How is the lifespan of a photocoupler determined? ~Considerations for CTR degradation, low IF drive, and isolation device selection~

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In recent years, in addition to the increased lifespan of industrial equipment, the shortage of maintenance personnel and the lengthening of equipment replacement cycles have led to an increase in "devices used for 20 years or more." Therefore, design that considers not only the failure rate but also deterioration over time has become more important than ever. Photocouplers, in particular, which are widely used as insulating elements, have a unique challenge in that signals are not transmitted correctly due to "deterioration of characteristics" rather than element failure.

This article focuses on the aging of the Current Transfer Ratio (CTR), an indicator of performance degradation, and organizes the degradation mechanism and design considerations. Furthermore, it explains the concept of life design using low interface (IF) drive as an approach to ensure long-term reliability.

Basic principles of "low IF drive" and lifespan design in photocouplers

A photocoupler is a device that achieves electrical isolation by combining an input light-emitting element and an output light-receiving element to transmit signals via light. It is widely used as a safety component in industrial equipment and automotive equipment.

As shown in Figure 1, the input-side light-emitting diode (LED) and the output-side phototransistor are enclosed in transparent resin, and the input and output sides are electrically isolated from each other. Signal transmission is achieved by detecting the light emitted from the LED with the phototransistor.

Figure 1: Internal structure of a photocoupler

Source: How to Use Photocouplers | Renesas

Thus, because photocouplers have a structure that "converts electrical signals into light and then back into electrical signals," the internal light transmission efficiency greatly affects their operating characteristics. A typical indicator of this is the CTR (Current Transfer Ratio). CTR is the ratio of output current to input current (IF) and is a fundamental characteristic that represents signal transmission capability.

The essential point to consider when thinking about the lifespan of a photocoupler is that the CTR (Current Temperature) changes over time. In particular, the input LED gradually loses its luminous efficiency due to heat generated during operation and deterioration of its internal materials. As a result, even when the same IF (Internal Frequency) is applied, the amount of light generated decreases, the output current decreases, and consequently the CTR also decreases.

Furthermore, the transparent resin filling the space between the light-emitting element and the light-receiving element also deteriorates over time due to the effects of heat and light. The decrease in the transmittance of the resin worsens the light transmission efficiency, which also contributes to the decrease in CTR. Thus, the decrease in CTR in photocouplers is characterized not by a single factor, but by a combination of "decreased luminous efficiency" and "decreased light transmission efficiency."

What becomes important here is the setting of the LED driving conditions, especially the forward current (IF). Generally, increasing the IF provides sufficient output current and makes it easier to secure a design margin, but on the other hand, it increases heat generation, which is detrimental from the perspective of long-term reliability. Conversely, by keeping the IF low, the burden on the LED is reduced, and as a result it is possible to suppress the decrease in CTR.

Figure 2 shows the change in CTR with respect to input current (IF). CTR does not simply increase proportionally to IF; it also tends to decrease after reaching a peak under certain conditions. Therefore, setting IF excessively high to ensure sufficient CTR is not always optimal.

Figure 2: Forward current (IF) vs. Current transfer coefficient (CTR)
Source: PS2761B-1 Data Sheet | Renesas, p7

Thus, in photocouplers, there is a trade-off between designing to "increase the IF to ensure sufficient transmission characteristics" and designing to "decrease the IF to ensure long-term reliability." Therefore, it is important not only to meet the initial characteristics but also to design the lifespan with long-term characteristic changes in mind.

Why do photocouplers need to be designed with lifespan in mind?

Risk of malfunction due to CTR degradation

One often overlooked aspect of photocoupler design is the decrease in operating margin due to the aging of the CTR (Current Transformer). The CTR listed in the datasheet is the initial value and guaranteed range, and does not directly represent the characteristics after long-term use. Therefore, even if sufficient output current is obtained in the initial state, the"margin assumed during design"will gradually be lost as the CTR decreases over time. As this margin decrease progresses, the output current will eventually fall below the required threshold, leading to a state where the signal is not transmitted correctly. For example, if the output of the photocoupler is connected to the input of a subsequent IC, insufficient output current due to CTR decrease may fail to meet the input threshold, potentially leading to logic instability.

Furthermore, overlapping temperature conditions can temporarily worsen performance, potentially leading to malfunctions occurring only under specific conditions. Unlike component failures, these problems progress gradually, making them difficult to detect during initial evaluations. In many cases, they only surface as issues in the field environment. Moreover, their occurrence varies depending on environmental conditions, making it difficult to pinpoint the cause.

Thus, CTR degradation in photocouplers gradually erodes the operating margin, increasing the risk of malfunction. Therefore, life design that takes into account long-term characteristic changes, rather than designing based solely on initial characteristics, is essential.

Factors affecting the lifespan of photocouplers and design considerations

Relationship between low IF, temperature, and aging degradation

To accurately estimate the lifespan of a photocoupler, it is crucial to understand the key parameters that affect the aging degradation of the CTR and to design accordingly. Particularly influential factors include input current (IF), temperature, and operating time.

First, the input current (IF) is one of the factors that most significantly affects the degradation of the light-emitting element. Setting a high IF makes it easier to secure sufficient output current, but on the other hand, it increases the heat generated by the light-emitting element, which accelerates the decrease in luminous efficiency. Conversely, keeping the IF low can suppress heat generation, and as a result, it is possible to slow down the aging degradation of the CTR.

