AN-1059 Datasheet, PDF (5/11 Pages) International Rectifier – Application Note

Rating Calculator
The DirectFET Rating Calculator is available at:
www.irf.com/product-info/hexfet/thermalcalc.html
The Rating Calculator applies the equations from
Appendix A. It uses a simple tabular entry format to
facilitate fast and accurate calculations.
Notes on use
ƒ ^{For high-frequency applications, use the value}
returned for maximum permitted power.
ƒ ^{For low-frequency applications (where switching}
losses are not significant), enter a worst-case
value for R_DS(on) and use the value returned for
maximum permitted current.
ƒ ^{Obtain values for the thermal resistance of the}
substrate from the manufacturer’s material data
but make sure it is correct for the device footprint,
board thickness and power rating.
ƒ ^{Obtain values for the thermal resistance of the}
heat sink from the manufacturer’s curves – include
the mounting interface as appropriate.
ƒ ^{Appendix B gives guidance for applications where}
the can is cooled through the chassis or case.
ƒ ^{Appendix C gives guidance for small and medium}
can devices that have no additional heat sinking.
ƒ ^{Where fan or forced cooling is used, thermal}
resistance tends not to vary with power. This
makes it easier to use the Rating Calculator.
ƒ ^{Where thermal resistance varies considerably with}
power, estimate the relative power flow through
the can and substrate. Run the Rating Calculator
and use the result to review the resulting power
flow, checking if thermal resistance values are still
correct for those power levels. If not, adjust the
inputs, repeat the calculation and check the power
flow again. Repeat until the power flows through
the substrate and heat sink align with their thermal
resistances for those flows. Appendix D
demonstrates this procedure.
Rating Calculator inputs
Enter:
ƒ ^{R1, R2 and R3 for the appropriate DirectFET}
device from the table on page 3
ƒ ^{Rth substrate-ambient for the appropriate cooling}
conditions and power (estimate the power flows
through this path for the first iteration)
ƒ ^{Rth heat sink if one is used, taking into account}
the cooling conditions and power flow (estimate
the power flows through this path for the first
iteration) – if no heat sink is fitted to the can, enter
the figures for Rth can-ambient from Appendix C.
ƒ ^{Ambient temperature of the environment}
ƒ ^{Maximum permitted junction temperature}
ƒ ^{The hot R}DS(on) ^{of the device – enter this only if a}
low-frequency current rating is required.
Rating Calculator outputs
The Rating Calculator returns maximum current or
maximum power. If Rth substrate-ambient or Rth can-
ambient varies significantly with power, adjust the
values to reflect the actual power returned by the
Rating Calculator. Refer to the last of the notes on
using the Rating Calculator (left) and to Appendix D.
DirectFET^® Technology
www.irf.com
AN-1059
Version 3, September 2010
Thermal Model and Rating Calculator
Page 5 of 11