Blown buck converter possibly from inrush current. - Page 1
Forum section: electronics / projects

author: skstarne
author_rank: Newbie
timestamp: November 29, 2020, 07:53:29 pm
content: This is my first 'real' PCB design where I am not using off-the-shelf modules to supply power. I've picked the TS30013 for it's current capacities as well as it's price. I've let the smoke out of 3 of these chips so far during my testing, and out of fear of destroying a channel on my scope, I haven't hooked it up. I've bodged in a 1N4002 (all that I have on hand, but I've ordered some schottkeys to put in it's place per the datasheets optional components), but it hasn't helped with the frying of the chips.As a recommendation from one of Adafruit's guideshttps://learn.adafruit.com/adafruit-neopixel-uberguide/basic-connections, I've put in a 1000uF cap across the supply voltage and I am thinking that is possibly the cause of the smoke.Any guidance would be more than appreciated.More details:Atmel 328PB microCANBus supporting chips25 sk6812 RGBW LEDssome optoisolators for input/output.project page:https://github.com/seanstar12/dance-pad

author: skstarne
author_rank: Newbie
timestamp: November 30, 2020, 10:48:13 pm
content: As an update to this adventure, I placed a 25 ohm 5w resistor in series from my 12v source. That seemed to work well for limiting the inrush, but after the circuit sets for an hour or two (in the off state), the ts30013 starts pulling 2A on startup. I'm going to do some more replacing and assume it's the large electrolytic cap that is causing the fried chips. This time I'll put a 47k resistor in parallel with the cap.I removed the 1000uF cap as a test, and the only out come was some flickering by the LEDs (figured that would happen). But it didn't destroy the ts30013 after several power cycles, so that's nice.I guess what I am trying to ask here is, am I doing something wrong? Is there a more elegant way to limit the current in? Is my circuit filled with 'traps for young players'?

author: KT88
author_rank: Frequent Contributor
timestamp: November 30, 2020, 11:44:23 pm
content: Inrush current shouldn't do any harm as the TS30013 is a current mode regulator. Circuit and layot don't show severe issues as well.What looks odd to me is the optional catch diode - haven't seen that in a sysnchronous monolithic buck yet... If the lowside FET is somehow overloaded it may cause issues. A litmus test for that would be to add a low Vf shottky as shown in the DS. Pick a small one that could heat up to reduce VF while operating (not too small thgough).CheersAndreas

author: skstarne
author_rank: Newbie
timestamp: December 01, 2020, 03:41:02 am
content: @KT88 I really appreciate the feedback. This thing has been driving me nuts.I was able to find a 'NTE578' in one of my parts boxes (If:1A, Vf: 0.79@1A). Attached that diode, removed the 2 Ohm resistor and kept the resistor in parallel with the cap. I've power cycled it several times and it hasn't done anything crazy yet. I was assuming that the sk6812s would drain that capacitor enough that it wouldn't be an issue, but since adding that resistor, I haven't had to replace the ts30013.I've attached a photo of the circuit as it stands with the catch diode in place. Please ignore the extreme flux build-up, this board has been through the ringer.

author: KT88
author_rank: Frequent Contributor
timestamp: December 01, 2020, 01:51:04 pm
content: Your comment on the resistor in parallel with the cap hints another possible issue: A synchronous buck operates as a synchronous boost in the opposite direction. When turned on the charge of the large output cap could possibly been dumped into the input cap which is significantly smaller in your design. That way the input voltage could exceed the max. allowed input voltage which then takes out the regulator. Not sure if this happens here but you might check...

author: skstarne
author_rank: Newbie
timestamp: December 01, 2020, 05:33:11 pm
content: Interesting, and sounds totally plausible. If that were the case, (from a circuit design standpoint) is there a better way to mitigate this on the next revision? My initial thought would be to put a diode after the inductor (to prevent the cap from discharging into it) and switch to an adjustable version of the buck converter to add back the 0.7 volts or put a zener in parallel (this one wouldn't prevent charging the coil). But that's just basing what I know from text books. And I'm not sure if those are even good ideas here.From simulating the circuit, it looks like spikes around 40A and 100V when that coil collapses. So if anything, I can't see how that catch diode is even remotely optional. Since VSW to PGND max is 26.4V, that would explain the fried chips. And looking at the input cap, it peaks at 157V but averages around 18v (which is the higher-end of allowed)(and not going through the buck/boost, just natural response). That simulation probably isn't the greatest for the component though.I'll solder some test leads to the bypass cap and the diode and get a proper scope measurement. Now that I've done some simulation, I'm less afraid of hitting the 300v limit of my scope.