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2016August

Using high pressure

From: www.szbako.com Data:10/08/2016

in determining how best to let users with line voltage isolation, we need to carefully weigh the pros and cons. We can achieve isolation in the power supply, but also in the LED installation process to carry out such isolation. In some low power design, LED physical isolation is a common method, because it allows the use of a lower cost of non isolated power supply.

use LED as a light source to replace the screw thread rotary incandescent bulbs have a lot of benefits. In general, we will trumpet LED in series, the use of a power supply to the line voltage is converted to low voltage (usually dozens of volts), when the current is about 350 to 700mA. In determining how best to let users with line voltage isolation, we need to carefully weigh the pros and cons.

we can achieve isolation in the power supply, but also in the LED installation process to carry out such isolation. In some low power design, LED physical isolation is a common method, because it allows the use of a lower cost of non isolated power supply. Figure 1 shows a typical LED lamp replacement method. The power supply of the example is a non isolated power supply, which means that the isolation of the high voltage protection of the user is embedded into the package instead of the power supply. Obviously, the space of the power supply is very small, so the package is a challenge. In addition, the power supply is buried inside the package, thereby impeding the heat, affecting the efficiency of.

Figure 1 light bulbs to replace the power space becomes very small

Figure 2 shows a non isolated circuit through the 120 volt AC power supply for the LED. It contains a rectifier bridge for the buck power level. The buck regulator is a “ an inverted version of ” the power switch Q2 is in the loop, and the loop diode D3 is connected to the power supply. During the power switch, the current is regulated by a source resistance. In spite of the efficiency of doing so (80%-90%), there are several limitations of this circuit. When the power is switched on, the power switch must bear all the output current, and the output current flows through the loop diode when the power switch is closed. In addition, the current sense resistor R8 and R10 voltage is about 1 volts. Compared to 15 to 30 volts LED voltage, all of the three voltage drop are great, and the power supply efficiency constraints. More important is that these losses will promote the bulb temperature rise. LED's luminous power will slowly decrease, and this ability is closely related to the working temperature of LED. For example, under 70oC conditions, the LED light output is reduced by 30% for 50000 hours, while under 80oC conditions, this time is only 30000 hours. Because the bulbs are installed in some “ tube ” in the “ tube ” often hinder the heat, is not conducive to convection cooling, so the heating problem is further complicated.

Figure 2 blood pressure regulator via a simple offline LED driver

LED manufacturers will be a plurality of LED series on a common substrate, to create a more high voltage of the luminous body. These high voltage light emitting bodies are either at lower cost or higher power efficiency. Using the high voltage products, we only need to use a set of rectifier and a constant current resistor, so as to achieve a lower cost of the power method. Although this power supply can produce very good power factor, but the efficiency is very low. The reason is that input voltage to the great part is used in the steady flow resistor, resulting in 30% - 50% of the LED power loss. However, it can be used in some small volume low power applications. However, in some high power applications, low efficiency to no avail. Figure 3 shows another alternative: the use of a boost power supply. Most of the circuit is the same as the method mentioned above. However, switching, diode, and current detection loss are much smaller, bringing efficiencies as high as 90% to 95%. In addition, the circuit also has a good power factor of 97%.

Figure 3 the use of boost power to improve the efficiency of LED drive

Figure 4 Figure 1-2 schematic diagram described the power of the photos. Even if the output power of the power supply is approximately the same, there are some significant differences in the size of the power supply. The inductor size of the boost power supply is significantly smaller because of its low energy storage requirement. Compared to the boost power supply, the buck power supply has a larger resistor. The resistor is a simulation of a load resistor (shown in Figure 2), which is used to determine when to turn on a silicon controlled rectifier (SCR). The reason for this is that the regulator has an electromagnetic interference (EMI) suppression capacitor at the three end of the two-way thyristor switching module, which is relatively high in the case of no load. This disturbs the power supply, resulting in unstable dimming. When using the boost power supply, it does not need to do so, because the LED is connected to the input through a boost inductor, which provides sufficient load, so the above problem is not a problem. The reverse side of the circuit board is not shown in the figure, but as the schematic diagram shows, the buck power supply has more low level circuits. Therefore, the boost power supply has a lower power consumption, which is very important in the application of space restrictions such as LED bulbs.

Figure 4 boost power supply smaller, more efficient

in short, the high voltage LED because of its low power consumption, low temperature rise, can help to increase the service life of rotary LED light bulbs. It is achieved through the use of booster power instead of the buck power supply, thus improving the power efficiency. The power loss is about half of the buck regulator. In addition, the power supply of the component is less, better power factor, smaller size, and the use of three - end two-way SCR switch components to achieve more easy to adjust the light.

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