horror stories about switching power supplies?

[coppercone2]

Did they need paramedic to come in when someone saw the BOM cost of that part?

I tried to justify it once, on paper, but I believe I ended up deleting that document. 

Maybe 15$ unless your building the smallest fucking thing ever. 10$ because its a BGA and no one wants to inspect it.

[coppercone2]

At first I was like 

Then I saw the price, compared the solutions 

Then I thought about inspection and repairability 

[coppercone2]

Was it one of the LOWEMI ones? Like this one
http://www.analog.com/en/products/ltm4613.html#product-overview

Did it match

My main draw to it was that it would not be a noisy thing that requires careful layout since its all integrated.

I do find it appealing for my own use, but not for sale

[T3sl4co1l]

I don't remember what the spectrum was like.  I remember it needed a metal box (also for cooling, because efficiency as I mentioned) and a nice big ferrite bead looped on the cables, which sucked that much more because of the feed-thrus, and the wires being soldered in.  It was all rather tight and fiddly.

Package size was still as expected, though.

Tim

[dmg]

Once I got ultrasonic coupling from corner to corner of a PCB, coming from an inverter charge pump, as not every switchmode power supply requires inductors.

The design was RF related and the component side height-constrained due to some heatsinking and standard card format requirements.

There was an very low phase noise RF synthesizer, requiring a fairly high (uF's) capacitor for its loop filter, and the only capacitors meeting height constraints were class 2 ceramics, which are piezoelectric and a no-no for this application unless you want to make a very sensitive sismograph (as we discovered later). Area constraints also didn't allow for paralleling too many caps so in the end we just chose the lowest capacitance density class2 ceramic cap that met requirements.

There was an inverter charge pump on the other side of the board which again required ceramic caps due to height constraints. It wasn't strictly required but using it slightly improved (2dB or so) linearity of the ADC preamp and the overall  SFDR in consequence for some gain settings.

We power up boards and the output from the synthesizer shows -80 dBc spurs at the switching frequency of the charge pump. Touching the board at some places or installing/removing the heatsink varied the spur level, so we suspected mechanical coupling (the charge pump was separated 20cm from the synth, and shared no power rails with it). We changed the capacitor to something non-piezoelectric on one of the boards and the spurs dropped to -95dBc, but were still there. Turns out that the interference was also sneaking  through the decoupling capacitors of the synth's charge pump power rail.

In the end we just removed the charge pump and it was OK.

[coppercone2]

thats amazing

[coppercone2]

How do you dampen ultrasonic vibrations anyway? Like say you have a DC DC converter box in the same enclosure?

It sounds like you can't get away from it. Does it transfer on wires too if you broke the PCB in half with tiny cable or maybe flat flex (so you can still have a ~solid ground plane? Join the two PCB halfs together with a tight but flexible braid plane?

How about isolating it from the floor, even if you manage to break it up some how? All those springs and stuff are meant for low frequencies, will they help? Or transmit through?

Does anyone have any figures or graphs relating to ultrasonic energy transmission through a PCB so spacing guidelines can be established?

[james_s]

In my early days of repairing stuff, switchmode PSUs were somewhat mysterious and hard to diagnose. Once I had an ESR meter and knew how to use it, I started having much better success. By far the most common problems I see are bad electrolytic capacitors. The most tricky to diagnose types I've encountered are some of the older self-oscillating designs that lack any sort of controller. The one Tek used in the TDS400 series scopes is also a real bear, half the problem is the lack of good documentation and the stupid ceramic hybrid module.