Input Power versus Dissipated Power

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

Dissipated Energy

Thermal engineers are more concerned about the dissipated power of a device. Dissipated power is the waste heat that the component cannot convert from the input power to the action the device is conducting. Any sort of thermal calculation needs this input to estimate appropriate heat sinks for your application, which includes Aavid Genie.

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 output power and efficiency to get to the total heat load. Then you can input this value into Aavid Genie or other thermal calculations.

For Example:

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

Total Load of Devices

If you have multiple devices, add each device’s dissipated heat together. You can use this value for estimating or within Aavid Genie as the Total Load of Devices parameter in Project Conditions.

Total Heat Source Size and Heat Density

While the total dissipated is great for rough estimations, the heat source size is also a critical aspect in how heat is applied to your heat sink or other thermal management solution. The heat source size, in addition to the dissipated power, defines the density of a heat load. Heat Density impacts how well a thermal management solution can handle the heat absorbed into the component.

High density heat sources have highly focused heat loads in smaller areas as opposed to a more spread out heat load. Most thermal management solutions rely on conduction to help move heat away from the heat load first, but if there is too much heat applied in a single area, it can accumulate and significantly raise the temperature of the surface and into the device. Like heavy traffic, it can pile up and back up for miles.

Applying Heat Source Size in Aavid Genie

In the Project Conditions of Aavid Genie, you can spread this heat across the entire heat sink base or you can define a specific footprint size to reflect the actual heat density. If you plan on using graphite heat spreaders, vapor chambers, and embedded heat pipes in your heat sink, evenly spreading the total heat load would be a better representation than using the actual area your heat sources.

For applications with multiple devices, you can total up the surface area for multiple loads in the Project Conditions section. This is so Aavid Genie can use an equivalent surface area for the Technology Selection and help pick what material heat sink to use. You’ll be able to update the actual devices and sizes later in the Heat Source Definition page.

Between 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.

After you select and design your heat sink, input your flow rate and determine if you will have a duct or flow bypass, 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 where your heat loads contact your heat sink.

Applying Thermal Interface Material in Aavid Genie

As most applications benefit from using a thermal interface material, you can also add a thermal interface resistance from a thermal interface material (TIM).

In Conclusion

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.