Views: 1 Author: Site Editor Publish Time: 2018-07-12 Origin: Site
1. What is a filter capacitor
An energy storage device installed at both ends of the rectifier circuit to reduce the AC ripple ripple factor and improve the efficient and smooth DC output. This device is usually referred to as a filter capacitor. Since the filter circuit requires the storage capacitor to have a large capacitance. Therefore, most filter circuits use electrolytic capacitors. Electrolytic capacitors are named for their use of electrolytes as electrodes (negative electrodes). One end of the electrolytic capacitor is a positive pole, and the other end is a negative pole, which cannot be reversed. The positive terminal is connected to the positive terminal of the rectified output circuit, and the negative terminal is connected to the negative terminal of the circuit. In all circuits that need to convert AC power to DC power, setting the filter capacitor will make the performance of the electronic circuit more stable, and also reduce the interference of the alternating ripple ripple on the electronic circuit. The sign of the filter capacitor in the circuit is generally indicated by "C", and the capacitance should be determined according to the load resistance and the output current. When the filter capacitor reaches a certain capacity, increasing the capacitance of the capacitor will adversely affect other indicators.
2. Characteristics of filter capacitor
(1) low temperature rise
The harmonic filter circuit consists of a capacitor series reactor, which forms the lowest impedance at a certain harmonic order to absorb a large amount of harmonic current. The quality of the capacitor will affect the stable absorption effect of the harmonic filter. The temperature has a great relationship. The higher the temperature, the lower the life. The filter full-film capacitor has the characteristics of low temperature rise and can guarantee its service life.
(2) low loss
Dielectric loss tangent (tgδ): ≤ 0.0003
(3) Security
It complies with GB and IEC standards, and the internal single capacitors are equipped with protection devices. When the line or single capacitor is abnormal, the protection device will immediately act and automatically cut off the power supply to prevent secondary disasters. A discharge resistor is included to ensure safe use of electricity and maintenance. The outer casing is stamped from steel plate, and the interior and exterior are coated with high-temperature paint with good weather resistance.
(4) Convenience
Small size and light weight, easy to handle and install
3. The role of the filter capacitor
The filter capacitor is used in the power rectifier circuit to filter out AC components. Make the output DC smoother. Moreover, for precision circuits, this time, a combination of parallel capacitor circuits [1] is often used to improve the working effect of the filter capacitors.
The low-frequency filter capacitor is mainly used for filtering after mains filter or transformer rectification. Its working frequency is 50Hz consistent with the mains; the high-frequency filter capacitor mainly works after the rectification of the switching power supply, and its working frequency is several thousand Hz to several Ten thousand Hz. The filter capacitor plays a very important role in the switching power supply. How to correctly select the filter capacitor, especially the selection of the output filter capacitor is a problem that every engineering technician is very concerned about.
A common electrolytic capacitor used in a 50 Hz power frequency circuit has a ripple voltage frequency of only 100 Hz and a charge and discharge time of the order of milliseconds. In order to obtain a smaller pulsation coefficient, the required capacitance is as high as several hundred thousand microfarads. Therefore, the goal of ordinary low frequency aluminum electrolytic capacitors is to increase the capacitance, and the capacitance, loss tangent and leakage current of the capacitor are identified. The main parameters of its pros and cons. The output filter electrolytic capacitor in the switching power supply has a sawtooth voltage frequency of up to tens of thousands of Hertz or even tens of megahertz. At this time, the capacity is not the main indicator. The standard for measuring the advantages and disadvantages of high-frequency aluminum electrolytic capacitors is the "impedance-frequency" characteristic. It is required to have a lower equivalent impedance within the operating frequency of the switching power supply, and at the same time have a good filtering effect on the high frequency spike signal generated when the semiconductor device operates.
Ordinary low-frequency electrolytic capacitors begin to exhibit inductivity around 10,000 Hz and cannot meet the requirements of switching power supplies. The high-frequency aluminum electrolytic capacitor dedicated to the switching power supply has four terminals, and the two ends of the positive electrode aluminum piece are respectively taken out as the positive electrode of the capacitor, and the two ends of the negative electrode aluminum piece are also taken out as the negative electrode. Current flows from one positive end of the four-terminal capacitor, through the inside of the capacitor, and from the other positive end to the load; the current returning from the load also flows from one negative terminal of the capacitor to the negative terminal of the other.
4. Filter capacitor selection principle
In the power supply design, the selection principle of the filter capacitor is:
C≥2.5T/R
Where: C is the filter capacitor, the unit is F;
T is the period, the unit is S, T=1/f
f is the AC power frequency in Hz
R is the load resistance in Ω
Of course, this is only a general selection principle. In practical applications, such as conditions (space and cost), C ≥ 5T / R is selected.
Since the four-terminal capacitor has good high-frequency characteristics, it provides an extremely advantageous means for reducing the ripple component of the voltage and suppressing the switching spike noise. The high-frequency aluminum electrolytic capacitor also has a multi-core form, that is, the aluminum foil is divided into a shorter number of segments, and multiple lead-out sheets are connected in parallel to reduce the impedance component in the capacitive reactance. And the low resistivity material is used as the lead-out terminal, which improves the ability of the capacitor to withstand large currents.
