Shielded Optocouplers Prevent Off-line Switching Power Supply Upset

Focus:

Parasitic capacitance within optocouplers allows common-mode noise to propagate through them, a vulnerability characterized by common-mode transient immunity (CMTI). This noise can cause power supplies to malfunction as illustrated by a case study in which the turn-on of an offline power supply induced a common-mode transient voltage that disrupted the operation of a second, identical power supply. The operation of the power supplies and the fault are analyzed, leading to an explanation of CMTI and how common-mode noise is coupled through an optocoupler. The use of an electrically conducting shield within the optocoupler is described as a remedy to improve common-mode rejection. A simple circuit for measuring CMTI is presented and used to compare the performance of shielded and unshielded optocouplers, confirming that shielded optocouplers do offer better CMTI. Finally, a shielded optocoupler is installed in the malfunctioning power supplies from the case study, eliminating the fault.

What you’ll learn:

• How to understand the meaning of common-mode transient immunity in optocouplers and how to measure it
• How to eliminate power supply faults caused by common-mode noise propagating through optocouplers
• How to improve common-mode transient immunity in isolated power supplies by using shielded optocouplers

Notes:

This article appeared in the April 2013 print edition of PET.

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Author & Publication:

Vincent J. Spataro, Sr. Member of Technical Staff, Communications and Control Solutions, BAE Systems, Wayne, NJ , Power Electronics Technology, Mar 27 2013

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