A Python script has been written to simplify the tedious task of checking capacitor voltage ratings in design reviews. It works with schematics designed in Altium with the CERN component library.
By parsing the EDIF netlist that is generated from Altium, the script attempts to extract the voltage of each power rail from its name (e.g. P3V3, P12V) - the user can provide the voltages for nets whose name doesn't follow any of the recognized naming conventions.
The output of the script is a table (in text or OpenDocument spreadsheet formats) that juxtaposes the voltage rating of the design's capacitors with their actual voltage applied, easily pointing out if any capacitors of insufficient voltage rating are used.
More details can be found on the project's git repo.
Derating of Different Capacitor Technologies
Derating is the practice to either reduce the specific stress electronic components are exposed to during their operation or to increase the design strength towards this stress. In practical terms this means achieving a high ratio between the applied stress and the strength of the component to withstand that stress. For capacitors this is often times the applied voltage vs the rated voltage:
Derating factor = Applied Voltage/Rated Voltage
A variety of standards and literature exist specifying guidelines for maximum derating factors. For different capacitor technologies A. Birolini specifies in Reliability Engineering - Theory and Practice, 2014, 7th ed. (CERN library entry) the following derating factor limits:
Type | Voltage | Internal Temperature |
---|---|---|
Film, Ceramic | ≤0.5 | ≤0.5 |
Tantalum, solid | ≤0.5 | ≤0.5 |
Aluminium, wet | ≤0.8 | ≤0.5 |
Furthermore, please note a potential capacitance change over the DC and AC voltage range when designing, for example for multilayer ceramic capacitors, see OHWR: Electronics Design - Decoupling Articles and DC and AC Bias Dependence of MLCC Capacitors Including Temperature Dependence.
In general, the higher the applied derating, the lower is the capacitance change effect. In addition, X7R rated components and low tolerance components should be preferred over lower rated components such as X5R, Y5V.