harsh industrial environments. Non-hermetic components mounted on a PCB which is conformal coated could also be said to have pollution degree one. Similarly, designers can create pollution degrees two and three by reducing the possibilities of condensation, moisture, or other pollutants. This environmental control can encompass areas such as adding ventilation, applying continuous heat to prevent condensation, or applying hermetic sealing.

How does tHe Material group or tHe Cti of tHe optoCoupler Molding CoMpound iMpaCt on tHe Creepage requireMent?

The IEC 60664 insulation coordination safety standard roughly characterizes insulating materials according to their surface tracking voltages (breakdown voltages) or surface arcing properties in the presence of pollutants as follows:

Material group CtiI 600 V ≤ CTI II 400 V ≤ CTI < 600 VIII a 175 V ≤ CTI < 400 VIIIb 100 V ≤ CTI < 175 V

The test for CTI is specified under the IEC 60112 standard. It consists of dropping aqueous contaminant (50 drops of 0.1% ammonium chloride solution) on 3-mm-thick insulation placed on a horizontal surface and determining the voltage at which electrolytic conduction takes place. This test provides both a qualitative and quantitative comparison among insulation materials having a tendency to arc or form tracks in the presence of voltage. Tracking can be construed as an electrical breakdown phenomenon on the surface of the insulating material. At some voltage, carbonized tracks may form across the 3-mm-thick insulating material to produce a leakage path. The voltage producing this electrical conduction or breakdown is the CTI. Insulation material with high CTI will need less of a creepage clearance, all other factors such as working voltages and pollution degrees held constant. ■

distances needed to insure the safety of a product and the kind of material it can use. (i.e. the CTI or comparative tracking index of the plastic molding compound that encapsulates the optocoupler. CTI measures the electrical breakdown (tracking) properties of an insulating material.) The IEC 60664 insulation coordination safety standard defines four degrees of pollution. Pollution degree one is no pollution or only dry, nonconductive pollution. Pollution degree two is only nonconductive pollution and temporary conductivity caused by condensation. Pollution degree three covers conductive pollution or dry nonconductive pollution that becomes conductive from condensation as might arise in industrial settings. Pollution degree four refers to persistent conductivity caused by conductive dust, rain, or snow. In general, a higher pollution degree will necessitate more creepage and clearance distance. A molding compound material group having a higher CTI needs less creepage or clearance distance for a given pollution degree.

Can designers Control tHe environMent to reduCe tHe pollution degree?

Designers can potentially reduce the pollution degree by improving the enclosure, adding a coating, or hermetically sealing the equipment. For example, the conformal coating of printed circuit boards or hermetic sealing of a component can produce pollution degree one conditions. Hermetic optocouplers are commercially available to handle

(VIORM) and a transient over voltage rating (VIOTM). VIORM is the voltage that one can continuously apply across the optocoupler insulation barrier. In contrast, the VIOTM is a 60-second rating only. Exceeding it could cause the optocoupler to fail. The VIOTM rating is generally indicated as Vpeak waveform. To convert the Vpeak rating to Vrms, simply multiply the Vpeak × 0.7071.

wHat are Conditions of aCCeptability for ul, Csa, and ieC60747-5-5 optoCoupler safety standards?

Conditions of Acceptability are prerequisites for compliance with safety standards such as those from UL, CSA, or IEC. For example, IEC 60747-5-5 ensures reliable isolation from optocouplers. To meet this standard, optocouplers must first withstand a series of environmental and mechanical stresses, then pass a battery of electrical conformance tests. Stress conditions include temperature cycles, vibration, mechanical shock, dry heat, damp heat, and a low-temperature soak. After surviving these environmental stresses, samples must then pass two electrical tests to confirm their intact ability to isolate.

wHat are pollution degrees, and wHat optoCoupler paraMeter or paraMeters do tHey iMpaCt?

Pollution degree defines the level of dust and moisture pollution within the product’s operating environment. Pollution degree affects creepage and clearance

wHat is tHe differenCe between isolation witHstand voltage and working voltage for an optoCoupler?

Working voltage is not the same thing as the isolation withstand voltage. The working voltage is the voltage that one can continuously apply across the optocoupler insula-tion barrier, i.e. from the input to the output sides of the circuit. This work-ing voltage rating is certified through an international safety standard called IEC60747-5-5. On the other hand, the isolation withstand voltage rating (also called the input-output momentary withstand voltage rating) is specified in another safety specifi-cation called UL 1577, the main com-ponent standard for optocouplers in the U.S. The UL 1577 rating of 3,750 Vrms/1 min or 5,000 Vrms/1 min means devices can survive and isolate transient (1-min) voltage spikes with these values. In other words, this is a momentary insulation-withstand rating for one minute. Designers should not interpret the voltage levels in these ratings as the con-tinuous voltage that can be applied across the optocoupler insulation barrier. Thus an optocoupler with a momentary withstand voltage rating as defined by UL 1577 won’t stand up to a 5 kVrms working voltage.

wHat is tHe differenCe between working voltage (viorM) and transient over voltage (viotM) for an optoCoupler?

The IEC60747-5-5 optocoupler safety standard provides certification of both a working voltage rating

Safety and regulatory issues for optocouplers

Optocouplers are the only isolation devices that meet or exceed the IEC 60747-5-5 International Safety Standard for insulation and isolation.

Stringent evaluation tests show Avago’s optocouplers deliver outstanding performance on essential safety and deliver exceptional High Voltage protection for your equipment. Alternative isolation technologies such as ADI’s magnetic or TI’s capacitive isolators do not deliver anywhere near the high voltage insulation protection or noise isolation capabilities that optocouplers deliver.

