Capacitors of any type provide longer life below their maximum voltage rating. The performance of such parts will be degraded when voltages close to their nominal values are applied and when subjected to high temperatures. This effect can be reduced by choosing to limit the applied voltage.
Ceramic capacitors are currently the most common type of capacitors, and their packaging and assembly are compact. Their name comes from the building material, they consist of alternating layers of metal paste and ceramic powder, which are then fired to harden the ceramic material. Because they are non-polarizing components, they can be used in both AC and DC circuits and have a range of capacitances making them ideal for coupling, decoupling and filtering circuits.
One of the advantages of ceramic capacitors is their high maximum voltage rating. When their rated voltage is slightly higher than the rated voltage, their capacitance drops without any major failure. Ceramic materials tend to crack when subjected to a voltage well in excess of the maximum rated voltage, causing short circuits between the metal plates. With overcurrent protection, this failure mode will be relatively safe. However, it is important for the designer to select the correct voltage drop of the ceramic capacitor to ensure that this failure does not occur during operation and thus preserve the life of the new design.The Importance of Reducing the Voltage of a Ceramic Capacitor
An important factor to consider is that the capacitance value of ceramic capacitors will decrease as the voltage across the component approaches its maximum voltage rating. In some components, this reduction can significantly affect the operation of the circuit. This effect is strongly influenced by the physical size of the component. The rated capacitance of SM06D ceramic capacitors is much smaller than the rated capacitance of SM06 ceramic capacitors. This effect is also more pronounced in high dielectric components such as class II dielectric devices (eg B/X5R and R/X7R). This effect can be problematic when DC bias is present on ceramic capacitors in signal processing circuits. The bias voltage can significantly reduce the overall capacitance, which affects the performance of the underlying circuit. A signal voltage superimposed on a bias voltage can amplify or attenuate this change, depending on its polarity, resulting in a capacitance change proportional to the signal voltage. The effect of consolidation is non-linear due to the change in capacitance. This problem is solved by ensuring that the maximumCapacitor voltage, calculated from signal peak voltage and DC bias voltage, remained within the capacitance specifications of the component with minimal capacitance changes. This may require careful selection of components with dielectric properties that meet the designer's requirements.
Another influence on ceramic capacitors is exposure to fast transients within the rated voltage. When voltage is maintained within acceptable limits, the rate of change in voltage can damage the ceramic material over time, reducing component life and increasing the likelihood of failure.What ceramic capacitor derating should I use?
There is a general rule of thumb that ceramic capacitors should be derated by at least 25% as a standard, but this should be increased to at least 50% in environments where they will be subjected to voltage ripple. The maximum voltage rating of the component must be at least twice the maximum voltage that can be applied to the component during normal operation.
A more accurate calculation can be made by looking at the relationship between the breakdown voltage and the maximum rated voltage. Typically, manufacturers calculate the maximum voltage rating by adding a margin to the breakdown voltage based on experience and judgment. The breakdown voltage is determined by the properties of the material used in the construction of the ceramic capacitor and the defects present in the material. The higher the quality of the manufacturing process, the higher the breakdown voltage, which is limited by the materials used. Interestingly, the higher the capacitance value, the less the effect of manufacturing defects on the breakdown voltage.
The properties of the ceramic-based insulating material dominated in the calculations; the study found that metallic elements had little effect on the results. The breakdown voltage is usually determined by the process of polarization within the dielectric, and not by any electrical breakdown. Manufacturers determine the breakdown voltage by defining the component's performance area. The voltage-related quality remains within the requirements of the equipment, and its predictable reliability is within the established limits. Any derating applied by the designer is then used in addition to the derating factor set by the manufacturer to calculate the maximum voltage rating based on the breakdown voltage.
One thing to keep in mind is that at first glance over-derating components may seem like the safest strategy, but it will result in physically larger or more expensive components being selected. The required additional board space may beseem unfeasible or may create other board layout and layout problems. In environments where mechanical vibrations may be present, larger components also increase the risk of internal cracking within the component. As with all design decisions, some implications require careful consideration.