EMC anti-interference design of circuit boards

The basic task of anti-interference design is that the system or device will neither malfunction or lose function due to the influence of external electromagnetic interference, nor send excessive noise interference to the outside world, so as not to affect the normal operation of other systems or devices. Therefore, improving the anti-interference ability of the system is also an important part of the system design.

Circuit anti-interference design guideline:

The design of power wire

1) The selection of suitable power supply

2) Widen the power cord as much as possible;

3) Ensure that the direction of power line and ground is same as the direction of data transmission

4) Use anti-interference components

5) A decoupling capacitor is added to the power inlet.（10~100uf）

 

Ground wire design

1) Analog and digital separation;

2) Single-point grounding is used as much as possible;

3) Widen the ground wire as much as possible;

4) Connect the sensitive circuit to a stable ground reference source

5) The PCB board is zoned to separate the high-bandwidth noise circuits from the low-frequency circuits;

6) Minimize the area of the ground loop (All grounded devices are returned to power ground, which forms a path called the "ground loop").

                                                                                     

Configuration of the component

1) Don't have excessively long parallel signal wires;

2) Ensure that the clock generator, crystal oscillator of the PCB and CPU clock input are as close as possible, and away from other low-frequency devices;

3) Components should be configured according to the core components to minimize lead length;

4) PCB should have different zones;

5) Consider the position and orientation of the PCB in the chassis;

6) Shorten the leads between high-frequency components.

 

Configuration of the decoupling capacitor

1) For every 10 integrated circuits, a charge-discharge capacitor (10uf) should be added;

2) Capacitor with lead-out is for low frequency, and SMD capacitor is for high-frequency;

3) Each integrated chip should be arranged with a 0.1uf ceramic capacitor;

4) Weak anti-noise ability, Devices with large power supply changes during shutdown should be added with high-frequency decoupling capacitors;

5) Same VIA should not be used all capacitors;

6) Decoupling capacitor lead should not be too long;

                                                                                       

Noise and electromagnetic interference reduction guideline

1) Try to use a 45°polyline instead of a 90°polyline (to minimize the external emission and coupling of high-frequency signals);

2) The series resistance is adopted to reduce the rate at which the signal changes at the transition edge;

3) The quartz crystal oscillator shell should be grounded;

4)  Do not suspend idle circuits;

5) When the clock is perpendicular to the IO wire, there is small interference;

6) Try to keep the electromotive force around the clock nearly to zero;

7) The IO driver circuit is as close to the edge of the PCB as possible;

8) Any signal does not form a loop;

9) For high-frequency boards, the distributed inductance of the capacitance cannot be ignored, and the distributed capacitance of the inductor cannot be ignored;

10)  Usually, the power wire and AC wire should be on a different board from the signal wire.

                                                                                       

Other design guideline

1) The unused pins of the CMOS are to be connected to ground or power by resistor;

2) An RC circuit is used to absorb the discharge current from components such as relays;

3) Adding about 10k pull-up resistors on the bus helps resist interference;

4) The use of full decoding has better anti-interference;

5) Components connect with power supply through 10k resistance not through pins;

6) Bus should be as much short as possible, and bus should be as in same length as possible;

7) The wire layouts between two levels should be as vertical as possible;

8) Heat-generating components avoid sensitive components;

9) Horizontal wiring on the front side, longitudinal wiring on the back side, as long as space allows, the thicker the wire, the better (ground and power lines only)

10) To have a good ground layer, the wire should be laid from the front as much as possible, and ground layer should be on the back side;

11) Keep sufficient distances, such as the input and output of filters, the input and output of optical coupler, AC power wires and weak signal wires, etc.;

12) Long wire plus low-pass filter. The wiring should be as short as possible, and the necessary long wires should be inserted into the C, RC, or LC low-pass filter at reasonable positions;

13) Thin wires are preferred, except ground wires,

 

Wire width and current

1) Wires width is generally not less than 0.2 mm (8 mil)

2) On high-density and high-precision PCBs, the spacing and wire width are generally 0.3mm (12mil);

3) When the thickness of the copper foil is about 50um, the wire width is 1~1.5mm(60mil) = 2A;

4) Public ground wire width is generally 80 mil, which is especially true for applications with microprocessors.

                                                                                   

Power wire

The power cord should be as short as possible, in a straight line, preferably in a tree shape, not in a ring shape.

 

Layout

First of all, consider the PCB size. When the size of the PCB is too large, the printed wires are long, the impedance increases, the anti-noise ability decreases, and the cost also increases; If it is too small, the heat dissipation will not be good, and the adjacent wires will be easily disturbed.

After the PCB size is decided, the location of special components should be decided. Finally, according to the function units of the circuit, all components of the circuit should be laid .

 

The following guideline should be obeyed when the location of special components are decided:

1) The connection wire between high-frequency components is shortened as much as possible, and their distribution parameters and electromagnetic interference with each other are reduced. Components that are susceptible to interference should not be too close to each other, and input and output components should be as far away from each other as possible.

2) High potential differences between some components or wires may exist, and the distance between them should be increased to avoid accidental short circuits caused by discharge. Components with high voltage should be arranged in places that are not easy to reach during debugging.

