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.
"Can" size R1 R2 R3
Small (SB) 2.68 2.47 1.05
Small (SC)      
Small (SH) 2.96 3.48 0.98
Small (SJ) 2.05 2.22 0.98
Small (SQ) 2.43 3.48 0.98
Small (ST) 2.36 2.58 0.98
Small (S1) 4.18 3.43 1.53
Small (S2) 2.35 4.55 1.60
Medium (MA)      
Medium (MN) 0.91 0.97 0.80
Medium (MP) 2.26 2.58 1.54
Medium (MT) 0.71 0.97 0.80
Medium (MX) 1.04 1.18 0.98
Medium (MZ) 1.62 0.97 0.80
Medium (M2) 2.09 1.03 1.33
Medium (M4) 1.27 0.68 0.80
Large (L6) 0.80 0.44 0.56
Large (L8) 0.65 0.25 0.49

Required Inputs
Thermal Parameter "R1" oC/W
Thermal Parameter "R2" oC/W
Thermal Parameter "R3" oC/W
Rth substrate-ambient oC/W
Rth CAN-ambient (or heat sink) oC/W
Ambient temperature oC
Maximum Junction temperature oC
Hot Rdson (at Tj max.) Ohm

Maximum permitted power W
Maximum permitted current
(Low frequency only)
Power through substrate W
Power through heatsink W

    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

Part Number / Can Size Matrix

Part Number
"Can" size
IRF6613 Medium (MT)
IRF6614 Small (ST)
IRF6616 Medium (MX)
IRF6641 Medium (MZ)
IRF6643 Medium (MZ)
IRF6644 Medium (MN)
IRF6645 Small (SJ)
IRF6646 Medium (MN)
IRF6648 Medium (MN)
IRF6655 Small (SH)
IRF6662 Medium (MZ)
IRF6665 Small (SH)
IRF6668 Medium (MZ)
IRF6674 Medium (MZ)
IRF6691 Medium (MT)
Part Number
"Can" size
IRF6702M2D Medium (MA)
IRF6706S2 Small (S1)
IRF6708S2 Small (S1)
IRF6709S2 Small (S1)
IRF6710S2 Small (S1)
IRF6711S Small (SQ)
IRF6712S Small (SQ)
IRF6713S Small (SQ)
IRF6714M Medium (MX)
IRF6715M Medium (MX)
IRF6716M Medium (MX)
IRF6717M Medium (MX)
IRF6718L2 Large (L6)
IRF6720S2 Small (S1)
IRF6721S Small (SQ)
Part Number
"Can" size
IRF6722M Medium (MP)
IRF6722S Small (ST)
IRF6723M2D Medium (MA)
IRF6724M Medium (MX)
IRF6725M Medium (MX)
IRF6726M Medium (MT)
IRF6727M Medium (MX)
IRF6729M Medium (MX)
IRF6775M Medium (MZ)
IRF6785 Medium (MZ)
IRF6794M Medium (MX)
IRF6795M Medium (MX)
IRF6797M Medium (MX)
IRF6798M Medium (MX)
Part Number
"Can" size
IRF7665S2 Small (SB)
IRF7739 Large (L8)
IRF7749L2 Large (L8)
IRF7759L2 Large (L8)
IRF7769L2 Large (L8)
IRF7779L2 Large (L8)
IRF7799L2 Large (L8)
Automotive DirectFET®Part Number "Can" size
AUIRF7640S2 Small (S2)
AUIRF7647S2TR Small (SC)
AUIRF7648M2TR Medium (M4)
AUIRF7665S2 Small (SB)
AUIRF7669L2TR Large (L8)
AUIRF7675M2 Medium (M2)
AUIRF7736M2TR Medium (M4)
AUIRF7737L2TR Large (L6)
AUIRF7739L2 Large (L8)