FEATURE POWER ELECTRONICS Designing in the right MLC The demand for better economy and lower CO2 emissions is driving
automotive electronics in directions not previously experienced in this field. Electrical loads in automotive systems over the last few decades have evolved from simple lighting and battery-charging to engine management & control, sensors & safety and of course ‘infotainment’, making the car smarter and more sophisticated. All of which is driving Multilayer Ceramic Capacitors (MLCC’s) into higher voltage and higher temperature applications, says Peter Scutt from Knowles Capacitors Syfer Technology facility
W
hile this trend continues we see increased use of electronics in high
intensity lighting, safety systems, transmission & controls and power train systems for better propulsion. Incorporating electrical loads and replacing the conventional mechanical and hydraulic loads in the powertrain improves efficiency leading to more focus on electric vehicle concepts –hybrid (HEV) and pure (EV). However this increasing need and
demand makes the conventional 12V power system more challenging. As such, it is critical to have higher voltages in order to handle power train loads more efficiently – and with flexibility. Switched-mode power supplies (SMPS) provide the basis to do so. This is made possible due to advances in power electronics brought about by higher specified components such as MLCC’s. Implementation of power electronic circuits makes the system smaller and lighter and therefore provides the basis to improve the fuel efficiency as well. Advances in dielectric materials used on
20 OCTOBER 2014 | ELECTRONICS
Multilayer Ceramic Capacitors, such as the X8R family from Syfer, have resulted in increasing capacitance values along with increased voltage ratings (up to 3kV). Increased MLC chip sizes and the inclusion of the StackiCap has also given further increases in available capacitance values and added benefits of volumetric efficiency. Syfer has recently
Figure 1: AEC-Q200 chips from Syfer
“The
temperatures attained when a capacitor discharges
improved and expanded its range of AEC-Q200 automotive qualified capacitors to include voltage ratings to 3kV; safety rated class X&Y capacitors for AC charging circuits; X8R dielectrics to 150ºC; chip sizes to 3640; StackiCap capacitors with high volumetric efficiency and open-mode and tandem-cap options for improved reliability. However the limiting factor is often the
temperature performance of standard dielectric materials. The reliability of
multilayer ceramic capacitors is directly related to the voltage applied and the operating temperature. Both voltage and temperature have an influence on the reliability acceleration factor, but temperature has a disproportionate affect and the reliability factor increases significantly as temperature rises. Thermal stress alone is sufficient to cause electrical failure. Thermal breakdown takes place when heat is generated in the dielectric at a higher rate than can be conducted away. This leads to increased conductivity, more heat generation and eventually to instability in the form of an uncontrolled, often very rapid temperature rise. The temperatures attained when a capacitor discharges through a region of localised thermal runaway can be high enough to melt the dielectric material. When determining whether a particular
component is suitable for use at high temperatures, customers must consider the thermal stress, and the effect of the elevated temperature on basic electrical properties such as capacitance, dissipation factor and insulation resistance. As component reliability is detrimentally affected due to thermal stresses it is not recommended that standard components are used at temperatures over 125˚C. For temperatures up to 160˚C, most
standard components will give reliable performance, but it is recommended for the component user to select components with a voltage rating ≥30% higher than the component that would normally be selected. For example, if a 0805 50V 10nF
through a region of localised thermal runaway can be high enough to melt the dielectric material ...”
component would normally be used, the recommendation would be to use an 0805 100V 10nF part – NB the 0805 63V 10nF would not meet the recommendation as the voltage increase is only 26%. For temperatures >160˚C, Syfer
test data shows that the reliability is affected exponentially. This makes it very difficult to provide a simple set of rules for component users to apply for use between >160oC and 200˚C. Sonsequently, for
component use >160˚C, the company recommends the user contacts its technical team with details of the application.
Syfer Technology
www.knowlescapacitors.com/syfer 01603 723300
Enter 210 / ELECTRONICS
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