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Posted on 01 March 2019

Specifying Optocouplers for Safe and Robust Industrial Systems

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Isolators for IEC/EN/DIN EN 62019-5-2

In the industrial field the system level safety standards are IEC (International Electrotechnical Commission) 6041 for worldwide or international standards, UL5082 (from Underwriters Laboratories) for the United States and EN 501783 (from the European Union) for Europe. At the component level for optocouplers, the safety standards are IEC 62019-5-24 for international, UL 1577 for the United States and EN 62019-5-2 for Europe.

By Jeremy Seah Eng Lee, Alexander Jaus, Patrick Sullivan and Chua Teck Bee, Avago Technologies, Isolation Products Division

 

Equipment and component safety is a major aspect of designing a robust, reliable and user-safe industrial system, especially when high voltages (defined as above 48 VDC, 110 VAC) are involved. These type of systems are usually surrounded by motor starters, servo drives, programmable logic controllers and power converters, hence, providing a safe environment for personnel to work in plays a vital role in system design. In addition to this, system critical applications are expected to be failsafe since a breakdown in components, which result in machine failure, will be costly for the business.

As one example, the migration of Ethernet from operation in an office environment to the industrial environment has called for a change from hardware that only has to operate in an office to hardware operating in the harsh and rugged conditions of the factory floor. Adding this to the integration of Ethernet into Fieldbus and Device levels, the accuracy of data collected at the receiver end is now much more important than ever before.

Various forms of galvanic isolation—including isolation transformers, magnetic isolators using giant magnetic resonance and optical isolators (optocouplers)—are used extensively in industrial networking systems. They allow electrical circuits with highly diverse voltage levels to work together as a system and be interconnected while remaining electrically isolated or galvanically separated from one another. Galvanic isolators are also used to ensure error-free data transmission, retain data integrity and protect interconnected equipment for high-speed Fieldbus communications. Applications also include industrial input-output systems, sensors and temperature controlling systems, power supplies and regulation systems, electric motor control and drive systems, instrumentation and medical systems.

The three main applications for galvanic isolation are:

Protection against voltage transients: These are potentially high current or voltage surges that may damage components or cause potentially life-threatening electric shocks to the equipment operators. They are usually brief and intense surges between two circuits or systems.

Protection against ground loop currents: These are unwanted signals between interconnections of different ground potentials, which cause disruptive ground loops. They are usually found in communication networks having different grounds at various connecting nodes. The potential difference between these grounds can be alternating current (AC) or direct current (DC) with a combination of various noise components. If the voltage potential is large enough, it may cause damage to equipment (e.g. communication ports), transmission error or degradation of data signals. Long-term ground loop conditions can result in heating and burning of circuit boards thus damaging components and creating the potential for electric shock.

High-voltage level shifting: With the migration of digital ICs to lower operating voltages, the need for devices to separate sensitive electronics from high power electronics is growing. In order to ensure reliable information exchanges and preventing current flow between different ground reference voltages there is a need to use isolation. For example, in a motor control application, the electronic system of a motor consists of two stages, the low voltage controller and the power module. Within such a system, it is important to protect and insulate the two stages from switching transients and common mode voltage fluctuations. At the same time, it is necessary to provide level shifting and signal isolation of interface control and feedback circuits.

Safety Standards for Galvanic Isolation Devices

International safety standards are published to ensure consistent rules to make equipment safe. These standards are concerned about public safety in the areas of electrical shocks, mechanical hazards, fire and EMI. At system and component levels, there are many isolation safety standards varying both geographically and with equipment applications. In the industrial field the system level safety standards are IEC (International Electrotechnical Commission) 6041 for worldwide or international standards, UL5082 (from Underwriters Laboratories) for the United States and EN 501783 (from the European Union) for Europe. At the component level for optocouplers, the safety standards are IEC 62019-5-24 for international, UL 1577 for the United States and EN 62019-5-2 for Europe.

It is known that for future optocoupler standards, the IEC will take the lead and its standards will become more universal. For IEC 62019-5-2 approval, an optocoupler’s components undergo stringent qualification tests that include environmental, mechanical, isolation and electrical testing. The criterion for passing the component is the Partial Discharge (PD) test with a rigorous upper limit of 5 pC (picocoulomb).

