Electrolytic and polymer hybrid capacitors have almost the same design: they consist of a cathode side and an anode side, and both are made of aluminum film. The anode film is oxidized to form an aluminum oxide layer, which forms the dielectric. The two films are rolled up using an isolation paper to form the coiled element (P1, P2).
P1
P2. Basic design of electrolytic and polymer capacitors
The difference between the two capacitors is the material used in the filling process, which is where the name comes from: electrolytic capacitors are filled with an electrolyte, while polymer hybrid capacitors use a polymeric electrolyte or a combination of solid and liquid polymers.
Both capacitors offer many advantages, such as small size but high capacitance value, low cost, and suitability for a wide range of designs, such as SMD, THT or snap-in designs.
Polymer hybrid capacitors have a higher ripple current capacity than electrolytic capacitors, as well as lower internal resistance at low temperatures and more stable capacitance at high frequencies. The disadvantage of both capacitor technologies is their limited service life. During operation, the electrolyte or liquid polymer will shrink (P3).
P3. The electrolyte or liquid polymer diffuses during operation, which shortens the service life of the capacitor.
The Arrhenius equation can roughly estimate the service life of capacitor.
The biggest factor affecting the service life of electrolytic and polymer hybrid capacitors is the core temperature of the capacitor, which rises with the ambient temperature and the level of ripple current applied. In addition, mechanical stress due to high ripple current can damage the oxide layer, resulting in a self-healing effect that consumes additional electrolyte. Self-healing is the ability of electrolytic capacitors and polymer hybrid capacitors to restore the oxide layer through a chemical reaction between the electrolyte and aluminum. Electrolyte shrinkage can also lead to deterioration of electrical parameters such as capacitance and parameters such as equivalent series resistance (ESR) and loss factor.
End of life is usually the stage where the data sheet parameters (usually the increase in capacitance loss and loss factor percentage) are not met.
When identifying capacitor products that meet the electrical parameters during the target operation of the final product, the user can use the Arrhenius equation for an initial evaluation. As shown in P4, the service life as a function of the diffusion coefficient is largely analogous to the Arrhenius equation. Thus, as a rule of thumb, it can be expressed as follows: a 50°F (10°C) reduction in operating temperature doubles the service life.
P4. Both the Arrhenius equation and the empirical method show that a decrease in operating temperature of 50°F (10 C)
doubles the life of the capacitor, providing almost consistent results
The Arrhenius equation provides only a rough guide, as it does not take into account the significant effect of ripple current on the self-heating effect.
In order to obtain an accurate value for the lifetime calculation, it is recommended that the user work with the appropriate capacitor supplier. This calculation requires the customer to provide a task profile detailing the actual operating hours in the relevant temperature range.
P5. Sample task profile shows what parameters the vendor needs to accurately calculate the lifetime
Each supplier uses a separate calculation for its own products, which includes temperature profiles and ripple current loads. Therefore, suppliers can use the task profiles provided by the customer for detailed lifetime calculations.
This also prevents the use of over-specified and more expensive capacitors.
Increasing the surface area of the heat sink is a good way to improve heat dissipation and thus extend the life of the capacitor. For example, active cooling through the use of fans or water can ensure better heat dissipation. Users can consider this type of cooling concept when verifying components and calculating service life.
The connection of the cooling element to the capacitor also plays a key role.
Connecting the cooling element directly to the component is often more effective than placing it on the other side of the board. In addition, the peripheral unit of the capacitor needs to be considered, as it radiates and absorbs heat simultaneously through the pins, especially if power semiconductors or other heat-generating components are installed nearby. If empirical data (e.g., on-state temperature, current, voltage, and frequency) is available, this heat input can be incorporated into the lifetime calculation.
If the user uses thermally conductive pastes or pads, their thermal resistance is the decisive factor. The lower the value, the higher the thermal efficiency. If the cooling element needs to be electrically isolated, an insulating thermal paste or a suitable solder pad should be selected.
If the user wishes to perform his own calculations or simulations, thermal resistance models can be obtained from the supplier from the core of the capacitor (winding element) to the legs and the package.
If the heat dissipation and the thermal resistance from the top cover or PCB to the cooling element are fully understood, additional heat dissipation or supply can be deduced. Once the possible heat dissipation is verified, the supplier may allow the use of higher ripple currents for the board layout, provided that the maximum ripple current specified by the supplier is not exceeded, as this would impose a mechanical load on the capacitor.
P6. Thermal equivalent circuit diagram of capacitor
When selecting a capacitor product, it is recommended that the Arrhenius equation be used to determine initial guidance values. By using the task profile, the lifetime of the capacitor selected for the application can be accurately calculated, which also takes into account the degree of self-heating caused by the ripple current. In order to maximize the capacitor lifetime, the user should investigate possible cooling concepts and involve the supplier or distributor during the development phase.