The LED power supply of the constant current type control IC represented by the 9910 series is actually current-limiting, and the control is relatively simple. Strictly speaking, it does not belong to the mainstream mode of Switching Power Supply control. The mainstream mode of switching power supply control is definitely necessary. Benchmarks and op amps. But this kind of IC can only be used for LED, it is very difficult to be used for other things, just because LED has extremely low requirement to ripple. However, because it is only used for LEDs, the price is higher now. Basic is the use of 9910 plus MOS tube production, output electrolysis, generally I see a lot of people is to use the word inductance to do the power conversion inductance. This kind of power supply, the chip data of the general manufacturer has the picture, basically is the step-down type. I don't have much to say. There are more people who are better at it than I am.
The second is represented by me, that is, the constant current driver of the switching power supply control mode. This kind of switch device is based on the ordinary switching power supply chip. There are many such chips, such as PI's TNY series, TOP series, ST's VIPER12, VIPER22, Fairchild's FSD200, etc., even with only triode or MOS tube RCC, etc. can do it. The benefits are low cost and good reliability. Because ordinary switching power supply chips are not only expensive, but they are all classic products that have been used in large quantities. In fact, such ICs generally integrate MOS transistors and are more convenient than 9910 plus MOS. However, the control methods are more complex. It is necessary to add constant-current control devices, use triodes, or operational amplifiers. Magnetic components can be used with I-shaped inductors, but also with high-frequency transformer with air gap.
I love to use transformers because the cost of the inductor is low, but I think it's not capable of carrying loads. In addition, the sense of adjustment is not flexible. So I think the better choice of devices is that ordinary integrated MOS switching power supply chips plus high frequency transformers are the best choices in terms of performance and cost. There is no need to use any constant current ICs. Not useful, expensive.
Finally, to distinguish between these two types of power supplies, one of the most important methods is to see if the output has an electrolytic capacitor for filtering.
About power supply
Whether it is to do current limiting constant current control power supply, or op amp controlled constant current power supply, we must solve the power supply problem. That is, when the switching power supply chip works, it needs a relatively stable DC voltage to power its chip. The operating current of the chip varies from one MA to several MAs. One such as the FSD200, NCP1012, and HV9910 is a high-voltage self-fed power supply that is convenient to use. However, high-voltage power feeds cause the IC's heat to rise because the IC has to withstand about 300V DC, with only a small amount of current. Even if a MA, there is a loss of 0.3 watts. The general LED power supply, however, is about ten watts. A loss of a few tenths of a watt can pull down the power supply efficiency by several points. There is a typical QX9910. Pull down with a resistor, so that the loss is on the resistor, and it must lose about a few tenths of a watt. There is magnetic coupling, that is, using a transformer, add a winding on the main power coil, just like the auxiliary winding of the flyback power supply, so as to avoid losing the zero-watt power. This is one of the reasons why I did not isolate the power supply and use a transformer. In order to avoid losing the power of a few tenths of a watt, the efficiency will be increased to several points.
Non-isolated buck power supply design method
Non-isolated buck type is a commonly used power supply structure, which accounts for almost 90% of fluorescent lamp power supply. Many people think that they don't isolate the power supply. There is only a step-down type. When they say no, they think of a step-down type, and they think of saying that the lamp is not safe - that is, when the power supply is damaged. In fact, the buck type is only one, there are two basic structures, namely, boost, and buck-boost, that is, BOOST AND BUCK-BOOST, the latter two power even if damaged. Does not affect the LED, there are such advantages.
Now LED fluorescent lamp power supply, the manufacturer of the lamp is generally required to be placed in the lamp tube, such as inside the T8 lamp tube. Few parts are external. Do not know why this is the case. In fact, the built-in power supply is hard to do and the performance is not good. But I do not know why so many people have such requirements. It may be down with the wind. External power supply should be said to be more scientific and more convenient. But I also had to go down with the wind, what the customer wanted, and what I would do. However, it is quite difficult to do a built-in power supply. Because of the external power supply, there is basically no need for shape, and it doesn't matter what shape you want to make. Built-in power supply, can only be made into two kinds, one is the most used, that is to say under the light board, put the light board above.
The following is the power supply, so that the power supply is required to be very thin, otherwise it cannot be installed. And this can only fall down the components, the line on the power supply is only extended. I don't think this is a good idea. However, everyone generally likes this. I will do it. There is less use, put both ends, that is, on both ends of the lamp, so do something better, the cost is lower. I have also done it, basically these two built-in shapes.
