED lighting manufacturers use power for outsourced perhaps outsourced, lamp depicts the power to know little, to the LED cooling space larger, to the power of the cooling space smaller. Usually painted after the light and then find the appropriate power supply to match, so give the power supply to bring a certain degree of difficulty. Often encountered due to the smaller interior space of the lamp may be higher internal temperature, and the cost of manipulation is low, can not be equipped with the appropriate power. Some LED lighting factory has power development talent, in the beginning to describe the early evaluation of the lamp, the power of the description of the synchronization, will be able to deal with the above problems. In the description of the LED to sum up the heat and power of the heat, all control the temperature rise of lamps, so that the ability to draw a better lamp.

3, the power to describe the problem

A, power depiction. Although the LED light efficiency, but there are 80-85% of the heat loss, resulting in 20-30 degrees inside the lamp temperature rise, if the room temperature of 25 degrees, the lamp has 45-55 degrees, the power supply for a long time in high temperature environment Under the operation, to ensure that life must increase the power margin, usually left to 1.5-2 times the margin.

B, component selection. Lamp internal temperature 45-55 degrees situation, the internal temperature rise is still about 20 degrees, then the component adjacent to the temperature to reach 65-75 degrees. Some components in the high temperature conditions will drift parameters, and some life will be shortened, so the equipment to be selected at a higher temperature for a long time to use, pay special attention to electrolytic capacitors and wires.

C, electrical function drawing. Switching power supply parameters of the LED alignment, mainly constant current parameters, the size of the resolution of the brightness of the LED, if the bulk current error is large, the brightness of the entire batch of light is not uniform. And the temperature change can also cause the power supply output current offset. Usually within the volume error control within +/- 5%, to ensure that the brightness of the lamp common. LED forward voltage drop error, the power to describe the constant current voltage scale to include the LED voltage scale. Multiple LED series using the time division, the minimum voltage drop multiplied by the serial number for the lower limit voltage, the maximum voltage drop multiplied by the serial number for the upper limit voltage, the power supply constant current voltage scale slightly wider than this, usually the upper and lower limits of the Stay 1-2V margin.

D, PCB layout .LED lamps left to the power of the smaller scale (unless the power is external), so the PCB design on the higher demand, to think more elements. The safety interval to stay enough, the demand input and output barrier power supply, the primary circuit and the secondary side of the circuit requires voltage 1500-2000VAC, PCB at least enough to stay 3MM interval. If the metal shell of the lamp, the entire power supply board also think about some of the high pressure and the safety of the shell interval. If there is no room to ensure that the safety interval conditions will use other methods to ensure insulation, such as punching on the PCB, plus insulation paper, potting insulation, and so on. Other cloth also think about heat balance, heating elements to be evenly distributed, can not be set to prevent some of the temperature rise. Electrolytic capacitors away from heat, slow aging, extending the use of longevity.

E, certification problems. At present, China has not yet aligned with the norms of LED lamps, the relevant part of the country is being developed, the domestic sales of lamps and lanterns is a reference to the norms of lighting, export CE or UL certification is done, and some reference to foreign LED lighting specifications to do The So the alignment of this situation, the description of switching power supply to be satisfied with the above specifications are more difficult, we can only match the different needs of different certification is not the same.

4, the use of parameters.

Outsourcing power supply in the selection of the main constant current and constant current voltage scale. The constant current value is selected as the specification of the LED current drop. Voltage scale selection should be moderate, try not to choose a larger scale, to prevent the power of the spoil

1. Protection of the use of fuses in the LED circuit (tube)

As the fuse is a one-time, and the reaction is slow, the effect is poor, the use of trouble, so the fuse is not suitable for LED lamp products, because the LED lights are now mainly in the city of glorious engineering and lighting works. It requires the LED protection circuit to be very harsh: beyond the normal use of current can immediately start the protection, so that the LED power supply path was disconnected, so that LED and power can be protected in the normal light after the normal power supply can be restored, Does not affect the LED work. The circuit can not be too complicated volume can not be too much, the cost is even lower. So the use of the fuse to achieve it is very difficult.

