Saturday, January 1, 2011
【 Weak current College 】 prevent electrostatic discharge (ESD) problems of cable design 】
With the development of science and technology, polymer materials, microelectronics and exploding equipment widely used in various fields, static hazards also are being increasingly embraced. Electrostatic discharge can cause electric explosive and flammable, explosive gases, combustion and explosion, but also prone to interference and damage electronic equipment. Therefore, electronic equipment design and development, more attention should be paid to the protection against ESD design and technology. Usually we from internal software design, exterior shell, circuit design and PCB layout, connection cables, and other aspects to consider in the design of electronic equipment for protection against electrostatic discharge. Especially for electronic equipment and devices connect cable suitable for electrostatic discharge protection design, will be on electronic equipment against electrostatic discharge up to very good effect. 1. electrostatic discharge form electrostatic discharge can be divided into contact and contactless discharge, which is the current injection discharge and air breakdown discharge 2 form. Electrostatic discharge on the impact of electronic equipment is mainly divided into 3 types: ① discharge before the charged object generated by the electric field effect on electronic devices; ② discharge direct charge injection on the impact of electronic equipment; ③ electrostatic discharge current excitation of magnetic field effect on electronic devices. Electrostatic discharge on device noise forms can be divided into: radiation noise and conduction noise. Radiation noise from discharge electrostatic field before and discharge current excitation of magnetic places. Conduction noise include direct current injection and electromagnetic fields generated by the induced current. The actual circumstances, two effects are also exists, but for the sake of simplicity, I will separately from the noise and conduction noise suppression to discuss cable electrostatic discharge protection measures. 2. Suppression of conduction noise on direct current injection and the transmission of electromagnetic induction, better noise suppression is the equipment for the design of various connection cable. The cable can transmit noise provides low-impedance ground circuit to prevent their interfering with electronic circuits. And it will effectively reduce electrostatic discharge before the electrostatic field effect of electronic equipment. In order to produce a low-impedance ground loop, first of all required equipment cable has low impedance value. For most systems, the impedance of the cable has been relatively low, but high frequency, "skin effect" for the impedance of the cable will be significantly enhanced. Therefore, you can increase the surface area of the conductor (increases core RADIUS), to a certain extent weaken "skin effect". Another factor affecting the cable impedance of the cable connection point contact corrosion, its impact on the cable impedance than the "skin effect" more significant. There are 2 ways to reduce the effects of: (1) the connection point of the element of metal materials as close as possible to potential order. In a wet environment, metal will cause because one produces the electromotive force (EMF). Different EMF will cause corrosion, electro-erosion, and the connection point between EMF poor metal, corrosion, the greater the degree of. The following table lists some metal elements potential sequence. Common metal potential of sequence table + 1.66 Pb (lead) + 0.13 Zn (zinc) + 0.76 Cu (copper)-0.34 Cr (chromium) + 0.74 Ag (Silver)-0.80 IronorSteel (iron or steel) + 0.44 Pt (PT)-1.20 Cd (cadmium)
+ 0.40 Au (gold)-1.50 Note: metal of EMF and environment is very large, in some cases the value of metal of EMF may be different in the table. Different environment allows the connection point metal has some differences in the EMF. For example, in the harsh marine environment, different contact between metal materials do not allow more than 0.25V EMF. Typically, connection points between the metal does not exceed the difference between all EMF 0.75V is acceptable. (2) connection point anodes metal exposed area should be large metal than cathode. Contact corrosion, cathodic anode provides e-metal to, so if the anode cathodic metal than bare metal, corrosion is larger than the opposite situation is serious. In coated with paint, is not part of the coating is also in line with this principle. In the connection point in addition to contact corrosion, but also the electrolytic corrosion and oxidation of metal connection point. When the current through the electrolytic solution from a metal stream to another metal, electrolytic corrosion occurs. Connection point metal EMF difference is not a necessary condition for electrolytic corrosion occurs. In a wet environment, and the connection point has a current flow through the electrolytic corrosion occurs. Shell grounding and shielding ground loop typically does not have a current to flow, so in this case less guglielmi corrosion. Furthermore, it should be possible using cathode metal to reduce the oxidation of metal. Because the air in the anode oxidation of metal as obvious, cathodic anode metal is usually more stable than metal. If corrosion and skin effect have been effectively dealt with, then, another important issue is the effect of the transmission line cables. Ideally, we take on the cable carries on the precise impedance matching, allowing all ESD energy flow. However generally accurate impedance matching is not easy to achieve, so the connection cables are generally of impedance. Nevertheless, if the ground loop impedance is relatively low, still you can make the most of the ESD current flow, to a certain extent, you can prevent direct on electronic circuits of arc discharge. Because when the line length is 1/4 wavelength of odd times, even if a completely open Terminal would like a short circuit. Similarly, if the line length is a half-wavelength integer x, a short circuit terminal can also be seen as short-circuiting. So even though the ground loop Terminal impedance not match exactly, in a number of frequency components, still showed relatively low impedance. However, on other frequency components, ground loop will render the appropriate high impedance. The device's casing and internal circuit between the very possibility of arc discharge. Thus interference even harm the internal circuit. 3. Suppression of noise radiation problems facing first is shell – low-radiation loop yourself. As mentioned earlier, the shell — low loop impedance mismatch, and this impedance mismatch will produce a standing wave, in the event of a system of cables around the electric and magnetic field distribution。 So something must be done to reduce the production of electromagnetic fields on the inside of the system of other signal cables. (1) the use of shielded cables, and the shield connected to shell ground. Under the skin effect, electrostatic current (high frequency) will tend to be at the screen surface flow (this is cable in the high-frequency rendering high-impedance). Shield of RADIUS than ordinary cable RADIUS to large, so the screen is a relatively large surface area and thus reduces the skin effect impedance. If you assume that compared with thick shield, ESD current will only shield of outer movements, thus shielding layer inside can still play the shielding effect. In order to make the shield up to better effect, the grounding shield is usually taken at both ends of the Earth. And shield in the housing connection points and cable equipment import/export between should as far as possible within the 4cm. Cable length of the exposed part of taps should be less than 4cm.
(2) ESD current loop try not and other signal cable with ferrite (ESD current path does not flow through any ferrite better). However, if the cable is not very good shielding effect of shielding or not obvious, then, in the cable harness internal line of the induction equal ESD Noise (that is, common-mode noise ESD). At this point, the use of common-mode ferrite will have very good. (3) If on the cable, using ferrite magnetic ring and placed in the signal line accept Terminal, enhanced noise reduction effect. Short cable bundle is not used to lead, to shield them completely wrap. Bare wire tap (also known as the "tail") rather than in the shield of internal placement a induction external antenna, the noise of external noise into the internal shield. Another treatment tapped by including its parallel connections to other signal cables. But it is best not to connect to the screen, because that is the equivalent of the shield on the induction into the external noise into the shield, thereby interfering with other signal cables.