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FREQUENTLY ASKED QUESTIONS

Safety and regulatory issues for optocouplers

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WHAT IS THE DIFFERENCE BETWEEN AN ISOLATION WITHSTAND VOLTAGE AND WORKING VOLTAGE FOR AN OPTOCOUPLER?

The UL1577 rating of 3750Vrms per minute or 5000Vrms per minute means that devices can survive and isolate transient (1min) voltage spikes with these values. This is a momentary insulation withstand rating for one minute. The input-output momentary withstand voltage rating should not be interpreted as continuous voltage that can be applied across the optocoupler insulation barrier. In other words, the momentary withstand voltage rating as defined by the UL1577 should not be construed or interpreted as that a part could stand up to a 5KVrms working voltage (ie. steady-state DC or AC voltage throughout the life of the part.) Steady-state working voltage rating is specified by IEC 60747-5-5 rating. The working voltage (VIORM) is the voltage that one can continuously apply across the optocoupler insulation barrier. This working voltage rating is certified through the IEC60747-5-5 safety standard.

WHAT IS THE DIFFERENCE BETWEEN WORKING VOLTAGE (VIORM) AND TRANSIENT OVER VOLTAGE (VIOTM) FOR AN OPTOCOUPLER AS INDICATED IN THE IEC60747-5-5 OPTOCOUPLER SAFETY STANDARD?

The International optocoupler safety standard IEC60747-5-5 not only provides a working voltage rating (VIORM) but also a transient over voltage rating (VIOTM) in the certificates. Steady-state working voltage rating is specified by IEC 60747-5-5 rating. The working voltage (VIORM) is the voltage that

one can continuously apply across the optocoupler insulation barrier. This working voltage rating is certified through the IEC60747-5-5 safety standard. The transient over-voltage rating is defined as the VIOTM in the IEC60747-5-5 certificate, and this is a sixty (60) second rating.

WHAT ARE THE DIFFERENT POLLUTION DEGREES, AND WHAT OPTOCOUPLER PARAMETER OR PARAMETERS ARE IMPACTED FOR OPTOCOUPLERS TO BE USED IN A MORE ADVERSE POLLUTION DEGREE ENVIRONMENT?

For the purposes of evaluating creepage and clearance distances, the IEC 60664 (Insulation coordina-tion safety standard) defines four degrees of pollution degrees. These pollution degrees classifications are primarily based on the extent of dry pollution and condensation present. The primary optocoupler parameters that define if the optocoupler can be used in a particular pollution degree environment are: external creep-age/clearance distances and the material group (i.e. CTI or compara-tive tracking index of the plastic molding compound that encapsu-lates the optocoupler). The four pollution degrees are defined as follows:

a) Pollution Degree 1: No pollution or only dry, non-conductive pollution occurs, and pollution has no significant impact. Air conditioned office environment, and clean labs are an example of pollution degree 1 environment.

b) Pollution Degree 2: Only non-conductive pollution occurs except that occasionally a temporary

conductivity caused by condensation is to be expected. Home environ-ments can be considered pollution degree 2.

c) Pollution Degree 3: Conductive pollution occurs or dry non-conduc-tive pollution occurs which becomes conductive due to condensation which is to be expected. Industrial warehouses, or factory floors, or construction sites may be construed as pollution degree 3 environments.

d) Pollution Degree 4: The pollution generates persistent conductivity caused by conductive dust or by rain or snow or other wet conditions. Areas exposed to outside atmosphere can be classified as pollution degree 4 environments.

OTHER THAN THE MAJOR AND SIGNIFICANT INSULATION CAPABILITY DIFFERENCES BETWEEN THE OPTOCOUPLERS AND MAGNETIC AND CAPACITIVE ISOLATORS, ARE THERE ANY OTHER ELECTRICAL PERFORMANCE DIFFERENCES BETWEEN THE THREE GALVANIC ISOLATION TECHNOLOGIES?

Alternative technology isolators, particularly magnetic isolators have been found to be sensitive to magnetic field interference issues and power supply latch up issues. The magnetic isolators have also been found to radiate excessive EMI.

Both capacitive and magnetic isolators have been found to have relatively poor common mode noise rejection performance. The figure of merit for CMR is not only the min guaranteed dV/dt rating, but also the common mode voltage magnitude at which the CMR has been specified at.

WHAT IS THE WORKING VOLTAGE AND HOW IS IT DEFINED BY THE SAFETY STANDARDS?

The IEC 60664 defines the working voltage as the highest rms value of the AC or DC voltage across any particular insulation which can occur when the equipment is supplied at rated voltage, with transients being disregarded, in both open circuit conditions or in normal operating conditions.

The optocoupler safety standard falls back on the IEC 60664 for its insulation coordination guidance, and hence the rms value of the AC or DC is construed to be the working voltage across the optocoupler.

Also, according to the IEC 60664 standard, rms values are used to calculate the creepage distances, and peak impulse withstand voltages are used to determine the clearance distances.

WHAT ARE THE DIFFERENT TYPES OF INSULATIONS AND HOW ARE THEY CLASSIFIED?

The IEC 60664 insulation coordina-tion safety standard defines various levels of insulation as follows:

a) Functional InsulationInsulation necessary for correct operation of the equipment between parts of different potential or between ELV (extra low voltage ) or SELV (safety extra low voltage) circuits and conductive parts

b) Basic InsulationInsulation needed to provide basic protection against electrical shock between a part and hazardous voltage and an earthed ground

c) Supplementary InsulationIndependent Insulation applied in addition to basic insulation in order to ensure protection against electrical shock in the event of failure of basic insulation

d) Double InsulationInsulation comprising both basic insulation and supplementary insulation

e) Reinforced InsulationA Single Insulation system which provides a degree of protection against electric shock equivalent to double insulation