Filter choke or Build Inrush Suppression  - Page 1
Forum section: electronics / beginners

author: Jwillis
author_rank: Super Contributor
timestamp: August 17, 2019, 04:20:20 am
content: So far everything is working but still a work in progress.I'm intentionally holding down the maximum current while I'm still bread boarding.Short duration of 12.9 amps make the current carrying wires start to smoke. Also my electronic load is only god for 185W.I know I can get the full 20A but I don't want to set the board on fire again.Anyway .It shows 20000uf but I have 30000uF filter caps installed but the inrush current can be very high.Up to 100amps.Even with 20000uf its quite high. So I was wondering if I could insert a 20 amp choke and lower value of the capacitors.Not sure if my math is right but I'm thinking around 2mH for the choke.I can wire up a choke easy enough but would need to order the right thermistors.I didn't put this in the 0-30V 0-3A thread because I figured it's really a different circuit.

author: MosherIV
author_rank: Super Contributor
timestamp: August 17, 2019, 07:47:10 am
content: The normal practise to reduce in-rush current is to use a circuit with an in line resistor, relay and a time delay circuit. The time delay circuit turns on the relay and the relay bypasses (shorts out) the resistor.

author: SmokeEscape
author_rank: Newbie
timestamp: October 31, 2019, 09:51:56 am
content: Gents, I am a beginner in understanding and designing electronic circuits. I recently designed a circuit for a little project and I run into an issue of blowing my 12V to 3.3V DC-DC converter chip by connecting a LiPo battery. I had no issues using some thin wires on the bench testing the whole circuit up and down, but after adding some thicker wires with less resistance my TI TPS621672 converter chip catched fire ... tried multiple times with multiple boards of the sameI do think I have an inrush current issue here, charging the 10uF capacitor at the first place. This also creates some sparks while connecting the battery. What is the best method of reducing this inrush current? I did find different suggestions like the following:- PTC fuses- Simple series resistor- soft start circuit- Zener diodeTarget solution should draw only very little additional current (<50uA), as I am running for months on battery with this project. What holds me off using a resistor - SMD resistors in packages max. 1206 do only provide 1/4 watt (only limited PCB space available) and I struggle to calculate the right balance between max wattage and not to affect charging capability of the capacitor and may affect converter stability.Using PTC may also not work 100%, as they are very slow until they trip, as far as I can tell from reading the datasheets. Soft start circuit may draw additional power and is unknown to me how to design it appropriately. I did tried adding a 15V zener diode to clamp voltage spikes that may occur during current rush, but that didn't helped either, still magic smoke.Any suggesting is highly appreciated

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: October 31, 2019, 12:17:17 pm
content: Quote from: SmokeEscape on October 31, 2019, 09:51:56 amYou can get higher rated smd resistors. I have some 400mW 0805 ones. So keep you should find higher than 250mW on 1206 packages.Also have you actually measured the current? 10uF isn't insanely high.

author: mikerj
author_rank: Super Contributor
timestamp: October 31, 2019, 12:29:50 pm
content: Is the 10uF capacitor a ceramic type? If so consider adding a tantalum or electrolytic capacitor in parallel as outlined in thisapp note.

author: SmokeEscape
author_rank: Newbie
timestamp: October 31, 2019, 04:15:59 pm
content: QuoteThe 10uF is an MLCC, so yes, ceramic.QuoteNo, only tried to capture max Volt (measured ~19 volts from an average charged 11.4V LiPo on the terminal connectors near the buck converter). Not sure if my cheapy Uni-t multimeter is able to pick high speed inrush currents, but unfortunately it is only rated for 10A and I assume current is way higher ... even if only for milliseconds. Uncharged MLCC really acts like a short and I have beefy traces on my PCB too ... I may blow my Multimeter fuseChecked the app note, many thanks @mikerj , very helpful ... will try a combination with "high ESR" aluminium capacitor to see if that is able to dampen the spikes

author: schmitt trigger
author_rank: Super Contributor
timestamp: October 31, 2019, 04:50:24 pm
content: @JWillis:I would use both the inrush circuit (relay + power resistor + monostable) and inductor.The reason being that an inductor will also reduce the current's crest factor.I would suggest that you simulate the circuit, with and without an inductor.Having said this, you require to know (or measure) the ESR of your capacitors, the inductor and your transformer, to obtain a realistic current's inrush and crest factor values.Or if you have a high current probe and a scope, you can directly measure it.