1) Brackets should be used to fasten the components with more than 15 g weight, and then these components should be soldered. Those components that are large, heavy and generate a lot of heat should not be installed on the printed board, but should be installed on the chassis bottom board of the whole machine, and the heat dissipation problem should be considered. Heat-sensitive components should be kept away from heat-generating components.

2) For the layout of adjustable components such as potentiometers, adjustable inductance coils, variable capacitors, micro switches, etc., the structural requirements of the whole machine should be considered. If these components are adjusted in the machine, they should be placed above the printed board, and their locations should be convenient for adjustment; As for external adjustment, their locations should be appropriate to the position of the adjustment knob on the chassis panel.

3) The position occupied by the positioning hole of the printed board and the fixing bracket should be reserved.

 

 

The following guidelines should be obeyed when all components of the circuit are laid according to the function units of circuit:

1) The locations of each function circuit unit should be arranged according to circuit requirements, so that the layout is convenient for signal transmission, and all signal direction should be as same as possible.

2) The core components of each function circuit should be as the center. The components should be arranged evenly, neatly and compactly on the PCB to minimize and shorten leads and connection wires between components.   

                                                                                 

3) Distribution parameters between components should be considered on the circuits under high frequency. Components should be arranged in parallel generally for circuit, which is nice and easy for soldering and bulk production.    

4) The components located at the edge of the circuit board are generally not less than 2mm away from the edge of the circuit board. The optimal shape of a circuit board is a rectangle. The aspect ratio of circuit board is 3:2 to 4:3. When the board size is greater than 200x150mm, the mechanical strength of the circuit board should be considered.

 

Wiring

The guideline of wiring is as follows:

1) The wires used at the input and output terminals cannot be as parallel as possible. It is best to add ground wires between wires to avoid feedback coupling.

                                        

2) The minimum width of the printed camera wire is mainly determined by the adhesion strength between the wire and the insulating base and the current value flowing through them. When the thickness of the copper foil is 0.05mm and the width is 1 ~ 15mm. Through a current of 2A, the temperature will not be higher than 3°C. Therefore, wire width of 1.5 mm can meet requirements.

For integrated circuits, especially digital circuits, 0.02~0.3mm wire width is usually selected. Of course, use as wide wires as possible, especially for power and ground wires. The minimum spacing of the conductors is primarily determined by the inter-wire insulation resistance and breakdown voltage in the worst cases. For integrated circuits, especially digital circuits, as long as the process allows, the spacing can be as small as 5~8mm.

3) The bend of the printed wire is generally arc-shaped, and the right angle bend or acute angle bend will affect the electrical performance in high-frequency circuits. Also, try to avoid using large area copper foil. Otherwise, after large area copper foil is heated for a long time, the copper foil is prone to expansion and falling off. When it is necessary to use a large area of copper foil, it is best to use a grid, which helps eliminate volatile gases generated by heating the adhesive between the copper foil and the substrate.

 

 Pad

The pad center hole is slightly larger than the device lead diameter. Too large pad is easy to form empty soldering. The outer diameter of the pad D is generally not less than (d+1.2)mm, where d is the lead hole diameter. For high-density digital circuits, the minimum diameter of the pad can be (d+1.0)mm.

 

 PCB and circuit anti-interference measures 

The anti-interference designs of printed circuit boards are closely connected with specific circuits. This article only illustrates some common measures of PCB anti-interference.

 

Power wire design

According to the current amount of printed circuit board, power wire width should be increased, and loop resistance should be reduced. Meanwhile, directions of power wire, ground wire and data transmission should be all the same, which helps strengthen anti-noise ability. 

Ground wire design

The guideline of ground wire design is:

1) Analog and digital separation. If there are both logic and linear circuits on the board, they should be separated as much as possible. The low-frequency circuit should be connected to the ground at a single point in parallel as much as possible, and when the actual wiring is difficult, it can be partially connected in series and then connected to the ground in parallel. The high-frequency circuit should be grounded in multi-point series, the ground wire should be short and wide, and the grid-shaped large-area ground foil should be used around the high-frequency components as much as possible.

2) Ground wire should be as wide as possible. If the ground wire is tough, the grounding potential will change with the change of current, which will reduce anti-noise performance. Therefore, the ground wire should be thickened so that it can pass three times the allowable current on the printed board. If possible, the grounding wire should be more than 2~3mm.

3) The grounding wire forms a closed-loop, for the PCB composed by digital circuits, if the grounding circuit is laid as a loop, anti-noise ability can be enhanced. 

Decoupling capacitor configuration

One of the common practices of PCB design is to configure the appropriate decoupling capacitors in various critical parts of the printed board.

Configuration guideline of decoupling capacitors is:

1) The power input is connected to a 10~100uf electrolytic capacitor. If possible, it is better to connect to 100uF or more.

2) In principle, each integrated circuit chip should be arranged with a 0.01pF ceramic capacitor, if the printed board spacing is not enough, every 4~8 chips can be arranged with a 1 ~ 10pF capacitor.

3) For weak anti-noise ability components with large power changes when they are switched off, such as RAM, ROM memory devices, the decoupling capacitor should be connected directly between the power wires and the ground wires of the chip.

4) Capacitor leads should not be too long, especially the capacitors on high-frequency bypass should not have leads.