Have a look at the manufacturer's data sheets for the capacitors, and you'll see spec's or graphs indicating the % capacitance change vs. temperature. This can be substantial for ceramic cap's in particular. I don't know how much of an effect it is for the typical film-type X and Y cap's, but that is likely the source of the change in leakage current.
Keep in mind that the EMC performance of a SMPS may be related to many factors, not just X or Y capacitor performance. There are often capacitors located further inside the product that are not X or Y types (and aren't required to be) yet serve an EMC-reduction function, that could be ceramic types with significant value change vs. temperature. There will also be temperature-related changes in electrolytic filter cap's, magnetic components, snubbers, the power semiconductors themselves, etc. Many of these changes may seem subtle in absolute value, but given the impact that parasitics have on EMC, the change could make a big difference. Jim Eichner, P.Eng. Manager, Engineering Services Xantrex Technology Inc. Mobile Power phone: (604) 422-2546 fax: (604) 420-1591 e-mail: [email protected] web: www.xantrex.com -----Original Message----- From: [email protected] [mailto:[email protected]] Sent: Monday, December 17, 2001 7:59 AM To: [email protected] Subject: Thermal effects on EMC and Earth Leakage Group. I am interested to find out peoples opinions on a particular trend we've seen time and time again on most switch mode PSU's. It seems that once a PSU is warm its earth leakage decreases and its EMC emission performance decreases (i.e. it gets noisier). This would suggest that there is some sort of thermal effect, probabaly in the Y caps, that reduces their capacitance. Has anyone done any work in this area or can anyone explain the physics of this change. Is it as simple as expansion of the dielectric or is it more complicated than this? Any thoughts or opinions would be greatly recieved. Many thanks in advance, Duncan Hobbs ---------------------------------------------------------------------- The contents of this communication are confidential to the normal user of the email address to which it was sent. If you have received this email in error, any use, dissemination, forwarding, printing or copying of this email is strictly prohibited. If this is the case, please notify the sender and delete this message. ---------------------------------------------------------------------- ------------------------------------------- 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: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: [email protected] Dave Heald [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server. ------------------------------------------- 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: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: [email protected] Dave Heald [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
