shorting 2 voltage rails together - Page 1
Forum section: electronics / beginners

author: VEGETA
author_rank: Super Contributor
timestamp: December 15, 2024, 01:30:38 pm
content: Hello,I wonder what happens when shorting 2 voltage rails together.the rails are 5v and 3.3v using TI modern buck regulators. they have hiccup mode for short circuit to ground which i verified to work.another rail is an lm317 12v output.what do you think will happen if we connect 2 of these rails together? any damage? permanently broken rails?my thinking is that the bucks use feedback voltages and since there is 5v or 3.3v then the feedback voltage will change and deliver another output voltage. once the short is removed and power is off, then powered on, it should return to normal output.not sure about lm317 despite also having similar output resistor feedback.all assuming no real reverse polarity protection.I had a prototype which had its 3.3v rail permanently broken which outputs a wrong voltage even when power cycling, don't know if i shorted it to 5v or not tbh.please share your thoughts.

author: xvr
author_rank: Frequent Contributor
timestamp: December 15, 2024, 03:34:39 pm
content: In general if higher output rail voltage is in range of maximum output voltage of other rail it should be Ok. Lower voltage rail just turned off (because its FB will gone high above normal and regulation circuit just turn off regulator).But it could be problem if feedback sourced directly from output that internally protected by diodes. This configuration could feed lower rail source from higher rail output.

author: Gyro
author_rank: Super Contributor
timestamp: December 15, 2024, 05:38:24 pm
content: It really depends on many factors. The specific parts/variants used, the available supply current on each rail, the nature of the loads and load currents, distribution of filter capacitances etc. otherwise it is impossible to make a prediction.Most at risk are the loads - presumably you're not shorting the supplies in isolation. An LM317 (depending on manufacturer, package, and input-output differential) can source an instantaneous current in excess of 2A before thermal limiting cuts in. It's reasonable to assume that another rail shorted to it is likely to at least approach its 12V output. I would think that this would damage devices connected to the 3V3 rail for instance.The only way to be sure would be to take transient measurements on a sacrificial board. Better still, fit appropriately rated TVSs to each rail if you are accident prone in this way.

author: magic
author_rank: Super Contributor
timestamp: December 15, 2024, 05:43:47 pm
content: Such devices could include capacitors, for example.Some synchronous buck controllers have a "suicide overvoltage protection" mode which turns on the low side switch to short the rail to ground. Assumption is that the overvoltage was caused by the high side switch shorting out and the low side is typically beefier so it has a chance to protect the load.

author: Vovk_Z
author_rank: Super Contributor
timestamp: December 15, 2024, 06:06:31 pm
content: Quote from: VEGETA on December 15, 2024, 01:30:38 pmLower voltge (3V3) gets powered from 5V rail, at least for a short period of time. It doesn't add health for 3V3 rated parts.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 15, 2024, 06:11:50 pm
content: Well, unless you are willing to do careful analysis of circuit specifics, good initial assumption which holds true in most practical circuits is that the lower voltage rail rises up to the higher voltage, destroying parts on that rail. Unless those parts can survive the higher voltage, but usually they won't.

author: VEGETA
author_rank: Super Contributor
timestamp: December 15, 2024, 06:36:38 pm
content: buck regulators are tps 564247 while lm317 is limited to around 340 ma via an efuse, the "rail" is taken after the fuse. while the bucks are not fused but has good amount of capacitors on the output.what i am talking about is shorting these rails without load connected. actually even this is impossible to do but still nice to know.reading the datasheet of buck, it has many protection features like over voltage which has a certain threshold. meaning if 5v is connected to 3.3v, the 3.3v will exceed it and shut down while 5v will be unaffected. correct?as for lm317 output it is taken after the efuse which has max limit of about 350mA or so, what could happen here?after this is gone, powered off, then powered on... will it return to normal operation?