DirectFET® Rating Calculator

The DirectFET® Rating Calculator, shown next, uses the equations generated in the thermal circuit analysis and is designed to allow for fast and accurate rating calculations with a simple tabular parameter entry format.

DirectFET® Rating Calculator - notes on its use
1. For high frequency applications use the "maximum permitted power" box.
2. For low frequency applications, where the switching losses are not significant, enter the worst-case Rdson and use the "maximum permitted current" box.
3. Substrate thermal resistance values may be obtained from manufacturer's material data but must be assessed for the correct device footprint size, board thickness and power rating.
4. Heat sink thermal resistance may be taken from manufacturer's curves - include mounting interface Rth as appropriate.
5. Where chassis or case cooling to the "Can" is used, figures in Appendix "B" may provide some guidance. For cases where there is no additional heat sinking applied to the "Can", Appendix "C" provides guidance for the two "Can" sizes currently available.
6. With fan or forced cooling, Rth values tend to be constant irrespective of power. This makes easier use of the DirectFET® Rating Calculator. Where however Rth varies considerably with power, estimate the relative power flow through "Can" and substrate and run the calculator. With that result, review the resulting power flow, check if thermal resistance values are still correct for those power levels- if not, adjust, re-run calculation and re-check. Eventually, the power flows through substrate and heat sink will align with their thermal resistances for those particular power flows.

An example of this procedure is given in Appendix "D".

Return to Application Note 1059 - DirectFET® Thermal Model and Rating Calculator