kablooey

*I* didn't piss off the customer; I helped them find the problem.

Probably nothing. 8.8 volts isn't a reasonable rail in an IEEE-standard system with an abs max requirement of 5.25. The crate needs to be fixed.

We'll have to fix them. Don't know who pays, but that's not really important. It looks like we'll have to replace two big chips and retest/recal, maybe a day's work per board.

This is a wonderful customer. If you tell them the truth and work sincerely to solve problems, they're not in the blame business, or the penny-pinching biz either.

John

Lookup table, I presume.

--
  Keith

No doubt you cheated and actually tested the thing yourself first. :-)

Of course. ;-)

But I also because I designed it based on a sequence of states that could smoothly reverse at any transition without problems. There were no irreversible transitions or any loops that had to execute in one direction. Each state could progress to the next, if appropriate, or revert to the previous state, if necessary. The start up and shut down stepped along this well defined set of states in opposite directions.

This taught me a lot about things that happen so quickly, under normal conditions, that it is easy to sweep some of the the transitions under the rug and just cross your fingers and hope for the best.

Don't do it if you want it to always work right.

John,

Could you expound upon what some of these states/transitions/loops are? I've done a couple of small (~50W) switchers, and I pretty much relied on the controller IC "doing the right thing" in that even if you flipped the power switch rapidly, the feedback loop would insure that the nothing bad happened. And I did test it and insure that this was the case, but perhaps I was just lucky! What's the typical cause of exploding power supplies? I have a suspicion you were working on a considerably higher power device...

---Joel

Yikes! 12 is even worse than 8.8.

Now I'm beginning to wonder... how many unexplained, seemingly random, logic failures are caused by smps's and random-duration brownouts?

Many switchers have huge open-loop voltage margins, especially 85..260 volt universal-input types run at 240 volts input. A 5-volt supply, running wide-open with 240 in, could easily make 15, maybe even 20, volts out.

John

It has been about 30 years, so this is rough.

There was a small electrolytic capacitor that charged up from the rectified line through a high value resistor. The energy in this cap supplied the drive and logic power, once the supply tried to start. A successful start tapped power from the switcher to resupply this cap and keep the whole shooting match going.

The voltage of that cap was monitored by a low power circuit that began a start attempt when some relatively high voltage was reached (so the cap could discharge, considerable while the supply was getting started, and still have enough voltage for everything to work correctly).

When this upper voltage threshold was reached, a switching transistor connected the capacitor to the oscillator and drive logic which began banging away but through a soft start sequence.

If the power cap voltage fell below the lower voltage threshold, the switch opened, the supply logic stopped cleanly, and the capacitor recharged from the rectifier resistor. There was no dribbling off or grinding on. One or the other. In the off state, there was a fixed and minimal load on the preregulator capacitor (only the voltage monitor and start up switch).

There may have been a few other nuances ( a few ministates in the latch up and unlatching of the on off switch, which all had to happen cleanly with no hesitation or high current draw), but this is the main set of states that worked, no matter how slowly the capacitor changed voltage, or no matter how fast or often. The voltage monitor also functioned as a zener clamp across the capacitor if some other failure prevented the switcher driver logic from using any current, to protect the low voltage capacitor.

Under very low line voltage conditions, the startup would try and abort every second or so, (you could hear it chirp) but it could do this for weeks on end without harm (and did).

message

Yes. That's normal and acceptable with decent kit.

*** though, supplied a 5V power supply whose sole purpose in life would on the face of it, be in supplying said 5V, yet the product proved on demonstration incapable of meeting it's primary intended purpose. If *** had specified a large overshoot at power-on or sold it as "usually 5V" then they're clear. (I'm in grumpy mode having discovered yet another 'issue' with a UK manufactured DVM) john

Yeah, I can do grumpy.

