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The capacitor selection techniques and application trends

In 2011, China provided nearly one-third of the world's passive components. China Electronic Components Industry Association data show that, the 2011 capacitor sales totaled $ 6.7 billion, an increase of 5% compared to 2010. Resistors, inductors, capacitors and other passive components since 2011 has begun to steadily, Tablet PC, consumer electronics, as well as by the positive impact of the various favorable policies, white goods, automotive electronics, handheld devices, etc. The increase in market demand for passive components such as resistors, capacitors market prospects can be said to be very optimistic. The issue Banyuetan will focus on the selection and application of capacitance at different occasions, while Outlook capacitance presented in the application market trends.

Capacitance There are many naming method, in accordance with the function points, including the bypass capacitor, decoupling capacitors, filter capacitors; follow the media can be divided into ceramic, film media (including a variety of film), aluminum electrolytic capacitor ... But no matter what kinds of capacitor, its principles are the same, namely the use of the AC signal presents a low impedance characteristics - the higher the operating frequency, the capacitance value, the greater the smaller the impedance of the capacitor. If the capacitor from the primary role is to provide a low impedance path to the AC signal in the circuit, is referred to as a bypass capacitor; if mainly in order to increase the AC-coupling of the power supply and ground, to reduce the influence of the AC signal of the power supply, it may be referred to for the decoupling capacitor; if used in the filter circuit, then it can be called a filter capacitor; addition, the DC voltage, the capacitor also as a circuit storage, using the battery charging and discharging play.

Because of the capacitance of a wide range of applications, and how to choose and capacitor has become a topic of concern for many engineers.

The choice of power supply filter capacitor

The impedance of the capacitor is inversely proportional to the frequency, capacitance can block the low frequency through. Inductor opposite - so both appropriate combination can filter various frequency signal. As in the rectifier circuit, the capacitance and on the load or inductive series on the load, the AC ripple can be filtered.

Due to the role of the electrolytic capacitor is to filter out the low-frequency signal current, but even if it is a low frequency signal, and its frequency is also divided into several orders of magnitude. Therefore suitable for use in different frequency electrolytic capacitors are also divided into high-frequency capacitance and low frequency capacitance (high frequency here is a relative term).

Low-frequency filter capacitor is mainly used for filtering mains filter or transformer rectifier operating frequency and mains consistent for 50 Hz; work after switching power supply rectifier filter and high-frequency filter capacitor, the operating frequency of thousands of Hz to a few Wan Hz. When the filter capacitor is used for the low-frequency to high-frequency circuit, due to the poor high-frequency characteristics of the low-frequency filter capacitor, it is in the high frequency during charging and discharging large internal resistance, the higher the equivalent inductance. Therefore, in use due to frequent the electrolyte polarization arising from the greater heat. And higher temperatures will enable the capacitance of the electrolyte inside the gasification, the capacitor internal pressure increases, eventually leading to the drum of the capacitor package and the burst.

The size of the power supply filter capacitor, usually do the design, the former 4.7u for filtering the low frequency, two with 0.1u for filtering high frequency 4.7uF capacitor to reduce the output ripple and low frequency interference, 0.1uF's capacitance should be to reduce the high-frequency interference caused due to the instantaneous load current changes. Usually front that the bigger the better, the two capacitance values ​​a difference of about 100 times. Power filtering, switching power supply, depending on your ESR (equivalent series resistance capacitor) how the choice of the high-frequency capacitance is best in its self-resonant frequency on. Large capacitor to prevent surge mechanism like stronger than the flood control capacity of the large reservoir; small capacitor filter high frequency interference, any device can be equivalent to a resistance, inductance, capacitance, series and parallel circuits, and will have a self-resonant only in the self-resonant frequency, the equivalent of least resistance, so filtering best.

The capacitance equivalent model of an inductor L, a resistor R and capacitor C in series (below), the inductor L, capacitor lead Suozhi, resistor R represents a power loss of the capacitor. Which can be equivalent to the series LC circuit to find the resonant frequency, the series resonant conditions the WL = 1/WC, W = 2 * PI * f, to thereby obtain this equation f = 1 / (2pi * LC), a series LC circuit the minimum performance of the center frequency reactance is purely resistive, and therefore play a filtering effect at the center frequency. The size of the lead inductance because of its thickness and length, the inductance of the grounding capacitor is generally 1mm about 10nH, the frequency depends on the needs grounding.


Capacitor filter design parameters ESR, ESL, withstand voltage value and the resonant frequency to consider.

The choice of the filter capacitor motor drive

The circuit filter capacitor selection need to consider several aspects: capacitor voltage, operating temperature, capacity, and so on. In theory the power supply filtering capacitor is the bigger the better, generally large capacitor filter low-frequency waves, a small capacitor filter high-frequency wave. In order to obtain a larger filter band, a small two capacitors in parallel, the general requirements of a difference of more than two orders of magnitude. In the power supply design, the filter capacitor, the principle of selecting the C ≥ 2.5T / R, C ≥ 5T / R are generally selected in the actual application.

Motor drive applications, filter capacitor, for example, the input filter capacitor capacity of selection and drive control of the drive voltage, a direct relationship of the maximum power, needed for some calculate if the capacity of this capacitor is too small, the drive performance of lack of driving force; while the capacity is too large, the increased manufacturing costs. Engineering applications, there is a rule of thumb: the filter capacitance value equal to the value of drive power. But note that this is only for single-phase 220V AC drive application of the full-wave rectifier.

