What is a Heat Sink and its Importance

Every electrical and electronic component in a circuit generates some amount of heat while the circuit is executed by providing power supply. Typically high-power semiconducting devices like power transistors and the opto electronics such as light emitting diodes,lasers generate heat in considerable amounts and these components are inadequate to dissipate heat, as their dissipation capability is significantly low.

Due to this,heating up of the components leads to premature failure and may cause failure of the entire circuit or system’s performance. So,to conquer these negative aspects, heat sinks must be provided for cooling purpose.

What is a Heat Sink?

Heat sink is an electronic component or a device of an electronic circuit which disperses heat from other components (mainly from the power transistors) of a circuit into the surrounding medium and cools them for improving their performance, reliability and also avoids the premature failure of the components. For the cooling purpose,it incorporates a fan or cooling device.

Heat Sink Principle

Fourier’s law of heat conduction states that if temperature gradient is present in a body, then the heat will transfer from a high-temperature region to allow- temperature region.And, this can be achieved in three different ways,such as convention, radiation and conduction.

Whenever two objects with different temperature come in contact with each other,conduction occurs causing the fast-moving molecules of the high-heat object to collide with the slow-moving molecules of the cooler objects, and thus, transfers thermal energy to the cooler object, and this is termed as thermal conductivity.

Similarly, heat sink transfers the heat or thermal energy from a high-temperature component to a low-temperature medium like air, water, oil, etc.Usually air is used as a low-temperature medium; and, if water is used as medium, then it is termed as cold plate.

Heat Sink Types

The heat sinks are classified into different categories based on different criteria. Let us consider the major types, namely active heat sinks and passive heat sinks.

Active Heat Sinks

These are generally fan type and utilize power for cooling purpose.They can also be termed as Heat sink or fans. The fans are further classified as ball bearing type and sleeve bearing type. The ball bearing motor fans are preferred as their working span is longer and they are cheaper when it comes to a long span usage.The performance of these kinds of heat sink is excellent, but not for long term applications as they consist of moving parts and are a bit expensive as well.

Passive Heat Sinks

These do not possess any mechanical components and are made of aluminum finned radiators. These dissipate thermal energy or heat by using the convection process. These are most reliable than the active heat sinks; and, for efficient operation of passive heat sinks, it is recommended to maintain continuous air flow across their fins.

Aluminum Heat Sink

Heat sinks are generally made of metals; and, aluminum is the most common metal used in heat sink.We are aware of the fact that the thermal conductivity of each metal is different.The thermal conductivity of metal is proportional to the heat transfer in heat sink. Thus, if the thermal conductivity of the metal increases, then the
heat transferring capacity of the heat sink will also increase.

The thermal conductivity of the aluminum is 235 W/mK; it is the cheapest and lightweight metal. Aluminum heat sinks are also called as extruded heat sinks as they can be made using extrusion.

Stamped Heat Sinks

These are made of metals that are stamped to form a particular shape. This stamp creates the heat sinks whenever metal is moved through the stamping machine. These are cheaper compared to extruded heat sinks.
These are used for low-power applications and hence these are low in performance.

Machining Heat Sinks

These are manufactured by machining process; frequently gang saw is used for removing a block of material to make inter fins with precise spacing. These are expensive as a lot of metal may go as wastage in the manufacturing process.

Bonded-Fin Heat Sinks

These are frequently used for physically large applications that require sensible performance like electric welding and DC-DC brick applications. These are made by bonding individual fins of a metal to the base of a heat sink. This can be done in two methods namely thermal epoxy which is economical and the other is by brazing which is expensive.

Folded-Fin Heat Sinks

These folded-fin heat sinks have large surface area,and possess folded heat sink material, and hence, they have very-high performance and very-high heat-flux density. In these sinks,air is directed to flow directly into heat sinks through some kind of duct. This makes the whole thing expensive as the cost of manufacturing and ducting is included in the overall cost of the sink.

