Limiting inrush current into an electrolytic capacitor

[philpem]

I've got a project "in the lab", which is about to go into production... and there's a bit of a snag. Part of the power supply is done with DC-to-DC converters (the little black plastic potted things) to isolate critical parts of the circuitry. To reduce the noise these push back onto the incoming 24V power supply, there's a 100uF low-ESR capacitor across the 24V input, behind a reverse-polarity protection diode.

Total power consumption of this thing is 50mA, plus about 200mA for some motors and stuff which are usually running but can be switched off by the electronics block under command from a PC.

Problem is, there's been a power supply spec change. We've gone from a 24V rail sourced from a "dumb" power supply (a switcher with an output fuse and a crowbar OVP) to an all-singing-all-dancing "smart protected power supply" which does open and short circuit detection. For bonus points, the "smart" PSU has fast-switching outputs - essentially a relay driven from the output filters, via the open/short protection circuit. The dV/dt is essentially as fast as having a lab PSU on and ready, then switching the power to the unit with a mechanical switch, only without the contact bounce. There is no soft-start.

If the device being powered draws more than 400mA for 200us, the PSU will declare the output as shorted and trip. The 100uF capacitor and power supply components (DC-to-DCs) are taking it way over this limit - by the highly scientific method of "stick a 1R resistor in series and measure across it", I'm getting 7A (falling exponentially, but not fast enough) for a couple of hundred microseconds, more than enough to trip the PSU.

I can't make any modifications to the shiny new power supply or its wiring, but I can modify my circuit's power supply all I like within reason. I'm not keen on adding a series resistor to the low-ESR cap as it'd impede the filtering ability of the capacitor, and the voltage drop over a series resistor on the 24V input might cause problems for the DC-DCs (this device could potentially be on a long run of cable and needs to run down to about 20V). An inductor (I've tried up to 100uH)

Can anyone think of a way I could reduce the inrush current enough to get by the limit, without impeding my capacitor's ability to filter the DC rail?

Cheers,
Phil.

[Ice-Tea]

NTC

[acbern]

the standard solution for an inrush current limiter is a fet. you need to drive the on-resistance during power-on, e.g.  in the simplest implemantation by an RC combo
there are circuits on the web also. ntc and relais are not recommendable, mtbf is bad and limited number of cycles (overall/per given time)

[jpb]

the standard solution for an inrush current limiter is a fet. you need to drive the on-resistance during power-on, e.g.  in the simplest implemantation by an RC combo
there are circuits on the web also. ntc and relais are not recommendable, mtbf is bad and limited number of cycles (overall/per given time)

Is the mtbf of relays that bad? I'm curious as instruments such as my bench DVM are full of clicking relays and have been going ok for decades.

I'm in the process of building a linear power supply myself and was thinking of simply using a relay with perhaps an SCR to control the coil to switch out a series R.
It has the advantage of being fail safe - if the relay does fail then the resistance isn't switched out so the voltage drops.

[suicidaleggroll]

A fet with an R/C combo on the gate is really dead simple to implement and works quite well.  Something like this:

[jpb]

A fet with an R/C combo on the gate is really dead simple to implement and works quite well.  Something like this:

The problem I had when looking at FET circuits was the knee voltage. A relay presumably can pass several amps whilst dropping less than 0.1V, most FETs have knee voltages much higher than this.

[Ice-Tea]

the standard solution for an inrush current limiter is a fet. you need to drive the on-resistance during power-on, e.g.  in the simplest implemantation by an RC combo
there are circuits on the web also. ntc and relais are not recommendable, mtbf is bad and limited number of cycles (overall/per given time)

I'm a bit confused. How is a NTC not recommended? Why is mtbf bad? I don't recall they have a limited #cycles (I presume this is meant for the relay?).

Not doubting you, rather looking for more insight. I've used NTCs in designs quite a bit and would like to know if it's something that's gonna bite me in the ass one day

[ovnr]

Is the mtbf of relays that bad? I'm curious as instruments such as my bench DVM are full of clicking relays and have been going ok for decades.

I'm in the process of building a linear power supply myself and was thinking of simply using a relay with perhaps an SCR to control the coil to switch out a series R.
It has the advantage of being fail safe - if the relay does fail then the resistance isn't switched out so the voltage drops.

Relays are also noisy. And is your series resistor capable of being switched in continously if the relay does fail? Most in-rush limiters designed to be switched out will dissipate too much power to be safely enabled all the time.

I'm a bit confused. How is a NTC not recommended? Why is mtbf bad? I don't recall they have a limited #cycles (I presume this is meant for the relay?).

Not doubting you, rather looking for more insight. I've used NTCs in designs quite a bit and would like to know if it's something that's gonna bite me in the ass one day

Relays die. Often at very inconvenient times...

