I designed a heat sink for the first time. I am not sure if it is over designed or a well designed one?

I use IRFP460 in a two-switch forward converter. The maximum power of the converter is 1000 watts and the efficiency is 90 %. Each MOSFET dissipates 50 Watt. The maximum dissipation rating of the MOSFET is 280 Watts and the maximum Tj is 145 degree centigrade in its operating point. For the ambient temperature of 45 degree and the thermal resistance of the Tjc+Tcs=0.7 the maximum thermal resistance of the Tsa=1.3.

By the below link I need a heat sink with the below characteristics:
Aluminum material

  • height 4 cm
  • width 6 cm
  • length 6 cm
  • 10 fins; 2 mm thickness
  • 1 lit/sec fan

Is there any body to verify my design?

  • \$\begingroup\$ As the heatsink gets hotter so does Tj of the IRFP460 ,This will mean some increase in RDS on which will increase conduction losses which will increase heating .....There is at least one possibility of thermal runaway ,so unless you really know what you are doing then overdesign,Now double check your mosfet disipation , if you have 90% efficiency which is not unreasonable then you are blowing signifacant power in other parts like the output diodes ,transformer , Choke .I cant see each fet wasting 50W and 90% efficiency. \$\endgroup\$
    – Autistic
    Nov 14 '15 at 7:23
  • \$\begingroup\$ I had considered the dissertations in other devices and the extra power loss due to Rds increase. The 50 Watts of power is dissipated only in each mosfet at worst case. \$\endgroup\$
    – Rasool
    Nov 14 '15 at 9:57
  • \$\begingroup\$ I had mistake! By linear derating factor the IRFP460 can deliver 50 Watts at 102 degree centigrade of case temperature. So the maximum thermal resistance of case-heatsink can be 0.89 which may be implemented by a 6x6x4 cm^3 aluminium heatsink of 2 mm thickness, 10 fins and 4 liter per second fan. \$\endgroup\$
    – Rasool
    Nov 14 '15 at 10:54

You have to change the design of your heat sink or increase the capacity of the fan. Using the dimensions provided and the dimensions of a IRFP460, Tj = 163C in a 45C ambient based on calculations using the heat sink calculator (http://www.heatsinkcalculator.com). The Rcs (resistance case to heat sink) was estimated using a film of thermal grease of 0.1mm thick with a thermal conductivity of 1.35W/(m.K) with the contact area equal to the width X length of the IRFP460. This gives a Rcs value of 0.24 C/W.

Keep in mind that the maximum flow rating of a fan is based on no obstructions being in front of the fan. Once the fan is place behind the heat sink the flow provided will be less than the maximum flow. The exact value is dependent on the design of the heat sink and the fan flow curve.

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  • \$\begingroup\$ Great calculator. \$\endgroup\$
    – Rasool
    Nov 16 '15 at 20:09

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