Multiple Heat Sources on PCB

Dissipated Power and Defining Heat Sources

Using Genie

Input Power versus Dissipated Power

For those that are new to thermal management, most devices have two power values associated with them: input power and dissipated power. Input power is the product of the input current and voltage to the device. This is what electrical engineers are generally most concerned with.

Dissipated Energy

For us thermal engineers, we’re more concerned about the dissipated power. Dissipated power is the waste heat that the device cannot convert from the input power to the action the device is conducting. Aavid Genie needs this input to estimate adequate heat sinks for your application. An example would be an LED producing light. The LED converts its input energy into light, but some of that energy is lost as heat. Some industries, like computing, report their dissipated heat as “Thermal Design Power.”

Efficiency of a Device

The percentage of the power effectively used by the device to function is called the efficiency of the device, and it can vary greatly from product to product. Some products can be low in the 10%-15% efficiency range like thermoelectric devices or 30% for LEDs. Occasionally, you may need to work your way backwards from from the output power and efficiency to get to the total heat load. Then you can input it into Aavid Genie.

For an example:

Your device may effectively be experiencing 100W but is only dissipating 80W as heat. The 80W is what the heat sink will need to dissipate. Your input into Aavid Genie should be 80W in this case. Our example 100W device that dissipates 80W has a 20% efficiency.

Total Load of Devices

Dissipated Power Input into Aavid Genie Screenshot

 
If you have multiple devices on your heat sink in Aavid Genie, add all of that dissipated heat as your Total Load of Devices.

Total Heat Source Size

You also have two options of what surface area of the heat sink base this heat is applied to. You can spread this heat across the entire base or you can designate a specific footprint size you want this heat load to be applied to. As with the total load, you can total up the surface area for multiple loads. Just use an equivalent surface area for the technology selection. You’ll be able to update the actual devices and sizes later.

Between these the total load and heat source size, Aavid Genie can produce a good estimation for the technology selection portion of your heat sink design project.

Total Heat Source Size Definition in Aavid Genie Screenshot

 

After you select your heat sink and flow definition, you’ll have the ability to place individual devices in the “Heat Source” page. You can drag, copy, and edit your heat sources to more closely match how your board or heat loads actually interface with your heat sink.

Thermal Interface

Heat Source Definition in Aavid Genie Screentshot

 
Speaking of interface, you can also add a thermal interface resistance from a thermal interface material (TIM). For those that live and breathe TIMs, Aavid Genie is working on expanding its powers to help you with TIM selection. But for now, Aavid Genie only has an input for interface resistance.

Heat Source Position in Aavid Genie Screenshot

 

To Wrap It Up

Take time to consider the actual dissipation power and the actual size of your devices. This will help Aavid Genie give you a better idea of what option to continue customizing.

Happy Designing!