Help with series NTC sizing

Question

The inrush current of my 12V/10A rated SMPS exceeds the inrush current limit of a rocker switch so I need to dampen the inrush current by an NTC as shown below. (I know there are other methods but this question is only about using an NTC.)

Now I know that the SMPS has 85% efficiency and will source not more than 7A. So AC input power should be 100*(12V*7A)/85 = 99W. This means to me the AC nominal current must be 99W/230V = 0.4A AC nominal current. So I guess we can take the AC input current as 0.5A.That means, we know that the SMPS rated input AC current is less than 0.5A.

The SMPS inrush current at cold start however is 70A. SMPS capacitance is unknown. And the rocker switch is rated for 6A nominal and can only handle inrush current up to only 45A. So I want to dampen the inrush current by an NTC. But for that I need to size its resistance.

Can we calculate the NTC minimum resistance (R_NTC) as follows:

For 230 VRMS network Vpeak = 325 V

I = 45A = 325 / R_NTC

Rntc = 7.2 Ω;

So does that mean we need an NTC with a minimum of 7.2 Ω which can pass 0.5A continuous rms current? Is this calculation correct?

In NTC datasheets like here there are other parameters such as below:

In my case the ambient temperature can go up to 55C. Can I use this 1.8W 8Ω NTC for my case?

Answer 1

From this ...

Selection of inrush current limiters :
There are three major criteria for selecting the best inrush current limiter for an application:

• Rated resistance (R25)
• Maximum permissible continuous current under rated operating conditions
  (Imax, DC or RMS values for AC)
• Maximum capacitance CT to be switched

Example of simulation. Made with FREE microcap v12.

With a serial resistor

With an NTC