methods for limiting inrush current? (3.3V ~ 5V And such low voltages) - Page 1
Forum section: electronics / projects

author: JPortici
author_rank: Super Contributor
timestamp: January 04, 2017, 05:58:16 pm
content: Typical scenario: Board is run from an external (or local) 5V/3.3V source, operating current 5 to 250 mA.Usually i could just whistle it away pretending it didn't exist, but from time to time even a 200mA peak current can cause problems.A solution i liked was to slow down the supply voltage ramp by using two pmos, one cap and two/three resistors.R1 for additional current limitingM1 is slowly turned on by the voltage across R2 risingM2 is there for reverse polarity protection (I am also using this in a design which has two possible 5V sources, one disables the other..)Values not put on purpose.The resulting peak current and Vout risetime depends on a mix of C1R2 time constant, load current and load capacitance..I usually aim for a 5-10 ms rise time which is adequate for the product to start up reliably and without upsetting the external power source. In my case the peak power through the PFETs is about 250 mW for less than 1ms so (i think) i can be relaxed and that it is reliable.But what are other methods? maybe simpler or more robust ones..There are those who suggests a C-L-C pi network but i don't really like the idea of a resonant element

author: TheAmmoniacal
author_rank: Unknown
timestamp: January 04, 2017, 06:02:24 pm
content: Inrush Current Limiters (NTCs) have worked fine for my applications. Ametherm makes some great ones.

author: JPortici
author_rank: Super Contributor
timestamp: January 04, 2017, 06:05:05 pm
content: I also read about those but frankly i didn't put too much attention to them because i thought they were intended for larger loads?(low voltage, low current)

author: TheAmmoniacal
author_rank: Unknown
timestamp: January 04, 2017, 06:07:36 pm
content: Quote from: JPortici on January 04, 2017, 06:05:05 pmI have only used them for larger loads, so not sure how helpful they are for small currents. They do seem to have a miniAMP series that starts at 250 mA though.

author: JPortici
author_rank: Super Contributor
timestamp: January 04, 2017, 06:12:22 pm
content: maybe load in this case isn't the appropriate word, but energy? i will check them out anyway

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: January 04, 2017, 06:33:27 pm
content: inrush current to what???mains XFMR primary, Cap bank? , both?

author: JPortici
author_rank: Super Contributor
timestamp: January 04, 2017, 06:45:14 pm
content: good point. i'll edit the first post!let's say up to 5V, less than 250 mA.

author: Zero999
author_rank: Super Contributor
timestamp: January 04, 2017, 07:01:50 pm
content: It seems like a sensible suggestion. I've used a similar thing on a motor which would otherwise cause a switched mode power supply to shut down, when started.The LRC filter seems like it could be a good idea too, if you damp it enough so the resonance is a non-issue.Another option is a relay which bypasses a series resistor, after a predetermined time.

author: BrianHG
author_rank: Super Contributor
timestamp: January 05, 2017, 04:44:30 am
content: You may also add a resistor across the source and drain of each PMOS tuned to your maximum startup current, this allows you to have an initial predictable connection switch-on current charge before the avalanche effect of the PMOS turning on which may still be harsh even with your RC circuit. With a parallel fixed charge resistor, you may also slow down you gate RC circuit to even a longer time.I am not sure of how the second PMOS protects your circuit, can you please elaborate.

author: JPortici
author_rank: Super Contributor
timestamp: January 05, 2017, 08:52:51 am
content: ah yes, that's because what i posted is only part of the actual circuit (that's why it's not the usual zener diode on gate for polarity protection)I had to solve the problem of having to power a board from either 12V or 5V.12V always present (sometimes switched), 5V switched on and off.if only 5V is present the inrush current limit is enough so it doesn't upset the unknown regulator.if both 12V and 5V are present the board has to be powered from a 12V regulator when 5V is present BUT it doesn't have to also give back power to the 5V line.Regulator is then enabled by 5V (taken before current limit circuit).Missing part of circuit is a resistor from 12V to gate of M2. a voltage divider that pulls the gate up so VGS is 0, blocks current from flowing back.Regulator is enabled if enable input left floating so when switched 12V only 12V is necessary.or, to rephrase it:12V present: 5V line connection to circuit cut off. Board powered from regulator. Regulator enabled by 5V line active or absence of 5V line connection.12V not present: 5V line power the circuit. inrush current limiter.we wanted to have a single circuit that could work in the three scenarios so that once the board was potted we didn't have three article numbers. Simulated, tested, working.And looking for other ideas (for 5V only scenarios)

author: BrianHG
author_rank: Super Contributor
timestamp: January 05, 2017, 10:31:29 am
content: I'm guessing your circuit wont allow the 0.2v drop of a 1 amp schottky diode. I would have used one in place of the second mosfet from the 5v source.

author: JPortici
author_rank: Super Contributor
timestamp: January 05, 2017, 11:55:50 am
content: your guess is right

author: BrianHG
author_rank: Super Contributor
timestamp: January 05, 2017, 12:22:39 pm
content: For simplicity sake, stick with what you have, unless you are willing to add a 65 cent microcontroller. The microcontroller will allow software controlled delays before switching, as well as optional switching under controlled power source circumstances, but, you would require either open-drain outputs for each mosfet , or, an additional transistor for driving the gates on the PMOS.

author: David Hess
author_rank: Super Contributor
timestamp: January 06, 2017, 05:10:06 am
content: The solution you have which applies a voltage ramp to the gate of a p-channel MOSFET is a common one which can work well. I would add a diode to quickly discharge the gate capacitance if power is interrupted.Inrush current limiting is common for hot insertion applications and powered interfaces like USB so ASIC solutions are available.Another way to do it is to use a switching regulator which provides cycle by cycle current limiting which might have other advantages in your application.The improvement I would consider for your p-channel MOSFET solution involves adding a separate error amplifier to control the MOSFET and provide a very exact current limit. This can be done with a pair of matched PNP transistors.

author: KhronX
author_rank: Frequent Contributor
timestamp: January 12, 2017, 12:44:12 pm
content: One (further) objection is that you'll always have that series resistor on the input, regardless of how "open" that first PMOS is.That being said, the PMOS-based "soft start" / inrush limiters i've seen in laptop schematics might just be what you're looking for.See page 30 in the attached pdf.

author: bktemp
author_rank: Super Contributor
timestamp: January 12, 2017, 12:59:38 pm
content: Quote from: KhronX on January 12, 2017, 12:44:12 pmImportant note for this circuit:PQ2 is a digital transistor (build in base resistor). Otherwise when PQ3 gets turned on a large current will flow converting PQ3 into magic smoke.

author: EmmanuelFaure
author_rank: Regular Contributor
timestamp: January 12, 2017, 08:45:33 pm
content: Foldback limitation, with a PNP pass element.

author: TheDane
author_rank: Regular Contributor
timestamp: January 14, 2017, 08:26:48 pm
content: A gyrator circuit emulates a large inductor coil, which also could be used to limit the inrush current. Any back-emf can be dealt with through zener(diodes).http://www.epanorama.net/documents/telecom/gyrator.htmlCircuit can be tweaked to work reliably down to 3v3, and up at a decent current draw. Have done SMPS circuits with 0.0x ohm ESR caps, and 5-32Vdc input - hot pluggable, and no sparks