Skived Heat Sinks

Skiving process is used for manufacturing these sinks, which involves making very fine blocks of metals generally copper. Hence, these are called as skived heat sinks. These are medium to high-performing heat sinks.

Forged Heat Sinks

The metals like copper and aluminum are used to form heat sinks by using compressive forces. This process is termed as forging process. Hence, they are named as forged heat sinks.

Single Fin Assembly Heat Sinks

These are light weight and can be installed in tight spaces. They also possess low to high performing capability, and can be used for many applications. But major drawback is that they area bit expensive.

Swaged Heat Sinks

Swaging is a cold working forging process but sometimes can be done even as hot working  process in which the dimensions of an item are altered into a die. These are inexpensive, medium performing and are limited in air-flow management.

Importance of Heat Sinks in Electronic Circuits

• A heat sink is a passive heat exchanger, and it is designed to have large surface area in contact with the surrounding (cooling) medium like air. The components or electronic parts or devices which are insufficient to moderate their temperature, require heat sinks for cooling. Heat generated by every element or component of electronic circuit must be dissipated for improving its reliability and preventing the premature failure of the component.
• It maintains thermal stability in limits for every electrical and electronic component of any circuit or electronics parts of any system. The performance of the heat sink depends on the factors like the choice of a material, protrusion design, surface treatment and air velocity.
• The central processing units and graphic processors of a computer are also cooled by using the heat sinks. Heat sinks are also called as Heat spreaders, which are frequently used as covers on a computer’s memory to dissipate its heat.
• If heat sinks are not provided for electronic circuits,then there will be a chance of failure of components such as transistors, voltage regulators, ICs, LEDs and power transistors. Even while soldering an electronic circuit, it is recommended to use heat sink to avoid over heating of the elements.
• Heat sinks not only provide heat dissipation, but also used for thermal energy management done by dissipating heat when heat is more. In case of low temperatures, heat sinks are intended to provide  heat by releasing thermal energy for proper operation of the circuit.

Selection of Heat Sink

For selection of Heat sink we need to consider the following mathematical calculations:

Consider

Q: Heat dissipation rate in Watt

T_j: Maximum junction temperature of device in 0C

T_c: Case temperature of device in 0C

T_a: Ambient air temperature at 0C

T_s: Maximum temperature of heat sink located utmost neat to the device in 0C

Thermal resistance can be given by

R=∆T/Q

Electrical resistance is given by

R_e=∆V/I

The thermal resistance between the junction and case of the device is given by

R_jc=(∆T_jc)/Q

Case to sink resistance is given by

R_cs=(∆T_cs)/Q

Sink to ambient resistance is given by

R_sa=(∆T_sa)/Q

Thus, junction to ambient resistance is given by

R_ja= R_jc+R_cs+R_sa=(T_j-T_a)/Q

Now, required thermal resistance of Heat Sink is

R_sa=(T_j-T_a)/Q-R_jc-R_cs

In the above equation the values of T_j,Q and R_jc are fixed by the manufacturer and the values of T_a and R_cs are user defined.

Thus, thermal resistance of the heat sink for application must be less than or equal to the above calculated R_sa.

While selecting the Heat sink, various parameters are to be taken into consideration such as the thermal budget allowed for heat sinks, air flow condition (natural flow, low flow mixed, high flow forced convection).

The volume of the Heat sink can be determined by dividing the volumetric thermal resistance by the required thermal resistance. The range of volumetric thermal resistance is as follows in the table shown below.

The below graph shows the variation in size of aluminum heat sink and thermal resistance as an example of choosing the heat sink based on the thermal resistance.

 

This article discusses about heat sink, different types of heat sinks and importance of  heat sink in electronic circuits in brief. For more information regarding the heat sinks, please post your queries by commenting below.

Photo Credits:

• • Heat sink by ecnmag
  • Heat Sink Principle by streacom
  • Heat Sink Types by electronics-cooling
  • Aluminum Heat Sink by specialchem4polymers
  • Area vs Thermal Resistance of Heat Sink by designworldonline