The second is based on me, that is, the constant current driver of the switching power supply control mode. This is the common switching power supply chip as the core conversion device, such a lot of chips, such as PI's TNY series, TOP series, ST's VIPER12, VIPER22, Fairchild's FSD200, etc., even using only triode or MOS tube RCC, etc., can be done. The benefits are low cost and good reliability. Because the ordinary switching power supply chip is not only good price, but also a classic product that has been used extensively. Such an IC is generally integrated with a MOS tube, which is more convenient than the 9910 plus MOS, but the control method is more complicated, and it is necessary to add a constant current control device, and a triode or an operational amplifier can be used. Magnetic components can be used with I-shaped inductors or high-frequency transformers with air gaps.
I love to use a transformer, because the cost of the inductor is very low, but I don't think it has the ability to load, and the adjustment is not flexible. Therefore, I think the better device choice is that the common integrated MOS switching power supply chip plus high-frequency transformer is the most ideal choice in terms of performance and cost. It does not need to use any constant current IC, that kind of thing, Not easy to use, expensive.
Finally, to distinguish between the two power supplies, one of the most important methods is to see if the output has electrolytic capacitors for filtering.
About the power supply problem
Whether it is a power supply with current limiting constant current control or a constant current power supply controlled by an op amp, the power supply problem must be solved. That is, when the switching power supply chip works, it needs a relatively stable DC voltage to supply power to its chip. The working current of the chip varies from one MA to several MAs. There is a kind of FSD200, NCP1012, and HV9910. This kind of chip is high-voltage self-feeding. It is convenient to use, but high-voltage feeding causes the IC heat to rise because the IC has to withstand about 300V DC, as long as it has a little current. Even if it is an MA, it has zero damage of zero watts. Generally, the LED power supply is only about ten watts. If you lose a few watts, you can pull down the efficiency of the power supply by a few points. There is also a typical like QX9910. Use the resistor to pull down the power, so that the loss is on the resistor, and it will lose about a few watts. There is also magnetic coupling, which is to use a transformer to add a winding to the main power coil, just like the auxiliary winding of the flyback power supply, so as to avoid losing the power of a few watts. This is one of the reasons why I don't isolate the power supply and use the transformer. In order to avoid losing the power of a few watts, I will increase the efficiency by a few points.
Introduction to the design method of non-isolated buck power supply
The non-isolated buck is a commonly used power supply structure that accounts for almost 90% of the fluorescent lamp power supply. Many people think that there is only one type of step-down type that does not isolate the power supply. When it comes to isolation, it is thought to be a step-down type. It is thought that the lamp is not safe - after the power supply is damaged. In fact, the buck type is only one kind, there are two basic structures, namely, boost, and buck-boost, that is, BOOST AND BUCK-BOOST, even if the latter two power supplies are damaged. It does not affect the LED and has this benefit.
Now LED fluorescent lamp power supply, manufacturers of lamps are generally required to be placed in the lamp, such as in the T8 lamp. Very few are external. I don't know why this is the case. In fact, the built-in power supply is difficult to do, and the performance is not good. But I don't know why there are so many people asking for it. It may be down with the wind. The external power supply should be said to be more scientific and more convenient. But I also have to fall with the wind, what the customer wants, I will do. But doing the built-in power supply is quite difficult. Because the external power supply, the shape is basically not required, how much you want to do, and what shape you want to make does not matter. The built-in power supply can only be made into two types, one of which is the most used, that is, placed under the light board and placed on the light board.
The following is the power supply, so the power supply is required to be very thin, otherwise it will not fit. Moreover, the component can only be dropped, and the line on the power supply is only lengthened. I don't think this is a good idea. However, everyone generally likes to do this. I will do it. There is also less use, put the two ends, that is, placed on both ends of the lamp, so that it is better to do, the cost is lower. I have also done it, basically the two built-in shapes.
