Bullet proof industrial communications

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Bullet Proof Industrial Communications By Patrick Sullivan Industrial proc ess c ontrol applic ations use c ommunic ation signals to sync hronize and monitor proc ess equipment suc h as PLCs and Servos with c ommunic ation needs ranging from simple on/off status signals to high-speed data c ommunic ation protoc ol suc h as Devic enet and Profibus. In these applic ations the overall system integrity requires that the c ommunic ation signals should be error free. Interruptions and errors on the c ommunic ation links have the potential to c ause severe c onsequenc es in terms of produc tion line shut down and produc tion yield loss. Ensuring error free c ommunic ation requires c lose attention to EMC issue in partic ular the signal isolation requirements.

Transient disturbances

System c ommunic ation glitc hes tend to be random in appearanc e and often generated by parts of the proc ess being c ontrolled. For example the switc hing of a relay c onnec ted to a motor c an generate large transients bursts on the power line. These transient bursts c an disrupt the data transmission or even potentially damage the interc onnec ted equipment. Although the use of fiber optic is gaining popularity, c opper is still the primary c ommunic ation interc onnec tion medium. These c ommunic ation interc onnec ts are often unavoidably routed in c lose proximity to power lines, in whic h c ase there is a opportunity for power line noise to c ouple into the c ommunic ation lines by means of c apac itive and induc tive c oupling. Other sourc es of disturbanc es are the galvanic or ground noise resulting from the switc hing of large power loads. To protec t the interc onnec ted equipment and maintain data integrity the c ommunic ation links are typic ally galvanic ally isolated. Optoc ouplers, pulse transformers and fiber optic s are widely used to provide this isolation. Figure ## show s a typic al industrial c ommunic ations interfac e. In this c ase the c ommunic ation link is isolated using optocouplers. The optoc ouplers effec tively provide the barrier against the c oupled transient interferenc e and as suc h they are c ritic al c omponents in the fight against EMC problems. International standards

To assess the ability of a piec e of equipment to rejec t EFT ( Elec tric al fast transient burst) Various standards have been developed. In the c ase of industrial applic ations the international standard - IEC 801-4 - Electromagnetic compatibility for industrial – process measurement and control equipment applies. This testing spec ified in this standard e stablishes the immunity of a piec e of equipment when subjec ted to the types of transients expec ted in its intended applic ation. There are four test severity levels, the severity test level is selec ted based on the environmental c onditions expec ted in the intended applic ation. Figure ##. Level 1 test level is for a well-protec ted environment in whic h the following is assumed;

• • • •

Suppression of all relay switc hing transients Good separation between power line c ables and c ommunic ation lines Shielded power supply lines and screens earthed at both ends Power supply protec tion by filtering

This is in fac t the type of environment typic al to a c omputer or data storage room

In respec t to a typic al industrial environment, test level 3 is rec ommended in whic h c ase the following is assumed:

• •

No suppression of relay switc hing transients Poor separation between power line c ables and c ommunic ation lines

The IEC 801-4 burst test transient is c apac itively c oupled into the c ommunic ation lines as shown in figure. The shape of the burst transient applied is shown in figure.

If we c onsider the burst transient in the c ase of level 3 environment, the peak transient voltage is 1000V . This results in a c ommon mode transient ac ross the isolation of around 160Kv/uS on the rising edge and ~ 11KV/uS on the falling edge. The effic ienc y with whic h the isolation and external c irc uitry rejec ts these transients is measured in CMR ( Common Mode Rejec tion) . Where the maximum CMR is the maximum slew rate that c an be withstood before a malfunc tion occurs. The c ommon mode transient is typic al expressed as a ratio of the peak test voltage VCM ( Voltage Common Mode) to the transient time period dt. e.g. a Vc m test voltage of 1000V with a transient period of 100nS would result in a slew rate of 10Kv/uS. Isolation Performance

