The Influencing Factors in Aluminum Capacitor Lifetime

Key Takeaways

• There are two grades of aluminum electrolytic capacitors available and they are categorized based on reliability.

• Capacitor degradation and sudden failure are the two main reasons for an aluminum capacitor’s short lifetime.

• The lifetime of an aluminum capacitor is approximately halved for every 10℃ rise in the ambient temperature.

An aluminum capacitor’s lifetime is directly related to the reliable operation of the finished product

For high capacitance-voltage products (large energy storage) with low equivalent series resistance (ESR), power supply manufacturers rely on electrolytic capacitors. One such capacitor is the aluminum electrolytic capacitor, otherwise called an aluminum capacitor. These capacitors are widely used in power supply circuit design due to benefits such as high price/performance ratio, high volumetric efficiency, and availability.

However, the relatively short aluminum capacitor lifetime poses a challenge to the normal operation, reliability, and lifespan of power supplies. The short life expectancy of aluminum capacitors is one of the main reasons for the premature failure of power supplies.

Why do aluminum capacitors have such a short lifespan? There are various factors influencing their lifespan. In this article, we will explore capacitor grades as well as the causes and effects of various factors on the useful lifetime of aluminum capacitors. 

Aluminum Capacitor Grades

There are two grades of aluminum electrolytic capacitors available:

1. Aluminum capacitors for general purpose applications - As per IEC publications, general-purpose (GP) grade aluminum capacitors are less reliable, with limited operating life. 
2. Aluminum capacitors for high-reliability applications - These are long-life grade aluminum capacitors with materials specifically selected to ensure purity requirements are met. High-reliability aluminum capacitors come with the letters ‘LL’ stamped on them. 

Aluminum capacitors are vulnerable to failure and damage. Manufacturers specify the useful service hours in the aluminum capacitor datasheet at nominal voltage, current, and upper category temperatures, with limits specified on the capacitance variation, dissipation factor, and leakage current.

However, service life drops due to the influence of various environmental parameters such as humidity, atmospheric pressure, vibrations, etc. It is important to accurately estimate the aluminum capacitor lifetime to ensure the lifetime of the product that utilizes it. Let's take a look at a few factors that influence aluminum capacitor lifetime. 

Aluminum Capacitor Lifetime 

The useful lifetime of an aluminum capacitor is the service hours achieved by the aluminum capacitor without exceeding the specified percentage of failures. There is a permissible range of parametric variation allowed for each aluminum capacitor. When the circuit operation fails to adhere to the allowable parametric variations given in the datasheet, the aluminum capacitor deviates from the specified useful lifetime. Ambient conditions are crucial in aggravating the effect of failures on aluminum capacitors.

According to the failure rate versus time graph of aluminum capacitors, commonly referred to as the failure rate curve or bathtub curve, the lifetime of an aluminum electrolytic capacitor can be divided into five time periods:

1. Guaranteed lifespan - Maintains the specified performance that is given in the product transactions. 

2. Design lifetime - Targeted at the design stage.

3. Practical lifespan - Precursory period of time where the aluminum capacitor enters into the burnout failure phase.

4. Effective lifespan - The period of time where the aluminum capacitor overcomes the infant mortality stage and enters the burnout failure stage. 

5. Average lifespan - The average or mean of the lifespan prior to the burnout failure phase. 

Factors That Reduce Aluminum Capacitor Lifetime 

Failures due to parametric variation, otherwise called capacitor degradation, can end the useful life of aluminum capacitors. There are two main reasons for an aluminum capacitor’s short lifetime:

1. Capacitor degradation - Degrades due to changes in characteristics or parameters such as capacitance, equivalent series resistance, leakage current, etc. The aluminum capacitor becomes unsuitable for application with parametric variations. 

2. Sudden failures - Vaporization of the electrolyte, short circuits, and open circuits can all lead to the sudden failure of an aluminum capacitor. 

The Practical Value of an Aluminum Capacitor Lifetime

The actual lifetime of an aluminum capacitor is diminished by the effect of temperature voltage, current, etc. The practical value of aluminum capacitor lifetime can be given by:

L_X is the resulting lifetime, L₀ is the lifetime specified in the datasheet under nominal conditions, K_T is the temperature factor, K_R is the ripple current factor, and K_V is the voltage factor. 

Temperature, Voltage, and Current Ripple

The table below summarizes the effects of temperature, voltage, and current ripple on the useful lifetime of aluminum capacitors. 

To extend the lifetime of end equipment such as power supplies and power electronics converters, you must understand your aluminum capacitor’s lifetime. You can rely on Cadence’s PCB design and analysis tools to build reliable circuits using electrolytic capacitors. Cadence’s tools help designers identify reliable electrolytic capacitors for circuit design.

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