Common Myths and Excessive Myths About Isolation Technology

“With the growing need for reliable signal isolation in industrial, automotive, communications, and personal electronics applications, recent design trends have shifted from traditional isolation technologies such as optocouplers to digital isolators. Despite the prevalence of digital isolation, there are still several common misconceptions about its effectiveness compared to optocouplers. Likewise, there are some misconceptions or myths about the reliable performance and longevity of optocouplers. In this article, I will address common misconceptions about these two devices.

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TI dispels several common misconceptions about signal isolation technologies, including digital isolators and optocouplers.

With the growing need for reliable signal isolation in industrial, automotive, communications, and personal electronics applications, recent design trends have shifted from traditional isolation technologies such as optocouplers to digital isolators. Despite the prevalence of digital isolation, there are still several common misconceptions about its effectiveness compared to optocouplers. Likewise, there are some misconceptions or myths about the reliable performance and longevity of optocouplers. In this article, I will address common misconceptions about these two devices.

What is an optocoupler? What is a digital isolator?

Before exploring these, let’s review the differences between these techniques:

An optocoupler uses a logic input to generate input-side current, the LED converts the signal to light, which is then transmitted through isolation to the other side, where it is converted to an electrical signal by a photodetector.

Digital isolators use silicon-based CMOS technology with two integrated circuits and a high-voltage dielectric built into the silicon process. Digital isolators convert digital signals to the high-frequency domain and send RF signals through a capacitor-based high-voltage silica dielectric barrier.

Optocoupler Misunderstanding 1: Optocouplers are very reliable

There is a common misconception that optocouplers will always fail with an “open” circuit when high voltages destroy the device. Although optocouplers can fail in a number of ways, optocouplers can also fail in “shorted” circuits, depending on the different failure modes in high voltage systems.

In the first failure mode, the isolation barrier can short-circuit for both optocouplers and digital isolators when the voltage applied across the barrier exceeds the isolator’s rated limits. Texas Instruments (TI) tested the first failure mode in its lab; the white paper, “Understanding Failure Modes in Isolators,” specifically describes the observations of short-circuit results to failure.

The second failure mode, where high voltage and high current inside the isolator destroys the circuit, can cause a faulty open condition. These high voltage events can cause more circuit damage, making it no longer functional, but the isolation barrier is still intact.

Figure 1a shows a high voltage event on an optocoupler, and Figure 1b shows a similar event on a digital isolator. Depending on the type of high pressure event and the strength of the barrier, varying degrees of degradation can occur. To prevent short-circuit faults caused by the first failure mode, an isolator must be selected that meets or exceeds electrical safety standards.

Figure 1: Cross-section of (a) optocoupler side and (b) digital isolator side during high voltage accident. (Source: TI)

Optocoupler Misunderstanding 2: The life of an optocoupler is predictable with little change

As with all Electronic designs, it is critical to ensure that the IC will last throughout the life of the product. This is especially true for isolation devices, as they protect signals across multiple voltage domains. While you might expect two identical optocouplers to have very similar high-voltage lifetimes, in reality the high-voltage performance of different devices can vary significantly, usually because the optocoupler’s isolation barrier is created at the packaging stage.

Digital isolator manufacturers typically build their isolation barriers in the more tightly controlled silicon chip manufacturing process. Figure 2 illustrates the difference in high voltage life and variation, where the high voltage life is longer and the TI digital isolators used in this test are more closely spaced. To learn more about this topic, see the white paper “Improving System Performance by Replacing Optocouplers with Digital Isolators”.

Figure 2: Time-dependent dielectric breakdown characteristics of optocouplers and digital isolators. (Source: TI)

Optocoupler Misunderstanding 3: The optocoupler Datasheet specification will strictly conform to the service life of the device

You may not realize that the light output of an LED in an optocoupler decreases over time, which has a direct effect on parameters such as current transfer ratio (CTR). The plastic material inside the optocoupler turns yellow over time, causing less light to pass through the isolation barrier, further reducing transmission. Eventually, the CTR will drop to a level where the device is no longer functioning properly, resulting in high failure rates and low mean time between failures.

To counteract this, designers often design margins to account for expected degradation over time, which can lead to higher initial power consumption. These issues are not always mentioned in the optocoupler datasheet, so it is difficult to consider them in the isolation design. For example, TI’s digital isolators use a highly controlled manufacturing process, and the Datasheet takes into account minimum or maximum specification burn-in, helping to set performance expectations over the life of the device.