No galvanic isolation medium is perfec t, although it should be said that fiber optic c able c omes pretty c lose, for other solutions suc h as optoc ouplers and pulse transformers, one of the main CMR performanc e-limiting fac tors is the transient leakage c urrent. It is this transient leakage c urrent whic h disturbs the operation of c irc uitry c onnec ted on both the input and output side of the isolation boundary. The disturbanc e c ould c ause the mis operation of the line driver on the output side or the mic roproc essor on the input side, the later being the more serious c onsequenc e bec ause it c ould potentially c ause a total malfunc tion of the equipment. The amount of transient leakage c urrent flowing ac ross the isolation boundary may be expressed as Ileakage=Cleakage*dv/dt Where Cleakage is the parasitic leakage c apac itanc e. The c ompac t c onstruc tion of optoc ouplers results in a very small c oupling plane between input and output, c onsequently they have very low leakage c apac itanc e < 1pF. In the c ase of pulse transformers their c onstruc tion results in a relatively large c apac itive c oupling plane between primary and sec ondary winding e.g. a typic al Ethernet pulse transformer has a typic al leakage c apac itanc e of > 10pF. A c ommon mode transient of 10Kv/uS would result in a leakage c urrent less than 10mA through the optoc oupler and around 100mA through the pulse transformer.. Despite the fac t that the optoc oupler has a c lear advantage in terms of transient leakage c urrent, it is important to c onsider the effec t the transient leakage c urrent has on the isolation c omponent as well as the external c irc uitry. In the c ase of the transformer the operation of the transformer itself is unaffec ted by the leakage c urrent. This is however not nec essarily the c ase with an optoc oupler. It is an integrated devic e with c omplex c irc uitry and as suc h the leakage c urrent c an potentially disturb the operation of the internal c irc uitry. By definition the c oupling mec hanism used in an optoc oupler is optic al. The input signal is c onverted to light on the input and then on the output side its rec onverted to an elec tric al signal. The c onversion from light to an elec tric al signal requires the use of a photodiode and high gain amplifier. The photo c urrent being detec ted by the amplifier is typic ally in the range of uA. Compare this to the transient leakage c urrent during a 10Kv/uS transient of 10mA. There is c learly a high potential for false triggering of the output. In many applic ations it is c onsidered c ommon prac tic e to apply some form of elec trostatic shielding to EMC sensitive c irc uitry, whic h normally c onsists of some form of metal shielding.

However it should be apprec iated in the c ase of the optoc ouplers the elec tric al emissions behave in an almost identic al manner to light waves, so applying a simple elec trostatic metal shield would bloc k the transmitted photons as well as the elec tric al emissions. To maintain light sensitivity on the detec tor IC it is important that the elec trostatic shield does not absorb the lightwave radiation. To meet both of these requirements Agilent has developed a shielding proc ess, whic h is applied to the detec tor IC during the wafer manufac ture. This proprietary shielding tec hnology has the exc ellent properties of high absorption of elec tric al energy and very low absorption of lightwave radiation. The effic ienc y of this shielding proc ess c an be observed if we c ompare the measured performanc e of the HCPL-263N with that of the JEDEC standard part 6N137 ( figure ##) . Also interesting to note is the asymptotic type response at lower Vc m voltages. This rapid inc rease in CMR at lower Vc m voltages is as a result of the dt period bec oming less than that of the response time of the optoc oupler. Similar isolation performanc e may also be observed on other Agilent produc ts implement this shielding tec hnology suc h as the IGBT gate drive produc ts and high speed optoc ouplers.

Summary

If we c onsider the optoc oupler performanc e requirements for an IEC 801-4 level 3 environment. In respec t to the burst positive transient of 160kv/uS, at first this sound like a extreme and very hard to meet requirement, in fac t this is not of major c onc ern in most c ases bec ause the transient time of 5ns is in fac t shorter than that of the minimum detec tor response time of the optoc oupler and subsequent logic stages. On the other hand the burst negative transient of 11KV/us is well within the response time range of the optoc oupler and as suc h it is vulnerable to suc h transients. In the c ase of the HCPL-263N the internal elec trostatic shield satisfac torily rejec ts these transients and meets the requirement of the IEC 801-4 burst test.