How to choose a proper Heat Sink ?

Heat sinks are an important element in circuit design because they provide an efficient path for heat to be transferred into the ambient air and away from electronic devices (Solid State Relays).

For any electronics design, “Thermal Management” is the key element, which decides the optimal performance and efficiency of the system during its entire product life. This article will give more insights into the Selection of Heat Sinks.

 celduc’s range of Heat Sink or you own Heat Sink ?

You have 2 possibilities :

1. Use celduc’s range of heat sinks : on our products data-sheets, thermal curves give thermal resistance values and below these curves you can find the part numbers of the celduc’s heatsinks having these resistance values.
   
2. Use your own Heat Sinks solution : If you want to use your own heatsink you need to ask yourself the right questions in order to make the proper choice. Thermal resistance values of celduc’s heatsinks are measured by our R&D department making tests in our lab. It is very important to understand that the values given in the product thermal curves are those from heatsinks we have qualified with the condition of use of our products. The given thermal resistance is therefore a real value that guarantee an optimal Thermal Management. If you want to install our Solid State Relays on your own heatsink, you have to choose a heatsink having the correct thermal resistance value. The main problem is that thermal resistance values of the heatsinks you can find on the market should be interpreted with caution. In fact the calculation of the thermal resistance value depends on a lot of parameters. Here are the 2 main difficulties you will need to face with the values you will get from Heat Sinks manufacturers:
   • No information about thermal resistance versus the power dissipation
   • Thermal resistance value given by Heat Sinks manufacturers is calculated by the total surface, means the heat-producing device (SSR) should have the same dimensions than the Heat Sink.

   This article will give more insights into the Selection of Heat Sinks so you can make the proper choice. First let’s start with some reminders.
   

What are the major Heat Sink Design Consideration factors ?

 Thermal resistance versus the power dissipation

The most important information is the thermal curve giving the thermal resistance depending on the power dissipation. You can see Thermal resistance change with the power dissipation.

Thermal resistance of an heatsink depends on the power you are applying to.

 Impact of forced ventilation in electrical cabinets

Another curve gives you information about the impact of  forced ventilation inside your electrical cabinet. There are basically two types of air cooling: forced air cooling and free (natural) convection cooling. For both types of cooling, the thermal resistance of the heatsink (Rth) is optimized.

 Some reminders

Power Dissipation = PD = Power dissipated by the relay. Expressed in Watts (joules per second), corresponds to the thermal current or the calories to be discharged from the relay, mainly depends on the current crossing the relay.

Thermal resistance = Heat sinks are rated by their thermal resistance and measured in degrees Celsius per watt (°C/W). The lower the thermal resistance, the better the heat sink will transfer heat. Thermal Resistance = Temperature/Power Dissipation

 You have to go further than a simple selection with thermal resistance value

 Thermal resistance value of the heatsink

The necessary thermal resistance value of the heatsink could be define by calculation or using the curves given by celduc. In fact on our products data-sheets, thermal curves give thermal resistance values of the heatsink required for your application. It makes things more simple !

It is very important to understand that if you bases your choice for the heatsink on the value of thermal resistance we advise, this is not simple and a source of confusion. You need to take the below factor sinto consideration.

Important to know Thermal resistance versus the power dissipation

When visiting heatsinks manufacturers websites, you will find thousands of heatsinks with different shapes, sizes, parameters, …. Heat sinks generally are aluminum alloy extrusions often with fins that increase surface area and allow airflow.

Most of the time, there is no curve indicating the thermal resistance versus the power dissipation… An additional simulation should give you the real thermal resistance value depending on the power dissipation. You require a special software for this.

Example :

Let’s take as an example a Heat Sink given for 1K/W. An addition simulation will point out that  1K/W is reached with 100W. But, with 1K/W at 40°C, our SO965460 (50A SSR) dissipates 55W for 45A. By using this heatsink, its real thermal resistance would be 1.25K/W only, not 1K/W as you guess.

 Caution : Thermal resistance value is calculated by the total surface

Additional to the lake of information concerning the power dissipation, heatsinks manufacturers don’t specify that the thermal resistance value is calculated by the total surface. This is due to the general theory behind a heat sink which is to increase the surface area of the heat-producing device, enabling a more efficient transfer of heat into the ambient environment. It means that you need a Solid State Relay that has the same dimensions than the heatsink. Of course, it is often not the case.

The below 2 simulations show you the temperature sharing / variation on 2 different heat sink configurations :

CASE 1 : Heat Sink’s size is 45 x 200mm

In black you have the surface area of the heat-producing device = SSR

èThe simulation shows a temperature gradient, between the top and the bottom, of 10K. The thermal conduction is limited and therefore the good choice would be a heatsink having its surface twice that of the relay and spread around it.

If you make the calculation of this temperature elevation of 71°C/W with the power dissipation of your SSR, you will realize that the thermal resistance value is not the one announced by the heatsink manufacturer, so no more suitable to your application.

CASE 2 : Heat Sink’s size is 90 x 100mm

Temperature variation is lower so this heatsink shape is more adapted and allows a better temperature sharing.

Conclusion :

Making the selection of the right heatsink is not easy as it depends on the configuration. Lots of parameters should be taken into consideration.

Celduc’s advises :

• Take some margin : for 1K/W, select a 0.8K/W heatsink
• Select a proper shape, adapted to the size of our Solid State Relays : the more you will have a distance between the relay and the top of the heatsink, the less is the performance. This is due to the conduction / temperature sharing on all the heasink
• The best is to get a surface around the relay which is almost the double of the size of the relay in order to have no loss of power dissipation capability
• For a better selection, the best is to ask the manufacturer to make simulations. We can also do this and even perform tests in our lab but all of this will never replace the test in real conditions.

Additional Resources :

Thermal Interface Materials (TIMs) play an important role in transferring heat more efficiently and consistently between two surfaces. https://www.celduc-relais.com/en/thermal-interface-materials/