Limiting inrush current on supercaps - Page 1
Forum section: electronics / projects

author: philpem
author_rank: Frequent Contributor
timestamp: March 27, 2014, 10:55:39 am
content: Hi guys,I'm designing a circuit which uses a 5.5V supercapacitor (charged to ~5.45V) to allow an MCU to capture and store a few minutes of data when power is lost. My initial tests worked fine on the bench supply, but on a prototype the inrush current is overloading the power supply and causing it to enter a limiting fold-back state.After the supercap there's a high-efficiency dual-mode switchmode converter (it can adaptively switch between buck and boost modes depending on input voltage).The obvious solution is to add a series resistor - I calculated 120 Ohms to limit inrush current to ~50mA. The catch is, when the voltage starts to drop, the converter enters boost mode and the voltage drop over said resistor in a discharge condition reaches a point where the SMPS output becomes noisy. Eventually the voltage drop is so bad that the SMPS shuts down even though the supercapacitor voltage is still above the SMPS shutdown voltage.My next thought was to bypass the resistor with a Schottky diode -- but this didn't make much difference.How can I limit the inrush current to the capacitor while maintaining a low-resistance path to the SMPS and from then on to the load?I was thinking one of the MOSFET-based "reverse polarity protection" circuits might be a good starting point -- though I wonder if there's a better way?Thanks,Phil.

author: madires
author_rank: Super Contributor
timestamp: March 27, 2014, 11:45:05 am
content: Maybe something like the circuit below. You could also replace D2 with a wire.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 27, 2014, 12:01:55 pm
content: Active current limit.Ideally, from another switchmode controller, if you're real tight on efficiency. Otherwise, something simple like a PNP current source can provide inrush limiting while supplying enough current to allow the subsequent dual-mode controller to behave itself.There are two kinds of SMPS behaviors, I suppose; those that limit current softly (continuous, stable operation in current limiting mode) and those that don't (bang-bang limiting).The latter are often the offline flyback/foward converters, which are self powered; short the output, they stop being self powered, and they go putt-putt-putt all day.Onboard DC-DC type controllers are usually the "stable" kind, but always check the datasheet to make sure. That dual-mode converter could have the same instability, for the same reason?Tim

author: Kjelt
author_rank: Super Contributor
timestamp: March 27, 2014, 12:27:18 pm
content: I use this for backupbattery support for the RTC in my project.Make R3 somewhat lower if you want faster charge. In my case i don't need a fast charge since the pcb is 24/7 on.I thought and learned that it is good practice to put a series resistor for a goldcap since otherwise the too high inrushcurrent would also damage the goldcap.

author: Tinkerer
author_rank: Frequent Contributor
timestamp: March 27, 2014, 10:31:41 pm
content: Dont know the strength of the inrush, but would a thermistor be out of the question?

author: philpem
author_rank: Frequent Contributor
timestamp: March 29, 2014, 07:11:42 pm
content: The inrush strength is a few hundred milliamps - the power supply folds back at about 100mA.I've found a solution though - a low-Vgs(th) P-channel MOSFET and a couple of resistors. When the input to the supercap regulator is active, the FET is disabled and current flows through a resistor to the supercap. When the input is inactive, the gate is pulled low by a high-value resistor and the FET shorts across the limiter resistor, reducing the voltage drop from ~200mV to near zero. It's a bit like one of those FET-based reverse-polarity protection circuits.Ordinarily I'd have used a Schottky diode, but the forward drop would severely reduce the run time from the cap. That's why we're using a dual-mode (can switch between buck and boost depending on input voltage) SMPS controller -- so we can suck as much energy from the capacitor as possible. Several reasons (not least of all physical size) mean we can't fit a larger supercap.Thanks for the advice, guys