The useful life values stated in data sheets apply to aluminum electrolytic capacitors with natural cooling, i.e. the heat generated in the winding is dissipated through the casing and by natural convection. It is possible to increase the permissible ripple current and/or prolong the useful life by using additional cooling by heat sinks or forced ventilation. Conversely, impaired cooling (e.g. due to closely packed capacitor banks, thermal insulating, sealing, and vacuum) will reduce the useful life.

**Forced air cooling**

**Forced air cooling**

In the case of forced air cooling it should be kept in mind that the mounting position can influence only the thermal resistance between the case and the surrounding air. Under natural convection, this thermal resistance is greater than the inner thermal resistance between the capacitor winding and the case.

For a given ripple current load, the thermal resistance is proportional to the difference between the capacitor case temperature and the ambient temperature. The user can measure this temperature difference (T_{case} - T_{A}) under normal conditions (ΔT)and under forced air condition (ΔT*) by applying constant ripple current load conditions.

Hence the relative reduction or increase of the thermal resistance can be calculated from the forced cooling ratio ΔT*/ΔT. In turn, the forced cooling ratio can be used to determine the ripple current factor I_{AC}*/I_{AC}. The latter is a measure of how much the ripple current load can be increased without reducing the useful life if forced cooling is used.

The diagram below shows the effect of the forced cooling ratio, as determined by measurement, on the ripple current factor I_{AC}*/I_{AC} for various case sizes. In this diagram, the useful life of the capacitor with forced cooling (ripple current load: I_{AC}*) has been equated to the useful life rating of the aluminum electrolytic capacitor under normal operating conditions (ripple current load: I_{AC}).

**Figure 2:** *Effect of forced cooling on the ripple current capability; Temperature difference ΔT=(T _{case} - T_{A}); I_{AC }permissible ripple current under normal conditions (natural convection cooling); * Values for forced cooling*

The following table gives typical values for the forced cooling ratios that can be achieved by forced convection with the respective air velocities.

Note that the ripple current capability I_{AC}* of aluminum electrolytic capacitors with impaired heat dissipation is lower than the rated value I_{AC}.

**Base cooling with heat sink**

**Base cooling with heat sink**

As a large amount of heat is dissipated through the base of the case, the use of a heat sink connected to the capacitor base is the most efficient cooling method. For heat sink mounting, specially designed versions of high-voltage capacitors with screw or snap-in terminals are available in order to ensure an optimal heat transfer from the heat generation area via the base of the case to the heat sink.

If a cooling plate with cooling fluid (e.g. water or oil) is used that is colder than the ambient temperature, the forced cooling ratio shown in figure 2 may be reduced to zero or may even have a negative value. Due to the limited thermal capacity of such media, the linear laws assumed for the use of pure thermal resistances no longer apply. In such cases, the forced cooling ratio is also a function of the power dissipated in the capacitor itself. If such cooling measures are to be used, the maximum possible thermal load must be calculated. This is not necessary if only cooling elements and/or forced convection are used.

**For further information on aluminum electrolytic capacitors please read the following articles:**

Calculating the Useful Life of Capacitors

Aluminum Electrolytic Capacitors – Overview and Key Applications

Design of Aluminum Electrolytic Capacitors and General Features

Mounting Positions of Aluminum Electrolytic Capacitors with Screw Terminals