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Posted on 13 December 2019

Cooling Methods for Power Semiconductor Devices

pool boiling

 

 

 

 

 

 

 

Cooling methods can be classified according to the mechanism or medium used to transfer the heat during the cooling process. A commonly used method of cooling power semiconductors is air cooling, which includes natural air cooling and forced air cooling. Another type of cooling is liquid cooling. Liquid cooling is often accomplished by use of water or a water/glycol mixture to perform thermosyphon cooling or forced cooling. Other agents can also be used for liquid cooling such as oil and several other inert fluids. Cooling can also be achieved by taking advantage of the heat transfer that occurs when materials experience phase transitions.

Air Cooling

Air is not an outstanding thermal conductor (air has a thermal conductivity of 0.026 W/mK). There are however advantages in using air as a cooling agent which include its universal availability, its ability to insulate, as well as its non-corrosive nature.

Natural Air Cooling

Natural air cooling

Figure 1. Natural air cooling

It is common knowledge that air rises as it is heated due to its resulting decrease in density (convection). The air flow resulting from this convection process is referred to as laminar flow. This process provides a natural means of removing heat generated by power electonics components.

Advantages of applying natural air cooling include low to no maintenance requirements, no resulting wear and tear, and no noise emission during application. The most prevalent disadvantage of using natural air cooling is that it results in very low levels of cooling.

Forced Air Cooling

Air blowers/fans are used in forced air cooling in order to increase the air velocity. This increased velocity aims to produce turbulent air flow rather than a laminar flow, effectively increasing heat disipation to the surrounding atmosphere.

The advantage of using forced air is that it has a far better cooling effect than natural air cooling. Disadvantages include the incredibly high amount of noise produced during application as well as resulting wear and tear.

Axial fanning and radial fanning are two types of forced cooling named and classified according to how the fan is mounted in relation to the device to be cooled.

Forced cooling using an axial fan

Figure 2. Forced cooling using an axial fan

Forced air cooling using a radial fan

Figure 3. Forced air cooling using a radial fan

In the examples above, the insulated structure of the modules makes it possible to connect all the semiconductors to the same heat sink. This results in a clear structure with simple bus-bars.

Liquid Cooling

A more effective method of cooling than air cooling is liquid cooling, which normally involves water or a water/glycol mixture as the heat transfering medium.

Turbulent Flow to Improve Thermal Efficiency

One way of increasing the thermal efficiency of a water cooling system is is by placing coils inside the cooling channel to induce a turbulant flow of the cooling liquid.

Use of coils in liquid cooling system

Figure 4. Use of coils in liquid cooling system

The turbulent flow created by the coils can increase the thermal efficiency of the cooling system by 15 - 20%.

Turbulent flow can also be created by the presence of microchannels in the cooling channel.

Use of microchannels in liquid cooling system

Figure 5. Use of microchannels in liquid cooling system

A disadvantage of this method is the high risk of channel blockage by particles in the cooling liquid.

Thermosyphon Cooling

In thermosyphon cooling, the transfer of heat is accomplished by the natural convection of water due to gravity. This results from the fact that heated water is less dense than the cooler water and therefore rises to the top of the cooling system causing natural circulation of the cooling liquid.

Thermosyphon cooling system

Figure 6. Thermosyphon cooling system

Advantages of using thermosyphon cooling is that a minimum amount of maintenance is required, no wear and tear is caused, and the cooling process produces no noise. The main disadvantages of this method is that the system must always be positioned in a vertical direction and requires a large amount of space.

Phase Transition Cooling

The enthalpy of vaporization is the amount of heat that must be absorbed by a given quantity of liquid in order to transition to the gas state. The opposite of this is referred to as enthalpy of condensation. The same amount of heat is used up or dissipated in each process respectively.

The cooling fluid evaporates at the location of the heat source. The vapor carries the heat to a condensor (which acts as a heat exchanger), where the fluid is then condensed back to its liquid form. The enthalpy of vaporisation of cooling liquids is high (>2000 kJ/kg).

Examples of application of this method of cooling include pool boiling, heat pipes, spray cooling, jet impingement cooling and vibration induced droplet atomisation (VIDA).

Pool Boiling

In pool boiling, the cooling medium evaporates at the heat source, gas bubbles rise and condense on the cooler upper surface.

pool boiling

Figure 7. Pool Boiling

At high heat stream density, a layer of vapor might build up at the heat source. This reduces or prevents contact between the heat source and the cooling fluid which inevitably leads to a great reduction in cooling. This is referred to as the Leidenfrost effect.

Heat pipes

A very useful method of transfering heat away from semiconductor devices is the use of heat pipes. Heat pipes also rely on natural forces to transfer heat. Heat pipes are made of hermetically sealed copper filled up with a small amount of fluid under low pressure. The inner part of the heat pipe is lined with a capillary-structured wick.

