Electromagnetic compatibility design is closely related to specific circuits. In order to design electromagnetic compatibility, PCB designers need to minimize the radiation (RF energy leaking from products) and enhance their susceptibility to radiation (energy entering products) and anti-interference ability. However, for the common conductive coupling at low frequency and the common radiation coupling at high frequency, cutting off the coupling ways must be given full attention in design. This paper mainly explains the points that should be paid attention to in PCB design, so as to reduce the electromagnetic interference in PCB.
PCB design principles
With the development of electronic technology, the integration of circuit board and signal frequency are getting higher and higher, which inevitably brings electromagnetic interference. Therefore, the following principles should be followed when designing PCB, so that the electromagnetic interference of circuit board can be controlled within a certain range, meet the design requirements and standards, and improve the overall performance of circuit.
1. Selection of circuit board
The primary task of PCB design is to properly select the size of the circuit board. If the size is too large, the impedance value of the circuit will increase and the anti-interference ability will decrease due to the long connection between components. However, if the size is too small, the arrangement of components will be dense, which is not conducive to heat dissipation, and if the wiring is too thin and too dense, it will easily cause crosstalk. Therefore, the circuit board with appropriate size should be selected according to the required components of the system.
Circuit boards are divided into single-panel, double-panel and multi-layer boards. The selection of circuit board layers depends on the function to be realized, noise index, number of signals and network cables, etc. Reasonable arrangement of layers can reduce the EMC problem of the circuit itself. The usual selection principles are:
① When the signal frequency is medium and low frequency, the components are few, and the wiring density is low or medium, single panel or double panel is selected;
② Multilayer boards are used when wiring density is high, integration is high and there are many components;
③ For high signal frequency, high-speed integrated circuits and dense components, choose four or more circuit boards. A single layer of multilayer board can be used as power layer, signal layer and grounding layer in design. The signal loop area is reduced and the differential mode radiation is reduced. For this reason, the multilayer board can reduce the radiation of the circuit board and improve the anti-interference ability.
2. Layout of circuit board components
After determining the size of PCB, the position of special components should be determined first, and finally all components of the circuit should be laid out in blocks according to the functional units of the circuit. Digital circuit unit, analog circuit unit and power circuit unit should be separated, and high frequency circuit unit and low frequency circuit unit should also be separated. The layout principles of common circuit boards are as follows.
1) Principles for determining the position of special components:
① The heating element should be placed in a position conducive to heat dissipation, such as the edge of PCB, and away from the microprocessor chip;
② Special high-frequency components should be placed next to each other to shorten the connection between them;
③ Sensitive elements should be far away from noise sources such as clock generator and oscillator;
④ The layout of potentiometer, adjustable inductor, variable capacitor, key switch and other adjustable elements should meet the structural requirements of the whole machine, which is convenient for adjustment;
⑤ Components with heavier quality should be fixed by brackets;
⑥EMI filter should be placed close to EMI source.
2) According to the principle of layout of the umbrella components of the circuit by the circuit functional unit:
① Each functional circuit should determine the corresponding position according to the signal flow direction between them, which is convenient for wiring;
② Each functional circuit should first determine the position of the core element, and place other elements around the core element to shorten the connection between the elements as much as possible;
③ For high-frequency circuits, the distribution parameters between components should be considered;
④ The components placed on the edge of the circuit board should be no less than 2mm； away from the edge of the circuit board;
⑤DC/DC converter, switch tube and rectifier should be placed as close as possible to the transformer to reduce external radiation;
⑥ Voltage regulating element and filter capacitor should be placed close to rectifier diode.
3) Wiring principle between power supply and ground
Whether the wiring of PCB’s power supply and ground is reasonable is the key to reduce electromagnetic interference of the whole circuit board. The design of power line and ground wire is a problem that cannot be ignored in PCB, and it is often the most difficult design. The following principles should be followed when designing.
1) Wiring skills between power supply and ground
The wiring on PCB is characterized by distributed parameters such as impedance, capacitive reactance and inductive reactance. In order to reduce the influence of distribution parameters of PCB wiring on high-speed electronic system, the wiring principles for power supply and ground are as follows:
① Increase the spacing of traces to reduce the crosstalk of capacitive coupling;
(2) The power line and the ground line should be run in parallel to optimize the distributed capacitance;
③ According to the carrying current, try to widen the width of the power line and the ground line, reduce the loop resistance, and at the same time make the direction of the power line and the ground line in each functional circuit consistent with the signal transmission direction, which is helpful to improve the anti-interference ability;
④ The power supply and the ground should be routed directly above each other, so as to reduce the inductive reactance and minimize the loop area, and try to make the ground wire run below the power line;
⑤ The thicker the ground wire, the better. Generally, the width of the ground wire is not less than 3mm；;
⑥ Make the ground wire into a closed loop to reduce the potential difference on the ground wire and improve the anti-interference ability;
⑦ In the wiring design of multilayer board, one of the floors can be used as “the whole ground plane”, which can reduce the grounding impedance and play a shielding role at the same time.
