This has been my experience with Class II switching power supplies. The absence of the earth connection will make very little difference in the emissions.
But, without the earth connection the leakage current requirement is much lower. (For ITE, 3.5mA is allowed for Class I, but only 0.25mA is allowed for Class II equipment ). Therefore to meet the 0.25mA leakage current limit the cap value must be reduced. With the cap value reduced the power supply will no longer meet the conducted emission limits. Then to reduce the conducted emissions, the common mode choke inductance must be increased. I am only a product safety person with very little EMC experience , but it seems be a simple voltage divider issue. With these changes the power supply can meet the conducted emissions and also the leakage current limit. Therefore, the desire for a class II switching power supply must be great enough to justify the increased cost and physical size of the common mode choke. But I have gotten several of them through world wide safety approvals. A final word: I recommend that you NOT try to modify an existing Class I design to meet the Class II leakage current and EMC requirements. Regards, Lou Aiken 27109 Palmetto Drive Orange Beach, AL 36561 USA Tel ++1 251 981 6786 Fax ++1 251 981 3054 Cell ++1 251 979 4648 ----- Original Message ----- From: [email protected] To: EMC-PSTC Internet Forum Sent: Tuesday, April 23, 2002 3:55 PM Subject: EMC and Class 2 equipment Hi all, Ground connections is usually nice to have when dealing with EMC. But what about EMC and a class 2 (double insulation) power supply (shall comply with MIL-STD-461E) ..... I'm thinking on, what to do with the regularly Y-caps, does it really have any meaning with these caps when ground is absent? Best regards Amund Westin, Oslo/Norway ------------------------------------------- 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: Ron Pickard: [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: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
