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Power Tip 36: Higher-voltage LEDs improve light bulb efficiency

Power Tip 36: Higher-voltage LEDs improve light bulb efficiency

Technology News |
By eeNews Europe



(Editor’s note: to see a linked list of all entries in the Power Tip series, click here.)

There is much interest in replacing incandescent screw-in light bulbs with bulbs that use LEDs as the light source. Typically, a small number of LEDs—between five and nine—are 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 trade-offs 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.

 


Figure 1: A light bulb replacement has little room for a power supply.
(Click on image to enlarge)

 

Figure 2 illustrates a non-isolated circuit that powers LEDs from a 120-volt 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.


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

 

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 30 volts, and will limit the power supply efficiency.

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.

LED manufacturers have started making higher-voltage emitters by connecting several LEDs in series on a common substrate. These higher-voltage offerings allow for either reduced costs or improved power supply efficiency. With these higher voltage products, a cheaper approach for the power supply simply can be a set of rectifiers and a ballast resistor.

While this approach can produce a reasonably good power factor, the efficiency is poor, as a significant portion of the input voltage is applied to the ballast resistor, resulting in losses on the order of 30-50 percent of the LED power. This may be an option in lower-power applications where size is at a premium.

However, at higher powers, the reduced efficiency renders this unviable. Figure 3 presents another alternative using a boost power supply. Much of the circuitry is similar in the two approaches. However, the switch, diode, and current sense losses are much smaller, resulting in efficiencies in the 90 to 95 percent range. Additionally, this circuit has good power factor with measurements showing 97 percent.

 


Figure 3: Improve LED driver efficiency with a boost
(Click on image to enlarge)

 

Figure 4 is a photo of the two power supplies depicted in the schematics of Figure 2 and Figure 3.


Figure 4: Boost supply is smaller and more efficient than buck supply
(Click on image to enlarge)

 

Even though this power supply produces about the same output power, there are several notable differences which impact the size of the power supply:

•The size of the inductor on the boost is notably smaller, because its energy storage requirements are less.

•The buck design also has a larger resistor than the boost does. This resistor is a dummy load resistor (R20 in Figure 2) that is used to determine when a dimmer fires a silicon-controlled rectifier SCR.

This is needed because the dimmers have an electromagnetic interference (EMI) suppression capacitor across the triac w presents a relatively high voltage to the power supply without some loading. This confuses the power supply and results in erratic dimming.

This is not needed on the boost converter, as the LEDs are connected to the input through the boost inductor and provide sufficient loading for this not to be a problem. The back side of the board is not shown but, as shown in the schematics, the buck version has more low-level circuitry. So the boost provides the smaller power, which is extremely important in space-constrained applications such as the LED light bulb replacement.

To summarize:

•Higher-voltage LEDs are helping to improve the lifetime of screw-in LED light bulbs by keeping losses and the resulting temperature rise down. This is realized through improved power-supply efficiency, by replacing a buck converter with a boost topology.

•A boost regulator will have about half of the losses of a buck regulator. The boost also has fewer parts, better power factor, smaller size, and easier dimming with a triac.

Please join us next month when we will discuss ripple voltages and current in the capacitor of an offline power supply.

For more information about this and other power solutions, visit: www.ti.com/power-ca.

About the author

Robert Kollman is a Senior Applications Manager and Distinguished Member of Technical Staff at Texas Instruments. He has more than 30 years of experience in the power electronics business and has designed magnetics for power electronics ranging from sub-watt to sub-megawatt with operating frequencies into the megahertz range. Robert earned a BSEE from Texas A&M University, and a MSEE from Southern Methodist University.
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