PCB EMC47 principles

Differential current and common-mode current
Radiation Generation: Current causes radiation, not voltage. Static charge generates electrostatic field, constant current generates magnetic field, and time-varying current generates both electric field and magnetic field. There are common-mode current and differential-mode current in any circuit. The differential-mode signal carries data or useful signals, and the common-mode signal is the negative effect of the differential-mode mode.
Differential current: equal in magnitude and opposite in direction (phase). Because the distributed capacitance, inductance, signal impedance of the traces are discontinuous, and the signal return path flows through an unexpected path, the differential mode current will be converted into common mode current: the common mode current is not necessarily equal in magnitude and the same in direction (phase).
External interference of equipment is mainly common mode, and differential mode interference also exists, but the intensity of common mode interference is often several orders of magnitude larger than that of differential mode. Common-mode interference is the main external interference. Common-mode interference itself generally does no harm to equipment, but if common-mode interference is converted into differential-mode interference, it will be serious, because all useful signals are differential-mode signals.
The magnetic field of differential mode current is mainly concentrated in the loop area composed of differential mode current, while the magnetic lines of force will cancel each other out outside the loop area. The magnetic field of common-mode current is outside the loop area, and the magnetic field generated by common-mode current has the same direction.
Many EMC designs of PCB follow the above theory.
The ways to suppress interference on PCB are: reducing the area of differential mode signal loop.
Reduce high frequency noise backflow (filtering, isolation and matching).
Reduce the common mode voltage (grounding design).
PCB design principle induction
Principle: PCB clock frequency exceeds 5MHZ or signal rise time is less than 5ns. Generally, multilayer board design is required.
Reason: The area of signal circuit designed with multilayer board can be well controlled.
2: For multilayer boards, the key wiring layers (the layers where clock lines, buses, interface signal lines, RF lines, reset signal lines, chip select signal lines and various control signal lines are located) should be adjacent to the complete ground plane, preferably between the two ground planes.
Reason: The key signal lines are generally strong radiation or extremely sensitive signal lines. Wiring close to the ground plane can reduce the area of its signal loop, reduce its radiation intensity or improve its anti-interference ability.
Principle 3: For single-layer boards, both sides of key signal lines should be covered.
Reason: Two sides of key signal are covered with ground, which can reduce the area of signal loop and prevent the crosstalk between signal line and other signal lines.
Principle 4: For double-layer boards, the projection plane of key signal lines should be paved with a large area, or the same as single-layer boards should be punched.
The reason is the same as that the key signal of multilayer board is close to the ground plane.
Principle: In the multilayer board, the power supply plane should be retracted for 5h-20h relative to its adjacent ground plane (h is the distance between the power supply and the ground plane).
Reason: The shrinkage of the power plane relative to its return ground plane can effectively suppress the edge radiation problem.
6. Principle: The projection plane of the wiring layer should be in the area of its reflow plane layer.
Cause: If the wiring layer is not in the projection area of the reflow plane layer, it will lead to the edge radiation problem and increase the area of the signal loop, which will lead to the increase of differential mode radiation.
Principle 7: In multi-layer boards, the TOP and BOTTOM layers of single board should be free of signal lines larger than 50MHZ as far as possible.
Reason: It’s best to walk the high-frequency signal between two plane layers to suppress its radiation to space.
Principle 8: For the single board with board-level working frequency greater than 50MHz, if the second and penultimate layers are wiring layers, the ground copper foil should be laid on the TOP and BOOTTOM layers.
Reason: It’s best to walk the high-frequency signal between two plane layers to suppress its radiation to space.
Principle 9: In multilayer boards, the main working power plane of a single board (the most widely used power plane) should be close to its ground plane.
Reason: Adjacent power plane and ground plane can effectively reduce the loop area of power circuit.
Principle 10: In a single-layer board, there must be a ground wire near the power supply wiring, which is adjacent to and parallel to it.
Reason: Reduce the area of power supply current loop.
Principle 11: In a double-layer board, there must be a ground wire near the power supply wiring, which is adjacent to and parallel to it.
Reason: Reduce the area of power supply current loop.
