Ceramic capacitance

Power supply designers are usually aware that the most stable ceramic capacitance comes from materials dubbed COG material, also called NPO (for negative positive zero, referring to its near perfect temperature coefficient). But this is a low dielectric constant material, and unsuitable for modern miniaturization. So the common materials in use today are called X7R, X5R, and so on. There are others, starting with a Y or Z prefix, which no power supply designer worth his or her salt will ever use. 

Silicon piezoresistance Thin-film piezoresistance Ceramic capacitance Thin-film capacitance Ceramic piezoelectricity... 

Sensors with ceramic capacitive and hermetic technologies for a full range of automotive and heavy vehicle off-road systems... 

Porous dielectric ceramics (capacitance type) Porous semiconductors (capacitance type) Other materials... 

Capacitors. Ceramic materials suitable for capacitor (charge storage) use are also dependent on the dielectric properties of the material. Frequently the goal of ceramic capacitors is to achieve maximum capacitance in minimum volume. The defining equation for capacitance is given by ... 

In lead zh conate, PbZrOs, the larger lead ions are displaced alternately from the cube corner sites to produce an antifeiToelectric. This can readily be converted to a feiToelectric by dre substitution of Ti" + ions for some of the Zr + ions, the maximum value of permittivity occumirg at about the 50 50 mixture of PbZrOs and PbTiOs. The resulting PZT ceramics are used in a number of capacitance and electro-optic applicahons. The major problem in dre preparation of these solid soluhons is the volatility of PbO. This is overcome by... 

It is unusual to be able to And one capacitor to handle the entire ripple current of the supply. Typically one should consider paralleling two or more capacitors (n) of I/n the capacitance of the calculated capacitance. This will cut the ripple current into each capacitor by the number of paralleled capacitors. Each capacitor can then operate below its maximum ripple current rating. It is critical that the printed circuit board be laid out with symmetrical traced to each capacitor so that they truly share the current. A ceramic capacitor ( 0.I pF) should also be placed in parallel with the input capacitor(s) to accommodate the high frequency components of the ripple current. 

Though such capacitors usually lose their capacitance at high temperatures, their ESR typically improves. I picked a lOnF capacitor in Figure 4-13 and varied the temperature using Murata s design tool. The resonant frequency hardly changes, but the ESR does improve with temperature. That, like the previously reported dependence on frequency, is once again similar to an elko. But luckily, this does not cause the ceramic capacitor to evaporate as an elko would ... 

There are apparently customers who soldered on ceramic capacitors in their power supplies and found the clock was just too low. They figured the capacitance was above the guaranteed upper tolerance band (a rare event with commercial ceramics ), and shipped them right back to their manufacturers. But the problem was only that as soon as the PCBs went through the soldering process, age reset (or de-aging) occurred and so capacitance rose. If only they had waited for some more time, their clocks would have been right on However, I would have preferred SMD him capacitors if stability was so important. 

Note that in the Buck and the Buck-Boost, the input capacitor is included in the critical path. That implies we need very good input decoupling in these topologies (for the power section). So, besides the necessary bulk capacitor for the power stage (typically a tantalum or aluminum electrolytic of large capacitance), we should also place a small ceramic capacitor (about 0.1 to IpF) directly between the quiet end of the switch (i.e., at the supply side) and the ground—and also as close as possible to the switch. 

These cause dynamic issues to the switching power supply, and usually the only solution to that is to have enough bulk capacitance present on the 12V output rail. Luckily, since the main feedback loop is derived from the primary 5V/3.3V rails of the power supply, there is no minimum ESR requirement for the 12V rail output capacitance, and we can freely add several electrolytic capacitors in parallel. However, modern core processors can place very fast transient load demands on the primary regulated rail, too, and for that we need a whole bunch of ceramic capacitors sitting right at the point of load. In that case we must ensure the converter is designed to accept ceramic loads. Otherwise it will break up into oscillations. 

