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Power Tip: Higher-voltage LEDs boost light bulb efficiency

Posted: 04 Jul 2011 ?? ?Print Version ?Bookmark and Share

Keywords:LEDs? High voltage? efficiency?

Changing incandescent screw-in light bulbs with bulbs that use LEDs as the light source is gaining much interest. Typically, a small number of LEDsbetween five and nineare connected in series and a power supply has to convert the line voltage to a low voltage, typically tens of volts, at currents around 350 to 700 mA.

There are a number of tradeoffs in determining how to best isolate the consumer from the line voltage. Isolation can be accomplished either in the power supply or in the mounting of the LEDs. In these lower-power designs, physical isolation of the LEDs is a common choice as it allows the use of a cheaper, non-isolated power supply.

Figure 1 shows a typical LED light replacement. The power supply in this example is non-isolated, meaning the isolation that protects the consumer from high-voltage is built into the package rather than the power supply.

It is quite evident that there is very little room for the power supply, which makes it a challenge to package. Furthermore, the power supply is buried within the package, which hinders cooling and makes good efficiency key.

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Figure 1: A light bulb replacement has little room for a power supply.

Figure 2 illustrates a non-isolated circuit that powers LEDs from a 120V AC source. It contains a rectifier bridge that feeds a buck power stage. The buck is the "upside-down version" where the power switch, Q2, is in the return and the catch diode, D3, is connected to the source. Current is regulated during the on time of the power switch through a source resistor.

While fairly efficient (80-90 percent), this circuit has several efficiency-limiting drawbacks. The power switch has to carry the full output current when on, and when the power switch is off, the output current flows through the catch diode.

Also, the voltage across the current sense resistors, R8 and R10, is around one volt. All three of these voltage drops are significant when compared to an LED voltage of 15 to 30V, and will limit the power supply efficiency.

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Figure 2: A buck regulator makes a simple offline LED driver. (Click to enlarge)

More importantly, these losses contribute to the temperature rise of the light bulb assembly. An LED's ability to produce light diminishes in time and is a strong function of its operating temperature. For instance, an LED light output will diminish by 30 percent over 50,000 hours at 70oC, while at 80oC, you can only count on 30,000 hours. The thermal problem is further compounded since the bulbs get installed in "cans" that tend to trap the heat and are not conducive to convection cooling.

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