Current Ratings Application Notes
Capacitor current ratings provided by many manufacturers need to be carefully evaluated as they can be misleading; not because the information provided is incorrect, but because of the information that may NOT be provided. BOTH frequency and temperature at which these ratings apply need to be factored into the “capacitor choice decision”. The application’s electrical and thermal parameters are rarely the same as the “environment” specified for the ratings provided. The “expected design life” at the ratings provided is not necessarily the same for all capacitor types!
Ripple current is the RMS value of the capacitor current in an application where the voltage across the capacitor is small (less than ~5% of DC rating). For switching supplies the voltage change across the capacitor may be much less than this. RMS capacitor current typically is not specified at a particular frequency and thus should be carefully considered. [The term "ripple" originated with vacuum tube capacitor input power supplies, and may not have any meaning in the context of some modern capacitor applications].
ESR (Effective Series Resistance) is a mathematical construct that “lumps” ALL of the capacitor losses together as an appropriate resistance connected in series with the capacitor. It is a valid concept if the current is near sinusoidal OR the ESR is essentially constant over the frequency spectrum of the current (fundamental and significant harmonics). In general ESR is frequency, temperature, and (in some cases for ceramic capacitors) bias voltage dependent! For aluminum electrolytics there is also an “aging” factor determined by run time, current, and temperature.
Although the ESR concept is truly valid only for power dissipation calculations at a specific frequency and temperature, ESR can be reasonably constant over some frequency and temperature range. ESR usually begins to rise above 100KHz., reducing allowable capacitor current. For some capacitor types ESR rises with temperature. Be very careful not to envision a real capacitor as “an ideal capacitor in series with a fixed resistor of ‘ESR’ ohms”! [Although good polypropylene capacitors may behave nearly this way over a very wide frequency range]
In spite of the above qualifications, ESR remains a VERY useful construct to estimate dissipation with familiar formulas. Current ratings based on ESR and temperature are easy performance metrics to “digest”, but use them with care!
E.S.R. = D.F. / (2*π*f*C*100)
D.F. = Dissipation Factor expressed as a %
f = Frequency in Hertz
C = Capacitance in Farads
When choosing capacitors consider the enormous difference in expected lifetimes between film and electrolytics when used at or near maximum “rated” current and/or temperature. Verify with a proposed supplier performance parameters at frequencies above 100KHz if that applies.
Our suggested maximum ratings assume a lifetime longer than the useful life of the application. Electrolytic capacitor lifetimes at performance extremes may be only a few thousand hours, and they remain one of the highest failure rate components in an application, especially when carrying high frequency ripple current even at stress levels below their “max rating”.
It is our opinion that our published voltage performance curves contain more information than do ripple current versus temperature charts at fixed frequency. Maximum allowed current versus frequency can be estimated from our voltage curves for polypropylene capacitors with +85°C convection environment.
It is a more difficult matter to specify RMS ripple currents for polyester capacitors where ESR is more frequency and temperature dependent. Because of smaller size [for a given voltage and capacitance], and higher temperature ratings they remain a viable solution for some high frequency ripple current applications.
Current versus temperature charts do not take into account ESR variation with frequency nor do they address possible methods [other than "ambient" temperature] to optimize performance!
Please contact us for more information and for specific application notes. We have detailed methods to estimate capacitor current limits for different capacitors and thermal/electrical environments. We always welcome an engineer-to-engineer discussion of your specific application!