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Digital isolator and photocoupler technology
Structure and basic technology
photo coupler
Optocouplers come in several versions, from cheap 4-pin devices to expensive high-speed devices. The most popular photocoupler is the 8-pin type that requires a bias voltage on the output side. Most optocouplers consist of a light emitting diode (LED), a photo detector, and an output driver (usually a transistor or buffer). A photo detector (Photo Detector) receives the light emitted by passing a current through the LED, turns on the output transistor, and the output becomes low. Conversely, when no current flows through the LED, the output transistor is turned off.
This radiograph is of a single-channel optocoupler. You can see that the LED and output chip are mounted in a physically isolated frame. There is a gap between the frames, and the distance between the insulators is called [DTI: distance through insulation] insulation width, which is usually about 80 µm to 1,000 µm.
The insulation distance (DTI) between the input side and the output side is the image of this picture. Between the LED and the detector die is a transparent insulating shield, such as a polyimide-based film. Some recent photocoupler packages are filled with silicone filler.
SKYWORKS Digital Isolator
The basic working principle of this digital isolator is to send a signal by the presence or absence of a high frequency (HF) carrier signal instead of the optocoupler light. A carrier signal is sent to the secondary side only when there is an input signal, and it basically operates like a photocoupler, but it has better characteristics than a photocoupler in terms of power consumption and speed.
Here is an internal photo of the 150Mbps 6-channel digital isolator die and its bonding. It wire-bonds the transmitting and receiving dies with the same insulation structure. The capacitive isolation barriers on each die are configured and designed to be differentially connected. The logic level of the input determines the output state.
Digital isolators isolate with an isolation barrier that safely withstands high voltage stress. The optocoupler isolation barrier relies on a physical gap. That is, it relies on DTI (insulation distance insulated with polyimide tape or silicone filler and insulating plastic mold compound). It is difficult to assemble to maintain a certain physical distance and complicated to manufacture, so it tends to be expensive.
Older international standards sometimes set isolation barrier requirements based on optocoupler DTI.
However, most recent insulation standards agency testing is based on the "withstand voltage" of the isolation barrier and does not rely on DTI.
Insulation materials and insulation performance will be explained in a separate section.
Photocoupler and isolator structure and structural differences
・Uses CMOS process technology with high operational stability and reliability
Photocoupler LEDs usually use elements such as GaAs, but digital isolators use a CMOS process.
The CMOS process is more stable, more reliable, and more cost-effective than the GaAs process.
・Using high-frequency signals instead of light
The combination of a high-precision high-frequency carrier signal and a narrow-band reception filter makes it resistant to disturbance noise.
・Isolation using completely differential signals that are not affected by CMT
Optocouplers are single-ended devices and susceptible to CMT (common-mode transient) fluctuations.
However, this isolator uses a differential signal and is less susceptible to CMT due to the output die with high reception performance.
Resistant to disturbing high-frequency electric and magnetic fields.
(Example: CMTI over 50kV/us, electric field resistance over 300V/m, magnetic field resistance over 1000A/m)
・Unique EMI design
The digital isolator meets FCC partB standards for conducted and radiated EMI.
Please refer to the separate document “CMOS Isolators Supersede Optocouplers in Industrial Applications”.
Isolator product page
Here is a digital isolator product page with useful features.
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