SSZTAM9 november   2016


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    2.     Additional Resources

Neel Seshan

In this day and age, when we change gadgets every year, we hardly care about how many years these gadgets last. It has a long lifetime if it lasts until the unveiling of the next revision. But there are many applications, especially industrial such as factory automation and motor control, where a longer equipment lifetime is critical to guarantee lower system outages and higher throughput.

Most industrial circuits need some sort of isolation between the high- and low-voltage sides. The exact voltage levels that constitute these high- and low-voltage levels may differ from application to application, but the mere presence of different voltage levels necessitates isolation to protect the low-voltage side from the high-voltage side. In some cases, isolation may be needed for ground-loop elimination even if the voltage difference is not very high.

There are plenty of isolators to choose from: optical, magnetic or capacitive. But one key priority for industrial applications is the lifetime of these isolators. So how do you ensure that the circuit will not be damaged within its lifetime under specific voltage stress conditions?

TI isolators use capacitive isolation technology, where the data transmits across a capacitive barrier that is capable of blocking high voltages. The capacitive barrier is silicon dioxide, or simply glass. With its high breakdown strength (500-800V/µm), the stability over temperature and moisture is much better than dielectric materials used by optical and magnetic isolators.

Time-dependent dielectric breakdown (TDDB) is widely used to measure isolator lifetime. As Figure 1 shows, the isolator’s input and outputs form a two-pin configuration when shorted, and a voltage is applied across the isolator under elevated conditions. As the voltage increases, the circuit is monitored for any leakage current that would indicate the breakdown of the dielectric, and as a result the breakdown of the isolator. Based on the breakdown at elevated conditions, it’s possible to predict the lifetime of the isolator at the working voltage levels.

GUID-1B04BC75-2C8F-404D-9DC5-EB24ED76360C-low.png Figure 1 TDDB Test Setup

Figure 2 shows the projected lifetime of the latest family of TI isolators. As you can see, capacitive isolators have a very healthy lifetime curve extending to 135 years for a 1.5kVrms working voltage at 150°C with a large margin. Similar tests on optocouplers and magnetic isolators yield much lower lifetimes. This is not surprising, as lifetime is related to the dielectric breakdown strength of the material providing the isolation.

GUID-03E787D2-F46F-4244-BA97-FBD7F7D1CEBD-low.png Figure 2 Reinforced Isolation Capacitor Lifetime Projection

Another point to note is that optocouplers typically use partial discharge tests as an indicator of device reliability. But the test doesn’t capture all of the scenarios that may cause the breakdown of the dielectric. In addition to TDDB testing, TI isolators are checked for partial discharge to ensure quality and robustness.

With the latest process technology and architectural changes, the ISO78xx family of isolators has a working voltage of 1.5kVrms in the wide body (DW) package. When compared to magnetic isolators with a 600Vrms maximum working voltage and optocouplers with 1.5kVrms maximum working voltages, these families provide a significantly large margin for isolation robustness. The additional 14mm-extra-wide body (DWW) package option in the ISO78xx family provides a 2kVrms working voltage for applications in higher altitudes without breakdown, by enabling 1kVrms line voltages for equipment to connect directly to lines per International Electrotechnical Commission (IEC) 61800-5-1 (drives) up to 5000m.

So the next time you are designing boards that will be used in the field for a long time, choose your isolators carefully. After all, they could be the ones determining the fate of your boards many years after release. If you need help choosing the right isolator for your design, log in to leave a comment or visit our digital isolation page.

Additional Resources