Due to its low cost, aluminum electrolytic capacitors have always been a common choice for power supplies. However, they have a limited life span and are susceptible to extreme conditions of high temperatures and low temperatures. The aluminum electrolytic capacitor places a metal foil on both sides of the paper sheet impregnated with the electrolyte. This electrolyte evaporates during the life of the capacitor, changing its electrical properties. If the capacitor fails, it will react violently: pressure builds up in the capacitor, forcing it to release flammable, corrosive gases.
The rate at which the electrolyte evaporates is closely related to the capacitor temperature. For every 10 degrees Celsius drop in operating temperature, the capacitor life is doubled. The rated life of a capacitor is usually the result at its maximum rated temperature. A typical rating life is 1000 hours at 105 degrees Celsius. When these capacitors are selected for long-life applications such as the LED bulbs shown in Figure 1, the lifetime of the LED is 25,000 hours, and the life of the capacitor becomes a problem. To achieve a 25,000-hour life, this capacitor requires an operating temperature of no more than 65 degrees Celsius. This operating temperature is particularly challenging because in this application the ambient temperature can exceed 125 degrees Celsius. There are some high temperature rated capacitors on the market, but in most cases, aluminum electrolytic capacitors will become the bottleneck component of LED bulb life.
Figure 1. This 105°C capacitor may not reach its claimed 23-year life.
This lifetime temperature dependence actually affects how you reduce the capacitor's rated voltage. The first thing you might think of is to increase the capacitor's rated voltage to minimize the chance of dielectric failure. However, doing so will result in a higher equivalent series resistance (ESR) of the capacitor. Since capacitors typically have high ripple current stress, this high resistance introduces additional internal power dissipation and increases capacitor temperature. The failure rate increases as the temperature increases. In fact, aluminum electrolytic capacitors usually use only about 80% of their rated voltage.
When the capacitor temperature is low, the ESR increases sharply, as shown in Figure 2. In this case, the resistance increases by an order of magnitude at -40oC. This affects power performance in many ways. If the capacitor is used at the output of a switched mode power supply, the output ripple voltage is increased by an order of magnitude. In addition, at the zero or higher frequency formed by the ESR and the output capacitor, it increases the loop gain by an order of magnitude, thereby affecting the control loop. This produces an unstable power supply that oscillates. To accommodate this strong vibration, the control loop typically makes a huge compromise in space and works at higher temperatures.
Figure 2 ESR performance drops sharply at low temperatures
In summary, aluminum electrolytic capacitors are usually the lowest cost option. However, you need to determine if its shortcomings will adversely affect your application. You need to consider the length of life through its working temperature. In addition, you should reduce the voltage rating appropriately so that you can achieve the lowest temperature operation for the longest life. Finally, you need to understand the range of ESRs that must be used so that you can properly design the control loop to meet the ripple specifications of your design.
Next time, we will discuss a low-power offline reverse structure, so stay tuned.
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