At present, the printed circuit board is still the main assembly method for all kinds of electronic equipment and systems. Practice has proved that even if the circuit schematic design is correct and the printed circuit board is improperly designed, the reliability of electronic equipment will be adversely affected. For example, if the two thin parallel lines of the printed board are close together, a delay in the signal waveform will be formed, forming reflected noise at the end of the transmission line. Therefore, when designing printed circuit boards, attention should be paid to the correct method.

A, ground design In electronic equipment, grounding is an important method to control interference

If the grounding and shielding can be correctly combined, most interference problems can be solved. The ground wire structure in electronic equipment is roughly systematic, shell ground (shield ground), digital ground (logic) and analog ground. The following points should be noted in the design of ground wire:
1. The correct choice of single point grounding and multi-point grounding in the low-frequency circuit, the signal operating frequency is less than 1MHz, its wiring and the inductance between the device has less influence, and the grounding circuit formed by the circulation has a greater impact on the interference, so should use a point grounding. When the signal operating frequency is greater than 10MHz, the ground impedance becomes large. In this case, the ground impedance should be reduced as much as possible, and the nearest multipoint grounding should be used. When the operating frequency is 1 ~ 10MHz, if one point grounding is used, the ground length should not exceed 1/20 of the wavelength, otherwise the multi-point grounding method should be used.
2. Separate the digital circuit from the analog circuit. Both high-speed logic circuits and linear circuits on the circuit board should be separated as far as possible, and the ground wires of the two should not be mixed and connected to the ground wires of the power supply. The ground area of the linear circuit should be increased as much as possible.
3. Make the ground wire thicker as far as possible If the ground wire is very thin, the ground potential changes with the change of current, resulting in the timing signal level of the electronic device is unstable, and the anti-noise performance deteriorates. Therefore, the ground wire should be as thick as possible, so that it can pass three allowable currents located on the printed circuit board. If possible, the ground cable should be wider than 3mm.
4. When designing the ground wire as a closed-loop circuit system composed only of digital circuit boards, making the ground wire as a closed-loop circuit can significantly improve the anti-noise ability. The reason is that there are many integrated circuit components on the printed circuit board, especially in the case of power-consumption components, due to the limitation of the thickness of the grounding wire, it will produce a large potential difference on the ground junction, resulting in a decrease in anti-noise ability, if the grounding structure into a loop, it will reduce the potential difference value and improve the anti-noise ability of electronic equipment.

B, electromagnetic compatibility design

Electromagnetic compatibility refers to the ability of electronic devices to work harmoniously and effectively in various electromagnetic environments. The purpose of electromagnetic compatibility design is to enable electronic equipment to suppress various external interference, so that electronic equipment can work normally in a specific electromagnetic environment, and at the same time reduce the electromagnetic interference of electronic equipment itself on other electronic equipment.
1. Choose a reasonable wire width Because the impact interference caused by the transient current on the printed line is mainly caused by the inductive component of the printed wire, so the inductance of the printed wire should be minimized. The inductance of the printed wire is proportional to its length and inversely proportional to its width, so a short and fine wire is advantageous for suppressing interference. Signal lines for clock leads, line drivers, or bus drivers often carry large transient currents, and printed wires should be as short as possible. For discrete component circuits, when the width of the printed wire is about 1.5mm, the requirements can be fully met; For integrated circuits, the printed wire width can be selected between 0.2 and 1.0mm.
2. Using the correct wiring strategy The use of equal wiring can reduce the inductance of the wire, but the mutual inductance and distributed capacitance between the wires increase, if the layout allows, it is best to use the well pattern network wiring structure, the specific approach is one side of the printed board transverse wiring, the other side longitudinal wiring, and then connected with metalized holes at the cross hole. In order to suppress the crosstalk between the printed board wires, long-distance equal wiring should be avoided as far as possible when designing the wiring.

C, decoupling capacitor configuration

In the DC power supply circuit, the change of load will cause the power supply noise. For example, in a digital circuit, when the circuit is converted from one state to another, a large spike current is generated on the power line, forming a transient noise voltage. The configuration of decoupling capacitors can suppress noise caused by load changes and is a common practice in the reliability design of printed circuit boards. The configuration principles are as follows:
● The power input end is connected to an electrolytic capacitor of 10 ~ 100uF. If the position of the printed circuit board allows, the anti-interference effect of the electrolytic capacitor above 100uF will be better.
● Configure a 0.01uF ceramic capacitor for each IC chip. If the printed circuit board space is small and cannot be installed, every 4 to 10 chips can be configured with a 1 to 10uF tantalum electrolytic capacitor, the high-frequency impedance of this device is particularly small, the impedance is less than 1Ω in the range of 500kHz to 20MHz, and the leakage current is very small (below 0.5uA).
● For devices with weak noise capacity and large current change when turned off, and memory devices such as ROM and RAM, the decoupling capacitor should be directly connected between the power line (Vcc) and the ground (GND) of the chip.
● The lead of the decoupling capacitor cannot be too long, especially the high-frequency bypass capacitor cannot have a lead.

