Introduction

In the previous article, we introduced the insulation performance evaluation in IEC 60747-17, as well as key parameters such as VIORM and VIOWM.

This article will first summarize the degradation mechanisms that occur in insulating systems, and then discuss TDDB (Time-Dependent-Dielectric-Breakdown), a method for evaluating the aging degradation of insulating barriers.

I will explain.

Insulation lifetime evaluation according to IEC 60747-17

IEC 60747-17 lists the following factors as affecting the long-term reliability of isolators:

Surface tracking

Deterioration of molding resin and packaging materials

- Deterioration of the insulating barrier

We are taking this into consideration.

For surface tracking, creepage distance and CTI are measured, while for mold resins and packaging materials, reliability evaluations are conducted against temperature and humidity.

On the other hand, when considering the long-term reliability of an insulation system, the degradation of the insulation barrier itself over time is particularly important.

Insulation barrier failures can be broadly categorized as follows:

- Failure due to initial defects caused by the manufacturing process or materials

- The inherent aging degradation of insulating materials due to long-term voltage stress.

It can be classified into these categories.

IEC 60747-17 specifies that an insulated barrier abnormality should be confirmed by a dielectric strength test, and voids in the molding resin or insulating material should be identified by a partial discharge test.

We select products that have initial defects that affect long-term reliability. Furthermore, we perform a long-term insulation barrier degradation evaluation using TDDB (Time-Dependent-Dielectric-Breakdown).

This article focuses on the inherent aging degradation of insulating materials that progresses over time even after proper initial quality control, and explains how to evaluate TDDB (Total Data Degradation).

Life evaluation of insulating barriers using TDDB

Insulators are materials that do not conduct electricity, but they cannot maintain their insulating properties indefinitely. When an electric field is applied to an insulating film for a long period of time, tiny defects are generated and accumulate inside the film.

One way to explain this phenomenon is through the percolation model.

Initially, each defect is isolated. However, as voltage stress continues and the defect density increases, the defects gradually begin to connect with each other. Eventually, when the defect network penetrates the insulating film, a conductive path is formed, leading to a decrease in insulation resistance and an increase in leakage current, ultimately resulting in dielectric breakdown.

The following is a conceptual diagram of the percolation model.

Conceptual diagram of the percolation model

In TDDB evaluation, a constant voltage stress is applied to the insulation barrier of the completed isolator IC, and the time until the insulation resistance falls below a specified value is measured as TTF (Time To Failure). In TDDB, the criteria for dielectric breakdown are when the insulation resistance falls below 2 MΩ for basic insulation and below 4 MΩ for reinforced insulation.

This evaluation is performed on the finished device, not the insulating film itself. Therefore, it reflects the actual insulating structure and electric field distribution in the lifetime evaluation.

Predicting lifespan from TDDB

To directly verify the insulation life under actual usage conditions would require a very long testing period.

Therefore, in TDDB evaluation, TTF data is acquired under multiple voltage stress conditions, and the distribution of failure times at each voltage is statistically analyzed. From the results, a lifetime curve is created.

We extrapolate the lifespan in the actual operating voltage range.

The following is an example showing the relationship between applied voltage and predicted lifetime.

Life curves and insulation life prediction in TDDB evaluation

The life curve shows a tendency for the time until dielectric breakdown to decrease as the applied voltage increases.

IEC 60747-17 calculates VREF based on the following lifetime and failure rate conditions:

Basic insulation: 1000 ppm, 24 years

• Enhanced insulation: 1 ppm, 30 years

The VIORM is determined by applying a predetermined voltage margin to the calculated VREF. In TDDB, the VIORM is determined by applying a voltage margin of 1.2 times after calculating the VREF.

It is shown.

VIORM and VIOWM are insulation performance parameters defined based on life prediction using TDDB, rather than being mere withstand voltage test values.

Concepts of insulation life in real-world applications

in conclusion

IEC 60747-17 evaluates insulation reliability by considering multiple factors, including surface tracking, degradation of molding resin and packaging materials, and degradation of the insulating barrier.

Among these methods, TDDB is used to evaluate the aging degradation of insulating barriers. TDDB evaluates the generation and accumulation of defects within the insulating film and statistically predicts the time until dielectric breakdown.

Although this article focuses on isolators, the concept of insulation lifetime is common to other insulating elements, including photocouplers.

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