Vic - Your assumption that at least Basic insulation is required is correct. However, creepage distances are based on rms voltages, not peak.
The below assumes that the voltages you mentioned are present on the heatsink of the FET. Assuming that a true rms meter will indicate an rms voltage between 250 Vrms and 300 Vrms, the minimum require creepage >from Table 2L is 3.2 mm. Based on the peak voltage you mentioned, the minimum required clearance distance from Table 2H is 2.0 mm, plus the added distance from Table 2J of 0.2, gives a total minimum clearance distance of 2.2 mm. Without knowing the particulars of the FET and its heatsink in any great detail, an insulating pad is an appropriate and common means of complying with the standard. Unless you can demonstrate that the pad will not compress to less than 3.2 mm thickness in the application, you'll probably need a shouldered washer or something similar to add additional creepage between the screw shank and the FET's heatsink. Polymeric screws are available, but I can't speak regarding their use in an elevated temperature. Regards, Peter L. Tarver, PE Product Safety Manager Sanmina-SCI Homologation Services San Jose, CA peter.tar...@sanmina-sci.com > -----Original Message----- > From: Gibling, Vic > Sent: Friday, January 17, 2003 12:00 AM > > > We drive a FET based H-bridge from > rectified 230VAC mains; thus > 320Vpk across the bridge. We have assumed the > insulation between the > rectified power return and chassis (Class 1 > product) is basic, thus > requiring 4mm creepage (IEC950). The proposed > FETs need to be attached to a > heat sink and we would like to use the chassis > for this purpose. > > Our problem is that the FETs have around > 2mm creepage between their > exposed heatsink surface and the fixing screw, > insufficient to meet the > basic insulation creepage distance. > > Is our interpretation regarding basic > insulation correct/reasonable? > > If we use an insulative thermal pad between > the FET and chassis, > does the compression of the pad exclude the air > path thus offering > sufficient protection? I am aware of such pads > offering 4.5kV breakdown. > > Thank you > > Vic Gibling > Compliance Engineer > > e2v technologies Ltd > Waterhouse Lane > Chelmsford > ESSEX CM1 2QU > > Telephone: +44 (0) 01245 493493 > Direct Line: +44 (0) 01245 453352 > Facsimile: +44 (0) 01245 453410 > > E-mail: vic.gibl...@e2vtechnologies.com > Internet: www.e2vtechnologies.com > > > > ------------------------------------------- > 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 All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list" 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 All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
