What affecting the lifespan of electrolytic capacitors

2024-02-05

Electrolytic capacitors are widely used in various fields of power electronics, mainly for smoothing, energy storage, or filtering after rectification of AC voltage.

They are also used for non-precision timing and delay. When estimating the Mean Time Between Failures (MTBF) of switch-mode power supplies, model analysis results show that electrolytic capacitors are the main factors affecting the lifespan of switch-mode power supplies. Therefore, understanding the factors affecting capacitor lifespan is crucial. The lifespan of electrolytic capacitors depends on their internal temperature. Therefore, both the design and application conditions of electrolytic capacitors will affect their lifespan. From a design perspective, the design methods, materials, and processing techniques of electrolytic capacitors determine their lifespan and stability. For users, factors such as operating voltage, ripple current, switching frequency, installation form, and heat dissipation method all affect the lifespan of electrolytic capacitors.

Abnormal failure of electrolytic capacitors:  

 Some factors can cause electrolytic capacitors to fail, such as extremely low temperatures, capacitor temperature rise (welding temperature, ambient temperature, AC ripple), excessive voltage, transient voltage, very high frequency, or reverse bias voltage. Among these factors, temperature rise is the most significant factor affecting the working lifespan(Lop) of electrolytic capacitors.

The conductivity of capacitors is determined by the ionization ability and viscosity of the electrolyte. When the temperature decreases, the viscosity of the electrolyte increases, resulting in decreased ion mobility and conductivity. When the electrolyte freezes, the ion mobility is very low, resulting in very high resistance. Conversely, excessive heat will accelerate the evaporation of the electrolyte, and when the amount of electrolyte decreases to a certain limit, the lifespan of the capacitor will also end. When operating in high-cold areas (generally below -25°C), heating is required to ensure the normal operating temperature of electrolytic capacitors. For example, outdoor UPS units in cold are are equipped with heating plates.

 Capacitors are prone to breakdown under overvoltage conditions, and surges and transient high voltages are common in practical applications. Especially in China, with its vast territory and complex power grids, the AC power grid is very complex, and voltages exceeding 30% of normal voltage are often encountered, especially in single-phase input, which exacerbates the normal range of AC input. Testing has shown that commonly used 450V/470uF 105°C imported ordinary electrolytic capacitors can leak and vent gas after 2 hours at 1.34 times the rated voltage, with the top bursting open. According to statistics and analysis, the failure of PFC output electrolytic capacitors in communication switch-mode power supplies close to the power grid is mainly due to surges and high voltages. The voltage selection of electrolytic capacitors generally undergoes secondary derating, reducing it to 80% of the rated value for more reasonable use.

Factors affecting lifespan analysis 

Apart from abnormal failures, the lifespan of electrolytic capacitors has an exponential relationship with temperature. As non-solid electrolyte is used, the lifespan of electrolytic capacitors also depends on the rate of electrolyte evaporation, resulting in a decrease in electrical performance.

These parameters include the capacitance, leakage current, and equivalent series resistance (ESR) of the capacitor.

Referring to RIFA's predicted lifespan formula: PLOSS = (IRMS)² × ESR (1), Th = Ta + PLOSS × Rth (2), Lop = A × 2 Hours (3), where B = reference temperature value (typically 85°C), A = capacitor lifespan at reference temperature (varies with capacitor diameter), and C = temperature rise required to halve the capacitor lifespan.

From the above formulas, we can clearly see several direct factors affecting the lifespan of electrolytic capacitors: ripple current (IRMS) and equivalent series resistance (ESR), ambient temperature (Ta), and total thermal resistance (Rth) from the hot spot to the surrounding environment. The hottest point inside the capacitor, known as the hot spot temperature (Th), is the primary factor affecting the capacitor's operational lifespan. The following factors determine the actual application of the hot spot temperature: ambient temperature (Ta), total thermal resistance from the hot spot to the surrounding environment (Rth), and energy loss caused by AC current (PLOSS). The internal temperature rise of the capacitor is linearly related to the energy loss.

When charging and discharging, energy loss occurs as the current passes through resistance and as the voltage changes across the dielectric. This, coupled with energy loss due to leakage current, results in an increase in the internal temperature of the capacitor.

3.1 Design considerations 

In capacitors with non-solid electrolytes, the dielectric is an anodic aluminum oxide layer. The electrolyte serves as the electrical contact between the cathodic aluminum foil and the anodic aluminum oxide layer. The paper interlayer that absorbs the electrolyte acts as an isolation layer between the cathodic aluminum foil and the anodic aluminum oxide layer, with the aluminum foil connected to the terminal via electrode connection pieces.

Reducing the ESR value can reduce the internal temperature rise of the capacitor caused by ripple current. This can be achieved by using multiple electrode connection pieces or laser welding electrodes.

ESR value and ripple current determine the temperature rise of the capacitor. One of the main measures to achieve satisfactory ESR values for capacitors is to typically use one or more metal electrode connection pieces to connect the external electrode and the core package, reducing the impedance between the core package and the pins. The more electrode connection pieces on the core package, the lower the ESR value of the capacitor. With laser welding technology, more electrode connection pieces can be added to the core package, allowing the capacitor to achieve lower ESR values. This also means that the capacitor can withstand higher ripple currents and have lower internal temperature rises, resulting in a longer operational lifespan. This also helps to improve the capacitor's ability to withstand vibrations, otherwise it may lead to internal short circuits, high leakage currents, capacitance losses, increases in ESR values, and circuit open circuits.

