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DEW Home›Electrical & Electronics›Optocouplers
Electrical & Electronics · Category Intelligence

Optocouplers

Optically-coupled isolators that pass a signal across an isolation barrier with light — no electrical connection — for gate drive, isolated feedback, and digital signal isolation. The pick is driven by function, isolation rating, speed, and the current-transfer behavior over life.

◆ Deep category · engineer-grade decision intelligence

Product types

Where engineers draw the lines within optocouplers.

Phototransistor Output (general purpose)
LED-to-phototransistor isolation for slow digital and signal-level isolation. Cheap and simple; CTR-limited and slower.
High-Speed Digital / Logic-Gate
Logic-output optocouplers for isolated digital links at Mbit/s rates with defined propagation delay.
Gate-Drive (IGBT / SiC / MOSFET)
High-CMTI drivers — often with DESAT protection, Miller clamp, and fault feedback — for power-switch gates in inverters and drives. Broadcom's core strength.
Linear / Analog
Optically-linear couplers for isolated analog feedback in power-supply control loops.
Photovoltaic / SSR Drivers
Optical MOSFET drivers and solid-state-relay output stages for isolated switching.

Specs that matter

The numbers to compare first — and which are hard deal-breakers.

Function (gate-drive / digital / linear)
Sets the whole part family — a feedback linear coupler and an IGBT gate driver are different devices.
Deal-breaker
Isolation rating & type (VIORM/VISO, basic vs reinforced)
Working and withstand voltage plus basic/reinforced class is a safety gate for the design.
Deal-breaker
Creepage / clearance & CTI
Physical spacing and material group must satisfy the working voltage and pollution degree.
Deal-breaker
Common-mode transient immunity (CMTI, kV/µs)
Fast dV/dt at the switch node false-triggers low-CMTI parts — critical for SiC/IGBT drive.
Key
CTR (current transfer ratio) & its derating
For phototransistor types, CTR sets drive design — and it ages and falls with temperature and time.
Key
Data rate / propagation delay & skew
Digital links need adequate speed and matched delay for both edges.
Key
Safety approvals (UL 1577, IEC 60747-5-5, VDE)
Certified isolation is required for regulated and safety-critical equipment.
Key
Operating temperature & gate-drive current
For drivers, peak source/sink current and temperature set the switch you can drive.
Nice-to-know

How engineers decide

Rules of thumb behind the common trade-offs.

Optocoupler or digital isolator?
Digital (magnetic/capacitive) isolators win on speed, tighter delay, and no LED aging — prefer them for high-speed digital links. Optocouplers still shine for high-voltage gate drive (galvanic simplicity, high CMTI parts) and where a light-based barrier is preferred. Choose by speed, lifetime, and CMTI needs.
Basic or reinforced isolation?
Reinforced when the barrier is the sole protection between mains/high-voltage and the user or low-voltage side; basic where a second layer of protection exists. Match VIORM and creepage to the working voltage and pollution degree.
How do I handle CTR aging?
Design with margin: the LED output falls over life and temperature, so size drive current for end-of-life CTR — or move to a digital isolator whose transfer doesn't age. Don't design at nominal CTR.
Selecting a gate-drive optocoupler?
Match peak output current to the gate charge and switching speed, pick high CMTI for SiC/fast IGBT, and add DESAT + Miller-clamp + fault-out for protection. Layout the isolation gap and decoupling per the datasheet.
When a linear optocoupler?
For isolated analog feedback in a power-supply control loop where you need a simple, low-cost barrier — accept its accuracy/temperature limits versus a dedicated isolated amplifier.
▼ From the field

What goes wrong — War Stories

Expensive failure modes engineers design around.

⚠ CTR aging → drive falls out of spec
Cause: LED light output degrades over life/temperature
Design around it: Design for end-of-life CTR with margin, limit LED current, or use a non-aging digital isolator.
⚠ CMTI failure / false switching
Cause: Fast dV/dt at the switch node couples through a low-CMTI part
Design around it: Select high-CMTI gate-drive optos, tighten layout/decoupling, add Miller clamp.
⚠ Insufficient creepage for working voltage
Cause: Under-spaced part/layout for the isolation class and pollution degree
Design around it: Choose the right package/creepage, honor reinforced-isolation spacing, follow IEC 60664.
⚠ LED overdrive shortens life
Cause: Driving the input LED beyond recommended current for extra CTR
Design around it: Stay within rated LED current, design margin into the drive, thermally derate.

Key builders

Manufacturer-direct sources we anchor specs to.

Broadcom (Avago) — gate-drive, high-speed digital & linear optocouplers (industry benchmark)VishayToshibaonsemiRenesasIsocomLite-OnSharpDigital-isolator alternatives: Analog Devices (iCoupler), Skyworks (Si isolators), Texas Instruments

Standards we hold specs to

UL 1577Optical isolators — withstand isolation voltage
IEC 60747-5-5Photocoupler safety — reinforced/basic isolation
VDE 0884-11Magnetic & optical isolator reinforced-isolation qualification
IEC 60664-1Insulation coordination — creepage & clearance
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