Electronic Design

What's All This LED Power Stuff, Anyhow?

Recently, NSC put on a webcast with Howard Johnson, NSC’s Chris Richardson, and some guys from Philips and Future. As they had set it all up and presented it at the last minute, I didn’t know what points they were going to make. When they made their pitch, it all made perfect sense—but I wasn’t prepared to contribute very much. I sat there like a “fifth wheel,” not making many comments.

But I did set up one experiment, which wasn’t shown on the webcast. I put a pair of Hershey’s chocolate bars (without almonds) on a couple of little supports. A good, bright LED array shone on one, and a high-brightness incandescent lamp of the same lumens shone on the other.

The candy bar under the LEDs warmed up about 1° and would sit there forever. The one under the incandescent lamp melted and collapsed in about 55 seconds. So this was another convincing argument that the heat from an incandescent lamp goes out in a completely different mode than the heat from an LED, which comes out of the heatsink in the back of the LED and isn’t radiated at the objective, where the light is shining.

Later, a couple of engineers asked, “Okay, but how much more efficient are LEDs than incandescents?” Chris dug up some numbers to show that good white LEDs can provide white light with 40% to 60% less power than the incandescent bulb that did such a good job of melting the chocolate bar. But after I thought about this, I realized I had another good point.

Chris showed that to get good efficiency from LEDs, you have to use a switching regulator specifically designed for current regulation. It can accommodate any reasonable range of VIN and put out an ampere or two at a voltage such as 16 V, which is about the voltage of a stack of five white LEDs in series. But the switcher doesn’t regulate the voltage. It regulates the current.

The voltage can move around as conditions change, temperature or whatever. Fine. The efficiency of the switcher is about 86%, so even though there are some losses there, the LED system needs less power than the incandescent. Fine. But during the webcast, we talked in general of the nominal volts and amps. When the customer began asking about real applications, I made my point:

  • At “Nominal Line Voltage,” an incandescent lamp has a certain output in terms of lumens, perhaps 15 to 25 lumens per watt. This could be at 115 V ac or 117 or whatever. An automotive lamp might be defined with a typical voltage of 13 V dc in a car. A switcher for LEDs puts out a regulated output to the LED, which may provide 50 to 60 lumens/W.
  • Now let’s go to high line, such as 125 V ac or 14.4 V dc. The incandescent bulb puts out a lot more light—the voltage factor of (1.09 x) is taken to the ~fourth power. It also draws more power. Is that good for you? Fine. But beware that the life of the incandescent bulb is then decreased by the ~eighth power of 1.09. Can you live with that?

Many incandescent bulbs are already running so hot that their life is only five or 10 hours. Most modern (incandescent) flashlights run a 2.2-V bulb at 2.9 V, which gives you great efficiency and very good light output—and poor bulb life.

Meanwhile, the LED output is constant. The switcher IC may have to work a little harder. Its efficiency may swing a little, but the LED is regulated, and its light output is regulated within better than 1%.

  • Now let’s go to low line. The LED output (lumens) is still constant. The switcher IC has to change its duty cycle, and its efficiency may change a little—up, down, who cares? The incandescent lamp at low line has considerably better life than it did before. But its light output shrinks at the ratio of 0.924, which is a considerable shrinkage. Is that acceptable for you?

Incandescents are changing all over the place. The lumens/ watt change around grossly, though the LED is regulated. So for worst-case study, the incandescent bulb has a lot of variation, and the output lumens are going to have broad variances versus line.

The actually available lumen output is poorer at low line. The bulb life is poorer at high line. So the usable efficiency of the incandescent bulb is even worse than in typical conditions— unless you run it from a switching regulator, too!

See associated figure

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