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Avoiding Common Defects of Aluminum Electrolytic Capacitors

elecfans | 19/04/2021

Because of 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 at high and low temperatures. In aluminum electrolytic capacitors, metal sheets are placed on both sides of the paper sheet soaked in the electrolyte. This electrolyte will evaporate during the life of the capacitor, thereby changing its electrical properties. If the capacitor fails, it will react violently: pressure builds up in the capacitor, forcing it to release flammable and corrosive gases.

The speed at which the electrolyte evaporates is closely related to the temperature of the capacitor. For every 10 degree Celsius drop in operating temperature, the life of the capacitor doubles. The rated life of a capacitor is usually the result obtained at its maximum rated temperature.

The typical rated life is 1000 hours at 105 degrees Celsius. When choosing these capacitors for long-life applications such as LED bulbs as shown in Figure 1 (the life of the LED is 25,000 hours), the life of the capacitor becomes a problem. To achieve 25,000 hours of life, this capacitor requires an operating temperature not exceeding 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-rated temperature capacitors on the market, but in most cases, aluminum electrolytic capacitors will become the bottleneck component of the life of the LED bulb

This life temperature dependence actually affects the way you lower the rated voltage of the capacitor. Your first thought may be to increase the rated voltage of the capacitor to minimize the probability of dielectric failure. However, doing so will make the equivalent series resistance (ESR) of the capacitor higher. Since capacitors generally have high ripple current stress, this high resistance will cause additional internal power dissipation and increase the capacitor temperature. The failure rate increases with increasing temperature. In fact, aluminum electrolytic capacitors usually only use 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 orders of magnitude at -40°C. This will affect power supply performance in many ways. If a capacitor is used at the output of a switching power supply, the output ripple voltage increases by orders of magnitude. In addition, at frequencies above zero formed by the ESR and the output capacitor, it increases the loop gain by an order of magnitude, thereby affecting the control loop. This creates an unstable power supply with oscillations. In order to adapt to such strong vibrations, the control loop usually makes huge compromises in space and works at higher temperatures.

In short, aluminum electrolytic capacitors are usually the lowest cost option. However, you need to determine whether its shortcomings will adversely affect the application. You need to consider its life span through its operating temperature. In addition, you have to reduce its rated voltage appropriately so that you can achieve the lowest temperature operation and obtain the longest service life. Finally, you need to understand the range of ESR that must be used so that you can correctly design the control loop to meet the ripple specification requirements of the design.

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