Next, let's consider the effects of temperature. Photocouplers are affected by ambient temperature and self-heating, and in high-temperature environments, the degradation of the light-emitting element and encapsulating resin accelerates. In particular, when use in high-temperature conditions is expected, such as in automotive or industrial applications, it is essential to design the product with consideration for the progression of degradation due to temperature.

Furthermore, degradation over time, proportional to operating time, cannot be ignored. Since the characteristics of photocouplers gradually change due to prolonged energization, it is necessary to estimate the change in CTR throughout the entire service life. Temperature conditions greatly affect CTR degradation. Figure 3 shows the change in CTR over time under different temperature conditions, and it can be seen that the CTR decreases more rapidly at higher temperatures.

Figure 3: Long-term CTR degradation graph with temperature
Source: Current Transfer Rate (CTR) of General-Purpose Photocouplers | Renesas

Figure 4 shows that not only ambient temperature but also input current (IF) has a significant impact on lifespan. In particular, lifespan is greatly shortened in high-temperature environments and high IF conditions, so careful setting of conditions is required for equipment intended for long-term use.

Figure 4 shows the lifetime under temperature and current (CTR is half of the initial value) graph.
Source: Current Transfer Rate (CTR) of General-Purpose Photocouplers | Renesas

In actual design, what CTR margin should be included?

In the design of a photocoupler, in order to meet the required output current CTR It is common practice to design based on the minimum guaranteed value. However, in reality, CTR This can change not only due to product variations but also due to ambient temperature and aging. Therefore, even if the requirements are met in the initial state, the design margin may gradually be lost over time with prolonged use.

For example, even if there appears to be sufficient margin in the initial stages, if it operates in a high-temperature environment for a long period of time... CTR This can lead to a decrease in output current, potentially resulting in insufficient output current.

Especially in industrial equipment,
• High-temperature environment inside the control panel
・24-hour continuous operation
Long-term use of 10 years or more
Such conditions are not uncommon.

For such applications, initial CTR It is important to ensure sufficient margins by considering not only the voltage itself, but also temperature derating, aging degradation, and required output current.

Also, photocouplers CTR Since there is variation between products, it is recommended to consider minimum guaranteed values rather than typical values during the design phase. By considering both variations during mass production and changes over time, the risk of unexpected malfunctions in the field can be reduced. On the other hand, input current is used to ensure a margin. (IF) If you set it to a large size, LED Increased load on, CTR This can accelerate deterioration.

In other words,
We want to secure a CTR margin.
We also want to achieve a longer lifespan.
Responding to these conflicting requirements is a major challenge in photocoupler design.

One effective approach to this challenge is, Select a photocoupler that can ensure sufficient CTR even at low IF. However, with conventional photocouplers, low IF Operation and High CTR It was difficult to achieve a balance between the characteristics, and in some cases, it was necessary to increase the input current to ensure a sufficient margin. For this reason, in recent years, IF High CTR Products that achieve this are attracting increasing attention.

Features of the RV1S2x51A/RV1S2x55A series, which achieves long lifespan with low IF drive.

As mentioned above, low IF driving, which suppresses the input current (IF), is effective in ensuring the long-term reliability of photocouplers. On the other hand, lowering the IF presents a challenge: insufficient CTR can lead to insufficient output current, making it difficult to secure operating margins. Selecting a photocoupler with high CTR characteristics even at low IF is an effective solution to this trade-off.

The Renesas RV1S2x51A and RV1S2x55A series photocouplers are designed to achieve both low IF drive and high CTR. One of the key features of this series is its ability to operate with low input currents. It is designed to provide sufficient output even at low IF levels of tens of nA, allowing for operation with less stress on the LED than conventional models. This suppresses the degradation of the light-emitting element and contributes to a longer lifespan by slowing down the decrease in CTR.

For example, as shown in Figure 5, the RV1S2451A series has high CTR characteristics, guaranteeing a minimum CTR of 300% at IF = 50μA. This ensures sufficient output current even with low IF drive, making it easier to maintain design margins. In addition to DC input types, AC input types such as the RV1S2255A and RV1S2955A are available in the lineup to suit various applications, allowing for flexible adaptation to various circuit configurations. Furthermore, multiple package variations are available, including a high-insulation type with a surface length of 15mm, as well as 8.2mm and 4mm packages, allowing selection based on the required balance of insulation performance and mounting area.

Figure 5 Overview of the 　RV1S2x51A and RV1S2x55A series
Source: Insulation element solutions contributing to miniaturization and ease of design of industrial equipment, p3

Thus, this series is designed to satisfy both the requirement of "achieving a long lifespan with low IF" and the requirement of "ensuring stable operation with sufficient CTR," making it an effective choice for industrial equipment and automotive equipment where long-term reliability is required.

In addition to the RV1S2x51A and RV1S2x55A series introduced here, Renesas offers a wide variety of photocouplers for various applications, including high-speed communication photocouplers and couplers for IPMs and gate drive applications, in numerous package configurations. Please visit our website for more information.

Photocoupler (Optocoupler) | Renesas

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