This calculator helps determine the maximum temperature a device can withstand

Inputs

°C
°C
°C/Watt
°C/Watt
°C/Watt

 
 

Outputs

Watts
°C

Overview

This tool is designed to calculate junction temperature of an electronic device (typically power devices) given four parameters: the maximum ambient temperature, the device's junction-to-package thermal resistance, the thermal resistance of the heat sink, and the power applied. It can also calculate the maximum power that the device can handle given its junction temperature, the maximum ambient temperature, the junction-to-package thermal resistance, and the heat sink's thermal resistance. 

Equations

$T_{J}= P(R_{case}+R_{1}+R_{2})+T_{a}$

Where:

$T_{J}$ = junction temperature

$P$ = power dissipated

$R_{case}$ = thermal resistance of device junction to case

$R_{1}$ = thermal resistance of device junction to air (if no heat sink) or thermal resistance of heat sink

$R_{2}$ = thermal resistance of device junction to air

 

Applications

The highest operating temperature that a semiconductor in an electronic device can withstand is called its junction temperature. This temperature is commonly higher than the temperature of the device's exterior and its case. The heat transferred from the junction to case, multiplied by the junction-case thermal resistance, is equal to the difference between the junction temperature and the outside, plus, case temperature. 

The maximum junction temperature of a device is always found on its datasheet. It can be very useful when there is a need to compute for the required case-ambient thermal resistance, given the amount of dissipated power. The value of the maximum junction temperature is then used for choosing the right heat sink. 

Microprocessors often measure the temperature of its core through a sensor. When the core reaches its maximum junction temperature, a cooling mechanism is triggered. Also, if the temperature goes beyond the maximum junction temperature, an alarm will be triggered that warns the computer operator to discontinue the process that is causing the overheating of the processor's core. 

Notes

Typical values of Thermal Resistance for Various Electronics Packages

PackageJunction to Case (°C/Watt)Junction to Air (°C/Watt)
TO-3560
TO-3912140
TO-220362.5
TO-220FB350
TO-22330.653
TO-252  592
TO-263 23.550
D2PAK435

Thermal Resistance for PCB Copper

Heat SinkThermal resistance (°C/Watt)
1 sq inch of 1 ounce PCB copper 43
.5 sq inch of 1 ounce PCB copper 50
.3 sq inch of 1 ounce PCB copper 56
Aavid Thermalloy, SMT heat sink:  PN:573400D0001014