Inrush current limiting, PTC or NTC?

[reyntjensm]

Hello everyone,

I have a 2 * 1000uF capacitor bank( 2 in parallel) with an ESR of 120mOhm. The circuit will be powered from 230VAC grid trough a bridge rectifier. I'm trying to limit the inrush current but i have no idea what is the best way to go. I know it's possible with a NTC or a PTC but what is the best solution? On what does it depend? The circuit will draw a constant current around 16 amps. I'm trying to simulate this in LTspice but i can't find any ready made symbol files to do so. What is the best way to simulate this in LTspice? Can anybody help me out with this?

Thank you all!

[Vovk_Z]

NTC resistors are made to withstand inrush currents.

[eblc1388]

... The circuit will draw a constant current around 16 amps.

With that high current the NTC will waste a lot of power as heat during normal operation. A better option would be a series resistor and then bypassing it with a relay contact after the capacitors have nearly charged up.

[Circlotron]

... The circuit will draw a constant current around 16 amps.

With that high current the NTC will waste a lot of power as heat during normal operation. A better option would be a series resistor and then bypassing it with a relay contact after the capacitors have nearly charged up.

Yep. An even better way perhaps is to use an NTC bypassed with a relay.

[Siwastaja]

Inrush current limiter needs considerable design, modeling and calculation. You just don't put an NTC there and call it a day.

Optimum solution is a fixed-value resistor coupled with thermal fuse (or possibly PTC) in series, this combination bypassed with a relay, with control logic.

NTC is a cheap alternative suitable for small supplies which are "almost OK" without any surge limitation at all, and is based on the fact you can accept that the inrush current is just reduced, not eliminated, and you can accept high inrush current under some edge cases, or you do rigorous analysis comparing the capacitor discharge time constant (based on the applied load) to the thermal time constant of the NTC cooldown.

[trobbins]

Have you worked through the many NTC inrush manufacturer design guides, as they have your design and selection advise all nicely set out.

[eblc1388]

Yep. An even better way perhaps is to use an NTC bypassed with a relay.

No, it is not. Using a NTC is:
a. more expensive then a power resistor
b. less reliable than a power resistor
c. offer no inrush protection if power is off and on in quick succession as the NTC needs time to cool down.

[Siwastaja]

If the rectified DC voltage is, for example, 320V, and the load (say, a PFC boost controller, for example) goes into Under-Voltage Lockout at, say, 80V, then the quick discharging of the caps stop at 80V, the capacitor voltage stays there for long enough to the NTC cool down, and if the supply is re-plugged while the NTC is still hot, the inrush current is 80V/320V = 25% less than it would be to empty capacitor bank. Not a big difference, but at least some limitation...

Add the hot resistance of the NTC, and it limits the inrush current even if it's still hot, compared to not using one at all.

This hot resistance, obviously, adds power loss and heating all the time is device is on, which totally sucks. So you need to think how to thermally insulate the NTC so it keeps hot at lower power, and not destroy nearby electrolytic capacitors.

[Circlotron]

Yep. An even better way perhaps is to use an NTC bypassed with a relay.

No, it is not. Using a NTC is:
a. more expensive then a power resistor
b. less reliable than a power resistor
c. offer no inrush protection if power is off and on in quick succession as the NTC needs time to cool down.

Seeing the NTC is bypassed by the relay, if the psu had been running long enough it would have cooled down and be ready if the power went off-on.

[Siwastaja]

Yep. An even better way perhaps is to use an NTC bypassed with a relay.

No, it is not. Using a NTC is:
a. more expensive then a power resistor
b. less reliable than a power resistor
c. offer no inrush protection if power is off and on in quick succession as the NTC needs time to cool down.

Seeing the NTC is bypassed by the relay, if the psu had been running long enough it would have cooled down and be ready if the power went off-on.

You can use NTC there (and add even more complex control logic to catch one more edge case you create with it); or you can choose the NTC so that it never heats up during the precharge, only utilizing its cold resistance; or you can use a potato there. It still makes absolutely no sense to use NTC with the relay. The point of the NTC is that it's a cheap replacement for the relay + control logic as it's "self-controlling". If you do have the relay + control logic, you can just use a fixed-value resistor, and choose optimum value for it. Adding protection for failed relay (or relay control logic) is always a good idea, though; that would be a thermal fuse in series.

Such controlled precharge tends to cost at least a $2-3 in BOM so it isn't feasible in sub-$100 supplies. Small supplies do without inrush limiting at all, or, use NTC. The range of supplies using NTC is quite small, in the end, they are some 50-100W supplies. This power level is ubiquitous, though, so you see a lot of NTCs. And because they sell in millions, they have the resources to choose the NTC properly, design the thermal insulation and placement for it, and model and verify the thermals of the complete product.

