How to eliminate the electromagnetic interference of the single chip system? (II)

2. Factors affecting EMC

1 voltage.

The higher the supply voltage, the greater the voltage amplitude, the more emissions, and the lower supply voltage affects the sensitivity.

2 frequency.

High frequencies produce more emissions, and periodic signals produce more emissions. In a high-frequency single-chip system, a current spike is generated when the device is switched; in an analog system, a current spike is generated when the load current changes.

3 Ground.

Among all EMC issues, the main problem is caused by improper grounding. There are three ways to signal ground: single point, multiple points, and hybrid. At frequencies below 1 MHz, a single point grounding method can be used, but not for high frequencies; in high frequency applications, multipoint grounding is preferred. Hybrid grounding is a method in which the low frequency is grounded at a single point and the high frequency is grounded at multiple points. The ground layout is the key, and the ground loop of the high-frequency digital circuit and the low-level analog circuit must not be mixed.

4 PCB design.

Proper printed circuit board (PCB) routing is critical to preventing EMI.

5 Power supply decoupling.

When the device is switched, transient currents are generated on the power line and these transient currents must be attenuated and filtered out. Transient currents from high di/dt sources cause ground and trace "emission" voltages, high di/dt produces a wide range of high frequency currents, excitation components and cable radiation. Current changes and inductance through the wire can cause a voltage drop that can be minimized by reducing inductance or current changes over time.

3. Electromagnetic compatibility design of printed circuit board (PCB)

A PCB is a support for circuit components and devices in a microcontroller system that provides electrical connections between circuit components and devices. With the rapid development of electronic technology, the density of PCBs is getting higher and higher. The quality of PCB design has a great influence on the electromagnetic compatibility of single-chip microcomputer system. Practice has proved that even if the circuit schematic design is correct and the printed circuit board is not properly designed, it will have an adverse effect on the reliability of the single-chip system. For example, if the two thin parallel lines of the printed board are in close proximity, a delay in the signal waveform is formed, and reflected noise is formed at the end of the transmission line. Therefore, when designing a printed circuit board, care should be taken to adopt the correct method, to comply with the general principles of PCB design, and to comply with the requirements of anti-interference design.

3.1 General principles of PCB design

In order to get the best performance of the electronic circuit, the layout of the components and the layout of the wires are very important. In order to design a good quality, low cost PCB, the following general principles should be followed.

(1) Special component layout

First, consider the size of the PCB: when the PCB size is too large, the printed lines are long, the impedance is increased, the anti-noise ability is reduced, and the cost is also increased; if it is too small, the heat dissipation is not good, and the adjacent lines are susceptible to interference. After determining the PCB size, determine the location of the special components. Finally, according to the functional unit of the circuit, all the components of the circuit are laid out.

Observe the following principles when determining the location of a particular component:

Minimize the wiring between high-frequency components and try to reduce their distribution parameters and mutual electromagnetic interference. Components that are susceptible to interference cannot be placed too close together, and input and output components should be kept as far away as possible.

Some components or wires may have a high potential difference, and the distance between them should be increased to avoid accidental short circuit caused by discharge. Components with high voltage should be placed as far as possible in the hands of the hand when debugging.

Components weighing more than 15 g should be fixed with brackets and then soldered. Those components that are large, heavy, and have a lot of heat should not be mounted on the printed board, but should be installed on the chassis of the whole machine, and heat dissipation should be considered. The thermal element should be kept away from the heating element.

For the layout of adjustable components such as potentiometers, adjustable inductors, variable capacitors, and microswitches, the structural requirements of the complete machine should be considered. If it is adjusted inside the machine, it should be placed on the printing plate for easy adjustment; if it is adjusted outside the machine, its position should be adapted to the position of the adjustment knob on the chassis panel.

Leave the position occupied by the positioning holes of the printing plate and the fixing bracket.

(2) General component layout

According to the functional unit of the circuit, the following principles should be met when laying out all the components of the circuit:

The position of each functional circuit unit is arranged according to the flow of the circuit, so that the layout facilitates signal circulation and the signal is kept in the same direction as possible.

Center around the core components of each functional circuit and arrange it around it. Components should be evenly, neatly and compactly arranged on the PCB to minimize and shorten leads and connections between components.

For circuits operating at high frequencies, the distribution parameters between components should be considered. Generally, the circuit should be arranged in parallel as much as possible, so that it is not only beautiful, but also easy to load and easy to batch.

Components located at the edge of the board are typically no less than 2 mm from the edge of the board. The optimal shape of the board is rectangular. The aspect ratio is 3:2 or 4:3. When the board surface size is larger than 200 mm × 150 mm, the mechanical strength of the board should be considered.

(3) wiring

The principles of wiring are as follows:

The wires used for the input and output should be avoided as far as possible, and it is better to add the ground between the wires to avoid feedback.

The minimum width of the printed board wires is primarily determined by the adhesion strength between the wires and the insulating substrate and the current value flowing through them. When the thickness of the copper foil is 0.5 mm and the width is 1 to 15 mm, the temperature rise is not higher than 3 °C by the current of 2 A. Therefore, a wire width of 1.5 mm is sufficient. For integrated circuits, especially digital circuits, a wire width of 0.02 to 0.3 mm is usually selected. Of course, as far as possible, use wide lines, especially power and ground. The minimum spacing of the wires is primarily determined by the worst case interline insulation resistance and breakdown voltage. For integrated circuits, especially digital circuits, the pitch can be less than 0.1 to 0.2 mm as long as the process allows.