Problems with such power supply requirements and circuit structure
My opinion is that because the power supply is built into the lamp, and the heat is the biggest killer of LED light, the heat must be small, that is, the efficiency must be high. Of course, there must be a high-efficiency power supply. For the T8 one meter long lamp, it is best not to use a power supply, but to use two, one at each end to dissipate the heat. Never concentrate the heat in one place. The efficiency of the power supply depends primarily on the structure of the circuit and the components used. Let's talk about the circuit structure first. Some people still say that it is necessary to isolate the power supply. I think it is absolutely unnecessary, because this kind of thing is originally placed inside the lamp body, and people can't touch it. There is no need to isolate, because the efficiency of the isolated power supply is lower than that of the non-isolation. The second is that it is better to output a high voltage and a small current, so that the power supply can make the efficiency higher. Commonly used now is the BUCK circuit, which is a buck circuit. It is best to make the output voltage more than one hundred volts, the current is set at 100MA, such as driving one hundred and twenty, preferably three strings, each string of forty, the voltage is one hundred and thirty volts, the current is 60MA This kind of power supply is used a lot, I just think that there is a little bad, if the switch tube is out of control, the LED will be played. LEDs are so expensive now. I am more optimistic about the boost circuit, the benefits of this circuit, I have said repeatedly, first, the efficiency is higher than the buck, and second, the power supply is broken, the LED light will not be bad. This will ensure that nothing is lost. If you burn a power supply, you only lose a few dollars and burn an LED fluorescent lamp, you will lose the cost of hundreds of dollars. So I always push the booster power supply. In addition, the boost circuit makes it easy to make the PF value high, and the buck type is more troublesome. The advantages of my absolute boost circuit for LED fluorescent lamps are overwhelmingly stronger than buck. There is only one year's shortcoming, that is, in the case of 220V mains input, 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 sandwiched in the middle, It is not convenient to use. But now do LED fluorescent lights, generally 60CM long is the use of 100 to 140, one meter two, generally just two hundred to two hundred and six, so it is still possible to use. Therefore, LED fluorescent lamps are generally used without isolation of the buck circuit, and there is no isolation boost circuit. This circuit is used for LED fluorescent lamps, which should be considered as my first.
Personally, there are many times when these practices are really ruined. Now let me know what advantages LED has over traditional lamps. First, energy saving, second longevity, then not afraid of switching, right. However, the high-PF method currently used is to use a passive valley-filling PF circuit. The original driving method, that is, 48 ​​strings, 6 is changed to 24 strings 12, and in this case, the efficiency is lowered in the case of 220V. About five percentage points, so the LED fluorescent lamp power supply, the heat is higher, the lamp beads will also be affected.
Another problem is that the 24-string 12-in-one approach will make the wiring of the LED fluorescent lamp beads uncomfortable, and it is not easy to route. In my opinion, the best way is still 48 strings and a string of methods, mainly high efficiency, low heat, and easy wiring, not complicated.
What's more, there are still people who propose 24 and 12 strings. This method is only suitable for isolated power supply. It is not applicable at all. Some people who don't understand the common sense of power supply feel that they are not isolated power supply to achieve constant current 600MA output. In fact, he did not carefully try to put it in the lamp, as this is not hot. Therefore, what low-voltage and high-current current LED light power supply is really a practice.
Two constant current control methods
The following is to say, two switching modes of constant current control mode, resulting in two practices. These two approaches, whether they are principles, device applications, or performance differences, are quite large.
First of all, the principle. The first one is represented by the current constant current LED dedicated IC, mainly like the 9910 series, AMC7150. All the brands that currently use the LED constant current drive IC are basically this kind, and call him constant current IC type. But I think this kind of constant current IC is doing constant current, but the effect is not so good. The control principle is relatively simple. It is to set a current threshold in the primary circuit of the power supply. When the primary MOS is turned on, the current of the inductor rises linearly. 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 constant current should be a current limit. We know that when the inductance is different, the shape of the primary current is different. Although there are the same peaks, the current average is different. Therefore, when such a power source is generally mass-produced, the consistency of the constant current size is not well controlled. There is also a characteristic of such a power supply, generally the output current is trapezoidal, that is, the wave current, the output is generally not electrolytic smooth, which is also a problem, if the current peak is too large, it will affect the LED. This is basically the case if the output stage of the power supply does not have a power source that is electrolyzed to smooth the current. That is to determine whether this is the control method, it depends on whether the output is electrolytically filtered. This constant current I used to call it a pseudo constant current, because its essence is a current limiting, not the constant current value obtained by the op amp comparison.
The second constant current mode should be called switching power supply. This control method is similar to the constant voltage control method of the switching power supply. Everyone knows to use TL431 for constant voltage because it has a 2.5 volt reference inside and then uses a resistor divider. When the output voltage is a little higher, or lower, a comparison voltage is generated. After amplification, the PWM signal is controlled, so this control method can control the voltage very accurately. This kind of control requires a benchmark and an op amp. If the benchmark is accurate and the op amp is large enough, then it is accurate. Similarly, to do constant current, you need a constant current reference, an op amp, with resistance overcurrent detection, as a signal, and then use this signal to amplify, to control the PWM, but unfortunately it is not very good to find a very accurate reference signal, Commonly used are triodes, this is the benchmark temperature drift, and the diode can be used as a reference for the conduction value of about 1V. This can also be used, but it is not high. The best is to use the op amp plus TL431 as the benchmark, but The circuit is complicated. However, the constant current power supply with such constant current accuracy is still much better controlled. The constant current controlled by this mode must be electrolytically filtered, so the output power is smooth DC, not pulsating. If it is pulsed, it cannot be sampled. So to determine which one is only to see if its output has electrolysis.
