through hole diode dissipation vs package material - Page 1
Forum section: electronics / projects

author: coppercone2
author_rank: Super Contributor
timestamp: October 27, 2021, 02:51:44 am
content: For a through hole part, like a zener diode, I found the same part in like three different varieties, a plastic body, a hermetic metal body and a hermetic glass body. And I assume there is also ceramic versions.How do these components handle power in regards to each other? For the same size, what is the % derating for each package type (I know this gets more complicated with ambient).I assume the metal body diodes will run cooler and dissipate more heat then the glass ones that were present.Does glass vary alot from ceramic? There are no glass transistors that I know of for instance.

author: TimNJ
author_rank: Super Contributor
timestamp: October 27, 2021, 03:35:21 am
content: Hmm. If looking at the body alone (disregarding leads), I'm not convinced a particular body style would run much cooler than another style of comparable size. Possibly a glass body will tolerate higher temperatures (think of vacuum tubes) and can therefore safely dissipate more power. Don't quote me on that.Usable power rating is more closely related to how well you can pull heat out of the die and into some sort of heatsink (or into the air). Generally speaking, the lead length needs to be minimized as to reduce the thermal resistance between lead and heatsink (i.e. PCB plane). From that point, how well the PCB dissipates the heat is based on its own thermal resistance to ambient. More PCB plane surface area the better.Check out the old Motorola datasheet for 1N5283 - 1N5314, page 3:http://pdf.datasheetcatalog.com/datasheet/motorola/1N5298.pdfThese days, there is a trend away from semiconductors with long leads (or any "leads" at all) and instead towards leadless packages with very little internal thermal resistance from die to lead. For example, instead of a traditional SMA (DO-214AC) package, diodes may be purchased in the following new(ish) styles: SOD-123W, SOD-128, "Thin-SMA", "SMA Flat Lead", "SMAF" and others. The new styles use "flat lead" construction. The original SMA package uses a "U-shaped inward bent" lead, which has a rather high thermal resistance from die to PCB pad. Ironically, most surface mount TVS diodes have historically been made in SMA, SMB, or SMC packages, which all have rather poor thermal resistance.

author: bdunham7
author_rank: Super Contributor
timestamp: October 27, 2021, 03:36:05 am
content: What sort of hermetic metal body? I haven't seen that in diodes other than the stud-mount types. For TH diodes, I've always assumed the bulk of the heat is dissipated through the leads, although that might not be correct for some types with very fine bonding wires.

author: David Hess
author_rank: Super Contributor
timestamp: October 27, 2021, 03:38:31 am
content: Power dissipation for leaded diodes is primarily through their leads. Packaging affects the the maximum operating temperature to a small extent.

author: coppercone2
author_rank: Super Contributor
timestamp: October 27, 2021, 04:22:11 am
content: Hmm maybe its painted, its a 1n4737a diode. the outer surface looks metallic but its not conductive on a multimeter so its either coated or paint.I see, I used a pliers to crack it open. It looks like a ceramic thats kinda 'dirty' looking with a metalization outside, maybe its a form of glass. I need to do more tests with a file and find my magnifier. So when I snapped it in half, there was a shiney metal spot in the middle I guess thats the diode body surrounded/directly in contact with the body that fractures when its cut with sidecutters and would need to be considered a ceramic or a VERY glassy plastic. It has a different feel then a IC body that is hard plastic. It feels substantially conductive to my finger also.I know, I will try to burn a diode fragment with a torch outside layer to see if it melts or not. I feel like it might be a ceramic axial package, but I am not sure.

author: magic
author_rank: Super Contributor
timestamp: October 27, 2021, 05:11:08 am
content: Nitpick: metal may dissipate more power by heat flow to the surrounding gas/liquid, but it also dissipates much less by IR radiation to the environment due to low emissivity.

author: coppercone2
author_rank: Super Contributor
timestamp: October 27, 2021, 05:59:04 am
content: I think its ceramic that is metalized and then coated to be nonconductive on the outside and the ceramic is in direct contact with the diode body and it feels like a decent heat sink. I have a piece of aluminum nitride here and it feels similar in terms of how cool it is when you touch it... like stoneIt was a little confusing because when the diodes are in their baggy and you press them on your skin they feel cool like metal. I have the aluminum nitride plate in a similar ziplock bag (just came in), and it also feels cool like that when you press it to your skin, like metal. Plastic feels substantially different and much less cooling.Are those glass diodes in a tiny glass tube, or are they enveloped in glass? As far as I can tell this diode is enveloped in the ceramic. The lead is thick copper. I sheared it in such a way that I think the tiny block of material I see at the shear junction is the diode,The diode is about 1/4 of the surface area of the copper slug that terminates into the copper wire, so the construction of the conductive portion isthin conductive lead, I think copper, that goes into a thicker copper slug. There are two slugs that sandwich the diode between them. This is all surrounded by ceramic. So they sink heat into the ceramic and into the lead. So the diode is on a surface 4x wider then itself and many times deeper, so those structures must wick the majority of the heat out. Since the lead is thin, I am not sure how much heat it is dissipating in comparison to the ceramic body. If it was in free air it seems that a round miniature heatsink would help it run cooler, but I don't know if the junction could be ran at a significantly higher power.Interestingly since I don't think it requires a vacuum to run, its possible to chip away the ceramic and depending on how the diode is attached, solder it directly to a very large heatsink if it does not fall apart, and perhaps even put a drop of oil in there. Not that I would want to but I think that this might work.