Practice has proved, even if the circuit schematic design correctly, printed circuit board of power supply (smps) design undeserved, it also can produce adverse impact to electronic equipment reliability. For example, if the power supply PCB of two thin parallel lines get close too much, it will form delay of signal waveform, will form reflex noise in transmission line terminal. Therefore, it should adopt correct method in the design of printed circuit boards.
A, Landlines design in electronic equipment, and the grounding is the important method to control interference.
If you could proper combine grounding and shielding to use, it can solve most of the interference problems. General struture of landlines in electronic equipment are system ground, chassis ground (shield ground), and digital ground (logically) and simulation etc. In landlines design it should notice the following points：
1. Correct choice single point grounding and multipoint grounding
In the low-frequency circuit, when the signal working frequency is less than 1MHz, its inductance between the wiring and devices had less affect, while the circulation formed by grounding circuit produce a greater impact on the interference, therefore, it should use single point grounding of smps. When the signal working frequency is greater than 10MHz, the ground impedance becomes large, at this time, should try to reduce ground impedance and should use nearby multipoint grounding. When the working frequency within 1 ~ 10MHz, the ground wire length of the switching power supply should not exceed the wavelength of 1 / 20 if using the single point grounding, otherwise it should use multipoint grounding method.
2. Separate the digital circuit and analog circuits
There are both high-speed logic circuit boards and linear circuit on PCB, we should make them separate as far as possible, while their ground wire do not mix ,they are connected with the supply-side ground wire separately. We should try to increase the grounding area of linear circuit.
3. Try to overstriking grounding lines
If ground wire is very fine, grounding potential will changes with the current change, it can cause the timing signal level of electronic equipment instability and anti-noise performance become worse. Therefore, Grounding wires should be overstriking as far as possible, to make it through the allowed current which is located in PCB. If possible, the width of grounding line should be more than 3mm.
4. Constitute the ground wire to closed-loop
When designed ground wire system of the PCB that only composed by digital circuit, Ground wire made of closed-loop and it can obvious improve the resistance to noise. Its reason is: there are many integrated circuit components on PCB, especially in case of the components which consumed much, as it restricted by the thickness of grounding lines, so it can produce greater potential differences and cause anti-noise ability decline, if the grounding structure made into the loop circuit, it will narrow the potential difference, improve the anti-noise ability of electronic equipment.