D, the size of the printed circuit board and the layout of the device

The size of the printed circuit board should be moderate, too large when the printed line is long, the impedance increases, not only the anti-noise ability decreases, the cost is high; Too small, the heat dissipation is not good, and it is susceptible to interference from nearby lines. In terms of device layout, as with other logic circuits, the related devices should be placed as close as possible, so that a better anti-noise effect can be obtained. The clock generator, crystal oscillator and CPU clock input are prone to noise, to be closer to each other. Noise prone devices, small current circuits, large current circuits, etc. should be as far away from the logic circuit as possible, if possible, should be another circuit board, this is very important.

E, heat dissipation design

From the point of view of heat dissipation, it is best to install the printed plate vertically, the distance between the plate and the plate should generally not be less than 2cm, and the arrangement of the device on the printed plate should follow certain rules: • For the use of free convection air cooling equipment, it is best to arrange the integrated circuit (or other devices) according to the length; For devices that use forced air cooling, it is best to arrange the integrated circuit (or other device) in a horizontal way. • The devices on the same printed board should be arranged as far as possible according to the size of their heat and the degree of heat dissipation, and the devices with small heat or poor heat resistance (such as small signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) should be placed at the most upstream (entrance) of the cooling air flow. Devices with large heat generation or good heat resistance (such as power transistors, large-scale integrated circuits, etc.) are placed at the downstream of the cooling stream. • In the horizontal direction, the high-power device is arranged as close as possible to the edge of the printed board in order to shorten the heat transfer path; In the vertical direction, the high-power devices are arranged as close as possible to the printed board, in order to reduce the impact of these devices on the temperature of other devices when they work.
The device that is more sensitive to temperature is best placed in the lowest temperature area (such as the bottom of the device), do not put it right above the heating device, and multiple devices are best staggered on the horizontal plane. • The heat dissipation of the printed board in the device mainly depends on air flow, so the air flow path should be studied in the design, and the device or printed circuit board should be reasonably configured. When the air flows, it always tends to flow where the resistance is low, so when configuring the device on the printed circuit board, it is necessary to avoid leaving a large airspace in a certain area.

EMC skills: Rectification tips

1, 150kHz-1MHz, mainly difference mode, 1MHz-5MHz, difference mode and common mode work together, after 5MHz is basically common mode. Capacitive coupling and inductive coupling with differential mode interference. Generally, the interference above 1MHz is common mode, and the low frequency band is differential interference. The common mode interference can be estimated by measuring the voltage of the two pins of the resistance with an oscilloscope.

2. Add differential mode inductance or resistance after insurance.

3, the small power supply can be processed by PI filter (it is recommended that the electrolytic capacitor near the transformer can be selected larger).

4, the front end of PI EMI parts in the differential mode inductor is only responsible for low frequency EMI, the volume is not too large (DR8 is too large, can use resistance type or DR6 better) otherwise the radiation is not good, if necessary can string magnetic beads, because the high frequency will fly directly to the front end will not follow the line. 5, conduction cooler in 0.15MHz-1MHz exceeded, there is a 7dB margin when the heat engine. The main reason is that the primary BULk capacitance DF value is too large, the cooler ESR is relatively large, the heat engine ESR is relatively small, the switching current will form a switching voltage on the ESR, it will pressure a current LN line flow, this is differential mode interference. The solution is to use a low ESR electrolytic capacitor or add a differential mode inductor between two electrolytic capacitors.

6, test 150kHz total exceeded solution: increase the X capacitance to see if it can be down, if it is down, it is differential mode interference. If it does not have much effect, then it is common mode interference, or the power cord is wound around a large magnetic ring a few times, and it is common mode interference. If the interference curve is good behind, reduce the Y capacitance to see if there is a problem with the plate, or just add a magnetic ring in front of it.

7, can increase the inductance of PFC input part of the single-winding inductance.

8. The components in the PWM line will adjust the main frequency to about 60kHz.

9. Attach a piece of copper to the transformer core.

10, the two sides of the common mode inductance asymmetry, one turn less can also cause the conduction of 150kHz-3MHz exceed. 11, the generation of general conduction has two main points: 200kHz and 20MHz, these points also reflect the performance of the circuit; Around 200kHz is mainly caused by leakage spikes; Around 20MHz is mainly the noise of the circuit switch. If the transformer is not handled well, it will increase a lot of radiation, and it is useless to add shielding, and the radiation will not pass.