The curved corners of printed conductors generally take the shape of a circular arc, and the right angle or angle affects the electrical performance in high frequency circuits. In addition, try to avoid the use of large areas of copper foil, otherwise, when heated for a long time, copper foil expansion and falling off is easy. When a large area of copper foil is used, it is preferable to use a grid shape, which is advantageous for eliminating volatile gases generated by the heat of the adhesive between the copper foil and the substrate.

(4) pad

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

3.2 PCB and circuit anti-interference measures

The anti-interference design of printed circuit boards is closely related to the specific circuit. Here, only some common measures of PCB anti-interference design are explained.

(1) Power cord design

According to the current of the printed circuit board, try to increase the width of the power line and reduce the loop resistance. At the same time, make the direction of the power line and the ground line and the direction of data transmission consistent, which helps to enhance the anti-noise ability. ]

(2) Ground design

In the design of single chip microcomputer system, grounding is an important method to control interference. If the grounding and shielding are properly combined, most of the interference problems can be solved. The ground wire structure in the single chip microcomputer system is roughly systematic, chassis ground (shielded ground), digital ground (logical ground) and analog ground.

Pay attention to the following points in the ground line design:

Correct selection of single point grounding and multi-point grounding. In the low-frequency circuit, the operating frequency of the signal is less than 1 MHz, and the influence of the inductance between the wiring and the device is small, and the circulating current formed by the grounding circuit has a great influence on the interference, so a grounding method should be adopted. When the signal operating frequency is greater than 10 MHz, the ground impedance becomes very large. In this case, the ground impedance should be reduced as much as possible. When the operating frequency is between 1 and 10 MHz, if a grounding is used, the grounding length should not exceed 1/20 of the wavelength. Otherwise, the multi-point grounding method should be used.

The digital ground is separated from the analog ground. The circuit board has both high-speed logic circuits and linear circuits. They should be separated as much as possible, and the ground wires of the two should not be mixed, and they are connected to the ground of the power supply. The ground of the low-frequency circuit should be grounded in parallel with a single point. If the actual wiring is difficult, it can be partially connected and then connected in parallel. The high-frequency circuit should be connected in series with multiple points. The ground wire should be short and thick. Try to use a grid-like large-area foil around the high-frequency components, and try to increase the grounding area of the linear circuit.

The ground wire should be as thick as possible. If the grounding wire uses a very thin line, the grounding potential will change with the change of the current, causing the timing signal level of the electronic product to be unstable and the anti-noise performance to be lowered. Therefore, the ground wire should be as thick as possible so that it can pass three times the allowable current of the printed circuit board. If possible, the width of the ground wire should be greater than 3 mm.

The ground wire forms a closed loop. When designing a grounding system of a printed circuit board consisting only of digital circuits, making the grounding wire closed can significantly improve the noise immunity. The reason is that there are many integrated circuit components on the printed circuit board, especially when there are many power-consuming components, due to the limitation of the grounding wire thickness, a large potential difference will be generated on the ground wire, causing the noise resistance to decrease; If the ground line is formed into a loop, the potential difference is reduced, and the noise resistance of the electronic device is improved.

(3) Decoupling capacitor configuration

One of the usual practices in PCB design is to configure the appropriate decoupling capacitors at each critical part of the printed board.

The general configuration principle for decoupling capacitors is:

The power input terminal is connected to an electrolytic capacitor of 10 to 100 μF. If possible, it is better to connect 100μF or more.

In principle, a 0.01 pF ceramic capacitor should be placed on each integrated circuit chip. If the printed board gap is not enough, a 1~10 pF tantalum capacitor can be placed every 4~8 chips.

For devices with weak anti-noise capability and large power supply changes during shutdown, such as RAM and ROM memory devices, decoupling capacitors should be directly connected between the power and ground lines of the chip.

Capacitor leads should not be too long, especially high-frequency bypass capacitors must not have leads.

In addition, you should also pay attention to the following two points:

When there are contacts, relays, buttons and other components in the printed board, a large spark discharge will occur when operating them, and an RC circuit must be used to absorb the discharge current. Generally, R takes 1 to 2 kΩ, and C takes 2.2 to 47 μF.

The input impedance of CMOS is very high and it is susceptible to induction, so when it is used, it is necessary to ground or connect the power supply to the unused terminal.

(4) Oscillator

Almost all microcontrollers have an oscillator circuit that is coupled to an external crystal or ceramic resonator. On the PCB, the lead of the external capacitor, crystal or ceramic resonator is required to be as short as possible. The RC oscillator is potentially sensitive to interfering signals, it can produce very short clock cycles, so it is best to choose a crystal or ceramic resonator. In addition, the outer casing of the quartz crystal should be grounded.

(5) Lightning protection measures

The outdoor-use single-chip microcomputer system or the power line and signal line that is introduced into the room from the outdoor overhead should consider the lightning protection problem of the system. Commonly used lightning protection devices are: gas discharge tube, TVS (Transient Voltage Suppression) and the like. The gas discharge tube is a gas breakdown discharge when the power supply voltage is greater than a certain value, usually tens of V or hundreds of V, and the strong shock pulse on the power line is introduced into the earth. TVS can be viewed as two parallel and opposite Zener diodes that conduct when the voltage across the terminals is above a certain value. It is characterized by the ability to transiently pass hundreds or even thousands of A currents.