Kris, Most USB ports employ a PTC device as the over-current protection device, for their self-resetting property. But of course, you can't really ASSUME anything.
Most PTC's are UL Recognized, and comply with the IEC 60730-1 requirements listed in IEC 60950-1 cl. 2.5, 2nd bullet (tests for endurance and manufacturing deviation and drift). So while you can't assume compliance, one can generally show that the circuit is LPS by examination of the schematic and component data, and not need to perform the test. UL will generally accept PTC's that are Recognized without performing the test, unless their I-trip rating is 8.0 A, in which case they will want to perform the test, since the device rating is right at the 8 A limit of Table 2B; most designers use a lower value PTC on USB ports, I assume to comply with the USB specs you mentioned (most use a hold current rating of 2.5 A, and a trip current rating of 5 A). The PTC should also be able to interrupt the maximum fault current that is available to the USB circuit (I-max) from the power supply. PTC's also have temperature and voltage maximums that must be considered. Of course, the LPS test is pretty simple and quick for a USB port, so why not go ahead and run it? If there is a PTC, you don't need to do the test under fault conditions, so it only takes a couple of minutes to do. UL's online certifications database carries a wealth of information on PTC's that they have evaluated; the category is XGPU2. Be sure to check the guide info for that category, so you'll know what all of the parameters mean. The one bit of information that they will not have in their database is whether or not the component meets the IEC 60730-1 requirements; you'll have to get that info from the product specs, or from the certification from another agency, like TUV, who uses the IEC 60730-1 standard to evaluate the PTC. I don't know whether there is correlation between UL Recognition of a PTC device, and the requirements of IEC 60730-1. If the schematic is not available for some reason, it's pretty easy to verify that a PTC is being used, by performing the test. What you'll see on your ammeter, as you decrease the load towards zero, is that a PTC will pass current up to and beyond the I-hold rating, but trip at the I-trip rating; remove the load for a couple of seconds, do the test again, and see that it does the same thing - there is obviously a resettable device in the circuit, instead of a fuse. Of course, the PTC is a safety-critical component, so you will need to have the specs and certifications for the PTC in order to get an agency certification for the product. The reason for the current limit on USB ports is because the manufacturer of the product incorporating a USB port has no way of knowing what peripherals will be attached, and what the flame rating of the plastics on that peripheral will be; so by limiting the available energy on the output to LPS levels, we are assured that if a fault occurs in the peripheral, the energy will not be sufficient to cause ignition. I've never ran across a USB port that did not use a PTC device, but I don't think it's safe to assume that a designer did not make a mistake or oversight. Regards, Doug Massey, NCE Manager, Product Safety Engineering Advanced Compliance Solutions, Inc. From: [email protected] [mailto:[email protected]] On Behalf Of Carpentier Kristiaan Sent: Friday, December 17, 2004 6:23 AM To: ieee pstc list Subject: USB interface Limited Power source ? Hi group, The USB standard 1.1 or 2.0 defines that a USB host device must have an overcurrent protection " for safety reasons". If the aggregate current exceeds a preset value, which is 5.0 A, then the over-current protection circuit shall remove or reduces power to the attached devices. The above definition looks quite similar to the requirements of LPS in IEC60950-1 but the wording LPS is not explicitely mentionned in the USB spec. Q: Can I assume that a USB interface is LPS ? Is there any other evidence available to support this view ? Thanks, Kris 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 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
