The rise in temperature between the operating temperature of the assembly and the location specified for thermal resistance units. The operating temperature is initially defined in the Calculation Data pane when the assembly is inserted. Normally, the operating temperature is the ambient temperature of the surrounding air, but it can also be a board temperature, the temperature of the heat sink rails, or any other specified temperature.

Assume that the operating temperature is the ambient air temperature and that the location for thermal resistance units is the case. The initial temperature rise is the difference between the air temperature and the case temperature. Because the temperature rise is quite often negligible, especially for low power devices, this field is often left blank. If no value is entered in this field, the initial temperature rise is assumed to be 0 in all temperature calculations.

The power dissipation of the device in watts, which is normally available in the manufacturer's databook. Be sure to use the actual rated (rather than maximum rated) power dissipation. For integrated circuits and semiconductors, operating power can generally be calculated as:

Operating Power = Icc * Vcc + Iol * Vol

Where each of the parameters above are found in the databook tables.

The resistance to the movement of temperature between the junction and the location specified for thermal resistance. When a device is operating, it generates heat at its junction. This heat is transmitted through the device to its surface and is then dissipated into the air (or into a heat sink). The lower the thermal resistance, the easier it is for the heat to escape or dissipate. Thermal resistance, which is often referred to as ThetaJC, is generally found in the manufacturer’s databook.

The location at which thermal resistance values are recorded. This field is not used in any temperature calculations. It is a five-character text field that is used for informational purposes only. Thermal resistance can be specified as Junction-Case, Junction-Air, Junction-Board, or whatever. If thermal resistance is being specified as Junction-Case, entering Case in this field points out that the thermal resistance value is the actual resistance to temperature movement between the device junction and the device case. Because consistent use of the same location for all thermal resistance values is assumed, you do not have to use this field to specify the location. You can use this field to enter any information you want.

The rise in temperature between the operating temperature and the junction. When this field is left at #.#, the temperature rise is calculated using the values specified for the initial temperature rise, power dissipation, and thermal resistance:

Temperature Rise = Initial Temperature Rise + Power Dissipation * Thermal Resistance

If you enter a value for this parameter, this value is used in place of a calculated temperature rise. To calculate the junction temperature of the device, the temperature rise (whether calculated or entered) is added to the operating temperature. If you prefer, you can directly enter the value to use for the junction temperature for Junction Temp Override.

For components inside a hybrid, if the temperature rise from case to junction cannot be determined, use the following defaults:

The maximum junction temperature, which is normally available in the manufacturer's databook. For silicon integrated circuits, the stated maximum junction temperature is normally 175 degrees C. The device is flagged as overstressed if the sum of the temperature rise and subassembly temperature is greater than the maximum junction temperature.

Generally this field is left at #.# so that the junction temperature is calculated. If you enter a value for this parameter, this value is used in place of a calculated junction temperature. (Perhaps you want to use the temperature value obtained from an infrared scanner or a thermal engineer.) Using this method, changes to the subassembly temperature or to factors contributing to the temperature rise are not taken into account.

The minimum rated lower limit operating temperature for a semiconductor. This field is shown for several semiconductor subcategories when the 299B or 299C model selected is selected. It is not required by the 299C model when Foreign is selected.