First, from the capacitors, resistors and the RC time constant τ Speaking larger: τ R at both ends of the voltage, the more stable, less pulsating power, then the ripple voltage. In engineering, when the RC time constant of the following conditions are met, to meet the ripple requirements. T is the cycle of the pulsating power source, for the 50Hz mains cycle after full-wave rectification and T is: 10ms.

Therefore, you can get by on two type; R is the equivalent load resistance; C filter capacitor capacity. Therefore, as long as the equivalent load resistance of the motor drive, can calculate the required size of the filter capacitor.

The capacitance choose the op amp circuit

Capacitor when in actual use may not be desirable, taking into account the size and cost, the most practical, large values ​​of the capacitors are electrolytic type, visible, such a capacitor is particularly applicable to filter out low frequency noise:
Z = RC + jωLC +1 / jωC.

When the frequency increases, the capacitive reactance of the electrolytic capacitor will decrease, until the inductance equal capacitance, will depend on the exact model of the electrolytic capacitor, it may appear at a frequency of approximately 1MHz. Require the selection of different types of capacitor for high frequency bypass usually recommend a cheap, small ceramic capacitor (0.01 to 0.1 μF). At 1 MHz, the capacitance of 0.1μF capacitor is 1.6Ω, but should pay attention to the most inexpensive ceramic capacitors are just the nature of the "content".

Ceramic capacitors have many functions, excellent NPO (negative - positive - zero temperature coefficient) devices have a temperature coefficient of ± 30p pm / ℃, and relatively cheap. However, ceramic capacitors are not a panacea, depending on the ceramic dielectric composition as a structural material, they are 0.1% to 1% or more dielectric absorption (DA) role. Dielectric absorption in fact, when the rapid charging and discharging, sucked into the charge within the media can not be immediately applied to the capacitor or leave from the capacitance. This effect in many channel data collection system is harmful. Before the conversion, these channels by sampling - maintain a variety of data acquisition circuit, and in the worst case of the largest step, to absorb the error caused by the retention capacitor dielectric is equal to the dielectric absorption capacity.

Do a quick correction is not possible due to a difference between the voltage is being measured, and the previously measured voltage determines the dielectric absorption index is multiplied by a coefficient, so this error source. The only solution is to use a dielectric absorption capacitor is lower than the maximum permissible error.

Capacitor application trends

Capacitance development reflects the following trends: major proportion of chip multilayer ceramic capacitors, tantalum electrolytic capacitor sharp growth, decline in the proportion of aluminum electrolytic capacitors and thin-film dielectric capacitors, variable trimmer capacitor on the decline, mica paper dielectric and organic thin film medium, tubular ceramic dielectric phased out; capacitor decline of the high-frequency characteristics and the temperature is not easy to solve and winding structure ... applications, due to the continuous development of the wireless communications, 3G 4G capacitor products higher requirements; medical electronics towards miniaturization and portability, so that the capacitor to usher in a new development in the medical electronic anti-jamming, regulator, etc. ...

Development of miniaturized acceleration multilayer ceramic capacitors. In areas such as mobiles phoness, computers, digital cameras, car extensive use of multi-layer ceramic capacitors and ceramic capacitors, but the technological advances of recent years, digital products making new demands. For example, the rapid development of mobiles phoness require higher transmission rates and higher performance. Baseband, in order to meet the requirements of TV, video games, high-speed, low-voltage processor; LCD module requires low thickness (0.5mm), large-capacity capacitor; PA module is looking forward to the miniaturization of products. Since 2011, the high-end set-top boxes (STB), high-definition television and automotive electronics products are becoming the main driving force of the capacitor production lines. mobiles phoness, wireless LAN and SD cards and other small devices will stimulate the growth of multilayer ceramic capacitors (MLCC), to further promote the miniaturization trend.

Field of new energy vehicles, photovoltaic, LED lighting and other energy-saving flourish promote the wide application of the capacitor. Electric vehicles, wind power, solar power, to promote the use of resistive and capacitive. Wind power generation power management system, the main consideration when selecting high-voltage, large capacitor values ​​and generate a lot of demand for products. Such as wind energy system, three use a capacitor to this product, the first film capacitors can choose to do the application of high voltage absorption; There is also a DC-LINK, a filtering role of energy storage in the DC-DC conversion process; and network anti-interference capacitor.

Aluminum electrolytic capacitors, for example, in other hybrid electric vehicle battery charging voltage converter, inverter circuit, the growing demand for aluminum electrolytic capacitors, aluminum electrolytic capacitors is also wind power, photovoltaic applications inverse The variable transformer module integral part to the size of the market is expected to reach 55 billion by 2015.

LED lighting solutions, LED lamp life, the greatest impact of the device is the decoupling capacitors. Electrolytic capacitors LED lighting scheme has also been launched, but if you do not use electrolytic capacitors, the design of the LED drive will become very expensive. Can solve this problem in a low power level, by replacing electrolytic capacitors with ceramic capacitors, high quality light source lamps, electrolytic capacitor is difficult to remove. High quality light source lamps, electrolytic capacitors are difficult to remove. In fact, if the electrolytic capacitor of good quality, you can reach the working temperature of 105 degrees Celsius and rated operating time of 10,000 hours, there is no need to remove electrolytic capacitors from LED driver design.
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