EMC problems often occur in high-frequency conditions, except for individual problems (voltage drops and instantaneous interruptions, etc.). High-frequency thinking, in a nutshell, is the characteristics of the device and the circuit. In high-frequency situations, it is different from the regular low-frequency state. If you still judge and analyze according to ordinary control thinking, you will fall into a misunderstanding of design. such as:
High frequency equivalent characteristics of capacitorsCapacitor, in the case of medium and low frequency or DC, is an energy storage component, which only shows the characteristics of a capacitor, but in the case of high frequency, it is more than just a capacitor. It has the characteristics of an ideal capacitor and has leakage current. (Expressed as R on the high-frequency equivalent circuit), there is lead inductance, and the ESR (equivalent series resistance) that causes heat in the case of voltage pulse fluctuations, (as shown in the figure). Analyzing from this graph can help our designers draw many useful design ideas. First, according to the conventional thinking, 1/2Ï€fc is the capacitive reactance of the capacitor. The higher the frequency, the smaller the capacitive reactance and the better the filtering effect. That is, the higher the frequency, the easier it is to bleed off the clutter, but this is not the case. Because of the existence of lead inductance, a capacitor is only when the equation of 1/2Ï€fc=2Ï€f L is established, and the overall impedance is the smallest, the filtering effect is the best, and the filtering effect will be reduced when the frequency is high and low. It can be analyzed and concluded, why two capacitors are added to the VCC terminal of the IC, one electrolytic and one ceramic, and the capacitance difference is generally a little more than 100 times. It is that the resonance frequency points of two different capacitors diverged for a certain distance, which is good for filtering slightly higher frequencies as well as filtering for lower frequencies.
The second is the high-frequency equivalent characteristics of the cable or PCB wiring (as shown in the figure). No matter the high and low frequencies, the wiring resistance is objectively present, but for the wiring inductance, it can only be revealed at higher frequencies. In addition, there is also the existence of a distributed capacitor, but when there is no conductor near the wire, this distributed capacitor is useless, just like a woman can’t give birth without a man, it needs two conductors to play. effect.
However, the high-frequency equivalent characteristics of the magnetic ring and the magnetic beads have to be mentioned, because the magnetic ring’s absorbing effect on the high-frequency pulsation is similar to the performance of the inductance, so it is often regarded as the inductive characteristic, but in fact it is wrong. The magnetic ring is a resistance characteristic, but this resistance is a bit special. Its resistance value is a function of frequency R(f). In this case, when a signal with high frequency fluctuations passes through the magnetic beads, the high frequency fluctuations will be Because of the heat generated by the I2R, the fluctuation will interfere with the conversion process of electrical energy-magnetic energy-thermal energy. Therefore, when the fluctuations on the wire are relatively strong, the magnetic ring will feel warm to the touch.
EMC design technology under high frequencyThe above is the basic knowledge of high-frequency thinking in EMC major. With these, a series of design experience can be easily solved. For example, why two capacitors, an electrolytic capacitor and a ceramic capacitor, are installed on the VCC terminal of the IC? It is because of the high-frequency equivalent characteristics of the capacitor. The series connection of the lead inductance and the capacitor causes its comprehensive impedance to change with frequency. At the frequency point of WL=(1/WC), it is the point with the smallest impedance (as shown in the figure). Moreover, the two capacitors have their own minimum impedance points, respectively corresponding to different frequency points, so as to provide current for the power supply requirements of the IC in different frequency ranges.
The ground wire of the electrostatic workbench uses a wide copper belt and a metal wire mesh snake skin tube instead of a yellow-green round ground cable. The wiring inductance of the round ground cable is too large, which is not conducive to high-frequency electrostatic charges. Vent. The distance between the cable and the cable should not be too close, otherwise it will cause crosstalk between the signal cables due to the distributed capacitance of the wires. Of course, the coupling of the signal wire to the ground wire is better to be closer. In this way, The fluctuating interference on the signal line can be easily vented to the ground line.
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