Electronics > Projects, Designs, and Technical Stuff
Inrush current burns switches
ogden:
--- Quote from: wraper on June 14, 2019, 02:00:28 pm ---You either need closely matched MOSFETS for that to work well or prohibitively high resistance source resistors for low voltage applications if you just use random MOSFETS.
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Yes, sure. You better measure VGS for each MOSFET and make sure it's spread (difference) is way smaller than voltage drop on balancing resistors. If load does not activate during soft start, then single big linear MOSFET with resistor could be enough for soft start with bunch of parallel "runtime" MOSFET *switches* (without resistors) that are opened *fast* when nominal voltage reached.
OM222O:
Am I missing something here? You don't nees to use a fet in the linear region here :-// the only problem that OP pointed out was the contacts welding due to inrush current, so I assume the circuit itself is fine with that ???
I had a similar issue for a project (only suitable size switches (8x8mm latching switches) were rated for 100s of mA but I needed 1.5A) and just used the switch to turn on the fet (i.e. short out gate to GND since I was using a P channel fet and a 10K pullup resistor to source) you can use an N channel fet with a pulldown resistor and a switch to connect it to VCC. Then the fer carries all the current wich would be fine, and the switch no longer welds itself since it carries no real amount of power :-+ if the entire circuit requires inrush current limiting, then this would not work.
Edit: if you decide to switch the AC mains, you can still do the same with a triac rather than fets.just be sure to include an optocoupler since it's not wise to have your switch controll actual AC unless it's fused and tested for safety (i.e: not cheap chinese crap) but I would still use an optocoupler even with a proper switch just to be extra safe ;D
ogden:
--- Quote from: OM222O on June 15, 2019, 07:58:55 pm ---Am I missing something here? You don't nees to use a fet in the linear region here :-// the only problem that OP pointed out was the contacts welding due to inrush current, so I assume the circuit itself is fine with that ???
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Perhaps you are missing fact that we talk about FET-based current limiter from another post, answering question "why it just blows FETs":
--- Quote from: rachdatu on June 14, 2019, 09:56:50 am ---I built an inrush current limiter for my ebike and it works. I then tried to use the same circuit for these BPS units and it did not work. It just blows the FETs and I don't know why. There is a long thread in the endless-sphere.com forum about these limiters. There are a lot of versions... You can start here (https://endless-sphere.com/forums/viewtopic.php?f=3&t=40142&start=100
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H713:
Sounds like a rather mundane soft-start circuit would cure this problem. This is a common thing to see in power amplifiers, as many have such large capacitor banks (often exceeding 30,000uF per rail for the top-end PA amps). Lots of receivers from the likes of Marantz and Pioneer had big enough cap banks to destroy the power transformer on startup without a soft-start circuit, and an MC^2 MC1250 would instantly trip a 30 amp breaker.
When the power switch is flipped, cap banks are charged through a 10 ohm resistor (value depends on the exact application). A relay then shorts across that resistor after a few seconds. Big linear supplies need this kind of thing simply due to the insane capacitor banks. Without this you start welding contacts and tripping breakers. Since a switchmode power supply has a rectifier and caps directly off the power input (no transformer limiting the current), the inrush current is essentially limited by the ESR of those caps.
wraper:
--- Quote from: H713 on June 16, 2019, 07:20:05 am ---Sounds like a rather mundane soft-start circuit would cure this problem. This is a common thing to see in power amplifiers, as many have such large capacitor banks (often exceeding 30,000uF per rail for the top-end PA amps). Lots of receivers from the likes of Marantz and Pioneer had big enough cap banks to destroy the power transformer on startup without a soft-start circuit, and an MC^2 MC1250 would instantly trip a 30 amp breaker.
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It would fix what shouldn't be there in the first place. Placing switch on the mains side is easier and cheaper solution. And the most important of all, it actually switches all of the thing off.
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