Principle 12: In layered design, try to avoid the adjacent layout of wiring layers. If adjacent wiring layers cannot be avoided, the layer spacing between the two wiring layers should be appropriately widened and the layer spacing between the wiring layers and their signal loops should be narrowed.
Cause: Parallel signal traces on adjacent wiring layers will cause signal crosstalk.
Principle 13: Adjacent plane layers should avoid overlapping their projection planes.
Cause: When the projections overlap, the coupling capacitance between layers will lead to noise coupling between layers.
Principle 14: When designing PCB layout, we should fully observe the design principle of laying it in a straight line along the signal flow direction, and try our best to avoid circling back and forth.
Reason: Avoid direct signal coupling, which will affect signal quality.
Principle 15: When multiple module circuits are laid on the same PCB, digital circuits and analog circuits, high-speed and low-speed circuits should be laid out separately.
Reason: Avoid mutual interference among digital circuits, analog circuits, high-speed circuits and low-speed circuits.
Principle 16: When there are high, medium and low speed circuits on the circuit board, the high and medium speed circuits should be kept away from the interface.
Reason: Avoid high-frequency circuit noise from radiating outward through the interface.
Principle 17: Energy storage and high-frequency filter capacitors should be placed near the unit circuits or devices with large current changes (such as the input and output terminals of the power module, fans and relays).
Reason: The existence of energy storage capacitor can reduce the loop area of high current loop.
Principle 18: The filter circuit of the power input port of the circuit board should be placed close to the interface.
Reason: Avoid the filtered lines from being coupled again.
Principle 19: On the PCB, the filtering, protection and isolation devices of the interface circuit should be placed close to the interface.
Reason: It can effectively achieve the effect of protection, filtering and isolation.
Principle 20: If there are both filtering and protection circuits at the interface, the principle of protection before filtering should be followed.
Cause: The protection circuit is used to suppress external overvoltage and overcurrent. If the protection circuit is placed behind the filter circuit, the filter circuit will be damaged by overvoltage and overcurrent.
Principle 21: Make sure that the input and output lines of filter circuit (filter), isolation circuit and protection circuit are not coupled with each other during layout.
Cause: When the input and output traces of the above circuits are coupled with each other, the filtering, isolation or protection effect will be weakened.
Principle 22: If the interface “clean ground” is designed on the single board, the filter and isolation devices should be placed on the isolation belt between the “clean ground” and the working place.
Reason: Avoid the coupling of filter or isolation devices through plane layer, which will weaken the effect.
Principle 23: On the “clean ground”, do not put any other devices except filter and protection devices.
Reason: The purpose of the “clean ground” design is to ensure that the interface radiation is minimal, and the “clean ground” is easily coupled by external interference, so there should be no other irrelevant circuits and devices on the “clean ground”.
Principle 24: Crystal, crystal oscillator, relay, switching power supply and other strong radiation devices should be at least 1000mil away from the single board interface connector.
Cause: The interference will radiate directly or the current will be coupled out on the outgoing cable.
Principle 25: Sensitive circuits or devices (such as reset circuits, WATCHDOG circuits, etc.) should be at least 1000mil away from the edges of the single board, especially the interface side edges of the single board.
Reason: Places like single board interface are the most easily coupled by external interference (such as static electricity), while sensitive circuits such as reset circuit and watchdog circuit are easy to cause the system.
Misoperation.
Principle 26: Each filter capacitor for IC filtering should be placed as close to the power supply pin of the chip as possible.
Cause: The closer the capacitor is to the pin, the smaller the area of the high-frequency loop, and thus the smaller the radiation.
Principle 27: For the series matching resistor at the beginning, it should be put close to its signal output.
Reason: The design purpose of the series matching resistor at the beginning is that the output impedance of the chip output and the impedance of the series resistor add up to the characteristic impedance of the trace, and the matching resistor is placed at the end, which cannot meet the above equation.
Principle 28: PCB wiring cannot have right-angle or acute-angle wiring.
Cause: Right-angle routing leads to impedance discontinuity, which leads to signal emission, resulting in ringing or overshoot, and strong EMI radiation.
