NTC connection to the input
Hi I have designed a 37W SMPS and I connect NTC to the input neutral wire as I see in the reference designs,
But in PCB drawing I see that if I connect the NTC to the line side the PCB layout will be better...
Why we connect to NTC to the neutral wire side rather than line side? is there any reason?
Comments
Please suggest me any design calculations
Hi Mohan,
The inrush current is limited by the impedance of the input loop, that is the NTC, any series resistance and the input capacitor's ESR. As a quick calculation, the maximum inrush current is the maximum peak input voltage divided by the NTC resistance and the ESR.
Regards,
PI-Picard
How can we conclude what is the exact value of NTC resistor we need to use.Please provide any formula or document to select the prefered NTC value.
Please see this example design and let me know weather it can help for me in reducing inrush current
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Hi Mohan,
In your example, there is a series resistor in each line, hence it will reduce the inrush current but your efficiency will suffer.
Regards,
PI-Picard
Hi Mohan,
The equation is easy. It is basically any resistance along the path to the input capacitors. The maximum inrush current is equal to the maximum peak voltage divided by the impedance\resistance. The series resistance comes from the NTC, any series resistor, DC resistance of the inductor (CMC\differential choke), trace resistance and the ESR of the input capacitors. To make it simpler, majority of the resistance will come from the NTC if there are no other series resistors. Hence, the formula is Vinput(peak)/NTC resistance.
Regards,
PI-Picard
The NTC is used for the inrush current limiter for charging the input capacitor, and there is no difference between line and neutral wires for NTC connection.