The hidden cost of isolated RS-485 optocoupler design

Optocouplers, also known as opto-isolators or optocouplers, have been used to achieve galvanic isolation of Electronic circuits for more than 40 years. Optocouplers use LEDs and phototransistors to achieve signal communication without the need to transmit current. As a low-cost solution, optocouplers have always been popular. But considering the advancement of digital isolation technology, is optocoupler really the most cost-effective way to achieve galvanic isolation of RS-485 systems?

Author: Texas Instruments Anthony Viviano

Optocouplers, also known as opto-isolators or optocouplers, have been used to achieve galvanic isolation of electronic circuits for more than 40 years. Optocouplers use LEDs and phototransistors to achieve signal communication without the need to transmit current. As a low-cost solution, optocouplers have always been popular. But considering the advancement of digital isolation technology, is optocoupler really the most cost-effective way to achieve galvanic isolation of RS-485 systems?

Figure 1 shows a typical circuit in which an isolated RS-485 transceiver uses an optocoupler to achieve galvanic isolation. The solution requires a total of three optocouplers: two high-speed optocouplers (each for sending and receiving signals) and a low-speed optocoupler for direction control. The solution also requires a large number of external components, including Schmitt buffers, Schmitt triggers, resistors and bypass capacitors. All these components increase cost and board area.

The hidden cost of isolated RS-485 optocoupler design
Figure 1: Isolated RS-485 design using optocoupler

Many system designers today encounter the problem of board space limitations in various industrial markets, because their goal is to achieve a smaller overall solution size or enhanced functionality in each new generation of products. One example is the heating, ventilation and air conditioning (HVAC) system that regulates the temperature and airflow in the building. With the development trend of smart energy-saving buildings, new HVAC control panels need to be able to combine advanced monitoring and interfaces with intelligent constant temperature systems without increasing the size of the overall solution.

RS-485 is the universal communication interface in these systems because of its long-distance reliability. When RS-485 nodes are placed at positions with different ground potentials, common mode noise can cause communication failures, so isolation is required to prevent these ground potential differences. The use of a huge optocoupler solution to isolate RS-485 allows designers to make compromises only in other aspects of the system, so there is a need for a smaller isolation interface solution.

In addition to large-size solutions, many designers will also encounter these performance problems of optocouplers:

reliability. Optocouplers usually use epoxy resin as the dielectric; compared with other common dielectrics, epoxy resin breakdown occurs at lower voltages, as shown in Figure 2. Inconsistent components and LED degradation can also cause differences in isolation reliability and lifetime from device to device.

Insulation Materials

Dielectric strength

Air

~1 Vrms/μm

Epoxy resin

~20 Vrms/μm

Silica filled mold compound

~100 Vrms/μm

Polyimide

~300 Vrms/μm

SiO2

~500 Vrms/μm

Figure 2: Dielectric strength of ordinary insulator materials

Energy consumption: Each optocoupler needs 5-10 mA to drive the LED on the internal input chip.

Temperature range: Optocouplers are mostly limited to withstand a maximum ambient temperature of 85°C, with a few exceptions that can withstand temperatures up to 105°C.

Switch specifications: The rise/fall time and propagation delay of the optocoupler vary with the bias current, current transfer ratio, and device-to-device changes.

Noise immunity The typical common-mode transient immunity range of optocouplers is 15 kV/μs to 25 kV/μs. In the case of voltage transients above this level, data corruption is likely to occur.

To meet the growing demand for compact solutions without compromising performance, Texas Instruments has developed an ISO1500 isolated RS-485 transceiver. Figure 3 compares the size of the optocoupler solution shown in Figure 1, which is an industry standard 16-pin small outline integrated circuit (SOIC) isolated RS-485 transceiver and ISO1500. Please note that these designs only show the signal isolation of the RS-485 transceiver. The application manual, “How to isolate the signal and power of an RS-485 system”, provides a useful overview of isolated power in an RS-485 system.

The hidden cost of isolated RS-485 optocoupler design
Figure 3: Solution size comparison between optocoupler solutions (a); industry standard 16-pin SOIC isolated RS-485 transceiver (b); and Texas Instruments ISO1500 (c)

Compared with discrete optocoupler solutions, ISO1500 can reduce board space by up to 85%, and can save up to 50% compared to industry-standard 16-pin SOIC packages. In addition to minimizing the solution size, ISO1500 also solves many of the performance issues mentioned above. All Texas Instruments isolated RS-485 transceivers are manufactured using a semiconductor manufacturing process, using silicon dioxide as the dielectric to minimize differences between devices and provide more reliable high-voltage performance. ISO1500 consumption

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