Limiting inrush with large capacitor bank - Page 1
Forum section: electronics / projects

author: rx8pilot
author_rank: Super Contributor
timestamp: November 21, 2014, 02:55:40 am
content: I am searching for the simplest way to deal with putting a large amount of capacitance on the main voltage bus. This is a power management circuit that is regularly powered through a relatively long cable between 12-17 volts @ 10-15A max. It has boost and buck converters as well as filtered FET controlled outputs at the bus voltage. Because of the long input cable, the outputs benefit from having a solid amount of bulk capacitance to start up capacitive loads without the bus voltage dropping too much.CHALLENGE:The challenge is to control the inrush during device startup when trying to charge about 8200uF of low ESR caps (10x 820uF in parallel). I do not control the power sources or the loads that the circuit sees which makes this even more challenging. I have seen some really weak power supplies as well as some really terrible power cables. That all adds up to the need to have a nice soft start-up while still being able to start capacitive loads without rail dipping 10 volts.My first thought was to use 2 FETs where a low-current one feeds the capacitor bank through a resistor until it is at the bus voltage and then switches to a high-current FET connected directly. This would involve either using some analog comparator arrangement and a FET controller or maybe the ADC on the microcontroller already in the circuit to manage.This seems a little complicated and maybe someone that has faced this before knows a simple solution. Any ideas?

author: rx8pilot
author_rank: Super Contributor
timestamp: November 21, 2014, 04:13:47 am
content: Thank you, that is helpful info. An integrated solution is really the best in this situation since I would prefer to have the shortest learning curve possible as well as being in a VERY tight enclosure. The reverse current is an interesting point. Hopefully the controllers can help with that so I don't need an ideal diode taking up more space and adding to the already bloated BOM.I will go dig through the Linear options you linked. There is already about 12 Linear devices in this design which work well - electronic fuses, source prioritizer, FET control, etc. They are kind of expensive but so far bullet proof.

author: rx8pilot
author_rank: Super Contributor
timestamp: November 21, 2014, 04:17:47 am
content: Quote from: 3roomlab on November 21, 2014, 04:07:36 amInteresting paper. Taking a look....

author: rbola35618
author_rank: Frequent Contributor
timestamp: November 21, 2014, 06:08:36 am
content: You can use a p-channel MOSFET and add a miller cap to control the rise time. The external cap is connected between the gate and the drain.

author: rx8pilot
author_rank: Super Contributor
timestamp: November 21, 2014, 07:14:26 am
content: Will that type of configuration allow the charged capacitors to discharge at full speed? Slow charge followed by fast discharge is the goal....

author: rx8pilot
author_rank: Super Contributor
timestamp: November 27, 2014, 04:19:15 am
content: So I have been researching and reading about how to best achieve the goal of effectively pre-charging a capacitor bank to near the rail voltage before connecting it to the load. I have seen many sophisticated designs that are stand-alone and control the capacitor inrush. The problem is that they all seem to involve too many components and/or an expensive controller. All I really need to do is slowly build the charge in the capacitor bank and then connect it to the load. The inrush current on 10 parallel 820uF caps is huge and must be controlled.A word about the load itself:The final load is out of my control and can be very dynamic - which is why I am putting in a substantial capacitor bank. The power source is also out of my control and will not likely be able to deal with big capacitive loads. To make things worse, there will probably be a long-ish cable between the power supply and this circuit, further reducing its ability to deal with dynamic loads.I roughed in a very simple concept that takes advantage of the fact that I can control the startup time and sequence of the system. The goal is to have the capacitor bank isolated by a MOSFET and resistor. When the power is first applied and the caps are at 0 volts - they charge through a resistor to keep current low. The value shown is rough. After a pre-determined amount of time that would allow the bank to charge to nearly the input voltage - the MCU can turn on the FET and continue with the rest of the start-up sequence.Will having the resistor across the source-drain create any problems? Is this too simple, maybe I am missing something. All other attempts I have found on the web are FAR more sophisticated.

author: AG6QR
author_rank: Frequent Contributor
timestamp: November 27, 2014, 05:43:26 am
content: I'll point you to a commercial project that does a very similar job, intended for ham radio applications of 12 to 14 volts. Ham radios, especially Morse code transmitters, can have peak current demands that are much higher than their average demands. A large bank of capacitance can smooth the demands out a bit.http://www.mfjenterprises.com/Product.php?productid=MFJ-4403Click on the "downloads" link and you'll find a link to the owner's manual, which includes a simplified block diagram, plus a detailed schematic.It uses 4.16 Farads (not a typo) worth of supercaps. That's 6 supercaps at 25 Farads apiece, in series. It charges them through a resistor initially, and when the charge is sufficient, a relay trips and the input is connected directly to the capacitor bank. The output load is always connected across the capacitor bank.There are several other features, like status LEDs, bleeder resistors, and protection circuitry, which you can see in the schematic.It's not hugely different from what you've come up with, but sometimes it's interesting to compare ideas.