Wasting the lifetime of these LEDs is not a good thing 4shared.
shows lifetime in hours along the bottom and failure rate on the vertical axis. (We haven’t actually populated the vertical axis because the numbers don’t really matter for this discussion.)

The dark blue line is the lifetime expected for the power supply, and the brown line shows the lifetime expected for the LEDs. You can see that the LEDs last significantly beyond the blue line, which represents the point at which the power supply begins to fail. The red area beneath the blue curved area represents early failure of the power supply. That is the wasted life which the LEDs still possess at the end of the system’s operation.

Think of it this way: If you bought a car with drive chain that lasts one million miles, but the wheels fall off at 50,000 miles, you’ve paid for a drive train that you’re never going to get full use of. Similarly, the power supply is dying very early. While the LEDs are still good for a long time, the lamp life has reached an end and you throw it away because the power supply failed. Moreover, if we’re using an inadequate standard power system design, the rising temperature of the power supply will also dramatically reduce the power supply’s life, making it even worse than shown here gemscool.

So what’s the source of the power supply’s lifetime problem? I already suggested that the main culprits are the aluminum electrolytic capacitors. But first, let’s take a look the rated lifetimes of the other components.

If we look at an LED string, based on L70, you can get to 45,000 hours quite easily.

· The controllers last longer much longer than 100,000 hours, so they don’t wear out very quickly.

· The other semiconductors have lifetimes that exceed 100,000 hours.

· Diodes and transistors really last a long time. These components don’t have a lifetime problem.

· Ceramic capacitors last a very long time and are not a concern.

· Aluminum electrolytic capacitors, however, have a short life expectancy of perhaps 20,000 hours, which largely determines the LED system lifetime since this translates into only about one year of operation.

The structure of the aluminum electrolytic capacitors, subjected to a high ambient temperature, shortens the life of the power supply.

shows a typical two-stage LED driver, commonly seen in applications today. This is a standard power supply that has to face the harsh temperatures that exist in an LED environment. How will it hold up? You can see positions on the circuit where you have high-voltage electrolytics. On the primary side, there is a big, bulk storage, 10 microfarad, 400 volt electrolytic capacitor. We also have an electrolytic 22 microfarad, 50 volt output capacitor for ripple reduction. And we have an electrolytic 4.7 microfarad, 25 volt capacitor biasing the controller.

As each of these electrolytic capacitors fail, they will have a different affect on the circuit. We’ll talk more about that later. At the moment, let’s make sure we understand that these components are a standard part of most two-stage LEDs drivers in use today. Let’s look at what happens to these electrolytic capacitors in their high ambient temperature environments and we’ll see why they fail.

The capacitors always operate at full load and at maximum temperature, as mentioned previously. shows the lifetime expressed in thousands of hours for aluminum electrolytic capacitors based against the capacitor temperature in degrees C across the bottom. Then we have different kinds of capacitors: red is a low-cost, 2,000-hour capacitor; blue is a 5,000-hour capacitor; and green is a long lifetime, 10,000-hour capacitor.

If you want your capacitor reach 45,000 hours of life, first you need a very good capacitor because the ambient temperature, shown in the brown area, is what you’d see inside a lamp. Actually you can’t get to 45,000 hours with an electrolytic capacitor because it will fail at 85°C at 40,000 hours. What’s happening inside the capacitor is that the dielectric material — a combination of a liquid and a filler — evaporates over time and the capacitor starts to lose capacitances. After a certain period of time and much faster in an increased temperature environment, the capacitor fails.

What this shows is that in a high-temperature LED environment, even capacitors rated for 105°C can’t reach the lifetimes that designers need to make an effective LED power supply. This is a clear indication of why we have seen early failure in many power supplies. These LED power supplies are inflicting a very high ambient temperature on the power supply which means the standard the power supply solutions that many people are using simply cannot do the job. 4shared

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