[akimpowerscr]

Yep. An even better way perhaps is to use an NTC bypassed with a relay.

No, it is not. Using a NTC is:
a. more expensive then a power resistor
b. less reliable than a power resistor
c. offer no inrush protection if power is off and on in quick succession as the NTC needs time to cool down.

+1

NTC resistor is not a good solution.

The NTC solution is an inexpensive solution used in poor quality SMPS power supplies.

There is one essential point that the OP did not indicate: what kind of rush current is it?

Indeed, the inrush current can have 2 different causes:
- the charge of a capacitor
- the saturation of a transformer on power-up.*

These two types of inrush current can have different solutions.

[Siwastaja]

NTC doesn't necessarily indicate poor quality.

It's a low-cost solution that fits some particular cases just right, and if the design is done properly, it isn't a bad or unreliable, and not even colossally inefficient (i.e., most of the losses are elsewhere).

It's not optimal from efficiency viewpoint though, but neither is energizing a relay coil all the time. For really high-efficiency products, a lot of thinking needs to go in a product, one part of that is optimizing the precharge circuit.

[opampsmoker]

Good inrush limiter is use an isolated 1W SMPS module, ....say feeding off a low voltage dc bit...and then get this to turn on the fet that shorts out the inrush resistor.......this is good because no logic is needed, as the low voltage bit would power up after the pfc anyway...so your inrush has happened (thru the inrush resistor), and then you can switch out the inrush resistor as i describe.

[reyntjensm]

Thank you all for your feedback.
It's a one of a kind prototype so i'm not worried about heat/energy loss.
So i think a NTC would be the best option. Any idea's on how to simulate this in LTspice?

[jonpaul]

NTC inrush is very old and reliable solution, used in many well designed supplies, including Tektronix 2465.

NTC: e.g. Cold = 100 ohms, hot (from current) = 1 ohm

PTC is the opposite of what is needed, e.g. cold = 1 ohm, hot = 100 ohm.


Relays and such add cost and reduce reliability.

See the many fine app notes from the NTC manufacturers.

https://www.ametherm.com/inrush-current/selecting-an-inrush-current-limiter.html

If properly selected and designed, the losses during operation are reasonable.


Jon

[Siwastaja]

Properly designed relay doesn't reduce reliability any more than a properly designed NTC.

Cost difference is true. NTC is a good idea for a 50-100W, low cost, reasonably efficient power supply, nothing shady in it.

I say it once more but do remember that NTC inrush limiter typically does not limit inrush properly in some very real corner cases, it's used to limit average/typical inrush current but if worst-case behavior is important (for example, to protect switches, mechanical or semiconductor), NTC is usually out of question.

[bdunham7]

Thank you all for your feedback.
It's a one of a kind prototype so i'm not worried about heat/energy loss.
So i think a NTC would be the best option. Any idea's on how to simulate this in LTspice?

Is the output of this circuit loaded as soon as it is powered up?  Is there an NTC available that can handle a 16 amp load like that?  You may need to parallel several devices and your inrush is still likely to be in the hundreds of amperes.

[Siwastaja]

Sanity check:

I think I have never seen NTC used in a multi-kW supply with 2000uF input capacitance after the rectifier. Such large supplies use relay for precharge.

It is possible yes but I'm 99% sure the solution will suck, i.e. require excessively large NTCs, possibly many paralleled, which then dissipate a lot of heat during the normal 16A load and need to be located far away from the capacitors.

I also think the safety of the OP's project should be questioned. If you are not very much sure what you are doing, and not qualified to do it, don't do it. Mains is dangerous. Just a reminder, feel free to ignore.

If it's a one-off hobby project or prototype, why spend time trying to specify NTC? The first time I did a surge limiter as a kid with a large 2kVA 24V transformer with some tens of thousands of uF at the output, I used a 150W halogen light bulb bypassed by simple mechanical switch in series with the primary. Worked a treat and provided an interesting current-limitation mode where the light intensity showed the level of output current! I used a standard 6A resetable fuse, properly mains rated, so that if I accidentally forgot the bypass switch on, this fuse would pop instead of the 16A house fuse.

[coppercone2]

Put a relay and timer on it with a simple circuit for that big a capacitance.

[akimpowerscr]

Hello everyone,

I have a 2 * 1000uF capacitor bank( 2 in parallel) with an ESR of 120mOhm. The circuit will be powered from 230VAC grid trough a bridge rectifier. I'm trying to limit the inrush current but i have no idea what is the best way to go. I know it's possible with a NTC or a PTC but what is the best solution? On what does it depend? .....
...
...
...
Thank you all!

