Brian, I believe that forcing a FET failure would be a good test but should be in addition to the mechanical short method. If your power supply is safe for both failure modes that would be great. The fuse should open before the failed FET can cause a heat or fire problem. And it was already a shock hazard so that might not be an issue. The reason I would hesitate to use only the failed FET test is that changes in the type or manufacture or the FET could be an issue. And someone at UL might call you on not using the mechanical short test. If an actual failed component reveals a problem that the mechanical short does not, then the whole idea of a mechanical short test has a problem! Dave Cuthbert -----Original Message----- From: boconn...@t-yuden.com [mailto:boconn...@t-yuden.com] Sent: Wednesday, January 29, 2003 2:40 PM To: emc-p...@majordomo.ieee.org Subject: single fault conditions
Good People of PSTC 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. The Bad: some FETs fail very violently, and can actually be a fire hazard and/or shock hazard in open-frame switchers; but if the FET itself does not provide the short circuit, we will never know.... 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. 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. R/S, Brian
