From: "rene_debets" <[email protected]> Duane,
I have had good results splitting the power transistor into two lower wattage (and smaller) ones. I once designed a PCMCIA modem that had too much dissipation according to the rules when you include the power dissipated by the front-end. We ran some thermal tests on it together with our client Deutsche Telekom. The application was for leased line (PCMCIA form factor!) which means we couldn't count on the user typically using it for less than one hour. These things had to maintain a data connection all the time.Add to that the required MTBF and it starts to be amazing that it turned out to be not such a big problem. The difference to a regular application is that we had inefficient cooling, but probably not as inefficient as a laptop. For normal operation I think the front end will not get into current limiting very often. It will occur even less when more and more modern exchanges are installed. Does anybody know whether new exchanges in France have their own current limit or does France Telecom still count on the remote equipment to limit the current. Best regards, Rene Debets -----Original Message----- From: Duane J. Marcroft <[email protected]> To: [email protected] <[email protected]> Cc: Per-Olof Astrom <[email protected]> List-Post: [email protected] Date: Thursday, March 05, 1998 1:08 PM Subject: Re: 60 mA 2 W? Per-Olof, In my opinion and the opinion of other DAA designers the use of a pico fuse in any DAA design is not a good idea. The blown fuse is a hard failure. It's very hard to replace a fuse within a PC Card. A pair of low ohm metal film resistors in series with a & b, followed by a Sidactor will pass the surge test. Two of our PCMCIA DAA designs have passed TBR 21. But, careful attention must be paid to the 2W dissipation problem. The transistor we used was 20W @ 25 degree C and was chosen to fit inside the PC Card covers. Skimping on the transistor power dissipation will result in a certain percentage of field failures. Another caveat: The problem of current limiting is more complicated than just limiting at 60 mA. Regards, Duane From: "Per-Olof Astrom" <[email protected]> To: [email protected] Subject: 60 mA 2 W ? List-Post: [email protected] Date: Tue, 03 Mar 1998 13:02:48 CET MIME-Version: 1.0 >I have seen that you asked (Virtual Conference Hall, 22-Sep-96) a >question about dissipating 2W during 60mA. Have you got an answer, >cause that I have the same desing problem. My concirnings is if it is >possible to use a picofuse. >Do you have any suggestions, or comments? > >"Has the issue of limiting the current in the holding circuit to 60 mA >been addressed? The problem is dissipating 2W in a PCMCIA card during >current limiting." > > >Regards > >Per-Olof strm / TeleLarm Care AB >UmeB, Sweden > >______________________________________________________ >Get Your Private, Free Email at http://www.hotmail.com
