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calculating Inrush current limiter

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thannara123

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Hai ,

I require a inrush current limiter Design for an inverter with 1000 watt [230volt rms ,50HZ sinewave]

My calculations are follows [reference - https://www.ametherm.com/inrush-current/inverter-inrush-current-protection ]

Normal current [Ess current] - 1000/230 =4.5Ampere

Inrush Current [300% surge capability] 3000/230 = 12Ampre [approximatly]

Duration of inrush cycle - 1*1/50 = .02second

Energy of the Thermistor - 230 * 1.414 *.02 = 55 Jule

Minimum resistance -( 230 * 1.414 ) / 4.5 Amprer = 40 Ohm
Assuming ambient temperature is 50 degree C So minimum Resistance will be 80 Ohm


calculation gives The NTC with { 80 Ohm ,55Jule ,5 Ampere }

is this calculation is correct ?

If yes Approximately which one select from here https://www.digikey.in/products/en/circuit-protection/inrush-current-limiters-icl/151/page/2

- - - Updated - - -

Do there connect in parallel or serial for the correct value ?
 
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The main reason(s) for an inrush limiter are to prevent (1) diodes from failing and to (2)control the voltage on the capacitor bank. (The customer/end user might also get cranky if his breakers trip when energizing his power supply) Do you have another reason for limiting inrush? Inrush limiters can be bypassed with a relay or not. I have designed both 230v and 480v 10kW inverters without bypassing the inrush limiter. They get warm when running tho.

(1) Diodes have an I^2t rating that you want to pay attention to. An inrush limiter will limit the inrush current and prevent diode failure due to over current events. You can put a large diode in and your need for inrush limiter in this case is lessened.

(2) The line impedance (resistance and inductance) forms an LRC circuit with the bus capacitor. If the resistance is low the voltage on the capacitor can overshoot on power-up- BUT- Most electrolytic capacitors act like zener diodes so that as the voltage tends to overshoot the capacitor begins to get very leaky and the overshoot is limited.

I usually use We=1/2 CV^2 to determine joule rating of inrush limiter. Capacitance and expected bus voltage should get you close. I usually multiply calculated joule rating by 2 when choosing inrush limiter.

You can use LT Spice to model the whole circuit including line reactance. It produced very accurate models of what you will measure. If you would like to try that and need a hand with it just get with me.

Regards,
PowerSupplyGuy
 
What I don’t see in your calculations is the hot (operating) resistance.
That will give you an estimate of the power dissipated by the thermistor and therefore the efficiency loss.

Most modern thermistors have a very large cold vs. hot ratio, and most likely your can get away without the need to bypass it.
But you have to perform the calculations.

It’s been a while, but if I remember correctly, the thermistor vendors provide spreadsheets or other software tools which will allow you to perform the analysis quickly.
 

Re: calculating Inrush current limiter

Mainly I planed to avoid the following condition

The output of the inverter reducing instantaneously when the inrush current occur.

Or How can i provide a staedy state current up to 300% of the normal rating .

Or How can i provide a staedy state current up to 300% of the normal rating .

Sorry question is
How can I provide a surge current capability up to 300% of normal rating
 

Re: calculating Inrush current limiter

Are you talking about inrush current when the line voltage is applied to the power supply?

The statement where the inverter is reducing instantaneously when inrush current occurs is confusing.

Is there a specification that you are designing to? It might help to post.
 
I require a circuits for Managing high-current transient loads in battery. after googling the solution may be the Inrush Current Limiter.In my inverter loads takes sudden increase in load for a very short period while the voltage drops occur accordingly.sorry for my bad English
 

NTC no good when its hot (eg after an ON/OFF/ON)
You need relay or fet switched resistive limiter.
Or switch on at the mains zero x.

As far as surges.....have an overcurrent shut off......there is no need to suddenly supply massive overcurrents when charging a battery.
 
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