Capacitors Derating and Category Concepts

The article explains capacitor derating principle and introduce category concepts. Almost all major capacitor technologies need a certain derating at their corner operating conditions.

However, physical reasons for this may be specific to individual capacitor technology – reliability, stability of the main electrical parameters or protection against excessive surge current …

There are two most common derating parameters: voltage (that may include hidden current limitation) and temperature

The derating factors are typically in “OR“, “whatever is greater” logic relationship, so if the voltage derating rule says 20% and due to the temperature you have to derate 30% as well, “whatever is greater” condition applies – it means that the 30% derating is covering both voltage and temperature derating requirements.

Example of capacitor voltage – derating chart:

Figure 1. tantalum polymer capacitors derating example chart; source: Kemet Electronics

Category Concepts and Derating

Recommendation for voltage derating means that the actual capacitor shall be used in the application at lower voltage than rated voltage. Derating is expressed usually by percentage of rated voltage that shall be subtracted. For example 20% derating means that the capacitor shall be used at 80% of rated voltage at the specific applications (10V capacitor to be used on 8V maximum).

The purpose of the derating is to reduce amount of load accelerating factors to the capacitors. The two main accelerating factors are voltage and temperature.

As per the equation C1-20 energy content is depending to voltage squared, thus voltage reduction (voltage derating) has a significant impact to overall energy handling through the capacitor. Reasons for voltage derating can be various depending to the capacitor technology, construction and applications.  The main general reasons for voltage derating can be as follows, nevertheless it may be good to study the capacitor manufacturer’s application guidelines.

Capacitors designed for DC voltages produce no internal heating. Therefore they often can be used with more or less reduced voltages up to the so called upper category voltage where the temperature characteristics of the material put a limit. This occur at the upper category temperature, TUC, in other nomenclatures called maximum usage temperature. The connections are shown in the following Figure 2. Varying derating curves are shown in MIL-HDBK-1547.

Figure 2. Typical voltage derating at the upper category temperature.

Note: derating due to the high operating temperature (see below) is sometimes called “temperature derating”, but this may cause some confusion. Degradation mechanisms are usually accelerated by both temperature and voltage factors, just with different root coefficient and predominant impacts. Thus the term “temperature derating” should be left to “limitation of use of the capacitor at lower then rated temperature due to a predominant temperature driven physical degradation mechanism”. In other words, we need mostly to have a voltage derating in place to limit amount of overall energy in the capacitor, but in some cases degradation process is accelerated more by temperature factor (and we want to limit this by limitation of maximum exposed temperature “temperature derating”)

Some capacitors, such as tantalum solid capacitors, may have limitation in its maximum allowed current surge spike. Current surge overloading may cause in some cases even thermal destruction and fatal failures in some cases.  The practical method to increase the surge current load capability is to use higher voltage capacitor, in other words use higher voltage derating. The derating recommendation may be then dependent to circuit function, application or specific capacitor technology.

As an example of solid tantalum capacitors the basic rules are:

tantalum MnO2 capacitors: 50% derating in high current surge applications (such as input side of DC/DC converters or directly on battery), 20% for other applications (coupling, timing, DC/DC output)

tantalum polymer capacitors: 10% for all circuits for <= 10V capacitors, 20% for all circuits for >10V capacitors

These derating guidelines are typically specified to 105°C (temperature derating). Additional derating may be necessary up to 125°C.

voltage is one of the strongest accelerator for number of failure mechanisms and thus its reduction may significantly improve the component reliability.

As an example aluminium electrolytic or film capacitors life time is strongly influenced by applied voltage and voltage derating is the most effective way to increase life time and reduce MTBF rate.

voltage may play an important inhibitor role in number of mechanisms. High K ferro-dielectrics such as BaTiO3 used in Class II MLCC capacitors are featuring strong dependency of capacitance value to AC and DC voltage (DC BIAS voltage impact). Applied voltage is also condition for piezo-effect that may cause harmful audio noise generation by MLCC class II capacitors.

Voltage derating may significantly suppress these phenomenons and thus improve performance of MLCC class II capacitors.

Multiplication of derating requirements

Different voltage derating requirements are usually in “OR” logic,”whatever is greater” relation. It means that the greatest derating principle is applied only.

in example: 12V input side of DC/DC converter (high surge current load application). Maximum operated temperature of end devices:  125°C and 105°C. Can we use 16V tantalum polymer or tantalum MnO2 capacitors?

Exit mobile version