Optocoupler Misunderstanding 4: The maximum operating temperature of the optocoupler is higher

Typical optocouplers are rated for a maximum operating temperature of 85°C. Although there are higher temperature rated optocouplers on the market, the options are limited and they are generally more expensive. In contrast, digital isolators can easily support operating temperatures up to 125°C. For automotive designs requiring temperature support up to 150°C, digital isolators such as TI’s Grade-0 certified ISO7741E-Q1 help provide reliable system operation at high peak ambient temperatures. For high temperature designs where every component needs to operate reliably above 110°C, lower temperature ratings can be problematic. Otherwise, system performance or device lifetime may be affected.

Optocoupler Misunderstanding 5: No primary side power supply means lower power consumption

When configuring a system to reduce power consumption, it is important to consider how the input of the isolator is driven. Optocouplers are driven by current inputs, while digital isolators are driven by voltage inputs – CMOS or transistor logic.

Optocouplers can drive digital device inputs, such as microcontrollers, analog-to-digital converters, and digital-to-analog converters, through series resistors that control voltage and current. Input currents as high as 10 mA are required to activate the LEDs and meet product lifetime reliability, which can result in high power dissipation at the input.

Digital isolators such as TI’s ISO7041 typically require less than 10 µA of standby current at the input. Figure 3 shows the current consumption versus data rate for the ISO7041. In this test, all four channels of the device consume less than 20 µA.

Figure 3: Current consumption versus data rate for the ISO7041. (Source: TI)

Digital isolator myth #1: A digital isolator with a smaller DTI indicates weaker isolation

The distance to insulation (DTI) of an isolator refers to the distance or thickness of the dielectric used for insulation between the high and low voltage sides. For optocouplers, DTI is the distance between the LED and the photodetector. For capacitance-based digital isolators, DTI is the distance between the two plates of the capacitor.

Due to historical safety standards that set minimum DTI requirements based on optocoupler technology, there is a misconception that all isolators must have a DTI greater than 0.4 mm to meet today’s stringent reinforced isolation certification requirements. In reality, however, the strength of the isolator barrier is a combination of DTI and dielectric material.

Optocouplers have a much lower dielectric strength and therefore require a larger DTI. Capacitive-based digital isolators use higher dielectric strength silicon dioxide and can support reinforced isolation with DTIs down to 21 µm.

Over time, organizations governing safety standards for equipment operations have taken this into account and updated regulations to allow for thinner dielectrics based on the technology being evaluated. Table 1 lists the dielectric strength of different insulating materials.

Table 1: Dielectric strength of common insulating materials. (Source: TI)

Digital isolator misunderstanding 2: The cost of digital isolators is much higher than that of optocouplers

Despite the historical significance of this myth, digital isolator technology has advanced significantly over the past decade, enabling higher performance at lower cost. The ability to achieve multiple channel counts in the same package also helps reduce overall system density and cost. For example, the ISO6741 provides four channels of reinforced isolation in the same package, providing a robust isolation solution at a reasonable cost per channel.

Digital isolator myth #3: Digital isolator integration and saving board space comes at a price

One of the greatest advantages of digital isolators is their ability to integrate other system requirements into the same package. Isolated interfaces such as GE Controller Area Network, RS-485, I2C and LVDS signals are good examples. You might be concerned that buying a digital isolator with an integrated transceiver will impact your budget, but the truth is that there are many benefits to an integrated digital isolator, especially compared to similar discrete optocoupler solutions.

The biggest disadvantage of discrete optocoupler solutions is the cost of the many discrete components (resistors, capacitors, diodes, Schmitt buffers, transistors) and their PCB area. Figure 4 compares the dimensions of an optocoupler and an ISO1500 integrated RS-485 digital isolator. See the technical article “The Hidden Costs of Optocouplers for Isolated RS-485 Designs” for more information on this comparison.

Figure 4: PCB comparison between RS-485 discrete and fully integrated isolation schemes (Source: TI)

in conclusion

Digital isolation has come a long way over the past few decades and is now the isolation solution of choice for many designers. Keep the myths and facts described in this article in mind when your design requires a reliable signal isolation solution.