Transistor Junction Temp calcs - Page 1
Forum section: electronics / projects

author: max_torque
author_rank: Super Contributor
timestamp: December 18, 2016, 01:49:21 pm
content: Using an NPN transistor as a linear pass element, and the datasheet says simply "6.25 degC/W" for the thermal impedance junction to tab.This ties in with the 150degC max junction temp, and the 20W max rating at 25degC tab temp (infinite heatsink) ie (20 * 6.25) + 25 = 150So, i'm measuring the tab temperature using a small thermocouple glued onto the tab (obviously going to be some measurement error / offset, but should only be a few degC max when temps have stabilised) .I'm experimenting with different thermal coupling and heat sinking because i need to sink the heat into the aluminium enclosure of the device across the range of ambient temps the device needs to operate in. If i say measured 100degC on the tab at 5w dissipation, is it correct to assume the junction temp is around 130degC (100 + (5 * 6.25)) ?I also can't seem obviously find any life data for the part on the datasheet:http://www.st.com/content/ccc/resource/technical/document/datasheet/c0/55/07/19/ee/bc/43/4e/CD00000829.pdf/files/CD00000829.pdf/jcr:content/translations/en.CD00000829.pdfIs life usually quoted at max junction temp?

author: SeanB
author_rank: Super Contributor
timestamp: December 18, 2016, 02:26:58 pm
content: Normally quoted at 125C, so running it at 150C will lead to around a tenth of the lifetime. Better is to aim for a die maximum temperature of 70C maximum, which unfortunately does mean you need a larger heatsink. You probably will only be able to do this by using multiple devices in parallel, and by using a thick aluminium plate to spread the heat out over a large surface of your case. then use further thermal compound ( here likely a silpad for least mess, the large surface area between the 2 surfaces helps heat transfer) to bond to the case. You might need a small low profile flat base heatsink on the outside if your design has insufficient airflow or is in an area where you have poor airflow over it as well.

author: Benta
author_rank: Super Contributor
timestamp: December 18, 2016, 03:14:24 pm
content: Do you have space constraints?The DPAK is not very suitable for higher dissipation, it's optimised for using the PCB copper for heatsinking.Could you move to say, a TO-220 device, which has far better thermal characteristics and can be bolted directly to your case/heatsink?

author: max_torque
author_rank: Super Contributor
timestamp: December 18, 2016, 04:52:11 pm
content: The device i'm going to use is probably not going to be the device i am currently testing, because that was just something i found in my "spare parts" bin! I think i will have to move to a D2PAK, but i need to stay surface mount really. Plan to use generous top and bottom copper planes, and via link to get decent thermal transfer at pcb level. Then need to pic suitable large area SIL pad to get the heat out to the case, yet maintain electrical isolation etc.I pushed the limits a bit to see what happened with my test setup, and the device case melted and then it unsoldered itself (measured 230degC on the Tabbut still works after cooling down ;-) Of course i don't plan to let the actual production setup get anywhere near that.No way will i be able to stay below 70degC junction temp however, because this device has to run in up to 65 to 70degC ambient! Worst case is needing to shift around 15 watts, but average power will be around 5watts. 2of D2PAKs with suitable mounting, and a proper case design should be sufficient i suspect?

author: Benta
author_rank: Super Contributor
timestamp: December 18, 2016, 10:43:26 pm
content: Dunno what your application is. If it's consumer stuff, you can get by with designing with 150 C junction temperature.If it's industrial/automotive, I'd go for 125 C max.For 70 C ambient, 125 C junction, your thermal resistance budget is 3.67 K/W total, which is a challenge.Good luck!

author: max_torque
author_rank: Super Contributor
timestamp: December 19, 2016, 01:01:50 pm
content: Indeed! Looking at options for heat sinking. Currently the module is screwed to a large metal object (around 25 tonnes in fact!) but that metal object can get hot, under solar load, probably to around 50degC, but it's effectively an infinite heat sink above it's bulk temp!

author: SeanB
author_rank: Super Contributor
timestamp: December 19, 2016, 07:37:54 pm
content: So you are going Automotive, so design for 70C and size the heatsink resistance for 20C drop across the whole case to block of steel side. 1C/W is a good aim, so your copper will need to be large, your copper will need to be thicker and you will need to have a nice thick silpad to transfer heat as well.i remember working with some transistors that would heat up enough ( in a TO3 package) to actually unsolder themselves from the leads to base and emitter. Generally the transistors would survive the unsoldering, as the load would blow up, saving the transistors and the fuses as well. If the emitter unsoldered itself it failed safe, but a base lead unsoldering itself led to thermal runaway and detonated lamps, as the leakage current of the 200c junction was plenty enough to turn it on hard, and the 12v 100W lamps really did not appreciate running at 500W or so. Failure in design, they really needed a better heatsink on them, along with a pull down resistor on the BE junction, and some cooling air as well, as they had passive cooling only. Some however survived indefinitely, so I just suggested selecting the driver transistors ( not the big power ones, they actually really were not part of the solution providing they met the minimum spec for the type) for high gain at the running current, and low saturation voltage to reduce the on state voltage of the darlington transistor.Substituting a different type, or redesign, was not possible as it would need a recertification, and that was a big pile of paper and time. We just ordered 2n 3773 transistors by the tray instead, and selected those with best gain at 10A instead. The rest were used in less critical applications, I made a nice power supply using 10 of them and a 723. there were a lot of UPS repairs as well, they were perfect there with 30 in parallel.