The funny thing is that this is almost the same problem that happened in the power supply of the Philips chassis KM1 multistandart color televisions but the capacitor was only 470µF

https://onedrive.live.com/?authkey=%21ALsyL1wEiE%2Dmono&cid=D3F55CD5F0B2818B&id=D3F55CD5F0B2818B%216002&parId=D3F55CD5F0B2818B%216100&o=OneUp

The inrush current protection was done by a NTC.

I repaired dozens of these televisions which all still had the same problem: BY127 diodes (later replaced by BY227) really exploded.

It was enough to turn off the television and turn it on again a few seconds later and it was a disaster ...

So, those who say that CTN protection is reliable make me laugh. 

That's not a reliable protection, only a cheap solution.

And, with 100% certainty, totally unsuitable for a circuit that consumes 16A !!!!

[reyntjensm]

Thank you all for your feedback.
It's a one of a kind prototype so i'm not worried about heat/energy loss.
So i think a NTC would be the best option. Any idea's on how to simulate this in LTspice?

Is the output of this circuit loaded as soon as it is powered up?  Is there an NTC available that can handle a 16 amp load like that?  You may need to parallel several devices and your inrush is still likely to be in the hundreds of amperes.

The output isn't going to be loaded as soon as is powers up.

Sanity check:

I think I have never seen NTC used in a multi-kW supply with 2000uF input capacitance after the rectifier. Such large supplies use relay for precharge.

It is possible yes but I'm 99% sure the solution will suck, i.e. require excessively large NTCs, possibly many paralleled, which then dissipate a lot of heat during the normal 16A load and need to be located far away from the capacitors.

I also think the safety of the OP's project should be questioned. If you are not very much sure what you are doing, and not qualified to do it, don't do it. Mains is dangerous. Just a reminder, feel free to ignore.

If it's a one-off hobby project or prototype, why spend time trying to specify NTC? The first time I did a surge limiter as a kid with a large 2kVA 24V transformer with some tens of thousands of uF at the output, I used a 150W halogen light bulb bypassed by simple mechanical switch in series with the primary. Worked a treat and provided an interesting current-limitation mode where the light intensity showed the level of output current! I used a standard 6A resetable fuse, properly mains rated, so that if I accidentally forgot the bypass switch on, this fuse would pop instead of the 16A house fuse.

I've found a NTC that could be used, check the attachment. Everything has to be installed on a PCB and inside a box. I'm unable to install a 150W lamp inside the box....

[jbb]

“Max steady state current at 25C: 15A”

Nope, too small, sorry. That will likely mean something like “operating at maximum temperature of 185C I’m free 25C air”.

If you assume ambient goes up a bit and you’ve got a box, the air temperature inside would be, oh, 60C or more. So you’d have to derate it to like 10A or less.

And the PTC itself would be a destructive monster, leaning heat into everything nearby. I would worry about it wrecking it’s own solder joints which could then get very hot and literally set the board on fire. (Note: flame retardant does not mean non-flammable, and if you keep pumping electricity in it will keep burning).

[bdunham7]

The output isn't going to be loaded as soon as is powers up.

Then you can easily use a relay and power resistor to limit the inrush to arbitrarily low levels. 

These two parts will give you a robust solution that will be more reliable, generate a lot less heat and easily withstand repeated power cycles.  It will also limit the inrush to about 15 amperes maximum, which you can change if you wish by selecting a different resistor.  Your NTC will be roasting hot, will not reliably limit the inrush to any specific value and any momentary power interruption will give you that huge surge that you are trying to avoid. 

https://www.mouser.com/ProductDetail/TE-Connectivity-PB/1649341-2?qs=sGAEpiMZZMtGt%252Bn33CgIPwDNQq27pAXuXZwiRFgJ10g%3D

https://www.mouser.com/ProductDetail/Bourns/WS5M22R0J?qs=R%252BZOTiLH6ThNZ0lJxIR7uw%3D%3D

[bdunham7]

NTC inrush is very old and reliable solution, used in many well designed supplies, including Tektronix 2465.
Relays and such add cost and reduce reliability.

Yes- they work very well in designs where they aren't really needed--the one you cited is a perfect example of this.

In the OPs use case, the relay/resistor combo will cost less and likely be more reliable than any reasonable NTC "solution".  It will also actually do the job of reliably limiting inrush current.

[akimpowerscr]

Let's not forget that there is also the possibility of using a triac instead of a relay to short circuit the inrush current limiting resistor.

But with 16A of consumption, it will take a rather powerful triac with radiator and the power loss will be about 30W