Capacitive-based galvanic isolation vs. optocoupler technology – who wins?


TI’s capacitive-based galvanic isolation technology differs from optocoupler-based isolation technology in a multitude of fashions, the most glaring of which is the implementation of the isolation. First let’s make sure we understand what we mean when discussing “isolation.” Essentially, isolation is a form of protection that permits communication between two points but stops current from directly flowing between those points.

How it works

In optocoupler-based technology, an LED is used to transmit signal information to a receiver that sends the message to the remainder of the circuit (think as though using a flashlight to send Morse Code). Their isolated barrier comes from a combination of the LED and distance through the mold compound. So, essentially their isolation is tied to the components that make up their package. In TI’s capacitive-based technology, signal information is based through a series of circuits etched in silicon. The center is a silicon dioxide-fabricated capacitor used to block direct current flow by leveraging an edge-based detection scheme (think more like tapping Morse Code through a wall).

To touch on all points of difference could literally fill a semester of college level course work, but let’s cover two key areas: part-to-part variation, distance through insulation and quality and reliability.

Part-to-part variation

Variation is a way of life, the granularity of that variation is relative. Since TI’s isolation is a silicon dioxide capacitor, created on the micron level, where variations occur and therefore microscopic from part-to-part. An optocoupler, on the other hand, gets its isolation at the packaging level, which is more like the millimeter level. Variations and error in the assembly process would naturally be larger at that level. If you think about it, the variation and noise relative to each node may be similar and small compared to the baseline spec but when comparing optos to capacitive you’ll see a night and day difference contributed to the underlying DNA of these technologies.

Distance through insulator (DTI)

That assembly difference plays into why optos are larger than TI’s isolation technology: a result of the properties of materials used. Distance through isolator (DTI) is a common metric brought up as tantamount to strength of isolators. In the case of isolation, the true metric associated with isolation strength is voltage breakdown per micrometer not pure distance. Let’s compare the two ratings for silicone/PI/mold-compound (used in optocouplers) and silicon dioxide (used in TI’s capacitive isolation):

 

Typical Breakdown Voltage

Isolation of 16um

Isolation of 32um

DTI needed for 40kV

 Silicone/PI/mold-compound                   (Opto-coupler)

50kV peak/ um

800kV

1,600kV

0.8um

Silicon Dioxide (TI)

800kV peak/ um

12,800kV

25,600kV

0.05um

The table above clearly shows that bigger doesn’t mean stronger isolation. 

Quality and reliability

Manufacturability and assembly also play into failure rates.  When you look at published failure-in-time (FIT) data for both optocouplers and TI isolators, you see magnitudes of order difference in the reliability data. Failure In time essentially means: given a certain number of use hours over a given sample set of devices, how many fails should one expect to see due to wear-out when all working under the same conditions?

Product

Temp (C)

FIT (Fails/109 hours)

Capacitive-based isolator

55°

0.7

Optocoupler

55°

258.5

Again, TI isolators are the clear winner - barely scathed - with not even a full fail within one billion device hours of operation, whereas optocouplers are riddled with 369 times as many fails within the same operating window. Riddling end devices with faulty products, be it from assembly issues or products just not making the cut… after they leave assembly and test floor and arrive at you the customer’s door step.

Hopefully now you have a better idea on some of the more apparent difference between TI isolators and optocouplers. Be it architecture, materials used or reliability, TI isolators are a clear winner for high-performance needs. Be on the lookout for more articles and blogs pertaining to this subject.

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