Hello Gary, You are correct. I reviewed the USB 3.0 specification again and found that Section 11.4.1.1.1 requires overcurrent protection with a rating of 5.0A maximum. The specification requires the host to detect an overcurrent condition and report it to the software. The specification does not state how that protection is to be implemented. The specification does state that the value of the overcurrent protector is to be no more than 5A, not that it must cut off before 5A. Many 5A overcurrent protectors will hold at higher currents, particularly at cooler ambient temperatures. I wouldn't put much faith in the 5A limit imposed by the USB specification.
Bostjan's original question was in regards to a USB output on a power supply. Maybe I have missed it, but I haven't seen the USB output on any power supplies that carry the USB logo. I doubt they could since the power supply typically won't handle the data aspects required for USB certification. However, in my opinion, other considerations would make it prudent to consider current limits well below 5A, including connector and cable ratings. Ted Eckert Compliance Engineer Microsoft Corporation [email protected] The opinions expressed are my own and do not necessarily reflect those of my employer. -----Original Message----- From: Gary Tornquist Sent: Monday, May 27, 2013 10:18 PM To: Ted Eckert; [email protected] Subject: RE: [PSES] USB output limitation I've understood from the USB 2.0 spec quoted below the maximum current provided should be no more than 5A. Has this changed? I've also noted that it isn't hard to find hubs that allow more than 5A to individual ports. Gary Tornquist Director of Product Safety MICROSOFT CONFIDENTIAL: This email message may contain confidential and proprietary information. Any unauthorized use is prohibited. If you are not the intended recipient, please contact the sender by reply email and destroy all copies of the original message. ******* 7.2.1.2.1 Over-current Protection The host and all self-powered hubs must implement over-current protection for safety reasons, and the hub must have a way to detect the over-current condition and report it to the USB software. Should the aggregate current drawn by a gang of downstream facing ports exceed a preset value, the over-current protection circuit removes or reduces power from all affected downstream facing ports. The over-current condition is reported through the hub to Host Controller, as described in Section 11.12.5. The preset value cannot exceed 5.0 A and must be sufficiently above the maximum allowable port current such that transient currents (e.g., during power up or dynamic attach or reconfiguration) do not trip the over-current protector. If an over-current condition occurs on any port, subsequent operation of the USB is not guaranteed, and once the condition is removed, it may be necessary to reinitialize the bus as would be done upon power-up. The over-current limiting mechanism must be ! resettable without user mechanical intervention. Polymeric PTCs and solid-state switches are examples of methods, which can be used for over-current limiting. -----Original Message----- From: Ted Eckert [mailto:[email protected]] Sent: Monday, May 27, 2013 4:43 PM To: [email protected] Subject: Re: [PSES] USB output limitation The USB specification gives a lower limit, defining the minimum current that must be available. However, it has no upper limit. In addition, the USB specification is written to ensure minimum functionality and interoperability. It does not directly address safety issues. Compliance with the specification means that you will have a product that can be eligible for the USB logo; it does not mean you will comply with IEC 60950-1. Conversely, it is quite possible to have a product that violates the USB specification yet meets all of the IEC 60950-1 requirements. Ted Eckert Compliance Engineer Microsoft Corporation [email protected] The opinions expressed are my own and do not necessarily reflect those of my employer. -----Original Message----- From: [email protected] [mailto:[email protected]] Sent: Monday, May 27, 2013 9:58 AM To: Ted Eckert; EMC-PSTC Subject: Re: [PSES] USB output limitation All, Regardless of a connector's ampacity, you should always refer to the USB specification and its limits. I cannot vouch for how much current a manufacturer may decide to put on the port, but if they exceed the USB standard, they are outside the spec. Conversely manufacturers sometimes provide dual USB ports, with the available current split between the two. http://www.usb.org/developers/docs/ On a side note, I have seen a number of manufactures "push" the specs, especially when used as cell phone chargers. They will push the upper tolerance of the voltage spec and make a claim of fast charging. Doug Douglas E Powell http://www.linkedin.com/in/dougp01 -----Original Message----- From: Ted Eckert <[email protected]> Date: Mon, 27 May 2013 14:59:41 To: <[email protected]> Reply-To: Ted Eckert <[email protected]> Subject: Re: [PSES] USB output limitation Hello Bostjan, There are a number of considerations regarding power from standard USB ports. There are USB ports designed to supply power over an additional set of contacts. I would recommend using one of these connector types where you intend to deliver more power than normally supplied by a USB port. The powered USB ports are designed for up to 30W at 5V. http://en.wikipedia.org/wiki/Powered_USB The standard USB connector will typically be designed for a maximum continuous current of 1.5A. Drawing more current through a standard connector may raise temperatures beyond what IEC 60950-1 would allow. The temperatures of the plastics of the connector body may exceed their RTI, or the temperatures of the circuit board supporting the connector might exceed the board's maximum rating. Although the USB standard doesn't limit the size of conductors in USB cables, the specification recommends making cable conductors as small as possible to enhance the flexibility of the cable. If you have a standard USB port, you have no control of what the user connects beyond instructional safeguards. The designers of many USB devices presume a limited current from USB ports. Their designs assume that under fault conditions, the USB port will limit the current enough that a short circuit in their device will not result in overheating or other issues. This could be a problem if such a device experiences a fault when connected to the USB port you propose. Ted Eckert Compliance Engineer Microsoft Corporation [email protected] The opinions expressed are my own and do not necessarily reflect those of my employer. -----Original Message----- From: Boštjan Glavič [mailto:[email protected]] Sent: Monday, May 27, 2013 5:30 AM To: [email protected] Subject: USB output limitation Dear experts, What would be the limit of USB output? I know IEC 60950-1 requires only that it complies with limited power source, but there might be some other requirements to limit power from USB? In particular, I am talking about power supply with USB connector. Can it go up to 5Vd.c./3A? Best regards, Bostjan Glavic SIQ - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/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]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/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]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/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]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/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]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
