Nowadays, with the electronic device dimension getting smaller and smaller, thermal management is more and more important in the design phase.

The component temperature is a key factor to the performance reliability of electronic devices as well as device life, keeping the component operation temperature within the range set by device designer can well realize the reliability and long life.

Heat sink is to help the component to transfer heat to the cooling ambient, so the device temperature will not exceed the maximum operating temperature.

Common terms:

We will talk about some common terms before talking about heat sink selection, it’s necessary for people who are not familiar with heat sink design. If you know quite well, you can pass this section and go to the next part.

Q: total power of heat dissipation, unit is W

Tj: maximum junction temperature of the device, unit is °C

Tc: package case temperature, unit is °C

Ts: heat sink temperature, unit is °C

Ta: ambient temperature, unit is °C

Fan PQ curve: unit is CFM, P is pressure, Q is air flow.

Heat sink dimension: L(Length)*W(Width)*H(Height)

Thermal Resistance:

When heat is transferred inside a device by heat conduction, the resistance encountered is called thermal resistance. The resistance encountered by heat in the heat flow path reflects the heat transfer capacity, indicating the size of the temperature rise caused by 1W of heat, in °C/W

Three important thermal resistance:

1.  Rjc: thermal resistance from transistor junction to case

2.  Rcs: thermal resistance from case to heatsink

3.  Rsa: thermal resistance from heatsink to ambient

The total thermal resistance from junction to ambient is Rja = Rjc + Rcs + Rsa

Do you need a heat sink?

It depends, if the transistor heat can dissipate to ambient successfully, then you do not need a heat sink, if not, then a heat sink is definitely needed.

For example, if the thermal resistance from junction to ambient is 62℃/W, and the transistor dissipates 2.86W, this means the temperature is 177℃. Assuming the ambient temperature is 48℃,  junction temperature will be 225℃, this exceeds the maximum silicon temperature 150℃, and will destroy the transistor, then a heat sink is required to help the thermal dissipation.

Define heat sink thermal resistance

Before starting to select a heat sink, you need to define the heat sink thermal resistance, Rsa=Rja ((Ts - Ta)/Q)-Rjc-Rcs.

The ambient air temperature Ta depends on the device operating environment, the interface resistance Rcs can get from material manufacturers or heat sink manufacturers.

Choose a Heat Sink

You need to know the air flow is natural or forced convection when choosing a heat sink:

Natural convection is the flow caused by the uneven temperature field of the fluid itself without relying on external forces such as pumps or fans.

Forced convection refers to the movement of fluid driven by external forces such as fans, pumps, suction, etc.

If you have limited space, you can go for a small sized heatsink and fan assembly. If your space is big enough, then you can choose a convection flow heat sink, it will be cheaper than heatsink + fan assembly.

Types of Heat Sinks

The most common heat sink types is following:

To sum up, when we choose a heat sink:
1.  We need to decide if we need a heat sink.
2.  Define the thermal resistance
3.  Choose a heat sink