Isolation education 101


Are you new to the world of digital isolation?  Perhaps you are a seasoned veteran? No matter the level of your expertise we can all use a refresh every so many clock cycles. The subject of digital isolation is one that is a very popular field with a lot of interesting aspects. The basics alone could make your head spin if left in an open format for consumption. At TI, we’ve taken the liberty of laying out the fundamentals into an easy to understand format and subsequently built on that foundation, diving deeper into more advanced topics.

Isolation basics

If digital isolation sounds like a place you lock up unruly mal-ware then you may want to start here. Topologies of galvanic isolation can take on many forms, of which, the three most popular are: optocouplers, inductive (or magnetic transformer) and capacitive.

Now, how they are measured? First, most isolation ratings are covered in units of Volts, either root-mean-square (RMS), or in peak voltage (Vpk). To convert a Vpk into an RMS simply divide by the square root of two, or basically 1.4 and vice versa to go from Vrms into Vpk. Then there are three main isolation values that are key to all designs: standard isolation voltage rating (Viso), transient overvoltage (Viotm), input voltage or surge (Viosm) and repetitive peak voltage (Viorm (pk)) To learn more about these different topologies, isolation rating values and find out how they help you to find the right component for your application check out the first part in our three part series called the “Isolation Navigation Frustration Cessation Education” on EDN.

Creepage and clearance

Part two moves onto a more advanced topic covering creepage and clearances, which correlate more to design of a digital isolation system. Creepage is the distance between two conductive points (input to output) on the outside package. Typically this is the perimeter of the package between the bottom input pin and output pin. Clearance is the shortest distance (for example, external line of sight) between the input to output. This is not going through the package, but typically just under it. Generally, it is the smaller of the two. For system-level minimum creepage and clearances, you need to know at least three things: 1) your isolator’s working voltage 2) the material group and 3) the pollution degree in which the isolation circuit will be used. These variables play an important part in the overall system considerations. To learn more about how this is a good indicator of energy in your system, read the second part in our isolation series on EDN. Or, learn more via illustration on this Engineer It video about creepage and clearance.

Partial discharge

Part three is another advanced topic on partial discharge, which is a phenomenon that occurs when assembling and creating isolators. The concept of partial discharge is essentially when the mold compound filler has air (or worse, other contaminate particles) trapped in it. As a voltage is applied across the capacitor at large, all areas of the continuous compound act as normally prescribed. It’s only when we come upon these unfortunate discontinuous pockets of air bubbles that accumulate charge then discharge within the transient states, known as inception and extinction voltage levels of the dielectric’s overall AC phase. If a large enough “bubble” is trapped, these partial discharge sessions can chip away at the filler/mold compound/silicon dioxide, causing micro cracks and crevasses to appear. Over time, if left unchecked, these micro cracks can become larger and larger until the dielectric breaks into two pieces rendering it not just aesthetically unappealing, but useless as an isolator. To learn more about this issue, read our third edition to the isolation series on EDN.

Digital Isolation can be a very tricky topic to cover, there are many pitfalls and hidden gotchas that can come about.  What’s worse, you may think you have a topic down cold until your design stops working midway!  All three articles are technically sound and designed for easy consumption, and hopefully you’ll learn a thing or two along the way. 

If you are ready to get down and dirty, the next design resource available for on-demand information is our Digital Isolator Design Guide.  It reinforces basic principles of operation but covers more advanced topic for actual board design including: design tips, board layout and signal routing.

After reading our design guide you may get inspired to do some creative isolation designs, well allow me to present some interesting options:

Boost power efficiency in your isolated systems

  • The SN6501transformer driver pairs with any digital isolator for a high efficiency solution
    • This means less power wasted and less heat.
  • The ISO1176T is the whole package. It offers isolated RS485 communication with the transformer driver inside.

 Provide high-voltage protection

  • The ISO764xFM devices offer 5kV isolation rating protection (per UL 1577) and up to 6kV overvoltage transient protection.

 Small package 2.5kV isolation

  • The recently released ISO71xx family reduces package size by 30% while providing the same 2.5kV isolation.

The final stop in this digital tour that you can cruise is our Industrial Interface E2E forum. Drop us a line or read up on some previous questions to avoid future pitfalls.  If you have a specific question or feedback, we’d love to hear from you!