1. Lecture-6 Thermal Design-2 Dr. Tahir Izhar
Power Electronics
Lecture-6
Thermal Design-2
Dr. Tahir Izhar

2. Thermal Design Example-1:
A certain power transistor dissipate 2 watts. The thermal resistance from junction to case is 8oC/W and case to ambient is 20oC/W. The free air temperature is 25oC. Calculate the junction temperature and case temperature.

3. Thermal Design
Solution-1

4. Thermal Design Example-2:
The maximum permissible junction temperature of a certain power device is 150oC. It is desired to operate the device at 15W in an ambient temperature of 40oC. Rjc = 0.5oC/W and Rca = 10oC/W. Determine whether a heat sink is required for this application. If sink is required, determine the maximum thermal resistance it can have assuming Rcs nearly zero.

5. Thermal Design Solution-2
197.5 will exceed the maximum limit of 150oC, therefore, heat sink is required.

6. Thermal Design
Solution-2
Set Tj to maximum value of 150oC.

7. Device Derating
The maximum permissible power dissipation of a semiconductor device is specified by the manufacturer at a certain temperature (either junction or case).
The maximum device power decrease as a function of temperature at elevated temperature.
The decrease in device power dissipation at elevated temperature is called derating.
Manufacturer also specify a derating factor in W/oC.

8. Derating
Example-3: Suppose a device has maximum power of 20 W at 25oC and a derating factor of 100mW/oC at temperature above 25oC. The maximum permissible dissipation at 100oC is Pd=20W-(100oC-25oC)X100mW/ oC Pd=20-7.5=12.5W

9. Derating
Example-4: The temperature of a certain semiconductor device is 100oC when it is dissipating 1.2W. The device temperature can not exceed 100oC. If the total thermal resistance from device to ambient is 50oC/W, above what ambient temperature should the dissipation be derated.

10. Derating
Solution-4

11. Thank you For your attention Questions?