N'achmittag! On Sun, 01 Mar 2020 01:38:11 +0100 Matthias Welwarsky <[email protected]> wrote:
> On Samstag, 29. Februar 2020 20:08:13 CET Attila Kinali wrote: > > The bulk (ie a bit less than half) of the power is dissipated in U3, > > which is a 0.5A spec'ed LDO in D3PAK... if it's properly cooled. > > Your PCB doesn't show any heatsink and will dissipate into the board. > > Assuming that you can dump more than 100-200mW at a single spot > > into a densly populated PCB without some thermal design is asking > > for trouble. You will need a heatsink. If we are going by the above > > 4W number, you need to get rid of at least 2W in U3. Assuming a > > 20°C above ambient case temp is ok, this means a maximum thermal > > resistance of 10K/W, better 5K/W. That's doable with a large D3PAK > > heatsink, but even that is probably pushing it, unless you add a fan > > No heatsink, just a couple of thermal vias into the ground plane and it gets > a > bit warm, but not more than 35°C (case temperature). I just checked with a > thermocouple. Ok.. I'm surprised. Is the PCB able to dissipate this much heat? > > The next two LDO, U4 isn't any better off. It comes in even > > tinier SOT-23-5 cases, which I wouldn't trust beyond 100mW > > dissipation. That limits the current going through it to > > about 10mA. Yet there are quite a few components on it that > > definitely draw more alone. Heck, U8 alone probably draws 40-50mA. > > Guestimating, I would say there is a total 60-100mA on U4, > > which would result in something around 0.5-0.8W of dissipated power. > > Yes, but you're overestimating the power consumption. The complete digital > part has no more than, say 30mA total consumption on the 3.3V rail. U4 gets a > bit warm, but 40°C case temperature is OK'ish. Still a lot, but apparently > the > thermal conductivity is enough. However - this is with no load on the 10MHz > output. The output driver is connected to the same power rail. This might > push > U4 over the edge ;) 40°C is not just OKish, it's totally OK. I wouldn't worry until you hit something like 60°C. As long as the die is safely below 100°C it will be fine. And yes, the 10MHz output will add a lot of additional current. At 3.3V that is about 25-30mA going into the connector. > > I would also recommed using a DC/DC switched power supply to go > > down from 24V to 5V, to get around the big bulk of waste heat > > production. > > For the GPS pre-regulator definitely. For the rest of the electronics - > maybe. > But I wanted the power supply of U6 at more than 5V and I had to balance the > power dissipation somewhat to not burden everything onto U3, hence the 12V > intermediate voltage. Still, U3 could be replaced by a buck converter down > to, > say, 6V, that would take the stress off of all downstream LDOs. I just need > to > find something that has an appropriate footprint. I don't have a lot of PCB > area. I seem to remember that synchronous buck converters can be had in > SOT-23-6 package ;) I just need to find something with a high enough > switching > frequency so that the inductor can be very small. There are parts are meant as a replacement for 78xx. Ie fit in a TO-220 footprint, like e.g. the OKI-78SR series from Murata. They are usually available in 3.3V, 5V and 12V... some manufacturers also have values in-between. > > Going forward.. or rather backward. You have Q1 presumably as > > reverse polarity protection. Unfortunately it doesn't work that > > way. The way you put the FET in, it will act as a source follower. > > Meaning the voltage at the source will be the voltage at the > > gate minus the threshold voltage. Now the gate is being pulled > > up by a 8.4V Zener diode, which means the gate is supposedly > > 8.4V-ish below the source, but that's more than the thresold > > voltage, so the FET closes off and doesn't conduct. For this > > kind of thing to work, you would have to turn the FET around, > > the source pointing at the power source. But then, while > > the FET sure does not conduct during reverse polarity, > > the body/protection diode of the FET will conduct. Hence you > > don't get any protection there. It would be a better idea > > to just use a 1N4001 instead. Simpler, and can withstand > > 100V reverse polarity :-) D4/D5 are probably meant as over > > voltage protection. While this definitely works, it's kind of > > crude. At least at these voltages. If this would be a 1kV system, > > then using a triac would be fine, but for low voltage electronics, > > the time it takes to fire a triac and get it switching will not > > prevent the downstream electronics from getting fried. A better > > approach is to use an appropriately rated TVS diode. Overall > > simpler and better (aka faster) protection. > > You probably missed that Q1 is a p-channel mosfet. The circuit around Q1 is > basically a textbook approach to reverse polarity protection. GAH! Indeed I did! I take back everything I said! > On the idea of using a TVS diode instead of the SCR crowbar - forget it. I > tried. By the time the polyfuse trips, the TVS has released the magic smoke. > The LPRO-101 draws about 1.7A during initial heat-up, the fuse has to be > rated > accordingly. A TVS diode with a breakdown voltage of 27V would have to > dissipate, say, about 50 Watts for a couple of seconds at least. I used a > LDP24A, Besides from being of enormous size, I didn't trust it not causing a > fire when the protection trips. Oh...kay? The input circuitry is usually meant as a protection against surges, not against having a power supply with the wrong voltage attached. So I am a litte bit surprised that you try to protect against that. Do you see it likely that your power supply goes up to a voltage that would break the LDOs downstream? > Do you have some schematics online of your design? I've seen Tobias > schematics > on the list, but I only looked at the last month of postings so far. Not yet. I'm working on a write-up that explains all the components and what design decisions lead to them. For now, you can find the key components in the mails I wrote as an answer to Tobias: http://lists.febo.com/pipermail/time-nuts_lists.febo.com/2019-October/097962.html http://lists.febo.com/pipermail/time-nuts_lists.febo.com/2019-November/098207.html Attila Kinali -- <JaberWorky> The bad part of Zurich is where the degenerates throw DARK chocolate at you. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
