[time-nuts] New Subscriber, DIY GPSDO project (yes, another one)

[Attila Kinali]

N'achmittag!

On Sun, 01 Mar 2020 01:38:11 +0100
Matthias Welwarsky <time-nuts at welwarsky.de> 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.