Author Topic: Limiting inrush current into an electrolytic capacitor  (Read 24036 times)

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Offline dazz1

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Re: Limiting inrush current into an electrolytic capacitor
« Reply #50 on: October 05, 2018, 07:17:44 am »
Hi
I looked at the simplest option of a series inductor shunted with a reverse biased diode.
Basic simulation showed I needed a 1mH inductor to keep the in-rush current below 70Amps on  a 12V battery fed system.  The shunt diode does very little, with only about 5 amps flowing when it conducts.
In addition, even with the diode shunt, the L/C combination showed a relatively slow harmonic decay.

This was fixed in the model by adding a series resistor to damp the oscillation.  By itself, adding a resistor was sufficient to reduce the in-rush current below 70A (the battery trip current).  Adding a resistor would be simple but inefficient.

Installing an 1mH inductor that doesn't saturate at 70A is not an option.  Too big, too heavy and too expensive.  An active in-rush limiter is the most obvious solution.

I found this paper  http://www.mosaic-industries.com/embedded-systems/_media/pdfs/application-notes/motorola-an1542-active-inrush-current-limiter.pdf that describes simple circuits that do what I want.   I don't need overcurrent protection because the LiFePO4 battery includes a crowbar circuit as part of the internal BMS.  I already have a standard fuse as well.    I like the idea of the PTC thermally linked to the transistor but if the transistor is rated to take more power than the battery protection, I wouldn't need the PTC.

What I don't currently have is a battery fuel gauge so combining the in-rush current limiter with a fuel gauge on the same PCB would make sense.

Dazz
Dazz

Over Engineering: Why make something simple when you can make it really complicated AND get it to work?
 


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