PCB Tech - PCB design principles to reduce electromagnetic interference

The electromagnetic compatibility design is closely related to the specific circuit. In order to carry out the electromagnetic compatibility design, the designer needs to minimize the radiation (radio frequency energy leaking from the product) and enhance its resistance to the radiation (energy entering the product). Susceptibility and anti-interference ability. For the common conduction coupling at low frequency and the common radiation coupling at high frequency, cutting off the coupling path must be given full attention in the design.

PCB design principles

As the circuit board0 integration and signal frequency are getting higher and higher with the development of electronic technology, electromagnetic interference will inevitably be brought about, so the following principles should be followed when designing the PCB to control the electromagnetic interference of the circuit board within a certain range, Meet the design requirements and standards, and improve the overall performance of the 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 wiring between the components will be too long, which will increase the impedance of the circuit and reduce the anti-interference ability; and the size will cause the components to be too small. Dense layout is not conducive to heat dissipation, and the wiring is too thin and dense, which is easy to cause crosstalk. Therefore, the circuit board of the appropriate size should be selected according to the required components of the system.

Circuit board

Circuit boards are divided into single-sided, double-sided and multi-layer boards. The choice of the number of layers of the circuit board depends on the function to be realized by the circuit, the noise index, the number of signals and network cables, etc. Reasonable layer setting can reduce the electromagnetic compatibility problem of the circuit itself.

The usual selection principles are:

1. When the signal frequency is medium and low frequency, there are fewer components, and the wiring density is low or medium, choose single-sided or double-sided;

2. Multi-layer boards are used for high wiring density, high integration and more components;

3. For high-frequency, high-speed integrated circuits, and high-speed integrated circuits, choose 4 or more layers of circuit boards. When designing a multilayer board, a single layer can be used as a power layer, a signal layer, and a ground layer. 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 the PCB size is determined, the location of the special components should be determined first, and finally all the components of the circuit are laid out in blocks according to the functional units of the circuit. The digital circuit unit, the analog circuit unit and the power circuit unit should be separated, and the high-frequency circuit unit and the low-frequency circuit unit should also be separated. The layout principles of commonly used circuit boards are as follows.

1. The principle of determining the location of special components:

1. The heating element should be placed in a position conducive to heat dissipation, such as the edge of the PCB, and away from the microprocessor chip;

2. Special high-frequency components should be placed close to each other to shorten the connection between them;

3. Sensitive components should be kept away from noise sources such as clock generators and oscillators;

4. The layout of adjustable components such as potentiometers, adjustable inductors, variable capacitors, key switches, etc. should meet the structural requirements of the whole machine and facilitate adjustment;

5. Heavier components should be fixed with brackets;

6. The EMI filter should be placed close to the EMI source.

Layout of circuit board components

2. The principle of laying out the umbrella components of the circuit according to the circuit function unit:

1. Each functional circuit should determine the corresponding position according to the signal flow between them to facilitate wiring;

2. Each functional circuit should first determine the location of the core component, and place other components around the core component, and try to shorten the connection between the components;

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.

5. DC/DC converter, switch tube and rectifier should be placed as close to the transformer as possible to reduce external radiation;

6. Voltage regulating components and filter capacitors should be placed close to the rectifier diode.

3. Wiring principle of power supply and ground

Whether the wiring between the power supply and the ground of the PCB is reasonable is the key to reducing the electromagnetic interference of the entire circuit board. The design of the power line and the ground line is a problem that cannot be ignored in the PCB, and it is often the most difficult design. The following principles should be followed when designing.

1. Wiring skills for power and ground

The wiring on the PCB has characteristics of distributed parameters such as impedance, capacitive reactance, and inductive reactance. In order to reduce the influence of PCB wiring distribution parameters on high-speed electronic systems, the wiring principles for power and ground are as follows:

1. Increase the spacing of the traces to reduce the crosstalk of capacitive coupling;

2. The power line and the ground line should be routed in parallel to optimize the distributed capacitance;

3. According to the size of the carrying current, thicken the width of the power line and the ground line as much as possible, reduce the loop resistance, and make the direction of the power line and the ground line in each functional circuit consistent with the signal transmission direction, which will help improve Anti-interference ability;

4. The power and ground should be directly routed above each other to reduce the inductance and minimize the loop area, and try to make the ground wire under the power line;

5. The thicker the ground wire, the better, the width of the ground wire is generally not less than 3mm;

6. The ground wire is formed into a closed loop to reduce the potential difference on the ground wire and improve the anti-interference ability;

7. In the multi-layer board wiring design, one of the layers can be used as a "full ground plane", which can reduce the ground impedance and at the same time play a shielding role.

