Electronics > Projects, Designs, and Technical Stuff
OK to have heatsink "live" in linear power supply?
ejeffrey:
--- Quote from: james_s on April 19, 2019, 09:10:21 pm ---Adding a small heat spreader can solve one problem but creates another, it's yet another joint adding thermal resistance between the die and the air. It may improve transient response but still reduces the total heat transfer.
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Not if done right, at least not compared to using an insulating washer between the package and the heat sink. Yes, you have replaced one junction (package to heat sink) with two (package to heat spreader and heat spreader to heat sink), but the two can both have potentially much lower thermal resistance. The package to heat spreader can have less thermal resistance because you don't use an insulating pad. The heat spreader to heatsink can have much larger surface area.
Zero999:
--- Quote from: coppercone2 on April 19, 2019, 06:54:07 pm ---I think its basically a bad design decision from a repair/modify/test stand point
You can always make things easier for people that need to work on your circuit physically. This is not one of those things.
When you design something, this consideration goes against performance considerations, pretty much always. Testability/repairability/anti-repairman-fuckupability is all different design things you can work on. Some people will actually make demands for these factors if you are selling them equipment (i.e. some people might demand plexiglass plates over entire PCB's so tech's can work on the wiring harness or do subassembly-level repairs without exposing themselves to any potentially hot pins (or at least minimize potential exposure). So the chassis begins to look like a out door cafe with umbrellas over all the tables in case it rains. Why? Because a small plastic bit is worth less to some companies then a tech's finger or a unit that has odd damage occur to it.
I would recommend using a pad and grounding the heat sink to circuit ground so if something goes bad it blows a fuse. When I see it I think 'mother fucker'.
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Ease of servicing is a valid reason for having an isolated heat sink, but quite often there are other factors such as size and cost, which are much more important. This is especially true for a product produced in large volumes, at low cost, which isn't designed with serviceability in mind, as it goes against making it as small and cheap as possible. You may disagree with this, but nowadays few power supply designs are built for repair: bad PSU, junk the board and replace it with new one!
coppercone2:
if that's the angle your going for.. I thought it might be a home project etc where cost is much less important then arguing with counter trying to justify their job even moreso then trying to ensure reasonable savings (ensure company survival)
If you are putting a kit together for yourself.... you might not want to have a big corporate mindset . Home built thing, no boss to argue with, no real mass production cost savings.. might wanna make it serviceable. I doubt its a big business if you are selling 78 series linear regulators based power supply even if your designing the kit.. bet you won't sell more then 1000 of those and IMO with a damn kit people put together, the people will be happier if they don't blow up their kits if its some novice with non-jacketed multimeter probes monkeying around the circuit (your probobly gonna get at least one person using nail heated by a plumbers torch as a soldering iron probing it with loose wires from a harbor freight DMM connected in ammeter mode, the type of person that will defiantly try to abuse your warrente). Dealing with this one person might be more costly and time consuming and generally unpleasant then just absorbing a bit of cost. If I was in business for myself I would do as much as possible to remove customer interaction unless its for features that can make more sales. I.e. pre-solder the QFPNL on the PCB. Add some probe points rather then expect people to probe SOIC traces with some kind of fancy needle sharp probes in a place where you think they might tend to. Reduce skill level for manufacture and testing as usual to reduce fails and complaints.
Also some people might see a thermal pad/grease/etc as part of a kit as a 'cool' assembly step. Like if you read about how its made and you wanna buy it for some kid, its kind of interesting how you need to align up a little gasket for a electrical part. I always found screwing the PCB into the chassis and the mechanical assembly steps kind of interesting. Adds a bit of variety. So long your not having to screw down greasy thermal pads every day its kind of interesting. Only thing I would avoid is glue.... :scared: . The skill level for using it neatly without destroying furniture etc is too high. It's easier to learn how to solder to nasa standards lol :palm:
Zero999:
I have not made a linear power supply project, since I was an apprentice. The last one I made, was a bipolar tracking power supply. I fixed the LM217 and LM337 directly to small pieces of copper, with screws, nuts and an insulated sleeved washer, and fixed them to the case, with a thin piece of insulating material sandwiched in between. It worked quite well and the fact the pieces of copper float at different voltages didn't make it difficult to debug or repair the power supply.
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