Inrush current

[firozannan]

Hi I am new to this forum....and its my first post.

I am currently testing a power supply board which generates 3.3V,5V and 12V.I need to check for the inrush limits for these secondary voltages. Is there any thumbrule for inrush current limits? Or how do i calcuate the maximum current that a sink capacitor will draw from these volatges?

Thanks in advance.

 

[ericgibbs]

Hi I am new to this forum....and its my first post.

I am currently testing a power supply board which generates 3.3V,5V and 12V.I need to check for the inrush limits for these secondary voltages. Is there any thumbrule for inrush current limits? Or how do i calcuate the maximum current that a sink capacitor will draw from these volatges?

Thanks in advance.

hi and Welcome,
Which type of 3.3v/5v and 12v regulators are you using.?

 

[firozannan]

12v and 5V are generated via an OEM SPMS while 3.3V is generated using a switching buck reg....

 

[duffy]

The usual problem is it blows the fuse when you first switch it on and it charges the caps through the bridge on the high voltage side. I've found a good way to check to see if this is going to be a problem is to switch it on and see if it blows the fuse.

If so, you protect it with an NTC thermistor, also called an Inrush Current Limiter (ICL), also called an RTH. This has a high "cold" resistance, which drops as it heats up. You will find one in most switchmode supplies between the bridge and the cap.

 

[firozannan]

Thanks Duffy...

But my concern is how do i decide if the current is beyond the tolerable limits.....not all the ICs have this info...so i decided to check if i could find the max current that could be tolerated by the decaps...!

 

[duffy]

Is there any thumbrule for inrush current limits? Or how do i calcuate the maximum current that a sink capacitor will draw from these volatges?
...
...not all the ICs have this info...so i decided to check if i could find the max current that could be tolerated by the decaps...!

You need to explain what you are trying to do. "Sink capacitor" is not a real term. A decap is not an IC. A board full of decoupling capacitors will have considerably less capacitance than one power supply filter cap.