NTCs dissipate a fair bit of power and there is a fairly significant voltage drop. You may not be willing to put up with it; I rarely do. They also run hot (kind of the point), and having components running at 100+ deg C can introduce other issues (heat transfer to other components, etc).

[Ice-Tea]

Yeah, well, I wasn't proposing a relay. And you have a valid point about power consumption of an RTC. Can't be beat in terms of cost and simplicity though. And I'd argue that it's more reliable as well. Would prefer a surge on a NTC rather than a FET...

[mikerj]

I'm a bit confused. How is a NTC not recommended?

NTCs that provide a reasonable value of resistance when cold tend not to have very low "on" resistance
Their behavior change with ambient temperature
They have lengthy reset times.
Purely from my own experience they don't seem to be the most reliable of components.

They provide a perfectly adequate and cheap solution in many cases, but are not always appropriate.

Why is mtbf bad? I don't recall they have a limited #cycles (I presume this is meant for the relay?).

Mechanical relays have a finite number of operations they can make during their life, primarily limited by degradation of the contacts.  MTBF is not an appropriate measurement for a relay, since they may last almost indefinitely if held in one state, instead MCBF (Mean Cycles Between Failure)

[nuno]

Remove the electrolytic, check if the new better converter filters the noise...

[SeanB]

Mix both circuits. The NTC will limit the cold current, then the power FET will switch on and bypass the NTC. That way you have both low dropout voltage and if the FET dies short circuit ( the most common failure) you have the power supply overcurrent safely switching all of the unit off for repair. As well the NTC will never warm up unless the FET does not turn on. You can simply have a non resettable thermal fuse in series with the input, thermally bonded to the NTC and the FET, to disconnect the power if the unit gets too hot from this or other reasons. 110C fuses are cheap, reliable and not likely to fail at normal operating temperatures.

[suicidaleggroll]

The problem I had when looking at FET circuits was the knee voltage. A relay presumably can pass several amps whilst dropping less than 0.1V, most FETs have knee voltages much higher than this.

Could you describe a bit more what you mean by this "knee"?  I'm not following.  24V is so far beyond the Vgs threshold of any old P-FET that it's a non-issue, and 0.4 ohms of on-resistance is painfully easy to beat as well.

[rx8pilot]

I have recently had to deal with this and I went through many possibilities except a relay. My goal was to limit the inrush but not limit the outrush. This was to charge 8200uf capacitor bank which is 10 820uf caps in parallel, so the ESR is fairly low. The purpose of the capacitance is to ride through droops and deal with current spikes with minimal voltage drops. In the end I am using a FET with an ideal diode controller which blocks incoming voltage, but lets the caps discharge with VERY low resistance of about 3mOhms. To charge the caps I have a 50 Ohm resistor from the main bus. When the voltage of the caps approaches the bus voltage, the FET turns on to a Vgs of +15v. The ideal diode controller has a charge pump that will drive N-FETs which (in general) are lower loss. It also allows me to control the cap bank from a uController.

Conceptually, you have a similar setup but at much lower currents it seems.

I have never actually used an NTC, but from the little research I have done they seemed much too lossy and restrict current in both directions. I needed to be able to discharge the capacitors very fast to make my circuit work.

[jpb]

The problem I had when looking at FET circuits was the knee voltage. A relay presumably can pass several amps whilst dropping less than 0.1V, most FETs have knee voltages much higher than this.

Could you describe a bit more what you mean by this "knee"?  I'm not following.  24V is so far beyond the Vgs threshold of any old P-FET that it's a non-issue, and 0.4 ohms of on-resistance is painfully easy to beat as well.

The very simple circuit I was looking at had the FET in series with the smoothing cap before the voltage regulator which was a LDO one. As it was only for my own use in the UK I was trying to maximize efficiency (whilst staying linear) by having only a very low voltage drop from the rectifiers to the regulator. My total voltage allowance was therefore rather small and I didn't have much to drop across a FET.

My mistake I realise now was I was assuming I needed to drive the FET beyond the knee but of course it is fine to operate it in the linear region - I am used to small signal FET design so I intrinsically assume an above-the-knee bias point 

[rx8pilot]

what is the "knee voltage"?

[ajb]

Probably overkill for your current requirements, but there are hotswap-oriented high-side switch controllers that provide programmable soft-start and current limiting.

[jwm_]

How about a lm317 in current limiting configuration. Or a LDO version of the same if you can't handle that much of a voltage drop.

like so:
http://www.ae5d.com/images/LM317.png

[suicidaleggroll]

The very simple circuit I was looking at had the FET in series with the smoothing cap before the voltage regulator which was a LDO one. As it was only for my own use in the UK I was trying to maximize efficiency (whilst staying linear) by having only a very low voltage drop from the rectifiers to the regulator. My total voltage allowance was therefore rather small and I didn't have much to drop across a FET.

