1. Dr John Fletcher John.Fletcher@unsw.edu.au
Thermal Management
Dr John Fletcher

2. Thermal Management
Thermal management in power electronic converters is required to maintain semiconductor junction temperatures at design values.
Thermal management measures may include
Heatsinking
Forced-air cooled
Liquid cooling
Depending on the application and the environment that the equipment will be subjected to.

3. Thermal Basics
Heat energy can be dissipated by convection, conduction or radiation.
Conduction: Heat energy transfers from high to low temperatures in a material.

4. Conduction
The temperature rise is function of the thermal resistance of the material through which the heat conducts.
Where is the equivalent thermal resistance of the block and
This value sets the temperature rise across a given section of material for a given heat power.

5. Example - Conduction

6. Thermal Equivalent Circuits
Often heat flows through several different sections and materials. Each may have different areas, thicknesses and thermal conductivities. We can extend the resistance analogy to estimate temperatures in the system.

7. Thermal Equivalent Circuits
The chip is generating power loss, P, at the junction. What is the resulting junction temperature?
Thermal Equivalent Circuit

8. Thermal Equivalent Circuits
Therefore
Note: The heatsink thermal resistance includes the convection component from the heatisnk to ambient.
The thermal equivalent circuit procedure allows us to simplify steady-state thermal analysis to electrical circuit ‘analogies’.
Thermal power is the equivalent of current.
Temperature difference is the equivalent of potential difference.

9. Example
An IGBT in a power converter dissipates 40W of internal losses. The equipment must be capable of operating in ambient temperatures of between -40oC and 45oC. Calculate the required thermal resistance of the heatsink and the heatsink temperature. The junction temperature must be kept below 85oC.
The following information was extracted from the device date sheet

10. Solution

11. Using data provided:
With a heatsink of 0.4 K/W, the heatsink temperature can be calculated
This is as hot as is safe for an accessible heatsink.
NB: These calculations are only valid in the steady-state. Each element has some thermal capacity.

12. Heatsinks
A variety of shaped and sizes – in a few cases you may get away without a heatsink.
Eg A TO220 package has Rθ,ja = 65 K/W
[each watt of power dissipation raised the junction temperature by 65oC]
A TO220 clip-on heatsink has Rθ,sa = 30 K/W
A 5 K/W heatsink occupies a volume of 76 cm3. (4x4x5cm)
A 1.2 K/W heatsink occupies a volume of 640 cm3. (8x8x10cm)
A 0.5 K/W heatsink occupies a volume of 1700 cm3. (12x12x12 cm)

13. Forced-air cooling 0.5 K/W Compact volume but uses a fan.
Poor reliability

14. Liquid Cooling
Requires additional infrastructure: pipes heat exchangers, pump

15. Mounting Heatsinks
Are tabs on device isolated or are they connected to drain/source/cathode? May have to use isolating tab – increased thermal resistance.
If using isolating kit make sure that all burrs are removed from heatsink to avoid puncturing isolating pad.
Use heatsining paste to improve thermal conductivity between case and heatsink (if no isolating pad used).
Make sure heatsink is properly ventilated and fins oriented to achieve ‘chimney’ action.