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PCBD-Mar2016

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48 The PCB Design Magazine • March 2016 A year ago, my QuietPower column [1] de- scribed the possible large loss of capacitance in multilayer ceramic capacitors (MLCC) when DC bias voltage is applied. However, DC bias effect is not the only way we can lose capaci- tance. Temperature, aging, and the magnitude of the AC voltage across the ceramic capaci- tor also can change its capacitance. Finally, the initial tolerance needs to be considered as well. In the worst case, we may lose up to 90% of the capacitance for an X5R capacitor, and even for an X7R capacitor. This column will show you the details and also how the most advanced manufacturers are helping the users with new simulation models to take these ef - fects into account. As an actual example, let us look at one of the capacitors that was extensively tested [2] , where 1uF 0603-size 16V capacitors were tested from various vendors. We further assume that we want to use the part on a 12V supply rail, where the AC noise is low (this will be impor- tant later when we take the AC bias depen- dence into account). Some of the samples were chosen with X5R, some with X7R temperature characteristics. As showed with actual test data [2] , X7R capacitors are sometimes worse for DC bias sensitivity than X5R parts. If we take the part from Vendor B (labeled B7) in Figure 1 (this was Figure 5 [2] ), we see that at 12V DC bias we can lose 60% or 70% of the capacitance, dependent on which way the DC bias changes. But when we need to con- sider the worst-case capacitance loss, we have to consider the cumulative effect of all of the following factors: • Initial tolerance • Temperature effect • DC bias effect • AC bias effect • Aging The sample had +-10% initial tolerance. The X7R temperature characteristics comes with an additional +-15% tolerance window for the temperature variation. When the part is used with low AC excita- tion across the part, the capacitance may be up to 20–30% less than what the standard test pro- cedure provides. Since the vendors use the stan- dard test methods, the AC bias dependence has only a negative range: 0 … -20%, sometimes up to -30%. The aging in ceramic capacitors creates an exponential decay with a fixed percentage drop of capacitance for every decade of passed time. In case we take -2.5% per decade drop for X7R parts [3] and assume that the initial capaci- tance is measured 24 hours after manufactur- by Istvan Novak oracle Dynamic Models for Passive Components quiet poWer Figure 1: Percentage capacitance as a function of DC bias for all studied 1uF 0603 16V models, measured at 100 Hz and 10 mV AC bias.

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