LED short life electrolytic capacitor is not wrong

So what is the actual situation?

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? Of course it is defined in hours. However, if the life index of an electrolytic capacitor is 1,000 hours, it does not mean that the electrolytic capacitor will be broken after a thousand hours. No, it just means that the capacity of this electrolytic capacitor is reduced by half after 1000 hours. Originally, the capacity was reduced by half. It is 20uF, and now it is only 10uF.

There is nothing wrong with the short-lived electrolytic capacitors

In addition, there is another characteristic of the life index of the electrolytic capacitor, which is to indicate the life in the case of how many degrees of working temperature. And usually it is specified as the life at 105 ° C ambient temperature.

This is because the electrolytic capacitors we use today are electrolytic capacitors using liquid electrolyte. If the electrolyte is dry, the capacitance is of course gone. The higher the temperature, the easier the electrolyte will evaporate. Therefore, the life indicator of the electrolytic capacitor must indicate the life at what ambient temperature.

So all current electrolytic capacitors are marked with a lifetime at 105 °C. For example, the most common electrolytic capacitor has a life of only 1,000 hours at 105 °C. But if you think that all electrolytic capacitors have a life of only 1,000 hours. That's a big mistake.

Simply put, if the ambient temperature is higher than 105 ° C, its life will be less than 1,000 hours, if the ambient temperature is lower than 105 ° C, then its life is higher than 1,000 hours. So is there a rough quantitative relationship between life and temperature? some!

One of the simplest and easy to calculate relationships is that for every 10 degrees increase in ambient temperature, life is reduced by half; in turn, for every 10 degrees decrease in ambient temperature, life expectancy is doubled. Of course this is just a simple estimate, but it is also quite accurate.

Because the electrolytic capacitor used for the LED driving power supply is definitely placed inside the LED lamp housing, we only need to know the internal temperature of the LED lamp to know the working life of the electrolytic capacitor.

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

Because LEDs and electrolytic capacitors are placed in the same housing in many fixtures, the ambient temperature is simply the same. This ambient temperature is mainly determined by the balance of heat and heat dissipation between the LED and the power supply. And the heat and heat dissipation of each LED luminaire is different, so how can we know the ambient temperature?

In fact, this problem can be reversed, that is, a well-designed LED luminaire, which allows the internal ambient temperature to be constant. This is because the LED chip junction temperature of the LED chips is mainly determined light failure (lifetime) is, of course, the LED junction temperature by ambient temperature, so long as we know the allowable LED junction temperature, it can be deduced LED lamps inside Ambient temperature. However, there are at least three thermal resistances, that is, the thermal resistance θjc of the LED chip to the outer casing, and the thermal resistance of the LED casing to the surface of the aluminum substrate . In fact, it passes through the solder, the copper foil, and the insulating layer to the aluminum plate, but among them The most important is the thermal resistance of the insulating layer, collectively referred to as θlv, and the third is the thermal resistance θla of the air from the aluminum plate to the blister.

Take the 3014 LED, its thermal resistance θjc is 90 ° C / W, because its power is only 0.1W, so the internal and external temperature difference is 9 ° C. The thermal resistance of the aluminum substrate is 1 ° C / W. For a 10 W luminaire, since all 10 W LEDs are mounted on the same aluminum substrate, the total temperature difference is 10 ° C, a total temperature difference of 19 ° C, the last Θla is difficult to estimate because it is related to the circulation of air. In the case where the internal air does not flow, the temperature difference is only about 1 °C, so the total is 20 °C. That is, the LED junction temperature is equal to the ambient temperature plus 20 °C.

So can the ambient temperature inside the bulb allow 105 degrees? Just look at the picture below and you will know. That is the relationship between CREE 's LED chip junction temperature and light decay.

If the ambient temperature is 105 ° C, then at least 20 ° C is the junction temperature, so the junction temperature is about 125 ° C. It has not been found on this curve and can only be roughly estimated to have a lifetime of only 4,000 hours. This is absolutely unacceptable! In other words, the ambient temperature in the LED bulb must be much lower than 105 °C!

We can in turn see what the ambient temperature should be based on the required LED lifetime. Assuming we require LEDs to have a lifetime of 100,000 hours, then the junction temperature can only be below 65 °C, so the ambient temperature must be below 45 °C. In other words, the operating temperature of the electrolytic capacitor must be lower than 45 °C.