PRISM Forum - Message Replies
Topic: PRISM Questions and Answers
Topic Posted by: SRC
Date Posted: Wed Jan 12 8:33:33 US/Eastern 2000
Topic Description: Welcome to the PRISM forum! Please feel free to post your questions and comments about the PRISM assessment software here.
Posted by: Thomas Urbanczyk
Date posted: Fri Jan 28 5:22:40 US/Eastern 2005
Subject: Model Parameters
Under "Item Properties" it is possible to select different Temperature Calculation models.
The model "use ambient temperature + actual temperature rise" allows the input of a temperature rise value. Is this temperature rise supposed to be junction temperature rise or case temperature rise?
Furthermore the model "use case temperature + theta jc * p" allows the input of a case temperature value. Does it mean the absolute case temperature or only the delta (relative) case temperature when compared to the chosen "op temp" temperature.
Subject: Temperature Rise
Reply Posted by: (firstname.lastname@example.org
Organization: Reliability Analysis Center (RAC)
Date Posted: Mon Feb 7 9:29:11 US/Eastern 2005
What is actually calculated is the Component Temperature. In the case of the "use ambient temperature + actual temperature rise" model, the use supplied “actual temperature rise” is added to the operational temperature to yield a temperature for the Component. In the case of the “use ambient temperature + theta ja * p” model and the "use case temperature + theta jc * p" model, the junction temperature is calculated and deemed a worst case temperature for the Component. In the case of the later, the “ambient temperature” is also being used, but was not labeled in the PRISM program. More importantly, the “case temperature” label is meant to indicate the temperature differential (delta ca) between the case and the “ambient temperature”. Typically this is calculated as Tja = Ta + (theta jc + theta ca) * P but at some point in the development of PRISM the decision to use the input of delta ca instead was made. This is legitimate as delta ca = theta ca * P.