My mistake I realise now was I was assuming I needed to drive the FET beyond the knee but of course it is fine to operate it in the linear region - I am used to small signal FET design so I intrinsically assume an above-the-knee bias point 

I still don't know what you're talking about.

When you use a P-ch FET in a soft-start circuit, the goal is to gradually increase Vgs (well, decrease, from zero to negative), which gradually reduces Rds.

When voltage is first applied, Vgs is 0, Rds is infinite, and no current flows.  As the RC filter allows Vg to drop below Vs, Rds starts to decrease, and current starts to flow with almost the full 24V drop across Rds.  After some time (configurable with your R and C values, probably on the order of milliseconds), Vg will drop to its final voltage at or near ground, Vgs will be the full 24V, and Rds will drop to the nominal Rds(on) of just a few milliohms, and your voltage drop across the FET in steady-state will be on the order of a few mV at your operating current.

It essentially acts like a variable resistor that runs from infinite resistance to the FET's nominal Rds(on) over an amount of time you control with the R and C values.

[jwm_]

A FET configured in this way is often refered to as a 'constant current diode'.

I tend to use an lm334 or lm317 (depending on limit) for this though as i already have a bunch of those but may not have a suitable fet on hand and are equally easy to build. a caveat of the lm317 is that it doesn't like a reverse voltage so isn't quite a constant current diode so you may need to take that into account.

[SArepairman]

I don't think a relay timer is a bad solution so long you pick an appropriate relay. It's just more expensive.

I have seen quite a few failed NTC's before. They defiantly do fail into a brittle mess.

 I never had a product that used a relay timer though (well, I don't get to take apart gear that was not designed with dollar slashing in mind very often). I built my own though. I build an entire module that used a small transformer and a LPS / 555 timer and transistor to trigger a beefy relay which shorted out a beefy inrush current limiting resistor.

Are you guys sure that a power electronics solution at the same price range as a decent relay will be more reliable? In college my electromechancis professor (who had advanced in industry ranks as well) spoke about relay inrush current limiting circuits a few times in a manner that suggested they were used in the power industry and they were a respectable solution to this problem.

Keep in mind that a NTC is slow to cool down as well.

You guys got SERIOUS relay hate haha , sounds like you are specifying crap!

[jpb]

what is the "knee voltage"?

Where the IDS(VDS) curve leaves the linear region and levels off (though there is generally still a slope) - this is where the electron velocity in the device saturates. For  linear amplifiers the load line (class A) should stay in this saturated region.

Normally the knee voltage is a lot less than the overall voltage swing so it doesn't matter but for low voltage devices (battery operated) the knee voltage significantly reduces the potential linear voltage swing (class A amps). I was once involved in a project to design a FET to reduce the knee voltage to allow operation down to 1V (this was with GaAs devices at high frequencies).

[rx8pilot]

This is why I hang out in this forum. Nice.

[jpb]

I still don't know what you're talking about.

That makes two of us!

To try and explain my original thinking, I come from a background of working with GaAs FETs (schottky barrier gates) and on the whole, unless you're designing a mixer, you aim to operate the device where VDS is large enough for the electrons to be at saturated velocity (ideally the current curves are flat with VDS but in fact they keep rising but this is due to 2D E field effects rather than changes in electron velocity). The point at which the curve levels off (as far as it does) is the knee voltage and so I'm programmed to assume that you want VDS greater than this (around 0.5 to 1V for small signal devices).

Of course, in the case of using it as a variable resistor it is fine to operate it below the knee. I just wasn't thinking straight - I blame the fact that I've been out of the industry for more than 10 years and senility is setting in.

[AG6QR]

For a commercial product that does this, see

http://www.mfjenterprises.com/support.php?productid=MFJ-4403

This is a power conditioner, intended for the ham radio market, where radios nominally run off of 13.8V supplies, maybe in vehicles.  Vehicles can have lots of transients on their supplies, and radios can have transients in their power consumption as they switch between transmit and receive, or as SSB transmitters change output power with modulation. 

That power conditioner is basically a 4 Farad (notice no prefix on the Farad unit) capacitor bank, with some associated circuitry around it to limit inrush and provide a bit of protection.  There's a downloadable manual there with a block diagram and detailed schematic.  It uses a resistor to limit inrush.  Once the capacitor bank is charged sufficiently, a relay bypasses the resistor.

Not that that's the only or best way to do it, but here's hoping someone finds it interesting to look at how someone else approached a similar issue.