2019年11月11日星期一

PCB EMC design

  • PCB EMC design

1)Analysis of signal spectrum

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A1 = 2τ / T
A2 = 0.64 / T f
A3 = 0.2 / T tr f2
The shorter the rise (or fall) time, the richer the high frequency components of the signal.

(2)Analysis of interference sources

ΔI Noise interference

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Common mode interference and differential mode interference

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(a)Differential mode radiation
(b)common mode radiation

Crosstalk

When a high frequency current flows through a signal line, the interference voltage will be induced on the signal line adjacent to the PCB board.
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(3)Wiring design technology

 When wiring, make all signal loop areas (especially high-frequency signals and sensitive signal loop areas) as small as possible.
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 High-speed signal lines should not be crossed over the partition area or over the unrelated reference plane.
◊ In analog circuit and RF circuit designs, as well as in dual panels without a power ground plane, the guard wires are often used to protect critical signals from crosstalk from other signals. Generally, protection line is connected to the ground network, and the two ends of the line are connected to the ground. When the frequency is high, the protection line is grounded with multiple vias. For digital circuits with complete ground planes, protection lines are generally not used.
◊ The difference corresponds to parallel equidistant equi-corresponding traces,maintain symmetry, so that the circuit has a good suppression of common mode interference.
◊ Sharp lines and right angles are not allowed for high-speed signal wiring. Signals above 1 GHz should use circular arcs wiring as much as possible.
◊ In order to reduce the radiation and interference of high frequency signals, the high frequency signals are arranged in the inner layer as much as possible.
◊ Any adjacent signal layer should adopt vertical and orthogonal direction as far as possible.
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◊ Don’t lay a meaningless line on a single board; the test leads should be as short as possible.

(4)Ground wire design technology

◊ 20H principle
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◊ For multi-layer boards, the integrity of the ground plane should be guaranteed and there should be no large openings in the ground plane.
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Advantages:
① Provide a more stable electrical Level
② Provide a small signal loop area
③ Make the signal line have a certain and more uniform characteristic impedance
④ Can control crosstalk between signals
◊ Ensure that there are no isolated copper skin on the PCB; the copper areas, the metal inside the equipment (such as radiators, reinforcement strips, metal casings, etc.) must be well grounded.
◊ When there are multiple ground layers in PCB, the ground surface should be connected together with more dispersed through vias on the board, especially in the place where the signal concentrates to change the layer, so as to provide a shorter loop circuit for the signal of the change layer and reduce radiation.If the ground planes are connected together by using a via hole around the plane, the external radiation of the PCB can be effectively reduced.
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Design of grounding

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① Comb power supply and ground structure (double board). It is suitable for low-speed circuit and single signal wiring and low line density. To protect more important signals, after wiring is completed, the empty places are covered with copper sheet, and the two layers are connected together by multiple holes, which can improve the problem of large loop circuit area.
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② The raster ground structure (double board) is suitable for low-speed CMOS and ordinary TTL circuits, but attention should be paid to adequate protection of higher-speed signals.

(5)Power integrity (PI) analysis

Problems arising from switching current
① It produces strong radiation disturbance.
② Reducing VCC,affect the normal work of the chip.
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Solutions

◊ Setting up decoupling capacitor
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◊ Selection of Decoupling Capacitor
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Eg:I=20mA
△t=10nS
△V=100mV
So:C = 2nF

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