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From: Joerg on 21 Nov 2009 17:08 John Larkin wrote: > On Sat, 21 Nov 2009 13:18:08 -0800, Joerg <invalid(a)invalid.invalid> > wrote: > >> Mycelium wrote: >>> On Sat, 21 Nov 2009 12:59:52 -0800, Joerg <invalid(a)invalid.invalid> >>> wrote: >>> >>>> John Larkin wrote: [...] >>>>> Given the consequences of power supply failure, I'm leaning more >>>>> towards always incorporating transzorbs/clamps/crowbars on things like >>>>> this. >>>>> >>>> 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. > > You can buy a 3.3 volt transzorb. It will sink lots of current at > about 6 volts. > > http://www.vishay.com/doc?88940 > Ok, but at 6V it'll be way too late for whatever logic stuff hangs on that rail. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Joerg on 21 Nov 2009 17:14 krw wrote: > On Sat, 21 Nov 2009 13:30:00 -0800, Joerg <invalid(a)invalid.invalid> > wrote: > >> krw wrote: >>> On Sat, 21 Nov 2009 12:59:52 -0800, Joerg <invalid(a)invalid.invalid> >>> wrote: [...] >>>> The only thing that really works with such low voltage levels and >>>> finicky FPGA is precisely controlled crowbars. Either TL431-style or use >>> Use an FPGA that's not so finicky. They're all better than they were >>> but some have no sequencing requirements at all. >>> >>>> 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. >>> Comparitors with references are expensive. I'm surprised that you >>> even know they exist. ;-) ... >> >> 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 :-) > > They're too expensive as "protection devices" even for me. > That's why I usually opt for a TL431 plus transistor plus SCR. If I have the space which I sometimes don't. >>> ... With money to burn, the ones with the >>> variable hysterias input switch are pretty nice too (ADCMP343, etc.). >>> >>>> 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. >>> If you want belt and suspenders, what about a pass FET on the input? >>> >> That could also fail. I prefer sacrificial parts for this such as fuses >> or fuse-resistors. > > ANything can fail. How many belts do you wear with your suspenders? If a few hundred bucks worth of FPGA hang on it I'd say at least one :-) > 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. > It's the last resort. Still better than a unit belching smoke. >>>> 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. >>> Usually, the normal board variance is more than one that of one device >>> failing. Fuses, PTCs, and the like are pretty poor at this. >>> >> Fuses are excellent. SCR shunts the rail, holds it there for a brief >> moment ... phut ... fuse lets go. > > Fuses have very poor tolerances. A 2A fuse my let something go phut > at 5A. The electronics is there to protect the fuse. > You'd be surprised what a li'l DPAK SCR can do. It has no problem blowing at fuse at 5A or 10A. >>> 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. >> >> 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. > > Just sequence its power! Couldn't. In that case power was the actual control signal and things had to happen fast, within hundreds of usec. Now they do :-) -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Nico Coesel on 21 Nov 2009 18:25 Raveninghorde <raveninghorde(a)invalid> wrote: >On Fri, 20 Nov 2009 16:22:15 -0800, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>OK, I just got the first board from production this morning, for this >>spectroscopy controller thing. >> >>ftp://jjlarkin.lmi.net/First.JPG >> >>It gets 12 volts in, which runs an LTM8023 switcher brick to make 3.3 >>volts. The 3.3 runs most of the logic on the board (including a >>Spartan 6 and a PLX PCIe bridge, both BGAs) and also drives four >>secondary switchers and some LDOs to make 1.2, 1.5, 1.8, 2.5, and -5 >>for various uses. >> >>So when I powered it up everything went nuts. The PLX chip was >>obviously fried. After that was pulled, the Xilinx was running hot, >>and the 3.3 volt supply was bogged down to about 2.6. The LTM >>regulator was hot. >> >>Pulled the Spartan BGA next. >> >>Now the 3.3 volt rail wants to run at 5 or so. >> >>After much head scratching, I discovered this: >> >>ftp://jjlarkin.lmi.net/Swapped.jpg >> >>The resistor that's screened "R127" is actually R129. And vice versa. >>So the switcher was programmed wrong, told to run at an absurdly low >>frequency and an absurdly high voltage. The ref designators somehow >>got misplaced during layout. We usually check for this. >> >>Apparently our production people, when semi-auto placing dense parts, >>double-check the ref designator and plop the part into the "correct" >>place, even if the machine coordinates are a little off. I'll have to >>warn them to be suspicious about cases like this, especially on first >>articles. >> >>TGIF >> >>John >> >> > >Experience has taught me to power up new boards on a bench psu by >winding up the voltage from zero while monitoring the supply rails and >input current. 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. -- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... "If it doesn't fit, use a bigger hammer!" --------------------------------------------------------------