Wiring skills for power and ground

2. Grounding skills of each functional circuit

The grounding methods of each functional circuit of the PCB are divided into single-point grounding and multi-point grounding. Single-point grounding is divided into single-point series grounding and single-point parallel grounding according to the connection form, as shown in Figure 3 and Figure 4. Single-point series grounding is often used for protective grounding because of the different lengths of the grounding wires and the different grounding impedances of each circuit, and the electromagnetic compatibility performance is reduced. Single-point parallel grounding. Each circuit has its own grounding wire, so the mutual interference is small, but it may extend 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, with short grounding wires and low grounding impedance to reduce the interference of high-frequency signals.

In order to reduce the interference caused by grounding, grounding must also meet certain requirements:

1. The grounding wire should be as short as possible, and the grounding surface should be large;

2. Avoid unnecessary ground loops and reduce the interference voltage of the common ground;

3. The principle of grounding is to adopt different grounding methods for different signals, and it is not possible to use the same grounding point for all grounding;

4. When designing a multi-layer PCB, place the power layer and ground layer in adjacent layers as much as possible, so as to form layer-to-layer capacitance in the circuit and reduce electromagnetic interference;

5. Try to avoid strong and weak current signals, and digital and analog signals share the same ground.

Grounding skills of each functional circuit

24 tips to reduce noise and electromagnetic interference:

(1) Low-speed chips can be used instead of high-speed chips. High-speed chips are used in key places.

(2) A resistor can be connected in series to reduce the jump 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 is as close as possible to the device using the clock. The shell of the quartz crystal oscillator should be grounded.

(6) Enclose the clock area with a ground wire and keep the clock wire as short as possible.

(7) The I/O drive circuit should be as close as possible to the edge of the printed board, and let it leave the printed board as soon as possible. The signal entering the printed board should be filtered, and the signal from the high-noise area should also be filtered. At the same time, a series of terminal resistors should be used to reduce signal reflection.

(8) The useless terminal of MCD should be connected to high, or grounded, or defined as the output terminal, and the terminal that should be connected to the power supply ground on the integrated circuit should be connected, and should not be left floating.

(9) The input terminal of the gate circuit that is not in use should not be left floating. The positive input terminal of the unused operational amplifier should be grounded, and the negative input terminal should be connected to the output terminal.

(10) For printed boards, try to use 45-fold lines instead of 90-fold lines to reduce the external emission and coupling of high-frequency signals.

(11) The printed boards are partitioned according to frequency and current switching characteristics, and the noise components and non-noise components should be farther apart.

(12) Use single-point power supply and single-point grounding for single and double panels. The power line and ground line should be as thick as possible. If it is economical, use a multilayer board to reduce the capacitive inductance of the power supply and ground.

(13) Keep the clock, bus, and chip select signals away from I/O lines and connectors.

(14) The analog voltage input line and reference voltage terminal should be as far away as possible from the digital circuit signal line, especially the clock.

(15) For A/D devices, the digital part and the analog part should be unified rather than crossed.

(16) The clock line perpendicular to the I/O line has less interference than the parallel I/O line, and the clock component pins are far away from the I/O cable.

(17) The component pins should be as short as possible, and the decoupling capacitor pins should be as short as possible.

(18) The key line should be as thick as possible, and protective ground should be added on both sides. The high-speed line should be short and straight.

(19) Lines sensitive to noise should not be parallel to high-current, high-speed switching lines.

(20) Do not route wires under the quartz crystal or under noise-sensitive devices.

(21) For weak signal circuits, do not form current loops around low-frequency circuits.

(22) Do not form a loop in the signal. If it is unavoidable, make the loop area as small as possible.

(23) One decoupling capacitor per integrated circuit. A small high-frequency bypass capacitor must be added to each electrolytic capacitor.

(24) Use large-capacity tantalum capacitors or ju-cool capacitors instead of electrolytic capacitors for charging and discharging energy storage capacitors in the circuit. When using tubular capacitors, the case should be grounded.