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Power tip: Avoid common multilayer ceramic capacitor issues

Posted: 27 Jul 2012 ?? ?Print Version ?Bookmark and Share

Keywords:Multi-layer ceramic? electrolytics? ESR?

In these systems, the power source is connected to a load through a large interconnect inductance. The load is switched on through a switch and may be bypassed with ceramic capacitors. The bypass capacitors and interconnect inductance can create a high Q resonant circuit. Closing the switch at the load can create an overvoltage condition as the load voltage can ring to twice the source voltage. This can lead to unexpected circuit failures. For instance, in the POE, the voltage ratings on the load components can migrate as much as double the source voltage ratings.

A third potential pitfall lies in the fact that the ceramic capacitors are piezoelectric. That is, when the voltage changes on the capacitor, its physical size changes, which can result in an audible noise. Examples of problem applications are when the capacitors are used as output filter capacitors where a large load transient current is present or in "green" power supplies that go into burst modes at light-load conditions. Workarounds to this issue include:
???Changing to a lower permittivity ceramic material like COG
???Using a different dielectric such as film
???Using leaded versus surface mount technology (SMT) components which are tightly coupled to the printed wiring board (PWB)
???Using a smaller footprint device to reduce stress coupled into the board
???Using a thicker part to reduce applied voltage stress and physical distortion

Another issue with SMT ceramic capacitors is that their solder joints are prone to cracking if the PWB flexes, and due to thermal coefficient of expansion (TCE) mismatch between them and the PWB. There are a number of precautions you can take to mitigate this issue:
???Restrict package size to 1210.
???Keep capacitors away from high flexure areas such as corners
???Orient capacitors in the short direction of the board
???Keep board mounting points away from corners and edges
???Be mindful of potential board flexing during all assembly steps

To summarize, multilayer ceramic capacitors can provide cost savings, reliability and life improvements, and size reductions over electrolytic capacitors C if you are mindful of their pitfalls. They can have very wide capacitance tolerances, so you need to evaluate their performance over temperature and bias voltage. They are piezoelectric, meaning they can generate audible noise in systems with pulsating currents. Finally, they are prone to cracking so steps to mitigate the issue are required. All these issues can be worked around. Hence, the MLC capacitor is continuing to become very popular.

About the author
Robert Kollman is a Senior Applications Manager and Distinguished Member of Technical Staff at Texas Instruments. He has more than 30 years of experience in the power electronics business and has designed magnetics for power electronics ranging from sub-watt to sub-megawatt with operating frequencies into the megahertz range. Robert earned a BSEE from Texas A&M University, and a MSEE from Southern Methodist University.

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