Last night I stopped off at Popeye's Cajun Fried Chicken to pick up dinner. A block away, I pushed down on the clutch of my Golf, and the pedal disappeared into the darkness. So I had to drive home along busy streets with stop signs and stoplights and serious hills all the way, down Divisidero through the Castro, Noe Valley, Diamond Heights, finally home to Glen Park, all with no clutch.

Snarl.

But the chicken was mighty good.

John

The latest fad is digital power supplies. A custom chip has a few mux'd ADC channels and does everything, control loop, current limiting, pwm drive and all, DSP. That will either lead to perfect power supplies or, maybe, it won't.

John

It would be pretty ironic if the bootstrap power supply (that initially gets the DSP going and all) was poorly designed in a manner such that it could -- given the 'right' conditions -- fry the fancy DSP controlling the main power outputs. :-)

Hydraulic or cable ?:-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
     It\'s what you learn, after you know it all, that counts.

What I like about digital is that the MSB is usually just as likely to be in error as the LSB.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

It has a cable from the pedal to the transaxle housing, but that's not the problem. No such luck. The lever arm that goes into the bell housing moves fine, no resistance.

John

Ah! Such fun! I've changed-out only one clutch... that was quite sufficient ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
     It\'s what you learn, after you know it all, that counts.

Me too. Once in a lifetime is about right.

John

Recent posts described a very common startup circuit that has a negative aspect related to static power dissipation in the high-value res from the rectified line to the control supply. (I put a FET in there to shut off the R when the supply was running on mine, to reduce that heat.) Also, if the supply fails to start, that R can get real hot.

Anyway, another type uses a very flaky "teaser" circuit to blip on of the main power FETs on every once in a while. If these supplies fail to start for any reason, they just sit there squeaking and chirping. The idea, when everything is running right is that the FET is just triggered one time for a few microseconds every couple of seconds, and just one "tease" is enough to charge up the control supply and start the whole supply up. This design usually uses fewer parts, and no big resistor. But, it is a mighty UN-controlled way to start the supply! I'll almost bet that this is how the supply that started this thread works. If the variation of the components is too large, or the breakover device that triggers the FET is flaky and "sputters", it can result in an overvoltage condition. Even still, the crowbar should be a totally independent function, just a Zener and an SCR. Not having a crowbar on a multi-thousand $ VME crate that is guaranteed to have even more thousands of $ of boards plugged into it is totally unbelievable. Our VME crates run about $7000, and have probably twice that value of boards in them.

Of course, this may be a case of two points of failure. You'll never know if your crowbar is working unless you actually test it. So, the crowbar might just have been non-functional, and nobody noticed until the power supply regulation went haywire. I can only see one way a momentary power dip would cause an overvolt. The power supply probably has a slow turn-on arrangement that normally keeps it stable. The dip was enough that the switching transistors went to

100% (or full) duty-cycle, thus causing the regulation to go open-loop. Integral windup occurs, then the mains power comes back on, and the loop can't react fast enough to rising main supply voltage. Whew, that takes some pretty crude design of the control loop! If it didn't have slow-up, it would do this EVERY time it was turned on. Also, a load dump would liekly trigger the same situation. The maker certainly should test to a worse load-dump than any user could ever cause. Does this crate allow hot-plugging? If so, then the load dump of a hot-unplugging needs to be covered.

Jon

[...]

A lost skill. Modern youff just won't believe a car can be driven without a clutch, they've been ruined by synchromesh. None seem now to have enjoyed the sweet pleasures of using a motorbike gearbox. Point of honour on the Beezer A10 not to use the clutch. john

Of course I learned my clutchless driving skills from motorcycles (clutch cables were always breaking) and my Austin Healey Sprite (hydraulics ditto.) It's a nuisance on hills, in traffic, where stopping-strategy becomes crucial.

Now all I have to do is get it to the repair shop downtown. I think I'll wait til Sunday midnight to drive it down there and park it outside the shop. Unusual for largish US cities, San Francisco is a ghost town after about 10 PM.

John