Switching regulators thermal calculation - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: September 28, 2022, 07:55:59 pm
content: Hi,I usually use the manufacturer simulators to get the temperature for switching regulators at given loads/conditions.But when the manufacturer does not provide such simulators, are there any "general" online calculators - even if just approximated - to get at least junction temperature? I could not find any...I am interested for those with internal switch like this one:https://www.diodes.com/assets/Datasheets/AP64351.pdfThank you

author: redkitedesign
author_rank: Regular Contributor
timestamp: September 29, 2022, 10:08:27 am
content: That depends on a lot of things you don't mention in your post. E.g. internal or external switch? Type of regulator?But ususally the datasheet has useful information. You could try telling witch regulator you want to use.

author: ricko_uk
author_rank: Super Contributor
timestamp: September 29, 2022, 10:53:41 am
content: Thank you redkitedesign,for those with internal switch like this one:https://www.diodes.com/assets/Datasheets/AP64351.pdf(OP updated too)Thank you

author: redkitedesign
author_rank: Regular Contributor
timestamp: September 29, 2022, 12:00:46 pm
content: Well, thats a regulator with integrated switches. The largest contributor to heating will be the Rdson losses in the FETS, and those are specified. You might need some trail and error when the device gets significantly hot, and therefor the Rdson rises (also specified), but it is pretty straightforward.You can sanity-check afterwards against the efficiency numbers allthough they also depend on the inductor..

author: ricko_uk
author_rank: Super Contributor
timestamp: September 29, 2022, 12:52:49 pm
content: Thank you Redkitedesign,but it is still not clear how. I cannot see any Rdson losses specified in the tables or in the graphs.Initially I thought that for example, if at a specific load the efficiency is 80% then the additional 20% of the power generated at that specific load.So for example if the output is 8V at 2A = 16W output and the efficiency is 80% (at that 2A) then the SMPS loss is the extra 20% i.e. 4W. Is that correct?Somehow I got worried there would be more to it depending on factors like the frequency and coil specs?Thank you again

author: Picuino
author_rank: Super Contributor
timestamp: September 29, 2022, 03:31:47 pm
content: Quote from: ricko_uk on September 29, 2022, 12:52:49 pmYes, is correct. As long as the 80% efficiency figure is true.Quote from: ricko_uk on September 29, 2022, 12:52:49 pmAlso.At higher frequencies, efficiency decreases due to switching losses.At lower frequencies or with low inductance coils, current ripple increases and therefore conduction losses (R_dson).

author: redkitedesign
author_rank: Regular Contributor
timestamp: September 29, 2022, 11:14:12 pm
content: Quote from: ricko_uk on September 29, 2022, 12:52:49 pmYou'll have to do the math yourself. Given 2A load current, and about 0.1Ohm Rdson, you'll lose about 0.4W.I'll leave it to you to do the actual calculation for high- and low side, and for the rms current.To reflect on your original question: If there is no manufacturer-design tool for your switcher, you'll have to do the calculations yourself. They are not that hard, and it can help to put them in a spreadsheet (so you dont have to do them all over if you change parameters.)