5v from 12v and heat dissipation - Page 1
Forum section: electronics / projects

author: Morphology
author_rank: Newbie
timestamp: July 19, 2024, 08:25:47 am
content: Hi,An audio circuit I'm designing is pulling around 100mA at 5v and, for various reasons my only power source is 12v.In my prototype I used an AMS1117-5.0 which is a SOT-223. It is therefore dissipating around 0.7W. It gets pretty hot, though you can still touch it and I've not seen it go into thermal shutdown.I don't really want to use a buck converter, as I'm concerned about electrical noise.I wasn't terribly careful about the prototype PCB design, so I could doubtless improve the thermal layout of the SOT-223 footprint (larger 5v Pad, vias through to matching underside area etc).My question is: Is it better to go for a better thermal design, or should I look at, say, dropping the 12v to 8v or 10v via an LM2937 before hitting the AMS1117-5.0 that way I have two SOT-223 packages, the LM2937 dissipating 0.4W and the AMS1117 dissipating 0.3W (or thereabouts)?

author: Siwastaja
author_rank: Super Contributor
timestamp: July 19, 2024, 08:30:48 am
content: 0.7W is not that much - still easily dissipated with just PCB as a heatsink alone. Just get a regulator in a bigger package, like DPAK if you want to stay SMD, use a tad larger copper fill on both sides and add vias to thermally couple the sides.BTW 1117 is one of the crappiest (and most cloned) regulators out there and with such large voltage drop, it's not like you need LDO. While at it, you might find another regulator which is more forgiving against output capacitor requirements (1117 easily becomes an oscillator), or something with better PSRR at high frequencies which could be useful for you if you truly want to play this "low noise" game.Picking another regulator would also allow you to choose one where the thermal tab is connected to GND (not OUTPUT like in 1117), helping tremendously in heatsinking as you can just use your ground plane without need to cut it and add a separate copper pour area.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 19, 2024, 08:36:23 am
content: Quote from: Morphology on July 19, 2024, 08:25:47 amDo bothSpreading the heat load is always a good idea.I've also seen diodes used simply as thermal heat spreaders, to drop voltage.You can also 'throw more copper at the package', choosing TO-252 for example, that lowers the die temperature and spreads heat better than SOT223, for a few cents more.

author: moffy
author_rank: Super Contributor
timestamp: July 19, 2024, 08:38:10 am
content: If concerned you can drop the voltage down to the regulator with a series zener of suitable voltage and package.

author: Morphology
author_rank: Newbie
timestamp: July 19, 2024, 08:40:17 am
content: All excellent points, thank you. Especially picking a regulator with the thermal tab as ground, as I already have a large copper pour both sides.As you say: I definitely don't need LDO!Many thanks.Morph

author: Phil1977
author_rank: Frequent Contributor
timestamp: July 19, 2024, 08:44:22 am
content: As long as you can touch a power IC it´s perfectly healthy! Many high quality industrially qualified devices regularly operate at T_junction >100°CAnyhow, better cooling never harmed anyone, so if space is no constraint you can use a DPAK 7805 - as already mentioned, if you don't need low dropout voltage the old regulator designs are anything but bad. But be aware that there are lots of fake 78xx-ICs outside - though the original ones at reputable sources are so cheap thatno one understandsI don't understand why they are faked.