3 Isolator Lifetime Through TDDB Test

Time dependent dielectric breakdown (TDDB) test is an industry standard accelerated stress test for determining lifetime of a dielectric as a function of voltage. The test consists of applying various stress voltages across the isolation barrier of a device that are much higher than the typical working voltages and monitoring the amount of time it takes for the dielectric to break down. These voltage vs time coordinates are plotted on an appropriate graph, and the coordinates are extrapolated to lower stress voltages to determine expected dielectric lifetimes for the suitable working voltages.

Figure 3-1 compares TDDB plot of a TI digital isolator against a popular optocoupler, it can be noticed that the average TDDB line of optocoupler is about 2 divisions (100 times) lower than digital isolator average TDDB line. The primary reason for such a large difference in TDDB lifetimes of the two technologies is the large difference in dielectric strengths of the insulating material they use (refer Table 1-1). It can also be noticed that the lifetime of an optocoupler for a given stress voltage varies considerably from one sample to another while the same is consistent across samples for the digital isolator.