Can single fault condition testing between pins of a control IC and power switching MOSFET in a single package be omitted if there is inherent isolation between both complying with Basic Insulation requirements?
More specifically you asked: some AC/DC switching power supply circuits in IT/AV products are constructed using unrecognized off-line switcher IC components that incorporate the control IC and power switching MOSFET into a single IC package. Typically the chip is designed to provide isolation between the drain/source pins of the MOSFET, and the control pins. Is it always necessary to complete single fault testing between the control pins and drain/source pins of the MOSFET per B.4.1, or can omitting faults between the Drain/Source Pins of the MOSFET and the control pins of the IC be justified if the isolation between the Drain/Source pins and control pins (including PCB traces) meets creepage/clearance or electric strength requirements for BASIC INSULATION in accordance with B.4.4.1 and B.4.4.2?
In response, it is our understanding the switcher IC and MOSFET you describe function independently, but are isolated in the same IC package by equivalent to Basic Insulation.
Based on this understanding, in most cases, the short circuit between the pins of the switcher IC and MOSFET may not be required per clause B.4.4 of IEC 62368-1:2014 and IEC 62368-1:2018 since the isolation possesses a certain quality (clearance, creepage distance and electric strength) comparable to a Basic safeguard.
However, IEC 62368-1 is a hazard-based standard and the application of Annex B needs to be considered in the context of the Clauses (covering different energy sources/hazards) that reference it - Annex B is not applied independently.
Faulting the Basic insulation should not be required in the context of Clause 5, Electrically-cause Injury, since Functional Insulation only is required as a minimum for such an IC package.
However, in Clause 6, Electrically-caused Fire, if the “Reduction of the likelihood of ignition” method is chosen then, in accordance with the sub-clause 6.4.3 of IEC 62368-1:2014 or IEC 62368-1:2018, relevant single fault condition testing is required across a single safeguard in the context of risk of fire. In accordance with the definition of a Single Fault Condition (188.8.131.52) and B.4.1, Simulated single fault conditions – General, a basic safeguard failure should be considered if it affects the safety of the equipment. Therefore, we believe a single fault condition across the single safeguard is appropriate in this situation.
Your question points to an area of IEC 62368-1 that may need further refinement and we encourage you to engage IEC TC108 through your National Committee if you believe the Standard needs further clarity on this topic. Also, as this forum is a general forum and is not intended to analyze and provide guidance on specific designs, we suggest that you contact the UL office you work with if you have a specific design or construction that you wish to discuss.