It is often said that the short life of LED lamps is mainly due to the short life of power supply, while the short life of power supply is due to the short life of electrolytic capacitor.
There is some truth to this.
Inferior because the market is filled with a lot of short life of electrolytic capacitors, plus spell the price now, so some manufacturer regardless of the quality and inferior to adopt the short life of electrolytic capacitors, the result is the conclusion.
So what's going on?
The life of an electrolytic capacitor depends on the ambient temperature at which it operates
How is the life of the electrolytic capacitor defined?
By the hour, of course.
But if the life index of an electrolytic capacitor is 1000 hours, not after 1000 hours, the electrolytic capacitor is bad, no, I just said that the electrolytic capacitor capacity after 1000 hours capacity reduced half, was 20 uf, only 10 uf now.
In addition, the life index of electrolytic capacitors is also a feature, which must be explained in how many degrees of working environment temperature.
And are often prescribed for the life of the under 105 ℃ environment temperature.
This is because the electrolytic capacitors we use now are electrolytic capacitors of liquid electrolyte. If the electrolyte is dry, the electric capacity will be lost.
The higher the temperature, the easier the electrolyte will evaporate.
Therefore, the life index of electrolytic capacitance must be specified in which ambient temperature is the life.
So now all electrolytic capacitors are marked the life of the under 105 ℃.
Such as the most common electrolytic capacitor, life is only 1000 hours at 105 ℃.
But think of all electrolytic capacitors as having a lifetime of only 1,000 hours.
That's a big mistake.
Simply put, if the environment temperature is higher than 105 ℃, then its life will be less than 1000 hours, if the environment temperature is below 105 ℃, then its life span is more than 1000 hours.
Is there a general quantitative relationship between life and temperature?
One of the simplest and most easily calculated relationships is that for every 10 degrees of increase in ambient temperature, the life span is halved.
Conversely, for every 10 degrees of temperature reduction, the life span doubles.
It's a simple estimate, of course, but it's pretty accurate.
For surely used in electrolytic capacitor of LED drive power supply, is the inside of the shell on the LED lamps and lanterns, so we need to know the temperature inside the LED lamps and lanterns can know the working life of the electrolytic capacitor.
2. What is the ambient temperature in LED lamps?
Because leds and electrolytic capacitors are placed in the same housing in many lamps, the ambient temperature is simply the same.
The ambient temperature is mainly determined by the heating and cooling balance of LED and power supply.
And the heating and heat dissipation of each LED lamp are different, so how can we know the ambient temperature?
In fact, this problem can be calculated in reverse, that is, a well-designed LED lamp, which allows a certain internal ambient temperature.
This is because the junction temperature of the LED chip is to determine the LED chip droop (life), the main reasons for the LED junction temperature, of course, also by the environmental temperature, so as long as know that allow the LED junction temperature, also can calculate the internal environmental temperature of LED lamps and lanterns.
But in which there are at least three thermal resistance, is the thermal resistance of LED chip junction to case theta jc, shell and LED to the thermal resistance of the aluminum plate surface, after soldering, copper foil, and insulation to it to the aluminum plate, but is one of the main insulation layer of thermal resistance, collectively known as theta lv, the third is the thermal resistance aluminum plate to the air inside the blister theta la.
Take 3014 type LED, for instance, it is the thermal resistance of theta jc 90 ℃ / W, because it is only 0.1 W, so the temperature difference between inside and outside also is 9 ℃.
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 ℃.
That is to say, the LED junction temperature is equal to the ambient temperature and 20 ℃.
Can the ambient temperature inside the bubble allow 105 degrees?
Just look at the picture below.
That's the relationship between LED junction and light failure at CREE.
If the environment temperature is 105 ℃, there must be at least add 20 ℃ is the junction temperature, so the junction temperature is about 125 ℃.
You can't find it on this curve, you can only estimate roughly 4,000 hours of life.
This is absolutely unacceptable!
That is to say, LED blister in the ambient temperature must be much lower than 105 ℃.
We can in turn see what the ambient temperature should be according to the required LED life.
Suppose we require LED life is 100000 hours, then its junction temperature is below 65 ℃, so the environment temperature must be below 45 ℃. That is to say, electrolytic capacitor working environment temperature must be below 45 ℃.
3, all kinds of life of the electrolytic capacitor in 45 ℃ environment temperature of actual life
Now that you know the ambient temperature of the electrolytic capacitor in an actual LED lamp, it's easy to calculate its actual life.
The actual life of several common electrolytic capacitors is listed in the table below.
By the table as you can see, even the most ordinary life of 1000 hours of electrolytic capacitor, when the environment temperature 45 ℃ life can reach 64000 hours, for common LED lamps and lanterns of the nominal is 50000 hours is enough.
4. Methods to extend the life of electrolytic capacitors
4,1 is designed to extend its life
Actually, to prolong the service life of electrolytic capacitors, the method is very simple, because it dies a natural death is mainly due to the result of the liquid electrolyte evaporation, if improve its impermeability, don't let it evaporate, it is natural to extend the life of a.
Evox Rifa, for example, has designed a two-layer sealing system to slow the evaporation of electrolytes.
In addition, the loss of electrolyte can be greatly reduced by the use of a full-round phenolic plastic cover with electrodes and a double special sealing pad that is closely occluded with the aluminum shell.
4.2 extend its service life
If the ripple current is too large, it can be reduced by using two capacitors in parallel
4.3 prolong its life by selection
When choosing electrolytic capacitors, in addition to the quality assurance of the brand electrolytic capacitors, the voltage and capacity of the residual.
For example, after 220V passes bridge rectifier, its dc voltage will reach up to 300V, but at least 450V voltage-resistant electrolytic capacitors will be selected when electrolytic capacitors are selected.
If you figure out that you need 10uF, you'd better choose 20uF.
These measures can also extend the life of electrolytic capacitors.
Because the capacitance's equivalent resistance and ripple current will make its internal temperature higher than the ambient temperature, it is necessary to leave room.
5. Protect electrolytic capacitors
Sometimes, even if a long life electrolytic capacitor is used, it is often found that the electrolytic capacitor is broken. What is the reason?
In fact, if you think that this is the quality of the electrolytic capacitor is not up to standard, that can really bring the electrolytic capacitor to bear the injustice!
In fact, electrolytic capacitors are not the perpetrators, but the victims.
Why do you say that?
Because we know that in the mains ac online, often instantaneous high pressure generated by lightning surge, while on the large power grid for lightning have done a lot of lightning protection lightning protection measures, but inevitably there will be still leaking through the net to residents' home.
Power grid should be very advanced in the United States, but one thing happened in my home, after a lightning strike, I found my fax machine doesn't work, after carefully check, original because the power to be struck by lightning and completely broke, can only be scrapped.
If it is from the mains power supply for LED lamps and lanterns, so in the power of the lamps and lanterns must in mains input and surge prevention measures, including the fuse, resistance and over-voltage protection, often referred to as pressure sensitive resistance, to protect the back of the components, again long life electrolytic capacitors can also be breakdown by surge voltage.
I believe that if all the LED manufacturers can adopt normal electrolytic capacitors with high quality, and adopted the above relevant measures, then the electrolytic capacitor of time will be able to get clean!