It is often heard that the short life of LED lamps is mainly due to the short life of the LED driver, while the short life of the LED driver is due to the short life of the electrolytic capacitor. There is some truth in these arguments.Because the market is full of a large number of short life poor electrolytic capacitor, and now are competing in price, so some manufacturers regardless of quality and use these poor quality short life electrolytic capacitor, the results of the above conclusion.
So what is the reality?
1.The life of an electrolytic capacitor depends on the ambient temperature at which it operates
How is the life of an electrolytic capacitor defined? In hours, of course. But if the lifetime of an electrolytic capacitor is 1,000 hours, it doesn’t mean that the electrolytic capacitor is broken after 1,000 hours, no, it just means that the capacity of the electrolytic capacitor has been reduced by half after 1,000 hours, from 20uF to 10uF.
In addition, there is another characteristic of the life index of electrolytic capacitor, that is, the life of the operating environment temperature must be specified.And it is usually specified for life of at the temperature below 105℃.
That’s because the electrolytic capacitors that we use today are electrolytic capacitors of liquid electrolyte, and if the electrolyte is dry, the capacitance is of course gone. The higher the temperature, the more easily the electrolyte evaporates. Therefore, the life index of electrolytic capacitor must indicate the life at what ambient temperature.
So currently all electrolytic capacitors are marked with life at the temperature of 105℃ below. For example, the most common electrolytic capacitor has a life of only 1,000 hours at 105℃. If you think all electrolytic capacitors have a life of 1,000 hours only.That would be a big mistake.
Simply, if the ambient temperature is higher than 105℃,its lifetime will be less than 1,000 hours,If the ambient temperature is below 105℃, its life is over 1,000 hours. Is there a rough quantitative relationship between lifetime and temperature? Yes, sure.
One of the simplest and easiest relationships to calculate is that for every 10 degree increase in ambient temperature, the lifetime is reduced by half; conversely, life expectancy doubles for every 10 degrees drop in ambient temperature.It’s a simple estimate, of course, but it’s pretty accurate.
Because the electrolytic capacitors used for LED drive power must be placed inside the shell of LED lamps, we can know the working life of the electrolytic capacitors as long as we know the internal temperature of LED lamps.
2. What is the inside ambient temperature in the LED lamp?
Because leds and electrolytic capacitors are placed in the same enclosure in many lamps, simply put, the ambient temperature of the two is the same. The ambient temperature is mainly determined by the heating and cooling balance of LED and power supply.Moreover, the heating and cooling conditions of each LED lamp are different, so how can we know the inside ambient temperature?
In fact, this problem can be calculated in reverse, that is, a well-designed LED lamp, it should allow a certain internal environment temperature. This is because the junction temperature of LED chips is the main reason for determining the light decay (life) of LED chips. Of course, the junction temperature of LED chips is also related to its ambient temperature. Therefore, as long as the permitted junction temperature of LED chips is known, the internal ambient temperature of LED lamps can be calculated. But there are also at least three thermal resistances that are used to attach the LED chip to the shell, theta jc, and the LED shell to the surface of the aluminum substrate. In fact, it goes through solder, copper foil, and insulation layer to aluminum plate, but the main one is the thermal resistance of insulation layer, collectively known as theta lv, and the third is the thermal resistance of aluminum plate to air in the bubble shell.
Take the 3014 LED, for example, where the thermal resistance of theta jc is 90℃/W, and the internal and external temperature difference is 9℃, because of its power of only 0.1W. Aluminum plate thermal resistance is 1℃/W, for a 10 W since all 10 W LED lamps and lanterns is installed in the same block of aluminum substrate, so its total temperature is 10℃, a total of 19℃ temperature difference, the final theta la is hard to estimate, because it is related to whether the air circulation, in the case of internal air flow, the temperature difference is only about 1℃ or so, so add up to a total of 20℃. In other words, LED junction temperature is equal to ambient temperature plus 20℃.
So the ambient temperature inside the lamp can allow 105 degrees? If the ambient temperature is 105℃, then at least 20℃ must be added to the junction temperature, so the junction temperature is about 125℃, only a rough estimate of its lifespan is 4,000 hours (if 50,000). This is absolutely unacceptable. That is to say, the ambient temperature inside the LED lamp must be much lower than 105℃.
We can require LED lifetime to see what the ambient temperature should be. Assuming that we require the LED to have a lifetime of 100,000 hours, its junction temperature can only be below 65℃, so the ambient temperature must be below 45℃. In other words, the operating temperature of the electrolytic capacitor must be lower than 45℃.
3. The actual life of electrolytic capacitors with different life spans at ambient temperature of 45℃
Now that the ambient temperature of the electrolytic capacitor in the actual LED lamp is known, it is easy to calculate its actual life.Even the most common 1,000-hour electrolytic capacitor has a lifetime of 64,000 hours at an ambient temperature of 45 ° c, which is enough for a typical LED fixture with a nominal life of 50,000 hours.
4. Methods for prolonging the life of electrolytic capacitors
4.1 Extend its life from design
In fact, to extend the life of electrolytic capacitor, the method is very simple, because its life is mainly because of the evaporation of liquid electrolyte, if improve its sealing, do not let it evaporate, its life is naturally extended. The loss of electrolyte can also be greatly reduced by using a phenolic plastic cover with an integral electrode wrap and a double special seal pad that snaps tightly to the aluminum shell.
4.2 Extend its life from use
Reducing the ripple current can also extend its service life. If the ripple current is too large, the parallel connection of two capacitors can be adopted to reduce it.
4.3 Extend its life from Choice
In the choice of electrolytic capacitor, in addition to choose to ensure the quality of the brand electrolytic capacitor, but also in the voltage and capacity left margin.For example, the dc voltage of 220V after bridge rectification will be as high as 300V, but at least 450V voltage-resistant electrolytic capacitor should be selected. If you want 10uF, you’d better use 20uF. These measures can also extend the life of electrolytic capacitors. Since the equivalent resistance and ripple current of a capacitor can cause its internal temperature to be higher than the ambient temperature, some margin is necessary.
5. Protective electrolytic capacitor
Sometimes even if the use of long life electrolytic capacitor, but still often found that the electrolytic capacitor is broken, why?
In fact, if you think this is the quality of electrolytic capacitor is not pass, it is really let electrolytic capacitor suffered injustice. In fact, the electrolytic capacitor is not the perpetrator, but the victim. Why do you say that?
Because we know in the alternating current network of the mains electricity, often because of lightning strike and produce instantaneous high voltage surge, although in the large power grid for lightning strike has done a lot of lightning protection measures, but still inevitably there will be leakage into the residents’ homes.
If LED lamps are powered by mains, then in the power supply of lamps must be in the mains input end with anti-surge measures, including fuse, and overvoltage protection resistance, usually called piezors, to protect the components behind, otherwise the long life of electrolytic capacitor will also be broken by the surge voltage.
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