Insulation

Insulation is defined as the property of a material that resists the flow of current until it breaks down. The fundamental principle of designing for product safety is the separation of circuits that present a danger of electrocution from other circuits, or certain parts of the equipment which a user may come into contact or which connects to other equipments. The circuit must be safe not only during normal usage but also under fault conditions. Two main levels of insulation with clear distinction of safety are Basic Insulation5” and Reinforced Insulation6”. Reference 7 provides a good discussion concerning the definition of insulation categories by the IEC.

As of January 2004, the German safety standard certification for optocouplers VDE 0884 was replaced by IEC/EN/DIN EN 62019-5-2. This is now the safety standard directly applicable to optically isolated devices. Although this standard specifically pertains to only to optical isolators, devices using other isolation technologies such as magnetic or capacitive isolation barriers have also surprisingly and perhaps erroneously, obtained certifications to the optocoupler safety standard. However, their recognition is limited to Basic Insulation only, and this level of insulation may not provide failsafe operation. This means that devices that are certified and approved under IEC/EN/DIN EN 62019-5-2 with recognition for Basic Insulation only provide basic protection against electrical shock. They cannot be considered as failsafe8 and therefore such devices should not used in applications where parts of the equipment are accessible to an operator. Table 1 shows the IEC/EN/DIN EN 62019-5-2-related characteristics of an Avago HCPL-0720 CMOS Optocoupler9.

Optocoupler Basics

A basic optocoupler consists of a light emitting diode (LED), a photodetector and an optically transparent, electrically insulating dielectric (Figure 1). When a current drives the LED, it emits light, which is coupled to the photodetector through the dielectric. The photodetector generates a current that is proportional to the coupled light. This current can be manipulated by various circuitry to perform specific functions. The major function of an optocoupler is to prevent high voltages or rapidly changing voltages on one side of the circuit from damaging compo- nents or distorting transmissions on the other side. This is done by optically passing desired signals while maintaining electrical isolation between two systems.

IEC-EN-DIN EN 62019-5-2 Insulation Related Characteristics for Avago HCPL-772X and HCPL-072X Optocouplers with Option 060

Reliability of High Voltage Insulation

Optocouplers are often used in environments where high voltages are present. Though many safety standard regulations have been established to provide guidelines on the application of high voltages, the problem with high-voltage insulators is the uncertainty in reliability due to poorly understood ageing and failure mechanisms under electrical and thermal stress.

Electrostatic Discharge

One of the primary causes of component failure in high-speed logic circuits is Electrostatic Discharge (ESD)11. ESD occurs in various situations, during improper device or board handling, through improperly designed interfaces or if a lightning or other phenomenon that causes a large voltage spike on a device interface. When devices are damaged by ESD, the affected devices may cease to function, exhibit parameter degradation or demonstrate high failure rates. The only repair is the replacement of the damaged component.

Optocouplers are excellent for protecting against ESD problems especially in situations where two systems are being linked in electrically demanding environments. Optocouplers allow ground isolation making it possible for systems to remain electrically neutral within themselves even though they may be floating in an electrically noisy environment. Such areas include motor control, switching power supplies, industrial networks and medical applications.

Electromagnetic Interference (EMI)

Electromagnetic interference (EMI) can be defined as any electromagnetic disturbance that disrupts, degrades or otherwise interferes with authorized electronic emissions limiting the effective performance of electronics and electrical equipment. It can be induced intentionally, as in some form of electronics warfare, or unintentionally as a result of spurious emissions and responses, intermodulation products, atmospheric disturbances (including lightning) and extraterrestrial sources (such as sunspots). Radio Frequency Interference (RFI) is a special class of EMI in which radio frequency transmissions (usually narrow-band) cause unintentional problems in equipment operation. Radio frequency interference can originate from a wide range of sources such as mobile phones or power lines, transformers, medical equipment, electromechanical switches and many others unintentional emitters that can be found especially in the industrial environment.

There are two forms of EMI, radiated EMI and conducted EMI. While radiated EMI is interference that travels from a source through the air to the receiving source, conducted EMI travels along a conducting path. Both can lead to transmitting unwanted electronic signals, which propagates along with the desired signal, thus interfering with the proper operation of the equipment or device by alternating normal operating parameters. These failures are generally categorized as electromagnetic interference or EMI failures.

