What is the simplest way to limit inrush current?

[seyyah]

I have a capacitor of 220uF@400V and a dc bus of 310V. when system starts, due to inrush current the fuse at the input blows or sparks. At full load the current in the bus is below 0.2A. What is the simplest way to limit this inrush current?

 

[wizpic]

inrush current

we had this problem at work try puting a high value resistor across cap we did and it cured it

wizpic

 

[flatulent]

Re: inrush current

There are temperature sensitive resistors for just this.

**broken link removed** has several examples.

 

[IanP]

Re: inrush current

Temperature dependant resistors may have some delay and if the current is not sufficient will have certain resistance.
Pretty normal practice is to use a resistor of, say, 1Ω. In your case maybe more (up to 4Ω7 .. 10Ω) as the average current in your system is ≤200mA ..
This will significantly reduce the inrush current ..

 

[VVV]

Re: inrush current

The simplest way is to insert some series resistance.
Generally, however, a thermistor is preferred. Check out page 28 of this datasheet: www.avx.com/docs/masterpubs/tempNTC.pdf

 

[seyyah]

Re: inrush current

A tried 1-22ohm resistors at 0.5W all of them are burned with a spark. I also tried to connect resistors of 10k-100k accros the capacitor terminals. I think it did a little but didn't prevent the sparking also it gets hot.

 

[IanP]

Re: inrush current

This resistor should be rated for at least 5W ..

 

[pisoiu]

inrush current

This is a common problem for all SMPS. Use a 4,7 ohm/5w resistor, but pay attention to its tipe. It should be wire resistor rather than resistor made with deposition of resistive material onto a substrate. Wire resistors have a better tolerance to high peak currents. Another widely spread method is the use of NTC (negative temperature coefficient) thermistors. However, this method has one disadvantage: when you turn your equipment on, the resistance of the NTC is higher and the device is cold. After startup, it will increase its temperature and decrease resistance. If you turn your equipment off then on again before the NTC going to room temperature, you may blow it, because the inrush current will appear when it is hot and its resistance is low. You can check here , part 1, chap. 7 for more details.

/pisoiu

 

[seyyah]

Re: inrush current

ntc seems a better solution especially for space purposes. What value must i use or how to choose it? And the disadvantage of it, how much time does it takes to return its normal resistance? If it's not too long, i mean if it returns normal conditions before capacitor is discharged, there will be no problem. If not i must care not to turn on quickly after a turn off.

 

[pisoiu]

Re: inrush current

ntc seems a better solution especially for space purposes. What value must i use or how to choose it? And the disadvantage of it, how much time does it takes to return its normal resistance? If it's not too long, i mean if it returns normal conditions before capacitor is discharged, there will be no problem. If not i must care not to turn on quickly after a turn off.

It will be at room temperature in two or three minutes, depending on your case and placement (is the case ventilated or fully enclosed, how many components are around it?...etc). In either case, it will not be cold by the time the cap is discharged, I am sure of it. The type depends on what suppliers you have, choose one with the same parameters (at 25 degree Celsius) as a resistance used for the same purpose, ex 4,7 ohm/5W. But the most cheaper way to do it is to search for a defective piece of equipment, powered by a SMPS. You have big chances to find one piece there. Perhaps a monitor or a computer power supply. Take care at thermistors from monitors, they usually are two thermistors in the same package (3 terminal package). One of it is NTC used for inrush control, the other one is PTC used for degaussing.

/pisoiu

 

[seyyah]

Re: inrush current

Ok what about inductor? Can't we use it? I think with a proper inductor current rise will be limited. But will it suffice?

 

[pisoiu]

Re: inrush current

Ok what about inductor? Can't we use it? I think with a proper inductor current rise will be limited. But will it suffice?

I did not made calculations, but considering the charging slope of the capacitor, I think you will need a large value, and it will be impractical because of the size.

/pisoiu

 

[seyyah]

Re: inrush current

Yes you're right, inductor size muct be great. This explains why passice pfcs are big.

Added after 3 hours 33 minutes:

I tried ntc and it works great. I wonder is there any problem with connecting it to the ac side? (before bridge rectifier instead of output of the rectifier)

 

[VVV]

Re: inrush current

No there is no problem with that. Generally NTC's are conected to the input of the bridge, rather than the output.

 

[omore]

Re: inrush current

An elementary question.

What is the fuse you are using rated for ??

 

[seyyah]

Re: inrush current

An elementary question.

What is the fuse you are using rated for ??

10A

 

[omore]

Re: inrush current

Well mmm guess U are using slow blow fuse so there must be another reason. One could be that your circuit starts in a wrong way. Could you provide us with an schematic of the circuit. The rate of the fuse is too high, but you still get it blow.
???
Sorry for not being of too much help per now.

 

[seyyah]

Re: inrush current

Well mmm guess U are using slow blow fuse so there must be another reason. One could be that your circuit starts in a wrong way. Could you provide us with an schematic of the circuit. The rate of the fuse is too high, but you still get it blow.
???
Sorry for not being of too much help per now.

Very sorry. I gave you wrong info about fuse since i have 2 circuits similar. One is low power and the other is high power. I confused them since the message is old. Sorry again. I solved the problem in this circuit via ntc but i couldn't solve the high power one with ntc. Because i couldn't find a ntc that can stand that much of current. Is it available? Actually rated current of the circuit is ~5A. But transiently it can go over it.