Hardware EMC specifications that power adapter engineers must know

　　The three elements of electromagnetic interference of the power adapter are the interference source, the interference transmission path, and the interference receiver.

　　EMC conducts research around these issues. The most basic interference suppression techniques are shielding, filtering, and grounding. They are mainly used to cut off the transmission path of interference. The broad electromagnetic compatibility control technology includes suppressing the emission of interference sources and improving the sensitivity of interference receivers, but it has been extended to other disciplines.

　　This specification focuses on the EMC design of the single board, with some necessary EMC knowledge and rules. The consideration of electromagnetic compatibility in the design stage of the printed circuit board will reduce the electromagnetic interference of the circuit in the prototype. The types of problems include common impedance coupling, crosstalk, radiation generated by high-frequency current-carrying wires, and noise pickup through loops formed by interconnecting wiring and printed wires.

　　In high-speed logic circuits, this type of problem is particularly fragile for many reasons:

　　1. The impedance between the power adapter and the ground wire increases as the frequency increases, and common impedance coupling occurs more frequently;

　　2. The signal frequency is higher, and it is more effective to couple to the step line through the parasitic capacitance, and the crosstalk is easier to occur;

　　3. The size of the signal loop is comparable to the wavelength of the clock frequency and its harmonics, and the radiation is more significant.

　　4. Impedance mismatch problem caused by signal line reflection.

　　1. Overall concept and consideration

　　1. The five-to-five rule, that is, if the clock frequency reaches 5MHz or the pulse rise time is less than 5ns, the PCB board must be a multilayer board.

　　2. The planes of different power adapters cannot overlap.

　　3. The problem of public impedance coupling.

　　Shielding effectiveness SE (dB) = reflection loss R (dB) + absorption loss A (dB) The key to high-frequency radio frequency shielding is reflection, and absorption is the key mechanism of low-frequency magnetic field shielding.

　　2. When the operating frequency is lower than 1MHz, the noise is generally caused by electric or magnetic fields (interference when the magnetic field is caused, generally within a few hundred hertz), above 1MHz, consider electromagnetic interference. The shielding entities on the board include transformers, sensors, amplifiers, DC/DC modules, etc. The larger one involves the shielding between single boards, subracks, and racks.

　　3. Electrostatic shielding does not require the shielding body to be closed, only high-conductivity materials and grounding points. Electromagnetic shielding does not require grounding, but requires a path for the induced current, so it must be closed. Magnetic shielding requires materials with high magnetic permeability to be a closed shield. In order to make the magnetic flux generated by the eddy current and the magnetic flux generated by the interference cancel out to achieve the purpose of absorption, the thickness of the material is required. In the case of high frequency, the three can be unified, that is, sealed with high conductivity materials (such as copper) and grounded.

　　4. For low-frequency, high-conductivity materials, the absorption attenuation is reduced, and the magnetic field shielding effect is not good. High-permeability materials (such as galvanized iron) should be used.

　　5. Magnetic field shielding also depends on the thickness, geometry, and maximum linear size of the hole.

　　6. The noise voltage induced by magnetic coupling UN=jwB.A.coso=jwM.I1, (A is the area of circuit 2 when the loop is closed; B is the magnetic flux density; M is the mutual inductance; I1 is the current of the interference circuit. Reduce noise There are two ways of voltage. For the receiving circuit, B, A and COS0 must be reduced; for the interference source, M and I1 must be reduced. Twisted-pair cable is a good example. It greatly reduces the loop of the circuit. Road area, and at the same time produce opposite electromotive force on the other stranded core wire.

　　7. Empirical formula for preventing electromagnetic leakage: gap size <λmin/20. The coverage rate of a good cable shielding layer should be more than 70%.

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