2. Using a transient voltage suppression diode (referred to as TVS)

Transient voltage suppression diodes are a high-performance protection device in the form of diodes. When its poles are subjected to a reverse transient high-energy impact, the high resistance between their poles can be reduced to a low resistance at a rate of 10 times the negative time of 10 times the square, absorbing up to several kilowatts of surge power , The voltage between the two poles at a predetermined voltage value, the effective protection of the electronic circuit in the precision components. Transient voltage suppression diode has the advantages of fast response time, transient power, low leakage current, good consistency of breakdown voltage, easy control of clamping voltage, no damage limit, small size and so on.

However, it is not easy to find a TVS device that meets the required voltage value in actual use. The damage to the LED beads is mainly due to the overcurrent of the chip. TVS can only detect overvoltages can not detect overcurrent. To choose the appropriate voltage protection point is difficult to grasp, this device can not produce it is difficult to use in practice.

3. Select the self-healing fuse

Self-healing fuses, also known as polymer polymer temperature thermistor PTC, is composed of polymer and conductive particles. After special processing, the conductive particles in the polymer to form a chain conductive path. When the normal operating current passes (or the component is at normal ambient temperature), the PTC self-recovery fuse is in a low- impedance state; when there is an abnormal overcurrent in the circuit (or ambient temperature rises), the high current (or ambient temperature rises) The heat generated by the rapid expansion of the polymer, also cut off the conductive particles formed by the conductive path, PTC self-recovery fuse was high resistance state; when the circuit over- current (over-temperature state) disappeared, the polymer cooling LED drive power , the volume returned to normal, in which conductive particles and re- constitute the conductive path, PTC self-recovery fuse was the initial low resistance state. In the normal working state from the recovery of the fuse is very small, in the abnormal working state of its high heat resistance is great, it limits the current through it, which played a protective role. In the specific circuit, you can choose:

① Shunt protection. General LED lights are divided into many tandem branches. We can in front of each branch plus a PTC element were protected. The benefits of this approach are high accuracy and good reliability of protection.

② overall protection. In front of all the beads plus a PTC component, the protection of the entire lamp. The benefits of this approach are simple, not the volume. For civilian products, the results of this protection in practical use are satisfactory.

LED will certainly become the fourth generation of lighting, its development momentum more and more fierce, the main advantages are: energy saving, long life, no flicker does not hurt the eyes, do not attract mosquitoes. High-power LED lighting as long as the thermal design is good enough, 3-5 million hours of life is no problem (life is defined as: LED because it is a semiconductor component, the chip affected by the temperature will produce light failure, when the LED luminous flux because Light attenuation of less than 30% of the initial luminous flux, the basic LED lighting is no lighting significance, that is, life summary, although this time the LED lights can still light it, but the brightness has dropped significantly, so LED lighting is very Important is important).

You said the LED lights out of the object will be color cast, because the spectrum is missing a color, which is the wrong idea. Very simple reason, color TV (CRT) is not only three primary colors (red, green, blue) mixed to get a variety of colors. Lighting is the use of high-power white LED more, mainly blue-chip excitation phosphor mixed white light. I want you to say that the object will be exposed to color, mainly caused by two factors:

1. The LED color rendering index used is too low, causing the emitted light to not reflect the true color of the object. (Light source on the object's color rendering capacity known as color, color rendering index Ra up to 100, the best color) The current LED color rendering index of 80-85, then, has been very good, can be used indoor lighting.

2. The LED color temperature used is not good, the simple understanding can be considered the color of light. Color temperature is low, then the light will be out of the green partial red, too high will be blue, are not suitable for indoor lighting. So the indoor lighting on the LED lamp color temperature is a very important parameter. LED generally defined color temperature in the 2700K-4000K for the warm, 4000K-5000K for the natural white, 5000K-6700K for the white, warm white and natural white temperature will give a warm, warm feeling, suitable for use in hotels, hotels, and bedroom. While the white is to give people cool cool feeling, for the office, living room lighting more (fluorescent light is generally more of this color temperature).