By ensuring good mechanical contact between the capacitor core package and the bottom of the aluminum shell and through the heat sink in the middle of the core package, internal heat can be effectively released from the bottom of the aluminum shell to the connected base plate.

Internal thermal conduction design is extremely important for the stability and operational lifespan of capacitors. In Evox Rifa's design, the negative aluminum foil is extended to directly contact the bottom of the capacitor's aluminum shell. This bottom becomes the heat sink for the core package to release heat from the hot spot. By using bolt installation methods to securely install the capacitor onto the base plate (usually aluminum), a more comprehensive thermal conduction solution with lower thermal resistance (Rth) can be obtained. By using an injection-molded phenolic plastic cover with electrodes and double-special seals tightly fitting the aluminum shell, electrolyte loss can be greatly reduced.

Electrolyte loss through the sealing gasket determines the working time of long-life electrolytic capacitors. When the electrolyte of the capacitor evaporates to a certain extent, the capacitor will eventually fail (this result will be accelerated by internal temperature rise). The double-seal system designed by Evox Rifa can slow down the evaporation rate of the electrolyte, allowing the capacitor to achieve its longest operational lifespan. All these features ensure that the capacitor has a very long operational lifespan in the required fields.

3.2. Factors affecting lifespan in application  

According to the lifespan formula, factors influencing lifespan include: ripple current (IRMS), ambient temperature (Ta), and total thermal resistance from the hot spot to the surrounding environment (Rth).

Ripple Current

The magnitude of the ripple current directly affects the hot spot temperature inside the electrolytic capacitor. By referring to the capacitor's user manual, you can obtain the allowable range of ripple current. If it exceeds this range, parallel connection can be used to resolve it.

Ambient Temperature (Ta) and Thermal Resistance (Rth)

 According to the hot spot temperature formula, the application ambient temperature of the electrolytic capacitor is also an important factor. When applying it, consider factors such as ambient heat dissipation method, heat dissipation intensity, distance between the electrolytic capacitor and the heat source, and installation method of the electrolytic capacitor. The heat inside the capacitor always conducts from the hottest point, the "hot spot," to the relatively cooler parts. There are several pathways for heat conduction: one is through the aluminum foil and the electrolyte. If the capacitor is mounted on a heat sink, some of the heat will also be transferred to the environment through the heat sink. Different installation methods, spacing, and heat dissipation methods will all affect the thermal resistance from the hot spot to the environment.

Total thermal resistance

The total thermal resistance from the "hot spot" to the surrounding environment is denoted by Rth. Using clamp installation, installing the capacitor on a heat sink with a thermal resistance of 2℃/W, the obtained thermal resistance value of the capacitor Rth = 3.6℃/W; using bolt installation, installing the capacitor on a heat sink with a thermal resistance of 2℃/W and a forced air cooling rate of 2m/s, the obtained thermal resistance value of the capacitor Rth = 2.1℃/W (taking the PEH200OO427AM capacitor as an example, with an ambient temperature of 85℃). Additionally, directly extending the negative aluminum foil to contact the bottom of the capacitor's aluminum shell is also a good way to reduce thermal resistance. At the same time, attention should be paid to the fact that the aluminum shell will carry a negative charge and should not be connected to the negative pole. Capacitors must be properly installed to achieve their designed operational lifespan. For example, the RIFA PEH169 series and PEH200 series should be installed vertically upwards or horizontally. Ensure that the safety valve faces upwards so that the hot electrolyte and vapor can be smoothly discharged from the safety valve in the event of capacitor failure. When capacitors are arranged compactly, at least 5mm spacing should be left between adjacent capacitors to ensure adequate airflow. When using bolt installation, controlling the torque of the nut is very important. If it is too loose, the capacitor may not make good contact with the heat sink, and if it is too tight, it may damage the threads. At the same time, attention should be paid to not installing the capacitor upside down, as this may cause the bolts to break. Capacitors should be installed as far away from heat-generating components as possible, as excessively high temperatures can shorten the capacitor's lifespan, making it the shortest-lived component in the entire circuit. In cases of high ambient temperatures, forced air cooling should be used as much as possible, and capacitors should be installed at the air intake.

Frequency Influence If the current consists of fundamental frequency and multiple harmonics, the power loss values generated by each harmonic must be calculated and the results added together to obtain the total loss value. In high-frequency applications, the leads of the capacitor should be kept as short as possible to reduce equivalent inductance. The resonance frequency (fR) of the capacitor varies depending on the type of capacitor. For welded and bolted aluminum electrolytic capacitors, the resonance frequency is between 1.5kHz and 150kHz. If the capacitor is used at a frequency higher than the resonance frequency, its external characteristics become inductive.

 In conclusion, by selecting the correct application conditions and environment, the lifespan of electrolytic capacitors can be ensured, provided abnormal failures are avoided.