Osilloscope scoping problem - Page 1
Forum section: electronics / beginners

author: Olegkuskhov
author_rank: Contributor
timestamp: October 28, 2023, 01:54:24 am
content: Hi all, i'm new to electronic aka young player as dave said. Recently i make a sychronous buck converter for my mppt project. Firstly, i powered my buck converter with my bench power supply but it turns out that my bench psu is not capable of delivering even 1 amp when my buck is underload. So i switched to my transformer and full wave rectified dc with 2x10mF caps. Fyi, i use plug with only two leads (live neutral and no earth) (the dc volt that i get from this transformer psu is arround 23 to 24 volts). I start to wire things up and things were okay. I start to load it and play arround and the buck that i made is actually not bad, it can delivers 50 to 60 watts power (in my test). things were okay until i probe my oscilloscope (1x attenuation) into buck converter's output under the load. My idea is to measure the p-p ripple and i can see that under the load the ripple is a bit high( but that's another things). After sometimes maybe arround 20 seconds, my mosfet suddenly made a cracking sound and that magic smoke starts to appearing. I check the connection many times and i believe that my wiring is okay. I wonder if this has to do with earths problem or the 1x probe just has much impedance and causing osscilation in my circuit that destroy my lovely mosfet hahahaha. Or there is just other things that i didn't know but exist guys?. i fry about 10 mosfet because of rhis hahahahaha. Don't insult me guys

author: Kim Christensen
author_rank: Super Contributor
timestamp: October 28, 2023, 02:04:24 am
content: Post a full schematic of the circuit and show how you connected the scope probe. (ground clip and tip)This shouldn't normally be a problem. In the end, where you connected the scope's ground clip should be the negative/common rail of your battery system, so if connecting a scope there blows it up, then it's going to blow up when you try to use it in your system.

author: Olegkuskhov
author_rank: Contributor
timestamp: October 28, 2023, 02:40:43 am
content: Hi kim thanks for your reply. I still outside my city, i apologize for my drawing below. Here is the rough schematic and the real picture of it. The oscilloscope probe tip is connected to the + of my buck and the ground clip to negatif of my buck. I was hapilly loading like 5.4 amps at 11 volt roughly for about 5 minutes. I hook up the ossiloscope and play arround with it and several second afterwards the mosfet starts to blow up. This happen twice when the osilloscope hook up, when not, everything seems working fine.

author: Kim Christensen
author_rank: Super Contributor
timestamp: October 28, 2023, 03:45:53 am
content: Quote from: Olegkuskhov on October 28, 2023, 02:40:43 amWell, when you get back to the city, post the proper diagrams. I can't tell much from that.

author: Vovk_Z
author_rank: Super Contributor
timestamp: October 28, 2023, 07:43:51 am
content: Possibly it happened because of a grounding wire (it is connected with a common ground on a scope), but I can't say w/o seeing exact circuit.I use my scope ungrounded so it's less dangerous to devices I work with a scope.Anyways, usefull tip: use 1:10 divider on a scope probe only. Typically all work may be done with 1:10. Use 1:1 only if there is a strong need and if you are sure its large capacitance won't hurt.

author: Faringdon
author_rank: Super Contributor
timestamp: October 28, 2023, 06:59:28 pm
content: How did you do the hi side drive to the fet?Do you have proper syncronisation of the top and bottom fet?Do you have tight layout, including in the gate drives?Do you use diode to protect gate drive output?Do you use series gate drive resistor?What voltage is used to drive on the fet?What is the fet gate voltage rise time?Is there suitable dead time to fet gates of sync fet and top fet?When you do with synchronous buck, it is best to get it working without the sync fet in cct at first.....just have a diode there...be sure it is fast enough diode too.Thn when ok in non-sync mode...you bring in the sync fet.