Many commercial capacitors can be used as thermometers since they show a strong dependence of their capacitance on temperature. Figure 9.15 shows the low-temperature C(T) curve for a commercial smd capacitor (ceramic capacitor KEMET-Y5V). 

Fig. 9.15. Capacitance of an smd capacitor (ceramic capacitor KEMET-Y5V) as a function of temperature.

H20, humidity Capacitive polmyer sensors and ceramic sensors Thin film technology and thick film technology... 

Grain boundaries have a significant effect upon the electrical properties of a polycrystalline solid, used to good effect in a number of devices, described below. In insulating materials, grain boundaries act so as to change the capacitance of the ceramic. This effect is often sensitive to water vapor or other gaseous components in the air because they can alter the capacitance when they are absorbed onto the ceramic. Measurement of the capacitance allows such materials to be used as a humidity or gas sensor. 

A typical ceramic sample contains contributions from the bulk, the grain boundaries, and the electrode. Each of these is characterized by a semicircular arc with a maximum at RCu> = 1, where the values of resistance, capacitance, and frequency refer directly to the bulk, grain boundaries, or electrodes (Fig. 6.7c). The separation of resistance due to the bulk from that of the grain boundaries is thus easily achieved using impedance spectroscopy. 

The remarkable stability of the capacitance of the SIKO against variations in bias, temperature, frequency and time of operation is a consequence of the superior properties of its ONO dielectric. In contrast to aluminum and tantalum capacitors, the SIKO is a symmetrical device. It shows no significant voltage dependence of the capacitance, as the high s ceramic capacitors do. Only polymeric capacitors show a lower dependence of capacitance on bias than a SIKO. 

The change of capacitance in relation to frequency is a matter of the polarizability of the dielectric. This change is very large for ceramics and large for most polymer dielectrics, but very small for Si3N4 and Si02. 

For ceramic capacitors there is a continuous decrease of capacitance with time. The higher the dielectric constant, the higher is this decrease. In metal film capacitors the best values given for 63 V capacitors are changes of 0.5% in 2 years. The SIKO shows the same long-term stability of the capacitance as the trench capacitors in DRAMs. 

CASH CBM CBO CBPC CC CCB CCM CCP CDB CEC CFBC CFC CFR CMM COP CSH CT Calcium aluminosilicate hydrate Coal bed methane Carbon burn-out Chemically-bonded phosphate ceramics Carbonate carbon Coal combustion byproducts Constant capacitance model Coal combustion product Citrate-dithionate-bicarbonate Cation exchange capacity Circulating fluidized bed combustion Chlorofluorocarbon Cumulative fraction Coal mine methane Coefficient of performance Calcium silicate hydrate Collision theory... 

The ratio of permittivity with the dielectric to the permittivity in vacuum, e/eo, is called the relative permittivity, s, or dielectric constant. The dielectric constant is a material property. Some values of dielectric constants for common ceramic and glass insulators are given in Table 6.3. Since a polarizable material causes an increase in charge per unit area on the plates of a capacitor, the capacitance also increases, and it can be shown that the dielectric constant is related to the capacitance and displacement in vacuum and with the dielectric material as follows ... 

Figure 6.196 is an illustration of a DP cell which operates by varying the distance between the plates of two adjacent capacitors (see also Fig. 6.11)(24). The high and low pressure signals from the sensor are applied to ceramic diaphragms to which one plate of each capacitor is attached. The subsequent change in the separation of the capacitor plates produces a variation in capacitance which is detected by incorporating the cell into a capacitance bridge, as described in Section 6.5.3 (Fig. 6.316).

Electrolytic type sensors Uxt thick film techniques, e.g. capacitor coated in gl bonded on to a ceramic disc mounted on a thermoelectric (Peltier effect) cooler. Control is by a platinum resistance thermometer which adjusts the temperature of the cooler to regain equilibrium after a change in capacitance due to moisture deposit. Range depends on technique. Capable of high precision. Limitations are similar to those for AIjO) sensor. Capable of being direct mounted. Relatively cheap. Suitable for on-line use. 

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