Problems with EMC designs often occur at high frequencies

Electromagnetic compatibility problems usually occur at high frequencies, with the exception of individual problems (voltage drops, transient interruptions, etc.). To sum up, high-frequency thinking is a characteristic of devices and circuits, which is different between high-frequency and traditional low-frequency states. If we still judge and analyze based on general control thinking, it will enter a design misunderstanding.

First of all, a capacitor is an energy storage component that exhibits only the characteristics of a capacitor under medium to low frequency or DC conditions, but at high frequency conditions, it is not just a capacitor. It has ideal capacitor characteristics, including leakage current (represented as R on high-frequency equivalent circuits), lead inductance, and ESR (equivalent series resistance) that still generates heat when causing voltage pulse fluctuations, as shown in the figure. Analyzing from this diagram can help our designers get many beneficial design ideas. First of all, according to the conventional thinking, 1/2 π fc is the capacitive reactance of a capacitor. The higher the frequency, the smaller the capacitive reactance, and the better the filtering effect. That is, the higher the clutter, the easier it is to leak. However, this is not the case because of the existence of lead inductance. A capacitor has the lowest overall impedance only when the formula such as 1/2 π fc=2 π fl is established. In other words, separating the resonant frequency points of two different capacitors for a distance is not only beneficial for slightly higher frequency filtering, but also beneficial for lower frequency filtering.

The second is the high-frequency equivalent characteristics of cable or PCB wiring (as shown in the figure). Regardless of high or low frequency, wiring resistance is objectively present, but for wiring inductance, only wiring inductance is displayed. In addition, there is a distributed capacitor. However, when there is no conductor near the wire, this distributed capacitor is useless, just as it cannot have children without men and women. This is a role that requires two conductors.

The characteristics of inductance and resistance are relatively simple, easy to understand, and not repetitive.

However, it is important to mention the high-frequency equivalent characteristics of magnetic rings and beads. Due to the fact that the wave absorption effect of the magnetic ring on high-frequency pulsations is somewhat similar to the performance of an inductor, it is often considered to be an inductive characteristic, but in fact it is incorrect. The magnetic ring is a resistance characteristic, but this resistance is somewhat special. The magnitude of its resistance is a frequency function R (f). In this case, when the high-frequency fluctuation signal passes through the magnetic beads, due to the effect of I2R, the high-frequency fluctuation will heat up and interfere with the fluctuation.

The above is the basic knowledge of high-frequency thinking for EMC professionals. With these, a series of design experiences can be easily solved.

The reason why two capacitors, an electrolytic capacitor, and a ceramic chip capacitor are installed at the ICVCC terminal is that the high frequency equivalent characteristic line inductance of the capacitor and the series connection of the capacitor cause its comprehensive impedance to vary with frequency, while the frequency point with WL=(1/WC) is its minimum impedance point (as shown in the figure). In addition, the two capacitors have their own minimum impedance points, corresponding to different frequency points, to provide current for the power requirements of the IC in different frequency ranges.

The grounding conductor of the electrostatic workbench adopts wide copper tape and metal mesh coil instead of a yellow green circular grounding cable. The circular grounding cable has a large inductance, which is not conducive to the discharge of high-frequency electrostatic charges.

In EMC design, the spacing between cables should not be too close, otherwise crosstalk between signal cables can be caused by the presence of distributed capacitance in the wires. Of course, it is best for the signal line to be closer to the coupling of the ground wire, so that the fluctuating interference on the signal line can be easily discharged onto the ground wire.