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How to improve the anti-interference ability and EMC of PCB?
2020-09-12
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Author:Dag      Share

In the development of electronic products with processors, today ipcb to explain how to improve the anti-interference ability and electromagnetic compatibility?


1. The following systems should pay special attention to anti electromagnetic interference:

(1) A system with high clock frequency and fast bus cycle.

(2) The system contains high power, high current drive circuit, such as spark generating relay, high current switch and so on.

(3) The system includes weak analog signal circuit and high a / D conversion circuit.

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2. In order to increase the anti electromagnetic interference capability of the system, the following measures are taken:

(1) Low frequency microcontroller is selected

Choosing the microcontroller with low external clock frequency can effectively reduce the noise and improve the anti-interference ability of the system. For square wave and sine wave with the same frequency, the high frequency component of square wave is much more than that of sine wave. Although the amplitude of the high frequency component of square wave is smaller than that of the fundamental wave, the higher the frequency, the easier it is to emit and become a noise source. The influential high-frequency noise produced by the microcontroller is about three times the clock frequency.

(2) Reducing distortion in signal transmission

The microcontroller is mainly made by high speed CMOS technology. The static input current of the signal input is about 1mA, the input capacitance is about 10PF, the input impedance is very high, and the high-speed CMOS The output end of the circuit has a considerable load capacity, that is, a considerable output value. If the output end of a gate is led to the input terminal with high input impedance through a long line, the reflection problem is very serious, which will cause signal distortion and increase system noise. When TPD > TR, it becomes a transmission line problem. We must consider signal reflection, impedance matching and so on.

The delay time of the signal on the printed circuit board is related to the characteristic impedance of the lead wire, that is to say, the dielectric constant of the printed circuit board material. It can be roughly considered that the transmission speed of the signal in the printed circuit board leads is about 1 / 3 to 1 / 2 of the speed of light. The tr (standard delay time) of logic telephone components in the system composed of microcontroller is between 3 and 18 ns.

On the printed circuit board, the signal passes through a 7W resistor and a 25cm long lead, and the on-line delay time is about 4 ~ 20ns. In other words, the shorter the lead on the printed circuit, the better, and the length should not exceed 25cm. And the number of vias should also be as small as possible, no more than 2.

When the rising time of the signal is faster than the delay time of the signal, it should be processed according to fast electronics. At this time, the impedance matching of transmission line should be considered. For the signal transmission between integrated blocks on a printed circuit board, it is necessary to avoid the situation of TD " TRD. The larger the printed circuit board is, the less fast the system can be.

A rule of printed circuit board design is summarized with the following conclusions:

When the signal is transmitted on the printed board, the delay time shall not be greater than the nominal delay time of the device used.

(3) Reduce the cross interference between signal lines

A step signal with rise time of TR at point a is transmitted to terminal B through lead ab. The delay time of the signal on the AB line is TD. At point D, due to the forward transmission of the signal at point a, the signal reflection after reaching point B and the delay of line AB, a page pulse signal with width of TR will be induced after TD time. At point C, due to the transmission and reflection of the signal on AB, a positive pulse signal with a width of twice the delay time of the signal on the AB line will be induced, that is, 2TD positive pulse signal. This is cross interference between signals. The intensity of interference signal is related to di / at of C point signal and distance between lines. When the two signal lines are not very long, what is actually seen on AB is the superposition of two pulses.

The micro control made by CMOS technology has high input impedance, high noise and high noise tolerance. The digital circuit is superimposed with 100 ~ 200mV noise, which does not affect its work. If the first mock exam is a AB signal, the interference becomes intolerable. If the printed circuit board is a four layer board, one of which is a large area of ground, or a double-sided board, and the opposite side of the signal line is a large area of ground, the cross interference between signals will be reduced.

The reason is that the characteristic impedance of the signal line is reduced in a large area, and the reflection of the signal at the d end is greatly reduced. The characteristic impedance is inversely proportional to the square of the dielectric constant between the signal line and the ground, and is proportional to the natural logarithm of the dielectric thickness. If the first mock exam is AB, the interference of CD to AB will be avoided. There is a large area below the AB line. The distance from AB line to CD line is larger than that of AB line to ground. Local shielding ground can be used, and ground wires can be arranged on the left and right sides of the lead wire on one side with lead junction.

(4) Reduce noise from power supply

The power supply not only supplies energy to the system, but also adds its noise to the power supply. The reset line, interrupt line and other control lines of microcontroller in the circuit are easy to be disturbed by external noise. The strong interference on the power grid enters the circuit through the power supply. Even in the battery powered system, the battery itself has high-frequency noise. The analog signal in the analog circuit can not withstand the interference from the power supply.

(5) Pay attention to the high frequency characteristics of PCB and components

In the case of high frequency, the distribution of leads, vias, resistors, capacitors, connectors, inductance and capacitance on the printed circuit board can not be ignored. The distributed inductance of capacitance can not be ignored, and the distributed capacitance of inductance can not be ignored. When the length of the wire is greater than 1 / 20 of the corresponding wavelength of the noise frequency, the antenna effect will occur, and the noise will be emitted through the wire.

(6) The layout of components should be reasonably divided

The anti electromagnetic interference problem should be fully considered when the components are arranged on the printed circuit board. One of the principles is that the lead wire between the components should be as short as possible.

(7) Good use of decoupling capacitor

A good high frequency decoupling capacitor can remove high frequency components up to 1GHz. The high frequency characteristic of ceramic chip capacitor or multilayer ceramic capacitor is better. In the design of printed circuit board, a decoupling capacitor should be added between the power supply and the ground of each integrated circuit. The decoupling capacitor has two functions: on the one hand, it is the energy storage capacitor of the integrated circuit to provide and absorb the charging and discharging energy at the moment of opening and closing the door of the integrated circuit; on the other hand, it bypasses the high-frequency noise of the device. In digital circuits, the typical decoupling capacitance of 0.1uF has a 5NH distributed inductance, and its parallel resonance frequency is about 7MHz, which means that it has good decoupling effect for noise below 10MHz and hardly works for noise above 40MHz.