Principle 29: Try to avoid the laying of adjacent wiring layers as much as possible. When it is unavoidable, try to make the lines in two wiring layers perpendicular to each other or parallel to each other with the length of less than 1000mil.
Reason: Reduce the crosstalk between parallel traces.
Principle 30: If a single board has an internal signal wiring layer, the key signal lines such as clocks are laid in the inner layer (the preferred wiring layer is preferred).
Reason: The key signals can be shielded by laying them in the internal wiring layer.
Principle 31: It is recommended to cover the ground wire on both sides of the clock line, and the covered ground wire should be grounded through holes every 3000mil.
Reason: Ensure that the potentials of all points on the ground line are equal.
Principle 32: The routing of key signals such as clock, bus, radio frequency line and other parallel routing on the same floor shall meet the 3W principle.
Reason: Avoid crosstalk between signals.
Principle 33: The pads of surface-mounted fuses, magnetic beads, inductors and tantalum capacitors used in the power supply with current ≥1A should be connected to the plane layer through at least two vias.
Cause: Reduce the equivalent impedance of vias.
Principle 34: Differential signal lines should be on the same floor, of equal length, and run in parallel, keeping impedance one: zero, and there are no other lines between differential lines.
Reason: Ensure that the common-mode impedance of differential pairs is equal, and improve their anti-interference ability.
Principle 35: The routing of key signals must not cross the division area (including the reference plane gap caused by vias and pads).
Reason: Crossing the division area will lead to the increase of signal loop area.
Principle 36: When it is inevitable that the signal line crosses its return plane; it is recommended of adopt bridging capacitance near the signal crossing; and the capacitance value is 1nF.
Reason: When the signal is split across, it often leads to the increase of its loop area, so it is artificial to set up a signal loop for it by bridging.
Principle 37: There should be no other irrelevant signal traces under the filter (filter circuit) on the single board.
Cause: Distributed capacitance will weaken the filtering effect of the filter.
Principle 38: The input and output signal lines of the filter (filter circuit) cannot be parallel to each other or cross-routed.
Reason: Avoid direct noise coupling between the traces before and after filtering.
Principle 39: The key signal line is ≥ 3h from the edge of the reference plane (h is the height of the line from the reference plane).
Reason: Suppress the edge radiation effect.
Principle 40: For grounding elements with metal shell, the top layer of its projection area should be covered with grounding copper skin.
Reason: The distributed capacitance between the metal shell and the grounded copper skin can restrain its external radiation and improve its immunity.
Principle 41: In single-layer board or double-layer board, attention should be paid to the design of “minimizing loop area” when wiring.
Reason: The smaller the loop area, the smaller the external radiation of the loop, and the stronger the anti-interference ability.
Principle 42: When changing layers of signal lines (especially key signal lines), a via hole should be designed near the via hole of the layer change.
Reason: The area of signal loop can be reduced.
Principle 43: Clock line, bus line, RF line, etc.: The strong radiation signal line should be away from the outgoing signal line of the interface.
Reason: Avoid the interference on the strong radiation signal line from coupling to the outgoing signal line and radiating outward.
Principle 44: Sensitive signal lines, such as reset signal line, chip select signal line, system control signal, etc., should be kept away from interface outgoing signal lines.
Cause: The outgoing signal line of the interface often brings in external interference, which will lead to system misoperation when coupled to sensitive signal lines.
Principle 45: In single panel and double panel, the wiring of the filter capacitor should be filtered by the filter capacitor first, and then to the device pins.
Reason: The power supply voltage is filtered before supplying power to the IC, and the noise fed back by the IC to the power supply will be filtered by the capacitor first.
Principle 46: In single panel or double panel, if the power line is very long, the coupling capacitor should be added to the ground every 3000mil, and the value of the capacitor should be 10uF＋1000pF.
Cause: Filter the high-frequency noise on the power line.
Principle 47: The grounding line and power line of filter capacitor should be as thick and short as possible.
Reason: Equivalent series inductance will reduce the resonant frequency of the capacitor and weaken its high-frequency filtering effect.