Questions about this type of power supply and circuit configuration
My opinion is that since the power supply should be built into the lamp and heat is the biggest killer of LED degradation, the heat must be small, that is, the efficiency must be high. Of course there must be an efficient power supply. For the T8-meter-long lamp, it is best not to use a power supply, but to use two, one at each end, to disperse the heat. Concentrate on one place without heating. The efficiency of the power supply depends mainly on the structure of the circuit and the device used. First of all, the circuit structure, some people also said to isolate the power supply, I think it is absolutely unnecessary, because this kind of thing is originally placed in the lamp body, people simply can not touch. There is no need for isolation because the efficiency of isolated power supplies is lower than non-isolated efficiency. The second is that it is better to output high voltage and low current. Such a power supply can increase efficiency. Now commonly used is the BUCK circuit, the buck circuit. It is best to make the output voltage more than one hundred volts, and the current is set at 100MA. For example, one hundred and twenty drives, preferably three strings, each with forty strings, the voltage is one hundred and thirty volts, and the current is 60MA. This kind of power is used a lot, I just think that there is a little bad, if the switch is out of control, the LED will finish playing. Now LEDs are so expensive. I am more optimistic about the boost circuit, the benefits of this circuit, I repeatedly said, one is more efficient than the buck-type, and second, the power is bad, LED lights will not be bad. This will ensure that nothing is lost. If a single power supply is burned, only a few dollars will be lost and an LED fluorescent lamp will be burned, which will cost more than one hundred yuan. So I have been devaluing the boost power supply. There is, step-up circuit, it is easy to put the PF value high, buck-type on the trouble. The advantages of my absolute booster circuit for LED fluorescent lamps are overwhelmingly stronger than buck-type ones. There is a one-year shortcoming, that is, in the 220V mains input case, the load range is relatively narrow, generally only applicable to 100 to 140 strings or two strings of LEDs, for less than this number, or in the middle, but It is inconvenient to use. But now do LED fluorescent lamps, the general type of 60CM is the use of 100 to 140, one meter two of the kind, is generally used two hundred to two hundred sixty-six, can still be used up. So now LED fluorescent lamp is generally used is not isolated step-down circuit, there is no isolation boost circuit, this circuit is used for LED fluorescent lamps, should be regarded as my first.
Personally, there are many times when these practices are really done. Now, first of all, what are the advantages of LED relative to traditional lamps? First, energy saving, second longevity, and then not afraid of switching, right. However, the high PF methods used today all use passive valley filling PF circuits. The original driving method is 48 strings, 6 strings and 24 strings and 12 strings. In this case, the efficiency will be reduced at 220V. About 5 percent, so LED fluorescent power supply, heat is higher, the lamp beads will be affected a little.
There is also a problem, that is, 24 string and 12 together, will make the LED fluorescent lamp beads become difficult to accept the wiring, and bad wiring. In my opinion, the best way is to use 48 strings in a string, which is mainly because of its high efficiency, low heat, and easy wiring and complexity.
What is more, there are still 24 and 12 strings proposed by people. This method is only suitable for isolating power supplies. It is not applicable without isolation. Some people who do not understand the power supply common sense think that their non-isolated power supply achieves a constant current output of 600MA, which is better than he did. In fact, he did not try to put the lamp in his lamp carefully. So, what is the current low-voltage and high-current LED fluorescent lamp power supply?
Two constant current control methods
The following is a description of two types of switching power supplies in constant current control mode, resulting in two approaches. These two methods, whether it is the principle, the device application, or the performance difference, are quite large.
First of all, the principle. The first type is represented by current constant-current LED special-purpose ICs, such as the 9910 series and AMC7150. All brands that currently play LED constant-current drive ICs are basically this type of device, and they are called constant-current IC type. But I think this kind of so-called constant-current IC does constant current, but the effect is not very good. The control principle is relatively simple, that is, in the primary circuit of the power supply, a current threshold is set. When the primary MOS is turned on, the current of the inductor is linearly rising, and when it rises to a certain value, When this threshold is reached, the current is turned off and the next cycle is triggered by the trigger circuit. In fact, this kind of constant current should be a kind of current limiting. We know that when the inductance is different, the shape of the primary current is different. Although there is the same peak value, the current average value is different. Therefore, when the power supply is generally mass-produced, the consistency of the constant current size is not well controlled. There is a characteristic of this type of power supply. Generally, the output current is trapezoidal, that is, wave-like current. The output is generally not electrolytically smooth. This is also a problem. If the current peak value is too large, the LED will be affected. If the output stage of the power supply does not have a type of power source that is electrolyzed to smooth the current, this is basically the case. That is to say whether it is this kind of control method, it depends on whether the output is electrolytically filtered. This kind of constant current I have always called it a false constant current, because its essence is a kind of current limit, and it is not the constant current value obtained by comparison of op amps.
The second constant current mode should be called switching power mode. This control method is similar to the constant voltage control method of the switching power supply. Everyone knows to use the TL431 as a constant voltage because it has a 2.5 volt reference inside and then uses a resistor divider. When the output voltage is high, or when it is low, a comparison voltage is generated and amplified to control the PWM signal. Therefore, this control method can control the voltage very accurately. This kind of control method requires a benchmark and also requires an op amp. If the benchmark is adequate, if the op amp amplification is large enough, then it is very accurate. Similarly, to do constant current, you need a constant current reference, an op amp, using resistance overcurrent detection as a signal, and then use this signal to amplify, to control the PWM, but unfortunately it is not very easy to find a very accurate reference signal, Commonly used transistors, this benchmark temperature drift, there is the diode can be about 1V conduction value as a benchmark, this can also be, not high, the best is the op amp plus TL431 when the benchmark, but The circuit is complicated. However, in doing so, the constant current power supply is more accurate than the constant current accuracy. This mode of constant current control, the output must be added electrolytic filter, so the output power supply is smooth DC, not pulsating, pulse can not be sampled. So you have to decide which kind of electrolysis you want to see if you have the output.
