On 3/17/06, Russell McMahon wrote: > >> If there IS a fundamental package limitation its > >> Tjmax-Tcase)/Tj-c - > >> (Tc-25) x 1.5 > > > Which is the switching time ? > > Else how could you compute the power dissipated on this device ? > > I know you know this. Obviously just a communications anomaly. > > What I was looking at was a claimed "package limitation" for the > (presumably) TO247. Russel, let me tell you from my first 5 working years experience in 6kV to 30kV plant as autorised electrician. The limitation of this package are just the terminals. This transistor is able to switch 90A short pulses, but will not be able to drive continuous DC 90A. Compare the resistance of drain and source terminals (the wires comming from the package) with the internal Rds in conduction, and see which must be the current density through terminals. Take a normal density of 5 to 10A/mm2 (or change it for american upside down units) used for usual energy cables and see the discrepancy. The aplication was a twice a day switch from ON to OFF and viceversa, if I've understood well. The load is heavily inductive, so switching times of 20 to 500mS (or even more) are quite possible. For a 500mS switching time, this transistor will be blewed out even it will be mounted on infinite heatsink. That's why I've comment on this email question about the heatsink. For such applications, not the heatsink is the most complicated problem, but choosing of a device being able to switch the peak current of the load and to survive to the switching time (almost without heatsink or with a small one because switching occure just twice a day). When is turned ON this device will dissipate almost nothing (except the terminals and Rds heating). best regards, Vasile -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist