Use LED as light bulbs to replace the threaded screw-in incandescent bulbs has many advantages. In general, we will trumpet (5-9) of the LED in series, using a power supply to line voltage into low voltage (typically a few tens of volts), when the current is about 350 to 700mA. In determining how best to let users with the line voltage isolation, we need careful consideration, on balance. We can achieve power in isolation, can also be installed in the LED process in such isolation. In some low-power design, LED physical isolation is a common approach, because it allows the use of lower cost non-isolated power supply. Figure 1 shows a typical LED light alternative. The example of the power supply non-isolated power supply, the means to achieve user isolation voltage protection is embedded in the package rather than the power supply. Obviously, the power of the space is extremely small, and thus pose a challenge to the package. In addition, the power to be buried inside the package, thus preventing the heat, affecting the efficiency.

Shows a 120-volt AC power through the LED-powered, non-isolated circuit. It contains a power for the buck power stage rectifier bridge. The buck regulator is an “upside-down version”, the power switch Q2 in the circuit, while the circulation diode D3 is connected to power. The power switch conduction period, through a source resistance of the current regulation. Despite this very high efficiency (80% -90%), but there are several limitations of this circuit efficiency shortcomings. Turn the power switch must carry the full output current, while the power switch is off, the output current flows through the circulation diode. In addition, the current sense resistor R8 and R10 of the voltage of about 1 volt. 15 to 30 volts compared to the LED voltage drop across all three are great, and power efficiency will be a constraint. More importantly, these losses will promote bulb temperature. Ability of the LED light will gradually decrease, and this ability with the LED operating temperature are closely related. For example, 70oC conditions, LED light output by 30% of the time exceeded 50,000 hours, and in 80oC conditions, this time only 30,000 hours. As the bulbs are installed in some of the “tube” in which “tube” tend to impede the heat is not conducive to convection cooling, the heat problem is further complicated.

LED manufacturer LED series by the number of teams in a common substrate, to create a higher voltage of the light. The high pressure also brought light or have a lower cost or higher led power supply efficiency. Using these high-pressure products, we only need to use a rectifier and a ballast resistor, in order to achieve more cost-effective power method. Although this power can produce very good power factor, but is inefficient because a large part of the input voltage are used in a steady flow resistor, resulting in 30% -50% of the LED power loss. However, it can be used for small volume, low-power applications. However, in some high-power applications, the low efficiency allowed to no avail. Figure 3 shows another alternative: the use of a boost power supply. Most of the circuit the same way as above. However, the switch, diode, and current detection loss is much smaller, bring up to 90% to 95% efficiency. In addition, the circuit also has a good power factor of 97%.

Even if the power output generated roughly the same, but there are some significant differences in size affect the power supply. Power boost inductor size was smaller, because of its lower storage requirements. Compared to the step-up power supply, power supply with a larger step-down resistor. The simulation of the load resistor as a resistor (Figure 2 R20), used to determine when to turn dimmer silicon controlled rectifier (SCR). Need to do this because of the dimmer switch in the Triac next to a component of electromagnetic interference (EMI) suppression capacitor, in the case of no-load power supply voltage is relatively higher. This will disrupt the power supply, leading to instability dimming. Shique not necessary to use the power boost, because the LED through the boost inductor connected to the input, to provide sufficient load, so the problem will not be a problem. Not shown in the back of the board, but as shown in the diagram, there are more low step-down power supply circuit. Therefore, the step-up power with lower power consumption, this LED light bulbs replacement, such as limited space applications is extremely important.

In short, because of its low power consumption, high-voltage LED, a small temperature rise characteristics, can help increase the spin-LED lamp life. It is through the use of alternative buck-boost power supply, thereby enhancing the power efficiency to achieve. Boost the power loss is about half the buck regulator. In addition, fewer components boost power, power factor better, smaller, and the use of a triac dimmer switch assembly is easier to achieve. Next time, we will discuss off-line power capacitor ripple voltage and current, so stay tuned.