Hi Richard,
I didn’t intend to suggest that the IC would be appropriate for this particular application – just providing a tip for general design purposes. As to the spacings, I’m making an assumption (oops!, there I go again) that if the power control IC’s are UL Recognized as supplementary protectors, the spacings must meet the requirements, at the rated voltage. Guess I should take my own advice and not make assumptions! Another concern would be if the IC can provide LPS, and what method from cl. 2.5 should be considered as providing LPS. As to functional insulation requirements, one could apply 5.3.4 c) 1), assuming that the PWB is V-1 or better. I’ll look into it a little more when I get a chance and update this thread then. Thanks for stimulating the old brain cells! Regards, Doug Massey, NCE Manager, Product Safety Engineering Advanced Compliance Solutions, Inc. Ph. (770) 831-8048 FAX (770) 831-8598 ****************CONFIDENTIAL**************** This e-mail and any attachments may contain information which is confidential, proprietary, privileged or otherwise protected by law. The information is solely intended for the named addressee (or a person responsible for delivering it to the addressee). If you are not the intended recipient of this message, you are not authorized to read, print, retain, copy or disseminate this message or any part of it. If you have received this e-mail in error, please notify the sender immediately by return e-mail and delete it from your computer. _____ From: [email protected] [mailto:[email protected]] Sent: Tuesday, December 21, 2004 3:30 PM To: [email protected]; [email protected] Subject: Re: USB interface Limited Power source ? Doug, I mentioned using the very small type of surface mount fuse because it should just about be possible to locate it within the metal EMC connector body and because it is a totally series component. As such, the fuse is contained in mini-fire enclosure and the remaining electronics is protected by LPS and so doesn't need a fire enclosure. Of course, this may not be easy to achieve, but who said life was easy? While the product you mention no doubt has many uses, it does connect across the supply. My concerns would be that a) the internal circuitry could short out the supply and that b) the separation between the pins may not meet the creepage and clearance or hipot requirements for Functional Insulation (and if they don't, it would be necessary to short them out - which in this application would be a real no-no). Finally, there's no way that this device and its associated circuitry would fit into the space occupied by a SMT fuse. Hence, FOR THIS APPLICATION, I think that a regular SMT fuse located right up at the power inlet would be the best bet if current limiting was the chosen compliance route. As you can guess, I agree with your CONCLUSION. Al the best, Richard Hughes In a message dated 12/21/2004 16:53:18 GMT Standard Time, [email protected] writes: Jim, I'll second that pet peeve! Here's another design for compliance tip for USB ports...An alternative to the use of PTC's for USB port current limiting, is the use of a USB port power controller IC, such as a Micrel MIC2012 http://www.micrel.com/ These are UL Recognized as Low Voltage Solid State Overcurrent Protectors, category QVGS2. Micrel makes many models to suit your specific needs, and I'm sure that other IC manufacturers make comparable parts. And, while we're on the topic of I/O ports, don't forget current limiting of DVI ports, PS2 ports, and non-standard implementations of RS-232 ports. VGA, parallel, and Ethernet ports typically do not provide 5v outputs directly >from the unlimited power supply rail. I have seen DVI ports that did not provide overcurrent protection; the output overload test was quite interesting - they passed the fault testing (the resulting smoke, heat, etc was contained by the fire enclosure and did not ignite the cheescloth), but of course, our recommendation always is to provide overcurrent protection /LPS, since you can't know for sure that all monitors connected to your DVI port will have a fire enclosure. I have more concern about a DVI port than a VGA port; flat screen monitors are often powered by a desktop adapter power supply, which, I assume, may have LPS outputs; therefore, the flat screen display may not need a fire enclosure. VGA monitors have built-in power supplies, so I would naturally assume that a fire enclosure encompasses the VGA monitor. Of course, making assumptions like these can get you into trouble!! NEVER ASSUME THAT THE OTHER GUY WILL MITIGATE THE POTENTIAL HAZARD FOR YOU!! Doug Massey, NCE Manager, Product Safety Engineering Advanced Compliance Solutions, Inc. Ph. (770) 831-8048 FAX (770) 831-8598 ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [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://www.ieeecommunities.org/emc-pstc
