# Trouble with understanding heatsink datasheet

I am trying to understand the heatsink datasheet provided in the following link:

https://www.fischerelektronik.de/web_fischer/en_GB/PR/SK434_/datasheet.xhtml?branch=heatsinks

My problem is that the heatsink is available for lengths:

50 mm / 75 mm / 100 mm / 1000 mm

I am trying to find the thermal resistance for the 1000mm one. From the graph you can see that the thermal resistance is only given on the interval from 40-200mm. How can I find the thermal resistance for 1000mm?

If anyone know I would really appreciate the help:)

• 1000mm is quite a large heatsink, are you sure? Maybe you're meant to cut it up into smaller sections. Oct 16, 2020 at 11:07
• Yeah, I am not going to use the 1000mm heatsink I am currently designing a PFC and trying to minimize the volume of the PFC. Therefore, I have made a program that takes a given switching frequency then calculates the total volume, I just want to visualize the drastic change in volume that happens when the frequency is high. Oct 16, 2020 at 11:25
• If it doesn't have to be entirely accurate then I would approximate it as a reciprocal function (some number divided by the length). A 1000mm section is just 5 200mm sections in parallel. (In reality that's not true because the heat has to travel farther which adds extra thermal resistance) Oct 16, 2020 at 11:27
• Thats a good idea, thank you! Oct 16, 2020 at 11:31
• They don't expect you to use a 1000mm heatsink : they supply it in 1m lengths so you can cut it into N pieces of any length you want.
– user16324
Oct 16, 2020 at 12:19