Addressing EMI issues is a major challenge. When electromagnetic interference is suspected, the first step in resolving the problem is to determine the mechanism for energy transfer to the affected device(s): radiation, conduction, or induction. Improvements can be achieved by limiting the amount of induced energy either by removing the root cause (physical separation) or by protecting the failing device, e.g. by shielding in the telecommunication area. There are costs involved in this process too. The best way to avoid potential EMI problems is by choosing less sensitive or immune devices, by optimizing the layout to minimize coupling effects and proper shielding.

Looking at available isolators, most consist an integrated CMOS or bipolar IC. The coupling unit, which is the main differentiator between the different technologies available today, is optically coupled isolators (optocouplers), magnetic coupled isolators (magnetic couplers) and capacitive coupling isolators (capacitive couplers). Each of them behaves differently in the presence of strong electromagnetic fields. While the optocoupler LED/photodiode combination is known to be immune against electromagnetic interferences due to the optical coupling path, the magnetic isolators do have its limitations with respect to EMI due to their microstructure and the magnetic coupling. Failures of the magnetic couplers can occur when the magnetic field is at DC level (0 Hz) as well as at various frequencies at different levels of field strength.

The key factor for designers is to avoid potential future EMI problems in their applications, especially those used in the industrial environment and in close proximity to the motor control. Optocouplers are the best choice to use as they do provide superior EMI performance and can withstand much higher electromagnetic fields compared to all other isolators currently available in the market.

We have discussed four factors when designing a safe and robust industrial system. They are:

• The various safety standards for isolation devices, noting whether an optocoupler provides Reinforced Insulation, which provides failsafe operation.

• Reliability of high voltage insulation, which will minimize the frequency of component breakdown due to high voltage surges into the system. Note that Avago’s optocouplers can endure a high-voltage of 3.75 kV for a minimum of 168Hrs without failure.

• Electrostatic Discharge (ESD), which causes system degradation or malfunction. Taking note that even at an ESD voltage level of 11 kV, Avago’s optocouplers did not shows any dielectric breakdown failure.

• Electromagnetic Interference (EMI) is another factor that causes failure of industrial systems.

While designers may consider size, low power and cost in their initial selection for isolation products, it must not be forgotten that the basic requirement for isolation is actually to isolate unwanted signals while insulating against high voltages. Therefore, the four points highlighted above serve as a good selection criteria when you intend to design a safe and reliable industrial system.

Cross section and side view of an Avago optocoupler

 

References:

1) IEC 604: Industrial International standard for equipment and machines (http://www.iec.ch)
2) UL 508: US Industrial standard for machines (http://www.ul.com/)
3) 5 EN 50178: European standard for industrial equipments (http://www.newapproach.org/)
4) IEC/EN/DIN EN 62019-5-2: (http://www.cenelec.org/)
5) Basic Insulation: Insulation applied to live parts to provide basic protection against electric shock (http://www.601help.com/Disclaimer/glossary.html)
6) Reinforced Insulation: Single insulation system applied to live parts, which provide a degree of protection against electric shock equivalent to double insulation under the conditions specified in IEC 60601-1. (http://www.601help.com/Disclaimer/glossary.html)
7) Isolation and Safety Standards for Electronic Instruments, National Instruments Developer Zone (http://zone.ni.com/devzone/conceptd.nsf/webmain/6D1C1BE6590C0D4A86256C1A0078763C?opendocument)
8) Failsafe: A mode of system termination that automatically leaves system processes and components in a secure state when a failure occurs or is detected in the system.
9) Avago HCPL-0720/7720 and HCPL-0721/7721 40 ns Propagation Delay, CMOS Optocoupler Data Sheet Page 6 (Publication number: 5989-0790EN). A PDF version of the document can be downloaded at: http://cp.literature.avago.com/litweb/pdf/5989-2135EN.pdf
10) Avago Regulatory Guide for Isolation Circuits (Publication number: 5989-0342EN). A PDF version of the document can be downloaded at: http://cp.literature.avago.com/litweb/pdf/5989-0342EN.pdf

 

 

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