The two constant current control modes determine the use of two different types of devices, which in turn determines the use of the two circuit devices, their performance, and their costs.
The buck power supply also has its advantages. The main point is that it is suitable for 220, but it is not suitable for 110, because the original 110V voltage is low, even lower, the output current is large, the voltage is low, and the efficiency is not effective. Too high.
Step-down 220V AC, about three hundred volts after rectification and filtering, after the step-down circuit, the voltage is generally reduced to about 150V DC, so that high-voltage and small-current output can be achieved, and the efficiency can be increased. MOS is generally used as a switch, and this type of power supply is my experience. It can be done as much as 90 percent. It is also difficult to go up. The reason is very simple, the chip generally has zero to five W to one W, and the fluorescent lamp power supply is about ten W. So it is impossible to go further. Now that power efficiency is very imaginary, many people are blowing, and they actually can't reach it. Some people often say that 3W's power efficiency is 85%, and it is still isolated.
Tell everyone that even if it is frequency hopping mode, the no-load power consumption is the minimum, but also 0.3W, what also output 3W low pressure, can reach 85%, in fact, 70% is very good, anyway, now Many people bragging do not play drafts, you can flicker layman, but now do not understand the power of LED's. I said that in order to be efficient, first of all do non-isolated, and then output specifications but also high voltage and low current, you can save the conduction loss of power components, so the main loss of such LED power supply, one is the chip's own Loss, this loss is generally a few W to a W like, there is a loss of switching, and switching with MOS can significantly reduce this loss, with transistor switching loss is much larger. So try not to use triodes. There is to do a small power, it is best not to save too much, do not use RCC, because RCC circuit manufacturers generally do not do a good job of quality, its implementation in the chip is also cheap, ordinary switching power supply chip, integrated MOS tube, up to only two yuan Money, no need to save a little bit, RCC only saves material costs, in fact processing and repair costs are higher, but in the end it is not worth the loss.
The basic structure of the buck power supply is to string the inductor and the load into the 300V high voltage. When the switch is switched, the load achieves a voltage lower than 300V. There are many specific circuits and many on the Internet. I do not want to draw a picture again. Now 9910, there are general market constant current ICs are basically implemented using this circuit. But this kind of circuit is when the switch tube breaks down, the entire LED light board is finished, this should be regarded as the worst place. Because when the switch breaks down, the entire 300V voltage is added to the light board. Originally, the light board can only withstand more than 100 volts. Now it has become 300 volts. This happens. The LED must burn. Drop it. So many people say that non-isolated insecurity, in fact, that is buck, just because the vast majority of non-isolated buck, so that non-isolated damage must be bad LED. In fact, the other two basic non-isolated Structure, power damage, will not affect the LED.
Step-down power supply should be designed to be high voltage and low current, efficiency can be high, to elaborate, why? Because of the high voltage and low current, the pulse width of the switching tube current can be made larger, so that the peak current is smaller, and there is less loss to the inductor. Through the circuit structure, it can be known that the circuit is inconvenient to draw, and it is also difficult Then go on. To sum up, the advantage of the buck power supply is that it is suitable for 220 high-voltage input, so that the power device withstands a small voltage stress, suitable for large current output, such as 100MA current, easier than the latter two ways, efficiency To be high. The efficiency is relatively high, the loss of the inductor is small, but the loss of the switch is larger, because all the power through the load must be transmitted through the switch, but the output power, only part of the inductor, such as 300V input, 120 output Step-down power supply, only 300-120, that part of the 180 to go through the inductor, 120 part of the direct conduction into the load, so that the inductor loss is relatively small, but the output power, all through the switch tube conversion.
LED fluorescent lamp has bid farewell to the high price era
What is clear is that LED has already bid farewell to the era of high profits. Many people who originally wanted to take profits in the LED fluorescent industry could not play. I can give you an account.
LED fluorescent farewell to the main sign of the era of high prices, is the price of straw hat lamp beads, and now the brightness reaches more than 2000MCD 120 degree straw hat light (single luminous flux) 6 lumens, 0.2 yuan has been a lot of sales, when the original, the price At least 0.5 yuan. Let me calculate an account and calculate it by the traditional fluorescent lamp consisting of a PC tube and a straw cap LED.
1.2 meters LED fluorescent lamp, 264 beads, 15W LED fluorescent lamp. PC tube, plus light board, plus ordinary LED fluorescent lamp constant current power supply, these three add up, the cost can be up to 30 yuan to get. If the lamp bead is 0.2 yuan, then the maximum is 60 yuan. Adding up the material cost is about 90 yuan. Deducting other things and selling 150 yuan is reasonable. Therefore, LED fluorescent lamps are no longer the era of high prices. In another year, the price will exceed the 100 yuan mark, but it will take some time for marketing. People who do LED lighting must learn to adjust their mindset. The era of profiteering is gone.
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