Capacitance selection techniques and application trends

In 2011, China provided nearly one-third of the world's passive components. China's sales of capacitors totaled $6.7 billion in 2011, up 5 percent from 2010, according to the China association of electronic components. Represented by resistance, inductance and capacitance of passive components has started since 2011 are tablets of consumer electronics, as well as the positive impact of the positive policy of white goods, automotive electronics, hand-held devices, such as market increasing demand for passive components, such as resistors, capacitors, market prospect is very optimistic, so to speak. This half-month talk will focus on the selection and application of capacitors in different occasions, and look forward to the trend of capacitors in the application market.

There are many naming methods for capacitance, according to function, including bypass capacitance, decoupling capacitance, filtering capacitance; According to the medium can be divided into porcelain, thin film medium (containing a variety of thin film), aluminum electrolytic capacitor.. But the principle is the same for any kind of capacitance, which takes advantage of the low impedance of the ac signal -- the higher the operating frequency, the larger the capacitance, the smaller the resistance of the capacitance. In the circuit, if the main function of capacitance is to provide a low impedance path to the ac signal, it is called bypass capacitance. If the main purpose is to increase the ac coupling between power supply and ground, and reduce the influence of ac signal on power supply, it can be called decoupling capacitance. If used in a filter circuit, then can be called the filter capacitance; In addition, for dc voltage, capacitors can also be used as circuit energy storage, using charging and discharging to play the role of battery.

Due to the wide application of capacitance, how to select suitable capacitors has become a hot topic for many engineers.

Selection of power supply filter capacitance

The impedance of the capacitance is inversely proportional to the frequency, so the capacitance can block the passage of low frequency. Inductance is the opposite -- so a proper combination can filter frequencies. In a rectifier circuit, the alternating current ripple can be filtered by combining the capacitance with the load or by series inductance with the load.

Because the electrolytic capacitance is used to filter out the low frequency signals in the current, even the low frequency signals can be divided into several orders of magnitude. Therefore, in order to be suitable for use at different frequencies, electrolytic capacitors are also divided into high frequency capacitors and low frequency capacitors (the high frequency here is relative).

The lowfrequency filter capacitor is mainly used for the filtering after the municipal power filter or transformer rectification, and its working frequency is the same as that of the municipal power for 50Hz. The high frequency filter capacitor mainly works after the rectifier of the switching power supply, and its working frequency is from several thousand Hz to tens of thousands of Hz. When we apply the low frequency filter capacitor to the high frequency circuit, because the high frequency characteristic of the low frequency filter capacitor is not good, the internal resistance of the low frequency filter capacitor is large and the equivalent inductance is high when it is charged and discharged at high frequency. Therefore, in use, large amounts of heat will be generated due to the frequent polarization of the electrolyte. The higher temperature will gasify the electrolyte inside the capacitor, and the pressure inside the capacitor will rise, eventually causing the bulge and burst of the capacitor.

The size of power filter capacitance is usually designed with the former level of 4.7u for low frequency and the secondary level of 0.1u for high frequency. The capacitance of 4.7uf is used to reduce output pulsation and low frequency interference. The capacitance of 0.1uf should be used to reduce high-frequency interference caused by transient changes of load current. Generally, the larger the first one is, the better. The difference between the two capacitors is about 100 times. Power filter, switching power supply, depends on how big your ESR(the equivalent series resistance of the capacitor) is, and the selection of high frequency capacitors is best on its self-resonant frequency. Large capacitance prevents surge, just as big reservoirs are more powerful. Small capacitance filter high-frequency interference, any device can be equivalent to a resistance, inductance, capacitance of the series parallel circuit, there will be self resonance, only in this self-resonant frequency, the equivalent resistance is minimum, so the filtering is best.

The equivalent model of the capacitance is one inductance L, one resistance R and the series of capacitance C (as shown in the figure below). The condition of series resonance is WL=1/WC,W=2*PI*f, so this formula is f =1/ (2pi* LC). The minimum reactance at the central frequency of series LC loop is pure resistance, so the central frequency is filtered. The size of the lead inductance varies with its thickness and thickness. The inductance of the ground capacitance is usually about 1MM and 10nH, depending on the frequency to be grounded.

The design of capacitive filtering needs to take into account parameters such as ESR, ESL, voltage withstand value and resonant frequency.