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Balancing major elements of isolator for safety

30 Jan 2014  | David Krakauer

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To achieve the most robust isolation, polyimide thickness may be increased beyond what is reasonable for capacitors; therefore, capacitor-based isolation may be best suited for functional isolation where safety isolation is not required. In those cases, transformer-based isolation may make the most sense, especially when combined with a differential data transfer method that takes full advantage of the differential nature of transformers.

While each designer will choose an isolator with the appropriate balance of properties best suited to her or his application, three parameters tend to stand out: timing, power consumption, and, of course, isolation. To assess different technologies, consider the following chart which utilises a figure of merit based on timing divided by isolation capability plotted against power consumption. In this case, we have chosen to use surge withstand threshold (a high voltage pulse with a 2µs rise time and 50µs fall time used to establish suitability for reinforced insulation) to measure isolation capability.

Power consumption is the maximum power, in mW, per channel at 1Mbit/s data rates; we chose 1Mbit/s as a representative rate because most power sensitive applications run at modest data rates. For timing, we looked at the total timing delay for a signal to pass across the isolation barrier. Therefore, this includes not only the propagation delay, but also the jitter and output rise and fall times.


Figure 3: Combinations of the different isolator characteristics lead to different figure of merit positions. One aspect remains clear: optocouplers lag behind digital isolators.


When plotted in this way, one can see the presence of a performance frontier occupied by digital isolators. The optocouplers are positioned far behind the frontier, and while recent improvements to optocouplers have moved them closer to the performance frontier, they are still far behind digital isolators. One can also see that different technologies also occupy different positions along the frontier, with transformer/polyimide-based digital isolators with pulse encoding methods showing much lower power efficiency and on-off keying approaches showing much better timing performance.

Hidden within this chart are the subtle details of how different digital isolator vendors have moved along this frontier from one generation to the next. ADI's second generation approach moved to either side of the frontier by, in one case, reducing power consumption and, in the other case, reducing total timing delay. These changes were made without changing isolation capability. The Cap1 vendor only moved along the frontier in one direction and did so by increasing isolation capability; however, doing so increased the total timing delay. This is seemingly due to the fact that increasing SiO2 thickness to achieve better isolation reduced the coupling needed to transfer data; that, in turn, reduces performance.


Summary
When they developed their digital isolation technology, Analog Devices considered the various differences in the four elements of digital isolation, with a focus on insulating material, isolating element and the method for transferring data across the isolation barrier. They established that the core iCoupler technology would be based on polyimide insulation and chip-scale transformers because this combination affords the greatest flexibility not only to integrate other functions, such as isolated power, but also to allow the use of different methods for transferring data.

We have employed the pulse-based method for nearly 14 years. It is touted to provide excellent power efficiency and timing performance, yet still leaves open the possibility to employ other methods with their own benefits. This can all be done without compromising the isolation capability which, above all else, is the primary reason designers use isolators.


About the author
David Krakauer is the Product Line Manager for iCoupler Digital Isolator Group at Analog Devices Inc.


To download the PDF version of this article, click here.


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