I read in !emc-pstc that boconn...@t-yuden.com wrote (in <F7E9180F6F7F58 40858d3db815e4f7ad1f2...@cms21.t-yuden.com>) about 'single fault conditions' on Wed, 29 Jan 2003:
>The environment being considered is a switching power supply. The technique >that safety agencies use to simulate a SFC on a power FET does not seem, >IMHO, to simulate the actual failure mode of the device. To wit: when the >mosfet fails short, it blows itself open; so the amount of current "sucked" >out of mains, e.g., the PFC FET, would probably open the component after a >few input cycles. But if I apply a direct mechanical short (source to >drain), current is being forced to flow until the fuse blows, or until some >series trace or component opens. Agreed. > >The Bad: some FETs fail very violently, and can actually be a fire hazard >and/or shock hazard in open-frame switchers; Really? There doesn't seem to be enough combustible material to cause a fire hazard, and an open-frame switcher always has to be in some sort of outer enclosure, doesn't it? So it's not clear how a shock hazard could arise. > but if the FET itself does not >provide the short circuit, we will never know.... Indeed: the external short-circuit doesn't simulate the device failure. > >The Good: providing a "continuous" (mechanical) short will reveal if there >are other components in the current path that could be cause the unit to >fail in an unsafe mode. Agreed. >Although, according to QA records, these components >have never failed, so it can be both demonstrated by design equations and >empirical evidence that the SFC test does necessarily demonstrate anything >relevant... > >The Ugly: Safety testing results in design corrections that do not increase >product safety. > >So would it be legitimate to over-drive the gate, forcing short circuit >current to flow through the FET, but not to apply a mechanical short across >the component? Experiences & comments are appreciated. Doesn't the gate-to-drain short simulate this reasonably well? -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
