Capacitors high frequency

Magnesium titanate has many useful applications, for example, in dew sensors, in pigments, and in the electrical and electronic industries as a dielectric material for manufacturing on-chip capacitors, high-frequency capacitors, and temperature compensating capacitors. 

Capacitor manufacturers have only begun to specify the use of their capacitors in high frequency switching power supplies. One must use care when reviewing capacitors for use in one s power supply. The ESR should be specified at a frequency greater than 1 kHz. 

A high frequeney eapaeitor eould also be plaeed in parallel with the larger eapaeitors. This is beeause the aluminum eleetrolytie and tantalum eapaeitors eannot absorb the very high frequency current components being presented to them. A. 01 or. 1 pF ceramic capacitor is well suited for this purpose. 

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. 

The capacitors used for this function are high-voltage Aim or ceramic capacitors that exliibit very good high frequency characteristics. These capacitors... 

Place a high-frequency capacitor (ceramic or film) across the primary winding of the transformer, rectifier, or the element to be snubbed. Determine the capacitor value that produces an oscillation period which is three times the original period (Co). 

The next task is to determine the plaeement of the eompensating zero and pole within the error amplifier. The zero is plaeed at the lowest frequency manifestation of the filter pole. Since for the voltage-mode controlled flyback converter, and the current-mode controlled flyback and forward converters, this pole s frequency changes in response to the equivalent load resistance. The lightest expected load results in the lowest output filter pole frequency. The error amplifier s high frequency compensating pole is placed at the lowest anticipated zero frequency in the control-to-output curve cause by the ESR of the capacitor. In short ... 

Sometimes the high-frequency attenuation is insufficient to meet the specifications and a third pole needs to be added to the EMI filter. This filter is typically a differential-mode filter and will share the Y capacitors from the common-mode filter. Its corner frequency is typically the same as the commonmode filter. This filter is made up of a separate choke on each power line, and is placed between the input rectifiers and the common-mode filter. 

Structural binder A wide range of applications in electronics makes use of the plastics as a structural binder to hold active materials. For example, a plastic such as polyvinylidene fluoride is filled with an electroluminescent phosphor to form the dielectric element in electroluminescent lamps. Plastics are loaded with barium titanate and other high dielectric powders to make slugs for high K capacitors. The cores in high frequency transformers are made using iron and iron oxide powders bonded with a plastic and molded to form the magnetic core. 

Arc Plasma Method The principle of NPs synthesis in this method is based on evaporation by heating and condensation by cooling. The bulk metal is evaporated by heating with electrical resistance, electron beam, or high-frequency magnetics, and subsequently the vapor of metal atoms is condensed on a substrate as a sohd film or particles. In the AP method, electrical charge filled in an external capacitor... 

It is evident that the current still leads the voltage but that the "phase angle, a, will vary from close to 90° at low frequencies to close to 0 at high frequencies. Also, at low frequency Z — 1 /tuC and at high frequency Zf — R. In other words, at low frequencies, the circuit behaves like a pure capacitor but at high frequencies it behaves like a pure resistor. Moreover, by fitting the observed current data as a function of frequency to calculated values of Zj and a, an accurate estimate of both R and C can be made. 

In Figure 2-10, we Anally break up the input capacitance into a high-frequency capacitor and a relaAvely low-frequency bulk capacitor. The current distribuAons are shown, as well as how they all add up eventually. The mystery is clear now, and in the process we also understand how the decoupling capacitors are supposed to behave. Now we can also start to understand how this delicate balance can be easily shattered by lack of proper decoupling ... 

If the high-frequency decoupling is poor, the only way to check for it is to put a 0.1 pF capacitor right next to the pins of the IC and see if the problem goes away. There is almost no way we can ever really see the cause of that sort of problem on any of our instruments. We have to deduce it. 

We know that a 0.1 pF input capacitor takes care of the (high-frequency) noise. But it neither can nor do almost anything to smooth out the (low-frequency) ripple. However, we are now in a position to start calculating how much bulk capacitance we really need to ensure trouble-free performance (for typical ICs ). 

Yes, we could simply place a high-frequency ceramic capacitor directly across the output to kill the noise appearing there. But remember, many switchers (like the BJT-based switcher family mentioned previously) are actually relying on some minimum ESR at the output to... 

In such cases, we could try to reduce the high-frequency output noise by suppressing it at the input. So that could be a valid reason to place a small ceramic capacitor at the input of an older-generation switcher IC (i.e., one with a BJT switch). Its primary purpose is then not to ensure that the control does not go into chaos because of switch transient noise, but to reduce the output noise in noise-sensitive applications. 

Thus the high-frequency ESR is about half the low-frequency ESR. Frequency multipliers should always be used, or we will overestimate the heating and underestimate the life, possibly forcing us to move to a larger capacitor size (overdesign). 

We did it somehow, almost strangulating ourselves in the process. Now when I look back at this incident, I wonder why we didn t place a ceramic decoupling capacitor close to the switch, as shown in the lower half of the figure. The bulk capacitor could have successfully managed to provide the low-frequency current components, whereas the high-frequency capacitor could have really decreased the effective loop area in which the high-frequency components were circulating. 

In a Flyback, the high-frequency current loop encloses the transformer secondary, the output diode, and the output capacitor. This loop must be minimized as far as possible. 

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