Brightness equivalent to 100W energy-saving lamps, it depends on the luminous effect of the LED lamp used (lumens / watt, lm / w), the same wattage, the higher the luminous efficiency of LED lamp, the higher the brightness. General LED lighting efficiency of 70lm / w, as long as there are 50-60W will be able to more than 100W energy-saving lamp brightness.

Another indoor lighting I really have not seen to 100W energy-saving lamp brightness, At least our company is not producing more than 60W LED indoor, Finally to say is to buy LED lamps to pay attention to see light effect, color temperature, color rendering index, light failure. The first three are common parameters of all LED lamp, the last light failure is LED as a unique property of the semiconductor.

Also in the choice of LED lamp when some people have the following errors!

Misunderstanding 1, the actual life expectancy is too high.

LED manufacturers expect LED life to reach 100,000 hours MTBF (The average time between failures, the standard used by traditional lighting manufacturers to measure the life of the light source). But, like all basic light sources, like LED The luminous flux also fades with time. So although the LED light-emitting time can be very long, MTBF is not the only consideration to determine the service life. The attenuation of LED lumens is affected by many environmental conditions such as ambient temperature, humidity and ventilation. Lumen fading is also affected by control, thermal management, current levels and many other electrical design considerations (cited above for US CK).

Many salespeople in marketing LED for some reason may give customers to provide some fuzzy information, resulting in misleading, such as LED life of about 100,000 hours, where the 100,000 hours is the manufacturer expected LED life. As the light source of the light, the brightness of the lamp has a process of gradual decay and darkening (such as fluorescent lamps) , Energy-saving lamps), this process referred to as light failure. In the application-level LED, the light failure of more than 50%, the human eye can clearly feel the brightness of the change, then the product brightness we think is not suitable for lighting. Many manufacturers and engineers in the use of low-cost LED three months to six months or so, often found that the brightness of LED is not as bright as the installation of light, or even almost no, it is because the LED lamp failure did not take into account the reasons. In summary, we have to pay attention to the purchase of LED light decay of its speed, rather than the use of time.

Misunderstanding 2, the actual angle of light as an effective angle.

LED light angle is divided into effective angle and the actual light angle. The luminous intensity value is half the direction of the axial intensity and the direction of the light-emitting axial (normal) is the effective angle. Half of the half angle of the angle (or half power angle) is the actual light angle. In the axial strength of half the angle because the light is too weak in practical applications do not count the effective angle. (The picture below shows the side view of the piranhas particles, the angle between the solid line and the dashed line is not included in the effective angle)

So we should buy products in the product should pay attention to the actual angle of light, calculate the number of products used in the project, the actual luminous angle prevail, effective light angle can be used as a reference value.

Misunderstanding 3, the greater the power the higher the brightness.

The brightness of the LED is measured by the luminous intensity. The luminous intensity refers to the luminous intensity in the direction of the normal (for the cylindrical tube), that is, the luminous flux emitted by the unit solid angle, in candlelight (Candela, cd). As the general luminous intensity of LED is small, so the luminous intensity commonly used in Mendra (mcd) as a unit. In general, the light source will be different directions to different intensity of its luminous flux, in a specific direction of the unit angle generated by the visible light radiation intensity is called light intensity, referred to as axial brightness.

From the force in the same time to do the power of the number of the introduction of the concept of electric power, the current work fast, electric power is large; current work slow, electric power is small, or in the same time, the more power to do more power, On the LED, not the greater the power, the higher the brightness of the product. To the United States and the United States electronic light red LED, for example, axial brightness of 1200mcd, the current is 40ma, power 0.48W; three lights white LED at the same brightness, the current is 18ma, power is only 0.24W; The power of the two kinds of light at the same brightness is different. So when we buy LED to pay attention to its axial brightness, rather than power.

Misunderstanding 4, on the white LED module purchase.

1. High brightness (blue and green) white LED is very sensitive to static electricity, is determined by the characteristics of the LED itself. Exceeding the maximum forward DC current and maximum reverse voltage that it can allow, it may cause damage or breakdown of the LED. Such as three-lamp LED module, each lamp will have an anti-static resistance, coupled with the resistance for the current limit, a total of four. Anti-static resistance can effectively inhibit the electrostatic damage to the LED.