[FAQ-SF-5] Speedfit: Thermal Tab

Urvi_Ahluwalia

In this tab, you can define the cooling system’s thermal properties for your converter. The cooling system has all devices mounted on a common heatsink. The case-heatsink thermal resistance: Rth,ch, is updated automatically every time when the user changes their device selection. The click here link as shown in the Figure 1 below, redirect you to a list of acceptable values based on the package of the device and the TIM material type.

Figure 1: Specify TIM for each device based on defined range

Next, you have three options for the thermal system simulation; either have a varying heatsink temperature throughout the simulation or fix it to a specific value. You can also define a custom heatsink using a Foster or Cauer network.

Figure 2: Heatsink Configuration showing two different heatsink configurations

Figure 2 shows the comparison between the two configurations for the heatsink temperature. Each switch position in the topology will use the junction-to-case thermal impedance, Rth,JC inherent to the device selected, which is included automatically. In either configuration, you are required to enter the thermal interface resistance, Rth,ch. Rth,ch is the thermal impedance of the interface between each switch position in the topology and the heatsink regardless of whether a single discrete MOSFET, a half-bridge, or a six-pack module is used.

For a fixed heatsink temperature, you can define the fixed heatsink temperature, Th. While using the variable temperature heatsink, you are required to specify the the thermal impedance (Rth,ha) and time constant (τha) of the heatsink, along with the ambient temperature (Tamb).

An additional heat source to a common heatsink can also be coupled, such as an auxiliary converter. This can be specified as an additional power loss, Padd in Watts. 

Figure 2: Thermal Tab

For the custom heatsink, a Foster or Cauer chain can be defined by the user. The order of chain can be modified by clicking on + or – buttons. 

Figure 3: Custom heatsink table

As you edit the thermal chain of the custom heatsink, you can see the updated thermal impedance plot at the bottom of the page. You can select the option of plotting this curve in the log scale.

Figure 4: Thermal impedance plot for custom heatsink

Urvi_Ahluwalia

https://www.wolfspeed.com/tools-and-support/power/speedfit/