Achieving long-lasting insulation performance! What is a photocoupler emulator?

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Are you considering measures to maintain long-term reliability or mitigate the risk of product discontinuation?

In electronic devices, photocouplers are widely used to electrically isolate circuits.
On the other hand, photocouplers require consideration of the aging of LEDs, which presents challenges for long-term operation. Furthermore, there are concerns about the risk of production discontinuation.

One alternative that is attracting attention to solve these problems with photocouplers is the photocoupler emulator from Texas Instruments (hereinafter referred to as TI).
This article clearly explains the differences from conventional photocouplers, the benefits of introducing them, and the main product categories.

Why is a reassessment of photocouplers necessary now?

For long-term operation, it is necessary to consider the risk of LED degradation over time.

A photocoupler detects the light from the input LED using a photodetector and transmits the signal.
Since the luminous efficiency of LEDs is affected by usage conditions and time, devices used over long periods of time must be designed to account for changes in CTR (Current Transfer Ratio).

Specifically, these are as follows:

• Design margins are required that take into account temperature, input current, and usage time.

- Confirm whether the necessary output current can be secured even after long-term operation.

For industrial equipment with long maintenance periods, consider the potential increase in the burden of parts replacement and re-evaluation.

Cross-sectional view of a photocoupler
Source: https://www.ti.com/jp/lit/an/jajaa46a/jajaa46a.pdf

Risks of replacement selection and re-evaluation due to end-of-life (EOL)

If the photocoupler currently in use is discontinued or its new design is no longer recommended, it is necessary to compare not only the pin configuration but also the input/output conditions, insulation performance, propagation characteristics, temperature range, and other factors.

For products where long-term supply is a priority, it is effective to consider not only extending the lifespan of existing designs but also incorporating next-generation insulation devices.

What is a photocoupler emulator?

A photocoupler emulator is a device that operates on current input, similar to a photocoupler, but performs isolated signal transmission without using LEDs.

TI 's products use capacitive isolation technology with SiO₂ (silicon oxide) to isolate input signals and transmit them to the output side.

Cross-sectional view of a photocoupler emulator
Source: https://www.ti.com/jp/lit/an/jajaa46a/jajaa46a.pdf

Comparison item	Photocoupler	Photocoupler emulator
insulating materials	Air epoxy resin	Silicon dioxide (SiO₂)
Dielectric strength	~1 Vrms/um (air) ~20 Vrms/um (epoxy resin)	~500 Vrms/um (SiO₂)
Insulation method	Optical coupling method	Capacitive coupling method
lifespan	△	◎
power consumption	△	◎
data rate	0.1 Mbps ~ 1 Mbps	25 Mbps（ISOM871x）
Turn-on time	9 us	4.5 us（ISOM811x）
CTR	△ Deterioration due to temperature and time	◎ Stable CTR

Photocoupler emulator: 4 advantages

1. Easy to consider long-term stability

Since LEDs are not used, there is no need to incorporate CTR changes caused by LED degradation into the design.
For power supplies and industrial equipment intended for long-term operation, this is expected to reduce the design burden associated with characteristic fluctuations.

2. Easier to consider migrating from existing circuits.

Because some products have an input method similar to that of a photocoupler, and some have pin-compatible packages designed with replacement of existing products in mind, the consideration of replacement can be carried out smoothly.

3. Higher speed and lower power consumption are also achievable.

This allows for higher speeds than those previously possible with photocouplers.
Because there are no light-emitting elements, it leads to lower power consumption of the system.

4. New design and EOL measures can be implemented simultaneously.

Beyond simply searching for compatible parts, this allows you to organize the necessary insulation performance, response speed, and output format, enabling device selection that will lead to future designs.

TI's main lineup of photocoupler emulators

The product lineup can be easily selected by organizing it based on the output format and signal speed of the photocoupler being replaced.

item	ISOM811x	ISOM871x	ISOM861x (ISOM8610)
Positioning	- Analog transistor output Alternative candidates for transistor output photocouplers	• High-speed digital output - Alternatives for high-speed digital photocouplers	- Single-pole, normally open, isolated switch with built-in FET - Photo Relay / SSR alternative candidates
Current transfer coefficient (CTR)	ISOM8110/8115: 100~155% ISOM8111/8116: 150~230% ISOM8112/8117: 255~380% ISOM8113/8118: 375~560% ※IF=5 mA、VCE=5 V	Not applicable *Since this is a digital output product, it is evaluated based on data rate and propagation delay, not CTR.	Not applicable *Since this is a switch output product, there is no CTR specification. It is evaluated by the input trigger current.
Input Options	DC input: ISOM8110 ~ 8113 Bidirectional DC input: ISOM8115 ~ 8118	Single-channel diode emulator input Output: CMOS (ISOM8710) / Open Collector (ISOM8711)	Single-channel diode emulator DC input Output: Normally open, symmetrical 80V, 150mA
Insulation rating (kV RMS)	3.75 / 5.0 *May vary depending on the package.	3.75	3.75
Operating temperature range (°C)	-55 ~ 125	-40 ~ 125	-55 ~ 125
Main applications under consideration	• Feedback of switching power supplies PLC Factory automation Data collection - Motor drive I/O and position feedback	·power supply ・Power grid/wattmeter Motor drive Factory automation Building automation ·illumination ・Home appliances • Digital control signals	Factory automation Building automation ・Home appliances • Testing and measurement • Semiconductor testing Industrial controller I/O • Signal switching • Insulated switch

Clicking on the part number will take you to the TI datasheet. Applicable products, isolation ratings, data rates, packages, and certification status vary by part number. Please check the latest TI datasheet before adoption.

Points to check when considering replacement

Input current range and threshold

Output format, output voltage, required pull-up conditions

• Dielectric strength, creepage distance, clearance, surge requirements

Dynamic characteristics such as propagation delay, data rate, and CMTI.

Package, pin configuration, and mounting requirements

• Operating temperature range, safety standards and certifications

System failure modes and fail-safe operation

Especially when considering pin-to-pin replacement, it's necessary to evaluate the circuit, including worst-case conditions, rather than relying solely on the external dimensions and pin configuration.