Inrush current limiting

Thread Starter

dominik0801

Joined Jun 3, 2014
9
Hello!

I have an SMPS design, that I use pretty often. I designed it with EasyEDA and ordered the PCB via JLCPCB. The problem is, that when I power it up (by connecting it to a 20V DC laptop charger), it makes a huge spark. I guess its because the inrush current, since it has 4x470uf low esr caps on the main voltage line.

It works otherwise, but the sparking is annoying and I want to reduce it. I was thinking about having an inrush current limiting circuit on the main line, however, I am pretty dumb, and I don't know how to make it. I was thinking about an NTC thermistor (because that seemed to be the easiest way), but I couldn't find one that can carry 5A current, that my PSU needs from the 20V source.

Do you have any ideas? Maybe someone that can give me a good schematic?

Thanks in advance.
 

Papabravo

Joined Feb 24, 2006
12,763
Hello!

I have an SMPS design, that I use pretty often. I designed it with EasyEDA and ordered the PCB via JLCPCB. The problem is, that when I power it up (by connecting it to a 20V DC laptop charger), it makes a huge spark. I guess its because the inrush current, since it has 4x470uf low esr caps on the main voltage line.

It works otherwise, but the sparking is annoying and I want to reduce it. I was thinking about having an inrush current limiting circuit on the main line, however, I am pretty dumb, and I don't know how to make it. I was thinking about an NTC thermistor (because that seemed to be the easiest way), but I couldn't find one that can carry 5A current, that my PSU needs from the 20V source.

Do you have any ideas? Maybe someone that can give me a good schematic?

Thanks in advance.
A schematic diagram would help since the words may or may not convey the correct impression.
 

MisterBill2

Joined Jan 23, 2018
4,817
One way that I have seen used many times is to have a current limiting resistor and then a second switch or second switch position to bypass the resistor and feed the power in directly. OR, if you normally only connect the device to use it, just add a series resistor and a switch to bypass it. Perhaps ten to 20 ohms, but rated at several watts, since it will be carrying current for a few seconds. Cheap and simple, as long as you remember to use it.
 

Thread Starter

dominik0801

Joined Jun 3, 2014
9
The schematic is actually hella ugly, and you could barely read anything from it. Take my word. :D

But on the main input, it has 4x470uf 25V caps, then it connects to 5xLM2576 IC-s.

The best thermistor I could find for the purpose is an 5A with 16ohms. Could that help with my problem?
 

Papabravo

Joined Feb 24, 2006
12,763
The schematic is actually hella ugly, and you could barely read anything from it. Take my word. :D

But on the main input, it has 4x470uf 25V caps, then it connects to 5xLM2576 IC-s.

The best thermistor I could find for the purpose is an 5A with 16ohms. Could that help with my problem?
What difference does that make exactly? The problem is the words DO NOT CONVEY AN ACCURATE PICTURE!! They never will.
So take some time an make a pretty picture.

Also -- what exactly is this input you are referring to? Is it a battery, the mains, what exactly?
 

MisterBill2

Joined Jan 23, 2018
4,817
The words tell me exactly what that part of the circuit is like. The presumption is that first, the 20 volt supply is not being overloaded, and second, that the rest of the second supply that is being connected is not drawing much current at the instant of the spark happening. Based on those two guesses, and the presumption that the load is not connected at the time the supply is attached, a current limiting resistor that is then bypassed by a switch is a good way to work out a solution.
 
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