Liquid cooling with heat pipes

Figure 8. Liquid cooling with heat pipes

The cooling liquid is evaporated by the heat source at one end of the heat pipe. The vapor is transferred to the opposite end by convection where cooling fins are located. The vapor cools and condenses into liquid form and is carried back to the heat source through the capillary wick structure along the perimeter of the heat pipe.

To form the capillary structure of the heat pipe, a porous material is applied on the inner wall of the pipe. This can be done using either metal foams (such as steel, aluminium, copper or nickel) or using carbon fibres. Methods of creating the capillary structure include:

  • Using sintered powder (sintered wick) - offers the greatest cooling effect
  • Increasing the surface of the inner wall (grooved tube) - very weak capillary action
  • Screen mesh - most often used .

Different heat transfer media can be used within the heat pipes. The choice of medium depends on the application, in particular the required temperature range. Water, for instance, cannot be used below 0°C. Acetone or alcohol are commonly used.

Advantages of using heat pipes include:

  • Extremely high heat transfer ability (100 to 1000 times higher than copper at small temperature gradients
  • No parts need be moved mechanically, hence requires no maintenance
  • Heat pipes offer enough flexibility to be produced in all forms and sizes

Spray Cooling

Spray cooling and jet impingement cooling can be carried out either from the bottom or top or from both sides of the heat source. The cooling fluid vaporises upon coming into contact with the semiconductor chips and condenses when it reaches the cooler areas. Fluids used for spray cooling are mostly inert fluids such as fluorinert or other fluorinated hydrocarbons which cover a wide boiling point range. Conductive water can only be used when spraying on the bottom side of the module.

Spray cooling system

Figure 9. Spray cooling system

A great advantage of spray cooling is the fact that, upon spraying, the cooling fluid vaporizes directly on the chip. This leads to faster heat transfer. A big disadvantage, on the other hand, is the less than optimal thermal characteristics of the inert fluids. As an example, table one below shows a comparison of the thermal characteristics of fluorinert and water.

Comparison Fluorinert (FC - 72) Water
Latent heat of vaporisation 88 kJ/kg 2250 kJ/kg
Heat capacity 1.05 kJ/kg.K 4.18 kJ/kg.K

Table 1. Comparison of Thermal Characteristics of  Fluorinert and Water

Another disadvantage of spray cooling is high technical complexity. An large amount of equipment including pumps, spray pipes, condensors, storing tanks, microfilters, and a closed pressure system are required in order to carry out spray cooling. The process also requires very high pressures ranging from 3 bar to 15 bar. Wire bonds on the chip are also always in the way of the sprayed fluid, hindering optimum cooling of the chip. Also, due to their small diameter, there is always a chance that the spray pipes might become clogged. Cavitation of the pressure pump is also quite likely to occur.

Vibration Induced Droplet Atomization (VIDA)

Vibration Induced Droplet Atomization VIDA

Figure 10. Vibration Induced Droplet Atomisation (VIDA)

In vibration induced droplet atomization, the vibration of a Piezo membrane atomizes the cooling fluid. This atomized fluid then vaporizes at the hot areas under the chip. The vapor transfers the heat to the cooler walls of the cell where the fluid condenses and flows back to the Piezo membrane. A great advantage of VIDA is the high degree of cooling it provides and its very simple structure. On the other hand, VIDA is highly dependent on positioning.

Thermal-Acoustic Cooling

Thermal-Acoustic cooling is based on the principle of a Stirling machine that uses a loud speaker as a drive. The working medium is usually an environmental friendly rare gas such as Helium or Argon. This heat pump is powered by  very high acoustic pressure caused by resonant vibrations of the gas in the inner and outer board. Two cooling circuits (hot and cold) for heat transfer are required to complete the system. The advantage of using thermal-acoustic cooling is that there are no moving parts. Thermal-acoustic cooling however has lower than desirable effectiveness.

Cooling using the Peltier Effect

In this process, current flows through the connecting point between two different kinds of conductors (metal or semiconductors such as Bismuth Telluride Bi2Te3 ). This has either a heating or cooling effect depending on current flow direction (reversed Seebeck effect). Heat transfer occurs via electrons in n-semiconductors and via holes in p-semiconductors.

No noise is produced while cooling using the Peltier effect and the equipment used in this method requires no maintenance. Cooling using the Peltier effect, however, has very low cooling effect.

In conclusion, adequate cooling plays an important role in the reliability of power semiconductors and their lifetime. Choice of material as well as the structure of semiconductors determines the level of thermal resistance of any given semiconductor device. A wide range of processes with very different levels of effectiveness can be used for heat transfer in semiconductors.

 

For more information, please read:

Heat Transfer in Power Semiconductor Devices

Cooling Low Power Components

Heat Dissipation Using Cooling Plates

Heat Dissipation and Cooling for Aluminum Capacitors

 

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One Response

  1. avatar vinodhini says:

    good explanation.thanks

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