Optocoupler / isolator

01

Size the current transfer ratio and speed

Vishay's general description explains that the control circuit is on the emitter side and the load circuit on the detector side, with signals transmitted optically from DC up to the MHz band, “in most cases” by light whose wavelengths span the red to infrared range; compared with relays, optocouplers offer smaller size, shorter switching times and no contact bounce or arcing, and, in Vishay's words, they “are used to an increasing extent as safety tested components, e.g., in switch mode power supplies”. It lists high current transfer ratio (CTR) and low degradation among the essential requirements. [1] Its 4N25 family pairs a gallium arsenide infrared LED with a silicon NPN phototransistor in a 6-pin DIP; the DC CTR at VCE = 10 V and IF = 10 mA is 20% minimum and 50% (typ.) for the 4N25 and 4N26, and 10% minimum for the 4N27 and 4N28; rise and fall times are 2 µs at IF = 10 mA and RL = 100 Ω; and the LED is rated 60 mA forward current with a 5 V reverse voltage. [2]

Editorial calculation: with the 4N25's minimum CTR of 20%, 10 mA of LED current guarantees at least 0.2 × 10 = 2 mA of collector current. To pull a 5 V output low through a collector resistor, that resistor must be at least about 5 / 0.002 = 2.5 kΩ (ignoring the saturation voltage); using the 50% (typ.) CTR from the data sheet instead would under-design the circuit as the LED ages. The supply and LED current are assumed examples.

Give the supplier the input drive current and voltage, output type (phototransistor, Darlington, logic, triac or digital isolator), minimum CTR or data rate, switching time or propagation delay, isolation test voltage and working voltage, creepage and clearance, safety approvals (UL 1577, VDE), common-mode transient immunity, channel count, package, temperature range and lifetime or degradation requirements.

02

Specify the isolation barrier and noise rejection

The 4N25 data sheet lists an isolation test voltage of 5,000 V rms, creepage and clearance of at least 7 mm, isolation thickness between emitter and detector of at least 0.4 mm, a comparative tracking index of 175, isolation resistance of 10¹² Ω at 25 °C, coupling capacitance under 0.5 pF, operation from −55 to +100 °C and UL 1577 approval. [2] Vishay notes that low coupling capacitance suppresses interference spikes from steep pulse edges crossing the barrier, which is what is meant by common-mode rejection. [1] TI's ISO774x quad-channel digital isolators, which use a double capacitive silicon dioxide barrier, list 100 Mbps data rate, 10.7 ns propagation delay (typ.) at 5 V, up to 5,000 V rms isolation, ±100 kV/µs common-mode transient immunity (typ.), a projected lifetime of over 30 years at 1,500 V rms working voltage and certification to DIN EN IEC 60747-17 and UL 1577. [3]

Editorial calculation: a 2 µs rise time limits a phototransistor optocoupler to roughly 1 / (2 × 2 µs) ≈ 250 kHz for a square wave with equal rise and fall, before allowing any time at the logic levels, while a 100 Mbps digital isolator handles signals hundreds of times faster. The comparison is approximate; check the data rate in the actual circuit.

03

Name the right IEC component standard for optocouplers

Photocouplers (optocouplers) have their own IEC component standard, separate from the IEC 60747-17 standard that the ISO774x digital isolators above are certified to. IEC 60747-5-5 specifies the terminology, essential ratings, characteristics, safety tests and measuring methods for photocouplers. Its first edition (26 September 2007) replaced the photocoupler clauses of IEC 60747-5-1, 5-2 and 5-3 and stated that photocouplers certified to those earlier parts are to be considered compliant; it was withdrawn on 20 July 2020 when the second edition was published. The 2020 edition added an optional data-sheet basic insulation rating in accordance with IEC 60664-1:2007, 6.1.3.5, corrected the use of VIORM, and added an alternative surge-pulse (VIOSM) test method. [4] [5]

Ask which edition a part's certificate cites: a phototransistor optocoupler such as the 4N25 falls within the photocoupler scope of 60747-5-5, while TI certifies the ISO774x digital isolators to 60747-17. [2] [3] [1]

Editorial calculation: edition 1 was current for almost 13 years (September 2007 to July 2020).

Ask the supplier for the certificate number and the standard edition for the reinforced-insulation rating (VIORM, VIOTM, VIOSM), and whether the data sheet uses the optional basic insulation rating of the 2020 edition.

04

Phototransistor optocouplers or capacitive digital isolators, and the listed optocoupler maker

Orient Components lists phototransistor optocouplers alongside Darlington, triac, high-speed isolation and IGBT gate-drive optocouplers; the two device types below come from the Vishay and TI data sheets.

Types include phototransistor optocouplers with base connection such as the 4N25 family, and integrated digital isolators with capacitive barriers for multi-channel, high-speed data. [2] [3]

05

Input LED, photodetector, insulation barrier, package and capacitive barrier

The last row is the digital-isolator alternative: TI's ISO774x replaces the LED and phototransistor with a double capacitive silicon dioxide barrier. [3]

06

CTR, isolation voltage, creepage, coupling capacitance, speed and approvals for an isolator order

Size the CTR row on the minimum, not the (typ.) value: the 4N25's 20% minimum gives at least 2 mA of collector current at 10 mA LED drive. [2]

07

Send the signal, isolation and safety data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Signal: [type; data rate; drive current]
Output: [phototransistor; logic; triac; digital]
Isolation: [test/working voltage; creepage]
Noise: [CMTI; coupling capacitance]
Approvals: [UL 1577; VDE/IEC]
Package: [type; channels; temperature]

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