Inrush current limiting - Page 1
Forum section: electronics / projects

author: Rachie5272
author_rank: Regular Contributor
timestamp: March 04, 2020, 06:58:17 pm
content: I have a design which has about 480 uF of big electrolytic capacitors on the main 48V power rail. Plugging in the power connector arcs every time, and a 5A fuse keeps blowing. I already have a big P-FET for reverse polarity protection, and I'm hoping it can double as an inrush current limiter.Can someone help me modify the design? I'm guessing it just needs a capacitor and maybe another resistor, but I'm not entirely sure where.Thanks!

author: Zero999
author_rank: Super Contributor
timestamp: March 04, 2020, 07:24:38 pm
content: If you intend the MOSFET to provide any short circuit protection, then it's backwards. The bode diode will just conduct in the other direction. You probably wouldn't have made this error, had you used the correct MOSFET symbol.Connecting a capacitor between the gate and drain, in the current circuit will considerably slow the turn on time, thus provide some inrush current limiting.Because the MOSFET needs to block current flowing in both directions, it's not possible to use the same device for both inrush limiting and reverse polarity protection, unless you use something cleaver like a charge pump and an N-channel device.

author: MarkF
author_rank: Super Contributor
timestamp: March 04, 2020, 09:13:40 pm
content: First, 480uF is not a large capacitor.Most power supplies have a capacitor in 6800uF range after the bridge.Anyway, you could try adding a capacitor across R1 to slow down the gate voltage.Edit: Actually, I think the capacitor needs to be across the zener diode.Refresher:

author: Zero999
author_rank: Super Contributor
timestamp: March 04, 2020, 10:59:09 pm
content: Quote from: MarkF on March 04, 2020, 09:13:40 pmThat circuit is no good as a soft start, because the body diode will conduct, before the MOSFET turns on.Regarding the place of the capacitor: it will work with it across the zener diode, but between the drain and gate is better because the Miller effect boosts the capacitance, as the MOSFET turns on, so a smaller capacitor can be used for the same delay and it will discharge more quickly, when the power is removed.Here's a basic soft-start circuit, with an N-channel device to aid understanding. I don't have the simulation handy and will post it later, unless anyone beats me to it.

author: angrybird
author_rank: Regular Contributor
timestamp: March 05, 2020, 02:22:44 pm
content: You could also use a series resistor with the MOSFET across it, and turn the MOSFET on based on voltage across the capacitor. This would probably require 2 MOSFET, a little one and the big one you already have, but I've done it in a few different ways in the past and it works well. Just make sure you size the resistor such that if there is a short circuit on the output, you don't burn the resistor out before the fuse blows.

author: Ice-Tea
author_rank: Super Contributor
timestamp: March 05, 2020, 02:34:56 pm
content: Make sure to verify the SOA of your FET and derate plenty. You'll be spending some time in the ohmic region of the FET

author: Zero999
author_rank: Super Contributor
timestamp: March 05, 2020, 08:30:04 pm
content: As promised, here's the simulation comparing the capacitor between the gate and source and gate and drain.

author: MarkF
author_rank: Super Contributor
timestamp: March 05, 2020, 11:29:12 pm
content: Quote from: Zero999 on March 05, 2020, 08:30:04 pmGreat. Now redo it with the MOSFET on the high side like the OP wanted it.Personally, I have a need for a solution with the load tied directly to ground.