From: krw on 21 Nov 2009 19:14 On Sat, 21 Nov 2009 23:25:42 GMT, nico(a)puntnl.niks (Nico Coesel) wrote: >Raveninghorde <raveninghorde(a)invalid> wrote: > >>On Fri, 20 Nov 2009 16:22:15 -0800, John Larkin >><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >> >>>OK, I just got the first board from production this morning, for this >>>spectroscopy controller thing. >>> >>>ftp://jjlarkin.lmi.net/First.JPG >>> >>>It gets 12 volts in, which runs an LTM8023 switcher brick to make 3.3 >>>volts. The 3.3 runs most of the logic on the board (including a >>>Spartan 6 and a PLX PCIe bridge, both BGAs) and also drives four >>>secondary switchers and some LDOs to make 1.2, 1.5, 1.8, 2.5, and -5 >>>for various uses. >>> >>>So when I powered it up everything went nuts. The PLX chip was >>>obviously fried. After that was pulled, the Xilinx was running hot, >>>and the 3.3 volt supply was bogged down to about 2.6. The LTM >>>regulator was hot. >>> >>>Pulled the Spartan BGA next. >>> >>>Now the 3.3 volt rail wants to run at 5 or so. >>> >>>After much head scratching, I discovered this: >>> >>>ftp://jjlarkin.lmi.net/Swapped.jpg >>> >>>The resistor that's screened "R127" is actually R129. And vice versa. >>>So the switcher was programmed wrong, told to run at an absurdly low >>>frequency and an absurdly high voltage. The ref designators somehow >>>got misplaced during layout. We usually check for this. >>> >>>Apparently our production people, when semi-auto placing dense parts, >>>double-check the ref designator and plop the part into the "correct" >>>place, even if the machine coordinates are a little off. I'll have to >>>warn them to be suspicious about cases like this, especially on first >>>articles. >>> >>>TGIF >>> >>>John >>> >>> >> >>Experience has taught me to power up new boards on a bench psu by >>winding up the voltage from zero while monitoring the supply rails and >>input current. > >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. Assembling things in stages just pisses off the manufacturing folk. Our boards usually have far too many components to do manually. Sometimes not, but they're too simple to not work. ;-)
From: krw on 22 Nov 2009 11:10
On Sat, 21 Nov 2009 14:14:15 -0800, Joerg <invalid(a)invalid.invalid> wrote: >krw wrote: >> On Sat, 21 Nov 2009 13:30:00 -0800, Joerg <invalid(a)invalid.invalid> >> wrote: >> >>> krw wrote: >>>> On Sat, 21 Nov 2009 12:59:52 -0800, Joerg <invalid(a)invalid.invalid> >>>> wrote: > >[...] > >>>>> The only thing that really works with such low voltage levels and >>>>> finicky FPGA is precisely controlled crowbars. Either TL431-style or use >>>> Use an FPGA that's not so finicky. They're all better than they were >>>> but some have no sequencing requirements at all. >>>> >>>>> 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. >>>> Comparitors with references are expensive. I'm surprised that you >>>> even know they exist. ;-) ... >>> >>> 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 :-) >> >> They're too expensive as "protection devices" even for me. >> > >That's why I usually opt for a TL431 plus transistor plus SCR. If I have >the space which I sometimes don't. > > >>>> ... With money to burn, the ones with the >>>> variable hysterias input switch are pretty nice too (ADCMP343, etc.). >>>> >>>>> 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. >>>> If you want belt and suspenders, what about a pass FET on the input? >>>> >>> That could also fail. I prefer sacrificial parts for this such as fuses >>> or fuse-resistors. >> >> ANything can fail. How many belts do you wear with your suspenders? > > >If a few hundred bucks worth of FPGA hang on it I'd say at least one :-) 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 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. >> > >It's the last resort. Still better than a unit belching smoke. 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. >>>>> 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. >>>> Usually, the normal board variance is more than one that of one device >>>> failing. Fuses, PTCs, and the like are pretty poor at this. >>>> >>> Fuses are excellent. SCR shunts the rail, holds it there for a brief >>> moment ... phut ... fuse lets go. >> >> Fuses have very poor tolerances. A 2A fuse my let something go phut >> at 5A. The electronics is there to protect the fuse. >> > >You'd be surprised what a li'l DPAK SCR can do. It has no problem >blowing at fuse at 5A or 10A. > I just don't see the purpose, on small stuff. 100A, maybe. >>>> 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. >>> >>> 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. >> >> Just sequence its power! > > >Couldn't. In that case power was the actual control signal and things >had to happen fast, within hundreds of usec. Now they do :-) I meant sequence the power to the reference. ;-) |