Heatsink for outboard  power transitor. - Page 1
Forum section: electronics / beginners

author: fc3000
author_rank: Regular Contributor
timestamp: January 16, 2021, 03:30:23 pm
content: Hi All.I have an 7805 regulator to be used with an outboardTIP42A- PNP power transistor.The 7805 will supply 200mA and the TIP42A device about 2A.I looked at datasheet for the TIP42A but i cannot determine what rating heat-sink I need.The supply voltage is 10Vdc.Can you help please?RegardsFrancescoC

author: Benta
author_rank: Super Contributor
timestamp: January 16, 2021, 04:20:34 pm
content: The following presumes that your circuit is current limited to 2 A.Worst case power dissipation is when the output is short circuited: 10 V x 2 A = 20 WChoose your maximum junction temperature, for example +100 deg C.Specify the maximum ambient temperature, for example + 30 deg C100 - 30 = 70 KThis gives a total thermal resistance of 70/20 = 3.5 K/WTO-220 Junction-to-case thermal resistance: 1.67 K/W3.5 - 1.67 = 1.83 K/W, this is the spec for your heatsink.

author: drussell
author_rank: Super Contributor
timestamp: January 16, 2021, 04:35:15 pm
content: Well, you start out by calculating how many watts that transistor is going to dissipate at maximum dissipation...Be sure to take into account what the dissipation will be under any sort of fault condition, high input voltage, etc. and then leave yourself a little extra margin for safety / good design principles...Then select a heatsink that will dissipate enough °C per Watt that you need to dissipate while still remaining cool enough (in whatever the ambient temperature is) to keep the junction of the transistor below the "meltdown"temperature through whatever thermal resistance there is in your stack-up.There are many, many tutorials and application notes describing the whole path...Eg., a sample just from the first few responses from googling:Analog Devices Tutorial - Thermal Design BasicsInfineon Application Note AN-1057Heat Sink Design Facts & Guidelines for Thermal ... - DigikeyToshiba Application Note - Power MOSFET Thermal Design and Attachment of a Thermal Finetc. etc. ad infinitumI'm sure there are TI and Motorola application notes describing the basics that I probably have in paper form from decades ago but I have no idea what the AN- numbers are off the top of my head anymore...

author: drussell
author_rank: Super Contributor
timestamp: January 16, 2021, 04:38:19 pm
content: Awww,Bentabeat me to it...I was going to try to havefc3000try to show his work instead of just giving him the answer key...

author: fc3000
author_rank: Regular Contributor
timestamp: January 16, 2021, 04:40:43 pm
content: Hi Benta,thank you for the response.Is K/W different from C/W?And the smaller the number of K/W the larger the heat-sink. Is this correct?RegardsFrancesco C

author: Benta
author_rank: Super Contributor
timestamp: January 16, 2021, 04:42:22 pm
content: Quote from: fc3000 on January 16, 2021, 04:40:43 pm1: No2: Yes

author: fc3000
author_rank: Regular Contributor
timestamp: January 16, 2021, 04:44:25 pm
content: OK.Does K=Kelvin?If so just make conversion K>K?francescoCOuch that was mssunderstood.So K=C?

author: drussell
author_rank: Super Contributor
timestamp: January 16, 2021, 04:46:43 pm
content: Yes, the °C/W (or Kelvin / Watt, same unit magnitude, just start at absolute zero instead of freezing water at 0) is simply how much the heatsink will rise in temperature for each watt being dissipated.

author: fc3000
author_rank: Regular Contributor
timestamp: January 16, 2021, 04:50:50 pm
content: Hi,Thank you to all of you.Now lets buy the heatsink.RegardsFrancesco Cps, Great Forum. Regards to Dave Jones

author: Benta
author_rank: Super Contributor
timestamp: January 16, 2021, 06:01:48 pm
content: A tip for you:1.67 K/W is not a very good value, but that's the way TO-220 is.If you go to a transistor in TO-247 instead, the value is better.Heat sinks are expensive, transistors are cheap. With a TO-247, the heatsink will be smaller.

author: fc3000
author_rank: Regular Contributor
timestamp: January 16, 2021, 07:04:33 pm
content: Thank you I will considerRegardsFrancescoC