Polyimide Film Uses for Digital Isolators

PublishTime：2021-09-15

●Abstract:
Digital isolators provide compelling benefits over legacy optocouplers in terms of high speed, low power consumption, high reliability, small size, high integration, and ease of use. Billions of digital isolators using microtransformers have been widely adopted in many markets, including automotive, industry automation, medical, and energy. What are essential for the high voltage performance of these digital isolators are polyimide films deposited in between the top spiral winding and bottom spiral winding for the stacked winding transformers. In this article, digital isolator construction using polyimide films as isolation layers will be reviewed. To meet various safety standards such as UL and VDE, digital isolators need to satisfy various high voltage performances, such as short duration withstand voltage, surge voltage, and working voltage. Polyimide aging behavior under various high voltage waveforms such as ac or dc was studied, and isolator working voltage is extrapolated through a polyimide lifetime model. Structural improvements to improve polyimide high voltage lifetime will also be discussed.
●Introduction:
Isolation between circuit components is typically required for safety and/or data integrity considerations. For example, isolation protects sensitive circuit com-ponents and human interface on the system side from dangerous voltage levels present on the field side, where more robust components such as sensors and actuators reside. Isolation can also eliminate common-mode noises or ground loops that affect data acquisition accuracy. While optocouplers have provided isolation for decades, they present significant limitations in terms of low speed, high power consumption, and limited reliability. Their low bandwidth and long propagation delay present significant challenges in meeting the ever-increasing speed requirements for many isolated field bus communications such as RS-485 in industry automation systems.
Their high power consumption due to their LED puts a significant constraint on the overall system power budget in power limited industry systems such as process control 4 mA to 20 mA systems. As the current transfer ratio for optocouplers degrades over time, especially at high temperatures, it fails to meet the reliability needs for demanding applications such as automotive.
Digital isolators remove the penalties associated with isolation and they provide compelling advantages over optocouplers in terms of high speed, low power consumption, high reliability, small size, high integration, and ease of use. Digital isolators using microtransformers1,2 allow for the integration of multiple transformers and other necessary circuit functions. The stacked spirals used in digital isolators provide tight magnetic coupling between the top coil and bottom coil, and very little coupling between spirals side by side. This enables multiple channel integration with little interference between the channels. The magnetic coupling between the top spiral and bottom spiral depends only on the size and separation. Unlike the current transfer ratio for the optocouplers, it does not degrade over time, which leads to the high reliability for these digital isolators based on transformers. These transformers have a self-resonant frequency from a few hundred MHz to a few GHz and they can be used to realize digital isolators from 150 Mbps to 600 Mbps. With a high quality factor well over 10 for these transformers, the power consumption for these digital isolators is orders of magnitude lower than those of optocouplers.
The optocouplers shown in Figure 1 rely on a few mm thick molding compound between the LED die and photodiode die to achieve isolation. For the transformer-based digital isolators shown in Figure 2, isolation performance is mainly limited by 20 μm to 40 μm thick polyimide layers sandwiched between the top and bottom coils of the chip scale microtransformers. We will review the detailed construction of these isolators, deposition methods for these polyimide films, characterization of the polyimide films, high voltage performance, and aging behavior for the digital isolators.

Brand of Product：	ADI
Part#：
Product Category：	Polyimide Film Uses More
Application：	[ Digital Isolators ] More
Data Type：	Technical Documentation More
Safety & Environment Standards：	More
Operating Temperature：	Please see the document for details
Qualification Grade：	More
Qualification Approval：	More
Package：
Language：	English Chinese Chinese and English Japanese
Date of Creation：	3/21
Version Number：
Data Encoding：	TA22581-3/21
Data Size：	1.4 MB

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