day shot to hell

But then maybe I couldn't drive her jeep!

I did take the board home at night to do some of the placement and routing myself, for the analog front end, some fast stuff, and the switcher region. So it may well be my fault. She's learning fast, so I suspect this will go on her evolving checklist and it's not likely to happen again. There are probably 30 to 50 things one should check before releasing gerbers, and it's easy to forget a couple. We need a checklist.

All the supplies are working now. On Monday I'll have them put two new BGAs on the board and see what happens next. Bringing up a new board of this complexity is always, well, tense.

John

Yes. The only thing to do is to isolate the power section from the loads and bring up the supplies unloaded. That requires jumpers or whatever. I do have LC filters between the supply pours and the main pours, and in retrospect I should have removed the inductors and tested the supplies.

Given the consequences of power supply failure, I'm leaning more towards always incorporating transzorbs/clamps/crowbars on things like this.

John

This depends on the topology of the switchers. I normally use buck regulators but I admit that it doesn't work with other topologies.

Often doesn't work with buck regulators either. Depending on what they're driving startup can get quite nasty.

We generally put 0-ohm resistors on supply inputs. We do it mainly to be able to measure the current but this has also helped pass EMI a couple of times. Ferrites fit nicely where 0-ohm resistors once were. ;-)

Given that this sort of thing is a pre-production hazard, I don't see it worth spending much money or effort on. Have you had any bzzzt-BANGs in manufacturing? We do once in a while but rarely on supplies. There are just so many places this can happen that it doesn't seem reasonable to attack only a minor source of smoke.

For EMI it works but in Europe they have put some lids on that trick.

My problem is that's three parts for the oscillator plus one 0201 resistor per switcher (I've got three). No dice :-(

The board already looks like this:

Hey, didn't she pay that with college "savings" which ultimately came out of your pocket?

The only thing that really works with such low voltage levels and finicky FPGA is precisely controlled crowbars. Either TL431-style or use one of those newfangled comparators in DFN packages that have a reference built in. The ones with push-pull outputs ought to be able to fire an SCR directly. This also afford the chance to shut down when a sequencing rule is being violated.

Another option is to yank the shut-down or slow-start pin. Of course that won't help if the root cause is a blown FET in a buck.

Also, there must be something in front of the board that could gracefully blow open without causing the sirens to go off. With comparators be careful, I just unearthed an undocumented nastiness upon power-up in one. Do _not_ trust SPICE models.

I thought transzorbs were the current norm.

Too much lot tolerance, usually. A chip that works nicely at 3.3V might go kaputt at 3.7V so you don't have much wiggle room.

Use an FPGA that's not so finicky. They're all better than they were but some have no sequencing requirements at all.

Comparitors with references are expensive. I'm surprised that you even know they exist. ;-) With money to burn, the ones with the variable hysterias input switch are pretty nice too (ADCMP343, etc.).

If you want belt and suspenders, what about a pass FET on the input?

Usually, the normal board variance is more than one that of one device failing. Fuses, PTCs, and the like are pretty poor at this.

Unless a model specifically addresses an operational mode, assume it doesn't. If it says it does, be very skeptical. Manufactrurer's public models are pretty damned lousy.

Well, on a recent design I had to. Dozens of components that all had to fit into the space of a postage stamp. It was one of the few projects where the BOM budget was huge, at least from my usual perspective. I still ended up under 50% of it :-)

That could also fail. I prefer sacrificial parts for this such as fuses or fuse-resistors.

Fuses are excellent. SCR shunts the rail, holds it there for a brief moment ... phut ... fuse lets go.

This one has a problem that the reference shoot up to the VCC rail and then slowly comes back. IOW it isn't very suitable for power-up protection or sequencing.

You can buy a 3.3 volt transzorb. It will sink lots of current at about 6 volts.

John

They're too expensive as "protection devices" even for me.

ANything can fail. How many belts do you wear with your suspenders? Fuses are ugly. Customers with blown fuses even more so. THe owner got a ration a few weeks back because we made the television (official time out for equipment failure). ...and it wasn't even our fuse. They had the crate closed up so didn't hear the UPS bitching either.

Fuses have very poor tolerances. A 2A fuse my let something go phut at 5A. The electronics is there to protect the fuse.

Just sequence its power!

Ok, but at 6V it'll be way too late for whatever logic stuff hangs on that rail.

That's why I usually opt for a TL431 plus transistor plus SCR. If I have the space which I sometimes don't.

If a few hundred bucks worth of FPGA hang on it I'd say at least one :-)

It's the last resort. Still better than a unit belching smoke.

You'd be surprised what a li'l DPAK SCR can do. It has no problem blowing at fuse at 5A or 10A.

Couldn't. In that case power was the actual control signal and things had to happen fast, within hundreds of usec. Now they do :-)

If possible I assemble prototypes in different phases. Starting with the power supply. Another option is removing filtering components. I usually have beads at PSU outputs. Removing those isolates the PSU from the rest of the circuit.

Few hundred bucks? Jeorg, I'm ashamed of you. ;-)

I've really never been scared of taking out an FPGA, even when they were a few thou$and. The sort of thing we're talking about is a bring-up issue which is something I'd rather not spend any significant BOM cost on. That's a cost that keeps on taking.

Fuses to limit smoke, sure, but that's not going to save any electronics. The idea of fuses is to keep the damage inside the covers. Fuses just aren't accurate/fast enough to protect circuits.

I just don't see the purpose, on small stuff. 100A, maybe.

I meant sequence the power to the reference. ;-)

Any reference that "shoot(s) up to the VCC rail" should be replaced with one that doesn't. Where do you buy such crap?

As for fuses, I've always been fond of crowbar fuse blowers when over-voltage conditions occur. Why lose downstream electronics when a power supply goes bananas? ...Jim Thompson