Monday, 26 May 2014

Are too high capacitance capacitors "bad" for the circuit?


I am trying to learn basic circuitry and I've been looking into capacitors and their uses in different areas.


When looking at capacitance several different sources say that circuits might malfunction or burn with higher capacity capacitors than designed with. Unfortunately, but none of those sources go into detail.


How can a capacitor cause malfunction if capacitance increases? Wouldn't the capacitor simply take longer to fully charge? Can high capacitance capacitor really cause any sort of "burn"? I mean it cannot store or produce higher current than what is given by power supply, right?



Am I missing some important detail?



Answer




How can a capacitor cause malfunction if capacitance increases? Wouldn't the capacitor simply take longer to fully charge?



Capacitor is a charge reservoir. Switched-mode power supplies need to charge it first. Too large capacitors might make the internal power supply loop go unstable, which would create large voltage deviations across the capacitor and potentially burn it due to too large capacitor heating caused by its non-zero parasitic resistance called "ESR".



Can high capacitance capacitor really cause any sort of "burn"? I mean it cannot store or produce higher current than what is given by power supply, right?



Capacitor do burn quite often. Actually, the aluminum capacitor failure is the most common failure mechanism in large motor drives! Motor drives and other power electronics (solar inverter, wind inverter, car battery charger, ...) exhibit very large current ripples at various frequencies. These ripple currents cause capacitor heating (ESR), which degrades the capacitor capacitance and further increases ESR. It's like a positive feedback. Aluminum caps have limited lifetime measured in thousands of hours. Their lifetime also decreases with elevated temperatures.



The typical way to mitigate this issue is to use multiple parallel caps (splitting the ripple currents) or using higher quality capacitors. These methods, however, tend to increase cost of the final product. The electronics industry is very cutthroat nowadays, which gives rise to designing for full functionality and reasonable failure rate but not a bit more.


Other answers also list good examples of how not only the capacitor can burn but how the large capacitor can cause other components to burn.


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