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Power tip 50 is on aluminum electrolytic capacitors.
Aluminum electrolytic capacitors remain a popular choice in power supplies due to their low cost. However, they have limited life and are sensitive to both hot and cold temperature extremes. Aluminum electrolytic capacitors are constructed with foils placed on opposite sides of paper saturated with an electrolyte. This electrolyte evaporates over the capacitorís lifetime, altering its electrical properties. If the capacitor fails, it can be spectacular as pressure builds up in the capacitor, forcing it to vent a combustible and corrosive gas.
The rate at which the electrolytic evaporates is a strong function of the capacitorís temperature. For every 10 degree Centigrade decrease in operating temperature, the capacitor life is extended by a factor of two. Capacitor life ratings generally are specified at their maximum rated temperature. A typical rating might be 1,000 hours at 105 degree Centigrade. Selecting these capacitors for use in long-life applications, such as the LED light bulb shown in Figure 1, where the LEDís must operate for 25,000 hours is problematic. To achieve the full 25,000 hour life, this capacitor requires a temperature of no more than 65 degrees Centigrade. This is particularly challenging, as the ambient temperature in this kind of application can exceed 125 degrees Centigrade. There are capacitors available that are rated for higher temperatures, but in most instances the aluminum electrolytic capacitor is going to be the life-limiting component of LED replacement bulbs.
Figure 1: This 105oC capacitor probably wonít last 23 years as claimed.
This life-temperature dependence actually impacts how you should derate the voltage on the capacitor. Your first thought might be to increase the capacitorís voltage rating to minimize the possibility of a dielectric failure. However, doing so can lead to a capacitor with a higher equivalent series resistance (ESR). Because the capacitor typically has a high ripple current stress, this higher resistance leads to extra internal power loss and increased capacitor temperature. The failure rate increases with the increased temperature. In practice, aluminum electrolytic capacitors typically are used at about 80% of their rated voltage.
Cold temperature with these capacitors can result in significant increase in ESR as shown in Figure 2. In this case, the resistance can increase as much as an order of magnitude at -40oC. This impacts power supply performance in a number of ways. If the capacitor is used in the output of a switching power supply, output ripple voltage increases by an order of magnitude. It also impacts the control loop by making the loop gain an order of magnitude higher at frequencies above the zero formed by the ESR and output capacitance. This can result in an unstable power supply with oscillations. To accommodate this large variation, the control loop usually is severely compromised for room and higher temperature operation.
Figure 2: ESR degrades significantly at cold temperatures.
To summarize, aluminum electrolytic capacitors are usually the lowest cost option. However, you need to determine if their shortcomings will have a negative impact in their application. You need to consider their life expectancy as a function of their operating temperature. And you need to properly derate their voltage so that you can achieve the coolest running approach and maximize life. Finally, you need to understand the ESR range you have to work over so that you can design your control loop correctly and meet the ripple specifications of your design.
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