Is it the micro? Is it the power supply? Who knows!!!? - Page 1
Forum section: electronics / beginners

author: captainscarlet
author_rank: Contributor
timestamp: July 15, 2021, 06:44:06 pm
content: Hi,Weird one, I have a board with LT3971 buck converter and a STM32F745 micro.These micros are failing, seemingly at random (of course it's not random, I've just not found the cause!)The boards are powered at 12V from a Rigol DP832, through the 3.3V buck converter, which powers the micro. Usually it pulls about 40mA.Now I've found a weird failure mode.If I set my Rigol to 12V and current limit at 100mA, and power on the board, the Rigol's voltage drops to about 9V, but the PS doesn't hit the current limit. Almost like a zener is clamping the voltage.In this failure mode, if I set the current limit to 200mA, the board suddenly releases, the current drops back to 40mA, the voltage back to 12V.I don't understand it! There's no zeners that could be clamping the voltage, most of the circuitry is manufacturer's recommended.Any ideas?

author: JustMeHere
author_rank: Frequent Contributor
timestamp: July 15, 2021, 09:12:54 pm
content: I've seen a bad inductor selection cause something like this. I never figured out why. I just found one that worked correctly.

author: Kjelt
author_rank: Super Contributor
timestamp: July 15, 2021, 10:47:18 pm
content: Did you check for transient voltages at power up and down?I once had a dc converter that at powerup overshoot 20% , uCs don't like that at all.Replaced it with a Murata 78sr-3.3 no problems anymore.Could be a bit expensive for larger production runs though.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 15, 2021, 11:49:10 pm
content: Scope it? Sounds like the Rigol is lying, clearly it's not producing desired output voltage so it must be in a current limit. Perhaps the load is oscillating and you're reading the average, or median or something like that, hence the apparent confusion?Note it may be a good idea -- or mandatory -- to set the current limit a bit higher, to overcome the "hump" of startup current. Switching regulators have negative input resistance, i.e., once they start up, current drops as voltage rises. A buck converter will draw maximum current just above nominal output (or whatever it starts up at). A poorly compensated supply (don't know if the Rigol tends to do this) may oscillate under such load conditions.Tim

author: cortex_m0
author_rank: Regular Contributor
timestamp: July 15, 2021, 11:51:49 pm
content: Quote from: captainscarlet on July 15, 2021, 06:44:06 pmLab power supplies don't work like this. Either the Rigol power supply has a faulty design, your unit is broken, or this observation is not accurate.A "zener clamping" the power supply would only do so if the current limit were met, causing the Rigol to go into constant current mode. Chances are the Zener would boil off the PCB pretty quickly.

author: tonyh88
author_rank: Regular Contributor
timestamp: July 16, 2021, 12:29:18 am
content: Doesn't really make sense that your PSU drops to 9V without hitting the current limit in CV mode.Maybe as a first step just rule out if your Rigol PSU is not defective or related to the issue by using another PSU.Could be a simple 12V wallwart just to test see if you can reproduce the issue.

author: Faringdon
author_rank: Super Contributor
timestamp: July 17, 2021, 07:32:24 pm
content: T3slaco1l has got it..Your current limit is too low...and the inrush at startup, possibly along with the smps then pulling more current at low vin (like they do), is causing the problem...increase the current limit...thats all.This is a very very common problem of lab power supplies with current limit.You can buy expensive supplies that can get round this...but in truth, just set the current limit higher.This problem is so common that old engineers often use it to fool the heck out of the noob college electronics graduate.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 18, 2021, 12:00:32 pm
content: Yes, current limit set too low, can't supply the required inrush.Seemingly impossible indication is because the thing is likely oscillating. And I don't mean the control loop of the Rigol is oscilating, no, it's likely a relaxation oscillator around the CC-CV transition of the Rigol, and UVLO of the regulator.It happens like this:1 You enable the supply.2 Voltage exceeds the regulator startup limit.3 Regulator starts pulling current.4 Current exceeds Rigol's limit.5 Rigol goes into CC, as expected, dropping voltage.6 Voltage drops under regulator startup limit.7 Regulator turns off.8 Current goes to zero.9 Rigol goes into CV, as expected, voltage rises back10 goto 2"Impossible" indications are from "different points of time". Not great design, but not faulty per se. Voltage displayed is likely some kind of average, it goes both higher and lower than this. CC indicator does not apparently light up when it is in CC for short time only. A classical simple LED wired to the current limiter directly would at least light up dimly, but clearly the software has some delay of lighting up the indicator.On HP/Agilent 6632B, in such case, both CC and CV indicators light up, which is the correct indication of the situation.Scoping the supply reveals if this is the case.Finally, I don't think if this is a problem which even needs fixing, or is easy to fix at all. You just need a certain sized supply to overcome to current your circuit needs, otherwise it fails to start. By carefully playing around with current limit setting, you can find such oscillation from many designs powered by SMPS. In real world, when powered off a wall brick, the brick is chosen to supply enough current, or if not, it enters a hickup protection mode making the oscillation much slower (like 1Hz!) to be less of annoyance, clearly indicating even to a layman that "oh, it fails to start, maybe the supply isn't beefy enough".Sadly, I would hazard a guess that the micros failing has nothing to do with this behavior.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 21, 2021, 12:01:06 am
content: If your SMPS normally uses 40mA from a 12V power supply, then it would need four times that current from a 3V power supply (Same power, assume same efficiency).2*40 = 160mA and this is more then the 100mA current limit that you had set your power supply to.so when you turn the power supply on, it starts ramping up the voltage, and at some point the 100mA current limit is reached.This reminds me of the AVR "Dragon" (an AVR programmer I almost bought).It had a marginal power supply, and when the USB voltage was low, it starts pulling more current from the USB bus to pump up it's output voltage. And it does that to a point that it self destructs. Maybe the inductor used saturates or something. It was recommended to always run that thing from a powered USB hub to ensure it gets enough input voltage.

author: perieanuo
author_rank: Frequent Contributor
timestamp: July 21, 2021, 07:03:14 am
content: i guess everyone is right. the overcurrent your dcdc suck is maybe periodic and you haven't time to see this on rigol. the scope does see it.anyway, as implementing for some years dcdc's, i can bet your dcdc is not well layout-ed or some bad choice of capacitor or inductor. usually it's the layout to be improvedwe tested the prototypes with current protection on and rising the voltage from 0 to nominal, so you current limitation shouldn't destroy the dcdc if the dcdc is a solid design.leave alone the rigol (the only scenario rigol is the cause is if the rigol 'escapes' off-the-chart oscillation over the dc line, i doubt he's not respecting his specs, oscillocsope again tells you the naked true) and focus at your layout/C/L choiceas a testing procedure with prototypes we had also cutting the power supply with irregularity, i catched a bad design with this. it was the ground trace done incorrectly, the hw engineer responsible for the layout told me 'he overlooked the pdf', in the pdf they had examples and lots of explaining regarding how do you route the GND, he ignored that and cried why the boards were failing.

author: SteveyG
author_rank: Unknown
timestamp: July 23, 2021, 05:16:25 pm
content: It's definitely oscillating and the Rigol is just showing an average current. What do you have as soft-start on your DC-DC?