Design Guidelines for Optocoupler Safety Agency …

1 vishay SEMICONDUCTORSO ptocouplers and Solid-State RelaysApplication Note 43 Design Guidelines forOptocoupler Safety Agency ComplianceAPPLICATION NOTE Rev. , 07-Nov-111 Document Number: 83743 For technical questions, contact: DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENTARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT TO ELECTRICAL SAFETYT raditionally, electrical isolation from hazardous voltageshas been the most common application for optocouplerdevices.

2 Other applications for optocouplers includereducing EMI through the elimination of common-modecurrent loops, which are the greatest contributors toradiated emissions in high-speed digital systems. However,isolation is still the predominant role for optocouplers intoday's electronics isolation is important in modern electronics designas a way of minimizing the likelihood of exposing an enduserto injury from hazardous currents. The currents at whichharm or even death can occur are far lower than mostpeople think.

3 In certain invasive medical operations,currents as low as 80 A can be fatal and have anacceptable Safety limit of 10 A. These thresholds areoutlined in figure 1. Fig. 1 - Shock Hazardous LevelsElectrical isolation is typically achieved by one of threemethods: magnetic, capacitive, or electrooptical. All threehave their pros and cons. Magnetic isolation (using anisolation transformer) is probably the longest-establishedmethod of electrical isolation, providing high levels ofisolation at high frequencies in a robust package.

4 Among thedownsides of this method of isolation are a large devicefootprint when compared with other methods and suitabilityonly for AC signal coupling. Due to these characteristics,magnetic coupling is for the most part limited to high-powerAC applications. Fig. 2 - Magnetic IsolationThe second common method of electrical isolation iscapacitive coupling. The advantages of capacitive couplingare high switching speeds and a relatively small packagefootprint, but to eliminate the need for a floating powersupply on the secondary side, a large capacitance isrequired to transfer energy from the primary to thesecondary side.

5 Thus, the electrical isolation value of thistechnique is greatly diminished by the need for efficientenergy coupling. Consequently, most capacitive couplingisolation schemes have isolation values in the hundreds ofvolts rather than the thousands of volts achievable withother methods. Fig. 3 - Capacitive IsolationAnother potential isolation method involves the use ofmagneto-resistive sensors. These sensors are able to detectDC as well as AC magnetic fields. However, this is anemerging technology and is susceptible to induced noisefrom extraneous external magnetic fields.

6 17348 Pain, respiratory paralysisBurnsVentricular fibrilliationLet go currentPerception1 mA10 mA100 mA1 A10 A100 A60 Hz current RMS17349V-isolationV-primaryV-secondary1 7350 Voltagetofrequency+ Frequencytovoltage+ + CouplingcapacitanceSecondarysidevoltageP rimarysidevoltageDesign Guidelines forOptocoupler Safety Agency ComplianceAPPLICATION NOTEA pplication Note Semiconductors Rev. , 07-Nov-112 Document Number: 83743 For technical questions, contact: DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENTARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT isolation has many of the best aspects of the formermethods without the drawbacks.

7 Mainly, optical isolationoffers high electrical isolation values, an effective line in thesand barrier that hazardous voltages are incapable ofpenetrating. In the case of vishay s couplers, these valuesare as high as 8000 V, the highest level in the industry. Thisis achieved with small-footprint packages and high speed;moreover, it is equally effective with AC or DC signals. Fig. 4 - Electro-Optical IsolationSAFETY Agency STANDARDS OVERVIEWT here are several widely accepted industry standards thatgovern the manufacture and testing of electronicequipment.

8 Probably the most widely known of these inNorth America is Underwriter's Laboratories (UL). UL hastwo types of basic approvals: UL Listing andUL-recognition. The difference is simple but often a subjectof much confusion. The UL Recognized mark, optionallyinscribed on the devices themselves, refers to componentsthat have been evaluated to a certain extent by UL and willbe re-reviewed by UL for proper incorporation into theend-use equipment. A UL Listed mark is placed oncomplete equipment.

9 For example, a computer would be aUL Listed, while the component hard drive would be a ULRecognized part. As seen in the next page, UL wasconcerned enough about potential confusion between themeanings of these two marks that it intentionally made themdistinct from one another. Fig. 5 - UL Listed/UL RecognizedUL has Safety standards for everything that is or possiblycan be manufactured; however, to simplify things, thesestandards can be divided into two groups: systemstandards and component standards.

10 The systemstandards are beyond the scope of this document toaddress in their entirety. Arguably, the most commonlyapplicable system standard in the electronics industry isUL60950, which governs the electrical Safety requirementsfor the broad category of information technology equipment(ITE). In addition to information technology, there are alsostandards that deal with other specialty fields of productelectrical Safety . Of particular interest to Optocoupler designis IEC 60601-1, which governs the Safety of medicalequipment.