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Hi need help for inrush current limiter using inductor

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shelsd96

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I am trying to design a SMPS input inrush current limiter, and i wish to use inductor limiter. could you please help me on inductor current limiter design. at least please tell me how the inductor oppose the changes in current. what all equations comes in to picture. what all things we have to consider...? the inrush current rating is 100Amps/200uS. 277 AC inductive load
 

An inductor opposes current change by generating a voltage in opposition to the voltage driving the current into it. It does this by building a magnetic field where it stores energy. When the current flow stops, the inductor then generates an internal voltage to try and preserve this current flow by dissipating the energy in the magnetic field. The time constant for an inductor is T = L / R.
As the inductor reduces the inrush current at switch on, it will generate a voltage at switch off, unless this is handled properly, it can cause damage to solid state devices.
Frank
 

Hi Frank,

And how can we manage this voltage at switch off? I have a 33uH/3A inductor in series with the input in one design I am doing now. Using a simple RC snubber or an antiparallel diode would it be solved?
 

So if design a inductor for time duration less than the inrush current spike, it can limit the inrush current. Is that right..?
 

Hi shelsd,

I don know how you plan to limit the inrush current with an inductor, by the way, why don't you try to add a soft-start stage before the power stage? You could it with a Mosfet depending on the input power needed.

**broken link removed**

I mean something like this, adding a resistor and a capacitor to the gate you can control the RDSon dinamically so it will charge the power capacitors slowly until RDSon is nearly 0Ohm. You can use a n-channel, connectic D to S line in the negative input and the gate control to the positive line. Moreover, with this circuit you give an extra anti-reversal protection for the input voltage.
 

Gimlix said:
Hi shelsd,

I don know how you plan to limit the inrush current with an inductor, by the way, why don't you try to add a soft-start stage before the power stage? You could it with a Mosfet depending on the input power needed.

**broken link removed**

I mean something like this, adding a resistor and a capacitor to the gate you can control the RDSon dinamically so it will charge the power capacitors slowly until RDSon is nearly 0Ohm. You can use a n-channel, connectic D to S line in the negative input and the gate control to the positive line. Moreover, with this circuit you give an extra anti-reversal protection for the input voltage.
Click to expand...

Hi,

I have a similar situation, i need to control a 20 Amps, 12 Volts DC motor, and i am confused on what i should do with the inrush current. This is actually the auxiliary fan of a BMW, the original driver was fried, and i want to replace it. There is a PWM signal coming from the motor controller part of the car, i connected that to a CD4010, the output goes to an IRL3904, which i want to use to switch the power on/off with the duty cycle i get on the input. However, since that is a huge motor, i'm guessing, the inrush current will be great too, and even if that MOSFET can endure up to 79 amps, i highly doubt, this will work as i want it to. Any advises on this matter are welcome.

Thank you!

pityu
 

In the PSU and fan situations, a better solution is to use a low value NTC thermistor. Adding a series inductor into an existing PWM system is likely to cause all kinds of problems unless you have insider knowledge of BMW wiring. As Frank points out, inductors work just as well as spike generators if you don't use them properly and especially in automotive systems you could create other problems.

Brian.
 

Thank you for your help. I will add a thermistor then, i suspect it wont bother too much, if the signal is PWM, since it warms up at startup, when the spikes would be. I will let you know, if it worked or not.

Thanks again!

pityu
 

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