Two constant current control modes determine the use of two different types of devices, which determine the difference in the use of the two circuit devices, the difference in performance, and the cost.
The buck power supply also has its advantages. The first point is suitable for 220, but it is not suitable for 110. Because the voltage of 110V is low, the drop is lower, so the output current is large, the voltage is low, and the efficiency is not. Too high.
Step-down 220V AC, about 300V 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 realized, and the efficiency can be high. Generally, MOS is used as a switch tube to make power of this specification. My experience is that it can be as close as 90%, and it is difficult to go up. The reason is very simple. The chip generally has a zero point of five W to one W, and the fluorescent tube power supply is only about ten W. So it is impossible to go up again. Now that power efficiency is a virtual thing, many people are blowing, and the actual situation is not at all. Some people often say that 3W's power efficiency is 85 percent, and it is still isolated.
Tell everyone that even in the frequency hopping mode, the no-load power consumption is the smallest, it is 0.3W, and what is the output 3W low voltage, which can reach 85%, in fact, 70% is very good, anyway now Many people brag about not drafting drafts, can fool the layman, but now do not know much about LED power. I said that to be efficient, we must first do non-isolated, and then output specifications and high voltage and small current, which can save the conduction loss of power components, so the main loss of such LED power supply, one is the chip itself. Loss, this loss generally has a fraction of a few W to a W, and one is the switching loss. Using MOS as the switching transistor can significantly reduce this loss, and the loss of the triode is much larger. So try not to use a triode. There is also a small power supply, it is best not to save too much, do not use RCC, because the general manufacturers of RCC circuits simply do not do quality, in fact, the chip is also cheap, ordinary switching power supply chip, integrated MOS tube, up to two yuan Money, there is no need to save a little bit, RCC only saves material costs, in fact, the cost of processing and repairing is higher, and it is not worth the loss.
The basic structure of the buck power supply is to string the inductor and load into the high voltage of 300V. When the switch is switched, the load is lower than 300V. There are many specific circuits and many online. I will not draw a picture. Now 9910, there are generally constant current ICs on the market that are basically implemented with this circuit. But this kind of circuit is when the switch tube breaks down, the whole LED light board is finished, which should be regarded as the worst place. Because when the switch tube breaks down, the entire 300V voltage is applied to the lamp board. The lamp board can only withstand more than one hundred volts, and now it is three hundred volts. This happens once. The LED must be burned. Drop it. Therefore, many people say that non-isolation is not safe. In fact, it means bucking, just because the vast majority of non-isolated is buck, so non-isolated damage must be considered bad. In fact, the other two basic non-isolated Structure, power supply damage, will not affect the LED.
The buck power supply should be designed as a high voltage and small current, and the efficiency can be high. Explain, why? Because of the high voltage and small current, the pulse width of the switching tube current can be made larger, so that the peak current is smaller, and the loss of the inductance is also smaller. It can be known through the circuit structure that the circuit is inconvenient to draw, and it is difficult to be specific. Let's go on again. Just sum up, the advantage of the step-down power supply is that it is suitable for 220 high-voltage input, so that the voltage stress of the power device is small, suitable for high-current output, such as 100MA current, which is easier and more efficient than the latter two methods. Be high. The efficiency is relatively high, the loss on the inductor is small, but the loss on the switch tube is larger, because all the power through the load must be transmitted through the switch tube, but the output power is only partially passed through the inductor, such as 300V input, 120 output. The step-down power supply is only 300-120, that is, the part of 180 is going through the inductor, and the part of 120 is directly turned into the load, so the inductance loss is relatively small, but the output power is all converted by the switch.
LED fluorescent lamps have already bid farewell to the high price era
The point to understand is that LED has bid farewell to the era of high profits. Many people who originally wanted to take profits in the LED fluorescent industry did not play. I can give you an account.
The main sign of the LED fluorescent lamp farewell to the high price era is the price reduction of the straw hat lamp. Now the brightness of the 120-degree straw hat lamp (single luminous flux) of 6 lumens above 2000MCD has been sold. The original price is 0.2 yuan. At least 0.5 yuan. I will calculate the account, and calculate it from the fluorescent lamp composed of the traditional PC tube and straw hat LED.
1.2 meters LED fluorescent lamp, 264 LED lamp, 15W LED fluorescent lamp. PC tube, plus light board, plus ordinary LED fluorescent lamp constant current power supply, these three pieces add up, the cost can be fixed up to 30 yuan. The lamp bead is 0.2 yuan, then it is up to 60 yuan. The cost of the material is about 90 yuan. It is reasonable to deduct other things and sell 150 yuan. Therefore, the LED fluorescent lamp is no longer a high-priced era. In another year, this price will break through the 100-yuan mark, but the market promotion will take some time. People who do LED lighting must learn to adjust their mindset, and the era of profiteering is gone.
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