Recently, it was brought to my attention that Silicon Labs’ Isolators were part of a study at Aalto University in Finland. The study was conducted by Mikael Lumio as part of his Master’s program. The results are interesting and one of the most comprehensive, independent studies of isolation performance I have seen.

 

The full thesis is available here.

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Mr. Lumio approaches some of the concerns surrounding CMOS-based isolation straight on. In his thesis, he compares different types of CMOS isolation against each other and against the legacy optocoupler solutions. Capacitive-based isolators (the preferred technology from Silicon Labs), inductive (transformer)-based isolators, and magnetoresistive-based isolators are all included in the analysis. For many specs, the capacitive-based solutions outperform other technologies. Compared to optocouplers, Mr. Lumio’s analysis clearly shows advantages of CMOS-based isolation in common-mode transient immunity (CMTI), current consumption per channel, data rate, skew, and other timing specifications.

 

In addition to comparing datasheet specifications, Mr. Lumio also compares isolation performance. Mr. Lumio’s thesis does an excellent job of describing the different isolation parameters that are commonly specified by international certifying agencies. Then, the thesis uses those descriptions to aide in the comparison of different CMOS isolation technologies. For all technologies, the isolation capability is on par with legacy optocoupler devices. The only lagging parameter is in surge immunity, but CMOS-based isolators are catching up fast.

 

Mr. Lumio’s thesis is particularly focused on the application of isolation technology to variable frequency drive (VFD) systems. VFDs are increasingly used in all types of motor drive applications. Motor drive efficiency standards worldwide encourages the use of variable frequency drives wherever it is applicable to a motion application. The thesis lists, “blowers, compressors, conveyor belts, cooling and recirculating pumps, cranes, factory robotics, fans, elevators, hoists, mixers, paper mills, and printing presses,” as relevant applications where VFDs are prescribed for efficiency improvement.

 

VFDs are usually powered from rectified AC line inputs and are used to provide a three-phase, AC output to a motor. The supply provides a high DC voltage for the rest of the system. Isolation is necessary in several parts of the typical system. The VFD is converting the DC supply into an AC signal through a multi-phase inverter stage. The inverter has switches connected to the high voltage power rail and isolation must be utilized to protect the control circuit. The output of the drive is connected to the windings of the motor. Often each winding has an isolated sensor connected for current and voltage measurement. Both safety isolation, for protection of operators and people interfacing with the system, and functional isolation, for circuit protection, are necessary in the system.

 

Besides specific isolation requirements, Mr. Lumio looked at additional features of CMOS-based isolation to evaluate its suitability for use in VFDs. Electromagnetic compatibility (EMC) was of particular concern. ESD performance was also evaluated. Finally, the relative cost of the various CMOS isolators was considered.

 

On the subject of cost, the thesis found that at mass production volumes CMOS-based isolators are less expensive. In his analysis, Mr. Lumio included not only the device cost but also indirect costs including the reduction in PCB area and component count. For the typical fieldbus serial communication application, Mr. Lumio’s research indicates a close to 1/5 cost reduction in moving from a legacy optocoupler solution to a modern isolation solution. For a more specific IGBT drive function, the cost reduction wasn’t as large. It was still significant, though, at approximately 1/10.

 

EMC was a focus for Mr. Lumio’s thesis because of the environments in which VFDs operate. High voltage switching associated with motor drive means that the components used must be immune to a large amount of electromagnetic noise. Mr. Lumio found that modern CMOS-based isolators performed as expected even in the presence of large amounts of electromagnetic interference. Silicon Labs’ isolators showed great robustness in all test. For example, in the RF immunity evaluation, Silicon Labs’ isolator was free of errors even in the presence of 20 V/m RF energy.

 

In his own words, Mr. Lumio writes, “Digital isolators from Silicon Laboratories were in the top group of manufacturers in all of the tests.” His thesis is additional evidence for the suitability of CMOS isolators in demanding applications that require high reliability, like motor drives. We’re proud that Silicon Labs’ capacitive based isolation fared well in all of Mr. Lumio’s analysis and it’s our goal to deliver higher performance, more reliable isolated circuits than competing solutions. For more information, visit our Isolations Solutions page.

 

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