Download Heatsink calculations

 Heatsink calculations
Mounting a power resistor on a heatsink offers several design advantages over non heatsink mounted
devices.
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Low inductance compared to power wire-wound resistors.
As the parts are attached to a heatsink there is a reduced susceptibility to vibration.
Good thermal management leading to reduced heating effects on neighbouring components
PCB space saving.
Working out the heatsink requirements for power resistors is relatively straightforward. Careful
design and testing is needed to make sure the case temperature does not exceed its rating. The
allowable power rating (Pd, Watts) may be determined as follows:
Pd = (TRmax –TA) /(RθJC + RθCS + RθSA)
Where: TRmax is the maximum temperature for the resistor.
TA is the ambient or temperature reference
RθJC is the thermal resistance-resistor element to resistor surface
RθCS is the thermal resistance-ceramic element to heatsink surface (pad or thermal grease)*
RθSA is the thermal resistance of heatsink to ambient.
RθCS + RθSA = ((TRmax –TA) / Pd) – RθJC
Example of selecting a heatsink for a power resistor
Using a power resistor with a RθJC of 4.17°C/W at an ambient temperature of 25°C with a power
dissipation of 8 Watts, and a maximum resistor temperature of 150°C the total allowable thermal
resistance for the heat sink and the attachment interface is as follows:
Maximum total Rθ of the heat sink and interface =
((150°C – 25°C) / 8W) – 4.17°C/W ((150°C – 25°C) / 8W) – 4.17°C/W = 15.63°C/W – 4.17°C/W = 11.46°C/W
If 1°C per watt is allowed for the interface, then a heatsink of approximately 10.5°C/W is required.
Choosing a power resistor for your heatsink
If a heatsink is available and the designer needs to put the resistor on it, the calculation to determine
the power in a power resistor is as follows:
Pd = (TRmax –TA) / (RθJC + RθCS + RθSA)
However the quick way of doing both calculations is to use the calculators on the Rhopoint
Components website:
Maximum resistor power for given heatsink
Heatsink size calculator
www.rhopointcomponents.com Techtorial 10 – 2MT10.1