Buck converter shuts down if any wire is touched by probes or some other wire. - Page 1
Forum section: electronics / projects

author: nardev
author_rank: Unknown
timestamp: February 27, 2018, 12:00:44 pm
content: Strange thing happening,I tried to usehttps://www.mouser.com/ds/2/631/ACT4088... 345917.pdfFollowed the example guide for step down 12V to 5V. Than, connected LM1117 on the 5V out to get 3.3 and now i have an issue. When i power up the circuit with 12V, everything works until i touch it with probe or some other wire.The input is 12V 2.5A PSUWhat could i do wrong?p.s. here is the video how it happenshttps://goo.gl/byqtNKThe schematics in case it can help:http://www.hepek.ba/buck.pngp.s.Ah, i noticed something very strange,if i hold with my fingers the main 12V input (directly on the contacts), and touch some pads with wire in another hand, it doesn't happenMust me some mess with potentials, grounding .. etc...Any thoughts?

author: Wimberleytech
author_rank: Super Contributor
timestamp: February 27, 2018, 01:58:04 pm
content: Do you have any load on the lm1117? For it to operate properly, you need a minimum of 5mA. With no load, the buck is operating ai min duty cycle...maybe on the edge of stability. When you touch it with your body at elevated potential, I am guessing you are pushing the buck over the edge and it shuts down.

author: nardev
author_rank: Unknown
timestamp: February 27, 2018, 03:37:25 pm
content: Here is the thing, same issue doesn't appear on another board, with SMD resistors...But that setup gives 3.3V so i wanted to check how it would drive more devices for 3.3 and 5V.. SO i had to change resistors. Unfortunately i didn't have any 1% SMD which would work so i just soldered any 5% resistors and used one variable one.. And it was messy..Meanwhile, i changed the variable resistor to fixed one and it's very hard to cause same thing to happen.It's definitely something about the grounding, probably also poor PCB design since those bucks are quite sensitive .. etc..I just want to make it more stable..Lack of experience with those things....

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 27, 2018, 05:41:22 pm
content: You could double check all your solder joints.Is the EN (enable) pin properly tied to the input voltage like it's shown on the schematics?(The datasheet says you can leave it floating, but since it has a very weak internal pull-up, I think it would be pretty easy to unintentionally disable the buck regulator with some external perturbation if the pin is left floating.)

author: nardev
author_rank: Unknown
timestamp: February 27, 2018, 08:02:04 pm
content: It was only on the schematics.. i cut it off.Meanwhile, i used fixed resistors and the issue is almost gone.. so it's obvious, something shuts down the chip and probably the fact that i have been using regular 5% resistors with looong wires, instead of 1% SMDs adds to it. Since, same board, with SMD resistors, doesn't make any issue no meter what you do to it.

author: Niklas
author_rank: Frequent Contributor
timestamp: February 27, 2018, 09:40:41 pm
content: Long wires connected to the sensitive feedback pin on the Buck converter is not a good idea.Page 6, Setting the output voltageThe feedback resistor (RFB1) interacts with the internal compensation network, and plays an important in setting the ACT4088's transient response and ensuring stability.Page 10, PC Board layoutThe external feedback resistors should be placed next to the FB pin.Is the PSU made supposed to have a protected earth connection to mains but lack the PE connection? Some switchmode power supplies for laptops I have had charged the laptop enclosure and gave a tickling feeling if they were not connected to a main outlet with PE. By connecting only the wire to your circuit, you can create a path for discharging such charges from your setup and those current pulses can couple into the long wires on the FB network. When you put your fingers directly on the input, then your body will provide another discharge path.

author: nardev
author_rank: Unknown
timestamp: February 28, 2018, 09:41:14 am
content: Thank you very much....it's basically it..