Filter Capacitors

Introduction to the different categories of filter pieces:

During the EMC rectification process we need to use a variety of different filter circuits and components, generally in the following three categories:

1, simple inductive circuits work well when used in low impedance circuits, attenuating over 40dB, but may have no effect at all in high impedance circuits.

2, A circuit with a single capacitor works well in high impedance circuits, but poorly in low impedance circuits.

3, a multi-component filter will work well, but only if it is constructed correctly, so that the capacitor faces the high impedance and the inductor faces the low impedance.

Characteristics of ordinary capacitors:

Since the capacitor leads have parasitic inductance and resistance, the actual capacitor model is a structure of capacitance, inductance (equivalent series inductance ESL), and resistance (equivalent series resistance ESR) in series, with a self-resonant frequency, and the capacitor should be operated under its self-resonant frequency in order to function.

Table I: Ceramic Capacitors with 1.6mm Lead Lengths

It can be seen that the resonance frequency of general capacitors is very low and cannot be used in high frequency, microwave bands. In order to increase the resonance frequency, surface mount capacitors, three-port capacitors and through-center capacitors can be used.

Filter Capacitor-Three-terminal capacitor:

The three-terminal capacitor uses lead inductors and capacitors to form a T-shaped low-pass filter.

The lead inductors, together with the capacitors, form a T-shaped low-pass filter

Installation of three-terminal capacitors:

Keep the ground terminal as short as possible to reduce the lead inductance of the ground terminal.

Three-terminal capacitor mounting

Disadvantages of three-terminal capacitors:

The parasitic capacitance of the three-terminal capacitor causes coupling between the input and output, and the ground inductance causes reduced bypass effect.

Shortcomings of three-terminal capacitors

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Filter through-center capacitors:

Since its ground end can be in close contact with the ground and the lead inductance is smaller, the performance is superior, and feed-through filters based on feed-through capacitors are widely used in RF filtering.

through-center capacitance

Through the center of the capacitor input and output end of the line through the upper and lower two ground plates (so called through the center of the capacitor), equivalent to the upper and lower two capacitors in parallel into a total capacitance. Because of its grounded part of the circuit with the ground 360 ° connection, so the ground inductance is very small.

Also temperature and voltage have a great influence on the capacitance value

Effect of temperature and voltage on capacitance values

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