12. Change the input BUCk capacitor to a capacitor with low internal resistance.

13, for no Y-CAP power supply, wrap the transformer first around the primary, then wrap the auxiliary winding and the auxiliary winding close to the side, and then wrap the secondary.

14, the common mode inductor on the parallel a few k to dozens of k resistance.

15, the common mode inductor is shielded with copper foil and connected to the ground of a large capacitor.

16, in the PCB design should be the common mode inductor and transformer separated from each other to avoid interference.

17, condom magnetic beads.

18. The Y capacitance capacity of the two incoming wires grounded is reduced from 2.2nF to 471.

19, for a two-stage filter can be removed after the 0.22uFX capacitor (sometimes before and after the X capacitor will cause shock).

20. For π-type filter circuit, there is a BUCk capacitor lying on the PCB and near the transformer. This capacitor interferes with the L channel conducting 150kHz-2MHz. The improvement method is to wrap the capacitor with copper and shield it from the ground, or use a small PCB to separate the capacitor from the transformer and PCB. Alternatively, the capacitor can be stood up, or a small capacitor can be used instead.

21, for the π type filter circuit has a BUCk capacitor lying on the PCB and close to the transformer, this capacitor has interference with the L channel conducting 150kHz-2MHz, the improvement method is to replace this capacitor with a 1uF/400V or 0.1uF/400V capacitor, and increase the other capacitor.

22, add a small several hundred uH differential mode inductor before the common mode inductor.

23. Wrap the switch tube and radiator with a piece of copper foil, and the two ends of the copper foil are short-connected together, and then connect to the ground with a copper wire.

24, the common mode inductor with a piece of copper wrap and then connected to the ground.

25. Connect the switch pipe to the ground with a metal sleeve.

26, Increase the X2 capacitor can only solve the frequency band of about 150kHz, can not solve the frequency band above 20MHz, only in the power input with a nickel-zinc ferrite black magnetic ring, the inductance of about 50uH-1mH.

27. Increase X capacitance at the input end.

28, increase the input common mode inductance.

29, the auxiliary winding power supply diode back to the ground.

30. Replace the auxiliary winding power supply filter capacitor with a thin and long electrolytic capacitor or increase the capacity.

31, increase the input filter capacitance.

32, 150kHz-300kHz and 20MHz-30MHz are both conductive, and a differential mode circuit can be added before the common mode circuit. You can also see if there is a problem with the grounding, the grounding place must be strengthened, the ground wire on the main board must be straightened out, and the wiring between different ground wires must be smooth and not staggered.

33, in the rectifier bridge and capacitance, when considering the common mode component, should be adjacent Angle and capacitance, when considering the differential mode component, should be diagonal and capacitance.

34, increase the input differential mode inductance.

Product electromagnetic compatibility disturbance sources are:

1, the switching circuit of the equipment switching power supply: the main frequency of the disturbance source is dozens of kHz to more than 100 kHz, and the higher harmonics can be extended to tens of MHz.

2, the equipment DC power supply rectification circuit: power frequency linear power supply power frequency rectification noise frequency limit can be extended to hundreds of kHz; The upper limit of high-frequency rectification noise frequency of switching power supply can be extended to tens of MHz.

3, electric equipment DC motor brush noise: noise frequency limit can be extended to hundreds of MHz.

4, the operation noise of AC motor of electric equipment: high harmonics can be extended to tens of MHz.

5, frequency conversion speed control circuit disturbance emission: switching speed control loop disturbance source frequency from tens of kHz to tens of MHz.

6, switching noise of equipment operating state switching: The frequency limit of noise generated by mechanical or electronic switching action can be extended to hundreds of MHz.

7, intelligent control equipment crystal vibration and digital circuit electromagnetic disturbance: the main frequency of the disturbance source is tens of kHz to tens of MHz, and high order harmonics can be extended to hundreds of MHz.

8. Microwave leakage of microwave equipment: the main frequency of disturbance source is GHz.

9. Electromagnetic disturbance emission of electromagnetic induction heating equipment: the main frequency of the disturbance source is dozens of kHz, and the higher harmonics can be extended to tens of MHz.

10, the local vibration and its harmonics of the high-frequency tuning circuit of the television electroacoustic receiving equipment: the main frequency of the disturbance source is tens of MHz to hundreds of MHz, and the higher harmonics can be extended to several GHz.

11, information technology equipment and all kinds of automatic control equipment digital processing circuit: disturbance source frequency tens of MHz to hundreds of MHz (internal frequency doubling frequency can reach several GHz), high harmonics can be extended to more than 10 GHz.