2) Grounding skills of each functional circuit
The grounding modes of PCB functional circuits are divided into single-point grounding and multi-point grounding. According to the connection form, single-point grounding can be divided into single-point series grounding and single-point parallel grounding. Single-point series grounding is often used for protective grounding because of the different lengths of grounding wires, different grounding impedances of various circuits, and lower electromagnetic compatibility. Single-point grounding each circuit has its own grounding wire, so the interference between them is small, but it may prolong the grounding wire and increase the grounding impedance. It is often used for signal grounding, analog grounding and power grounding. Multi-point grounding means that each circuit has a grounding point, as shown in Figure 5. Multi-point grounding is often used in high-frequency circuits, which has the advantages of short grounding wire, small grounding impedance value and reduced interference of high-frequency signals.
In order to reduce the interference caused by grounding, grounding should also meet certain requirements:
① The grounding wire should be as short as possible, and the grounding wire should be large;
② Avoid unnecessary grounding circuit and reduce the interference voltage of common grounding;
③ The grounding principle is to adopt different grounding methods for different signals, and it is impossible to adopt the same grounding point for all grounding;
④ When designing multilayer PCB, the power layer and the ground layer should be placed in adjacent layers as much as possible, so that the capacitance between layers can be formed in the circuit and the electromagnetic interference can be reduced;
⑤ Try to avoid strong and weak signals, and digital and analog signals share the same ground.
24 Tips to Reduce Noise and Electromagnetic Interference
(1) If you can use low-speed chips, you don’t need high-speed chips. High-speed chips are used in key places.
(2) A resistor can be connected in series to reduce the jumping rate of the upper and lower edges of the control circuit.
(3) Try to provide some form of damping for relays, etc.
(4) Use the lowest frequency clock that meets the system requirements.
(5) The clock generator should be as close as possible to the device using the clock. The shell of the quartz oscillator should be grounded.
(6) Circle the clock area with a land line, and the clock line should be as short as possible.
(7) I/O drive circuit should be as close to the printed board as possible, so that it can leave the printed board as soon as possible. The signal entering the printed circuit board should be filtered, and the signal from the high noise area should also be filtered. At the same time, the series termination resistance should be used to reduce the signal reflection.
(8) The useless end of MCD should be connected to high, or grounded, or defined as the output end. The end connected to the power supply ground on the integrated circuit should be connected, and should not be hung up.
(9) The input of the unused gate circuit should not be hung up. The positive input of the unused operational amplifier is grounded, and the negative input is connected to the output.
(10) As far as possible, printed boards should use 45-fold line instead of 90-fold line wiring to reduce the external transmission and coupling of high-frequency signals.
(11) Printed boards are partitioned according to frequency and current switching characteristics, and the distance between noise components and non-noise components is further.
(12) Single-point power supply and single-point grounding, power line and ground wire should be as thick as possible for single-panel and double-panel panels. If the economy is affordable, multilayer boards should be used to reduce the inductance of power supply and ground.
(13) Clock, bus and chip selection signals should be kept away from I/O lines and connectors.
(14) The analog voltage input line and reference voltage terminal should be as far away from the digital circuit signal line as possible, especially the clock.
(15) For Class A/D devices, the digital part and the analog part would rather be unified than crossed.
(16) The interference of the clock line perpendicular to the I/O line is less than that of the parallel I/O line, and the pin of the clock element is far away from the I/O cable.
(17) Component pins should be as short as possible, and decoupling capacitor pins should be as short as possible.
(18) The key line should be as thick as possible, and protected areas should be added on both sides. The high-speed line should be short and straight.
(19) The line sensitive to noise should not be parallel to the high-current and high-speed switch lines.
(20) Do not run under the Shi Ying crystal and the noise-sensitive devices.
(21) Weak signal circuit, don’t form current loop around low frequency circuit.
(22) Don’t form a loop of signals. If it is inevitable, keep the loop area as small as possible.
(23) One decoupling capacitor per integrated circuit. A small high-frequency bypass capacitor should be added at the side of each electrolytic capacitor.
(24) Use large-capacity tantalum capacitor or condenser capacitor instead of electrolytic capacitor as circuit charging and discharging energy storage capacitor. When using tubular capacitor, the shell should be grounded.