Can Electrolytic Capacitance Really Lead To Short LED Lamp Life?

- Oct 26, 2018-

It is often heard 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.

These arguments have some merit.

Because the market is full of a large number of short life inferior electrolytic capacitors, and now are competing prices, so some manufacturers regardless of quality to use these poor short life electrolytic capacitors, the results of the above conclusions.




So what's actually going on?



1. The service life of the electrolytic capacitor depends on the ambient temperature at which it works


How is the lifetime of the electrolytic capacitor defined?

It's defined by the hour, of course.

But if the life index of an electrolytic capacitor is 1,000 hours, it's not that the electrolytic capacitor is broken after 1,000 hours, no, it's just that the capacity of the electrolytic capacitor is reduced by half after 1,000 hours, which was originally 20uF, now it's only 10uF.


In addition, the life index of the electrolytic capacitor also has a feature, which must explain the life under the working environment temperature of how many degrees.

And are often prescribed for the life of the under 105 ℃ environment temperature.


This is because the electrolytic capacitors commonly used nowadays are electrolytic capacitors in liquid electrolyte. If the electrolyte is dry, the capacitance is of course gone.

The higher the temperature, the easier it is for the electrolyte to evaporate.

Therefore, the life index of the electrolytic capacitor must indicate the life at what ambient temperature.


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 if you assume that all electrolytic capacitors have a life of only 1,000 hours.

That would be 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.

So is there a general quantitative relationship between life and temperature?

Some!


One of the simplest and most computable relationships is that for every 10 degrees rise in ambient temperature, life expectancy is halved.

Conversely, for every 10 degrees decrease in ambient temperature, the lifespan doubles.

This is a simple estimate, of course, but it's also pretty accurate.


Since the electrolytic capacitors used in LED drive power supply must be placed inside the LED lamp housing, we can know the working life of the electrolytic capacitor as long as we know the temperature inside the LED lamp.



2. What is the ambient temperature in the LED lamp?


Since leds and electrolytic capacitors are in the same enclosure in many lamps, the ambient temperature is simply the same.

The ambient temperature is mainly determined by the heat and heat balance of LED and power.

Moreover, the heat 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, namely, a well-designed LED lamp, which allows for a certain internal ambient temperature.

This is because the junction temperature of the LED chip is the main reason for the LED chip's optical failure (life). Of course, the LED junction temperature is also related to its ambient temperature, so as long as the allowable LED junction temperature is known, the ambient temperature inside the LED lamp can be calculated.


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 shell be 105 degrees?

Just look at the picture below.

That's CREE's connection to the mild light failure of the LED chip junction.



If the environment temperature is 105 ℃, there must be at least add 20 ℃ is the junction temperature, so the junction temperature is about 125 ℃.

It's no longer found on this curve, only roughly estimated to have a lifespan of 4,000 hours.

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 for 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 the actual LED light, it's easy to calculate its actual life.

The actual lifetime of several commonly used 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 capacitor


4,1 extends its life by design


In fact, to extend the life of the electrolytic capacitor, the method is very simple, because its end is mainly due to the liquid electrolyte evaporation results, if improve its sealing, do not let it evaporate, its life will naturally extend.

For example, a double-layer sealing system designed by Evox Rifa can slow down the evaporation rate of electrolyte.


Also, the loss of electrolyte can be greatly reduced by the use of a phenolic plastic cover with an integral wound around the electrode and a double special sealing pad that is closely interlocked with the aluminum case.


4.2 extend its life from use


Reducing the ripple current can also extend its service life. If the ripple current is too large, two capacitance parallel methods can be adopted to reduce it


4.3 select to extend its life


When selecting electrolytic capacitors, in addition to the brand electrolytic capacitors that ensure quality, there should be a margin on voltage and capacity.

For example, the dc voltage of 220V will be as high as 300V after the bridge rectification, but at least 450V pressure-resistant electrolytic capacitance is selected when the electrolytic capacitance is selected.

If you need 10uF, you'd better use 20uF.

These measures can also extend the life of electrolytic capacitors.

Since the capacitance's equivalent resistance and ripple current causes the internal temperature of the capacitor to be higher than the ambient temperature, a margin is necessary.



5. Protective electrolytic capacitance


Sometimes even with the use of long - life electrolytic capacitors, it is often found that electrolytic capacitors are broken. What is the reason?


In fact, if you think this is electrolytic capacitance quality is not pass, that can really make the electrolytic capacitors have a fair cause!

In fact, the electrolytic capacitor is not the perpetrator, but the victim.

Why?


Because we know that in the alternating current network of municipal power, there will often be instantaneous high voltage surge caused by lightning strikes. Although many lightning protection measures have been made for lightning strikes on the large power grid, it is still inevitable that the fish leaking from the grid will leak to residents' homes.


The electric grid in the United States should be very advanced, but there was an incident in my home, after a lightning strike, I found that my fax machine could not work, after careful inspection, it turned out that because of the lightning strike the power supply was completely broken, can only be scrapped.


If the LED lamp is powered by municipal power, it is necessary to add anti-surge measures at the municipal power input end in the power supply of the lamp, including fuse and over-voltage protection resistance, commonly known as pressure-sensitive resistance, to protect the components behind, otherwise the long life electrolytic capacitor will also be broken by surge voltage.


I believe that if all LED manufacturers can adopt normal high quality electrolytic capacitors and adopt the above measures, then the unfair treatment of the electrolytic capacitors will definitely be cleaned!