EMC rectification case

EMC rectification case 

Example of differential mode 

The connection between the power board and the main control board should be tightened or in some cases using the twisted pair, because the signal line is flowing to the ground, and the current in the two direction is better, the better the effect is, the pressure drop on the wire or the ground loop - > differential mode interference, such as the sound of the power amplifier, such as ADC sampling. 

Common mode example: if two adjacent wires are extracted in a computer‘s common flat feed line, the line length is 1 meters long, the wire is respectively common mode and differential mode current on the wire, and the harassment field strength is measured at the distance of 3 meters from the wire to GB9254. The experiment shows that if the field strength is to reach the limit of B class equipment (30~230 MHz is 40 dB mu V / M), the differential mode current is required to be 20 mA, and the common mode current is as long as 8 mu A, and the difference is 2500 times. It can be seen that the suppression of common mode current radiation is very important. 

Inductance and magnetic beads 

The composition and use of magnetic beads: it is made by sintering of ferrite materials with good impedance characteristics at high frequency section. It is specially designed to suppress the high frequency noise and peak interference of the signal line and the power line, and also has the ability to absorb the electrostatic pulse. 

The main parameters of the magnetic beads: 

Nominal value: because the unit of the magnetic bead is called according to the impedance that it produces at a certain frequency, the unit of impedance is also ohm. Generally, the 100MHz is the standard, such as the magnetic flux laminated patch 600R, which means that the impedance of the magnetic bead is 600 ohms at the time of 100MHz. 

Rated current: the rated current is the current that can guarantee the normal operation of the circuit, such as the magnetic flux rated current of the magnetic flux 600R is 0.3A (actual current capacity >0.3A). 

The difference between the inductor and the magnetic bead: 

Composition: the coils above one turn are used as inductance coils, and the coil habits of less than 1 turns (wire through magnetic beads) are used as magnetic beads; inductors are energy storage elements, and magnetic beads are energy conversion devices that convert high frequency into thermal energy; magnetic beads can be understood as series of resistance and electrical inductance; magnetic beads have a very high resistivity and a high resistivity. He is equivalent to a series of resistors and inductors, but the resistance and inductance values vary with frequency. He has better high-frequency filtering characteristics than ordinary inductors, and presents resistance at high frequency, so it can maintain a high impedance within a wide frequency range, thus improving the frequency modulation filtering effect. 

Uses: magnetic beads are mainly used to eliminate high frequency noise, such as some RF circuits, PLL, oscillating circuits, and ultra high frequency memory circuits (DDR SDRAM, RAMBUS, etc.) all need to add magnetic beads to the input part of the power supply, or the signal cable (the nearest C6 18001500), the inductance is used for the filtering of LC vibrating or low-frequency power circuits; EMC Magnetic beads are mainly used to suppress electromagnetic radiation interference, while inductors are mainly used to suppress conductive interference. 

Porous magnetic beads: there are many holes on some magnetic beads, which can increase the impedance of the component (through the square of the number of magnetic beads). However, the increase of the noise suppression ability at high frequency can not be as much as expected, and a few more magnetic beads will be better in series.