Design and Analysis of Non-Isolated Buck Power Supply

　　Non-isolated buck type is a commonly used power supply structure now, which accounts for more than 90% of fluorescent lamp power supplies. Many people think that non-isolated power supplies only have buck type. When it comes to non-isolation, they think of buck type, and they think of it as unsafe for lamps - that is, after the power supply is damaged. In fact, buck type is only one type, and there are two basic structures, namely boost, and buck-boost, namely BOOST AND BUCK-BOOST. Even if the latter two power supplies are damaged, it will not affect the LED, which has this advantage.

　　The step-down power supply also has its advantages. The first point is that it is suitable for 220, but not for 110, because the voltage of 110V is low to begin with, and it will be even lower once it is stepped down. In that case, the output current is large, the voltage is low, and the efficiency is not very high.

　　The step-down 220V AC is about 300 volts after rectification and filtering. After the step-down circuit, the voltage is generally reduced to about 150V DC, so that high-voltage and low-current output can be achieved, and the efficiency can be high. Generally, MOS is used as a switch tube. My experience is that it can reach about 90% when making a power supply of this specification, and it is difficult to go higher. The reason is very simple. The chip generally has a self-loss of 0.5W to 1W, while the power supply of a fluorescent tube is only about 10W. So it is impossible to go higher. Now the power supply efficiency is very virtual, and many people are bragging, but it is actually impossible to achieve it. It is common for some people to say that the efficiency of a 3W power supply is 85%, and it is still isolated.

　　Let me tell you, even in frequency hopping mode, the minimum no-load power consumption is 0.3W. What's more, the output is 3W low voltage, which can reach 85%. In fact, 70% is very good. Anyway, many people now brag without thinking and can fool laymen. However, not many people who work on LEDs now understand power supply.

　　我说过，要效率高，首先就要做非隔离的，然后输出规格还要高压小电流，可以省去功率元件的导通损耗，所以象这种LED电源的主要损耗，一就是芯片自有损耗，这个损耗一般有零点几W到一W的样子，还有一个就是开关损耗了，用MOS做开关管可以显著减小这个损耗，用三极管开关损耗就大很多。所以尽量不要用三极管。还有就是做小电源，最好不要太省，不要用RCC，因为RCC电路一般的厂家根本做不好质量，其实现在芯片也便宜，普通的开关电源芯片，集成MOS管的，最多不过两元钱，没必要省那么一点点，RCC只省点材料费，实际上加工返修等费用更高，到头到反而得不偿失的那样。

　　The basic structure of a buck power supply is to connect the inductor and the load in series with the 300V high voltage. When the switch tube is switched, the load realizes a voltage lower than 300V. There are many specific circuits, and there are many on the Internet. I will not draw pictures to explain them. Now 9910 and the constant current ICs on the market are basically implemented with this circuit. But this circuit is that when the switch tube breaks down, the entire LED light board is finished, which should be the worst part. Because when the switch tube breaks down, the entire 300V voltage is added to the light board. The light board can only withstand more than 100 volts, but now it has become 300 volts. Once this happens, the LED will definitely burn out. So many people say that non-isolated is unsafe, but in fact, it means step-down. It’s just that most of the non-isolated ones are step-down, so it is believed that the damage of non-isolated will definitely damage the LED. In fact, the other two basic non-isolated structures will not affect the LED if the power supply is damaged.

　　The buck power supply must be designed with high voltage and low current to be efficient. Why? Because high voltage and low current can make the pulse width of the switch tube current larger, so that the peak current is smaller, and the loss of the inductor is also smaller. You can know from the circuit structure that the circuit is not convenient to draw, and it is difficult to describe it in detail. Just to summarize, the advantage of the buck power supply is that it is suitable for 220 high voltage input, so that the voltage stress of the power device is small, and it is suitable for large current output, such as 100MA current, which is easier than the latter two methods and has higher efficiency. The efficiency is relatively high, the loss of the inductor is small, but the loss of the switch tube is larger, because all the power passing through the load must be transmitted through the switch tube, but only a part of the output power passes through the inductor, such as 300V input, 120 output buck power supply, only 300-120, that is, 180 part, must pass through the inductor, and the 120 part is directly turned on to enter the load, so the loss of the inductor is relatively small, but the output power must all be converted through the switch tube.