Hey everybody, I'm studying in detail how SMPS power supplies work and I've got quite a lot of stuff figured out already. However, I hit a wall
with a specific aspect of the inrush input protection when using an NTC thermistor.
I'm trying to "fix" a cheap 24V/15A power supply to make it more reliable. So far the project has been very succesful (replaced capacitors with good quality ones, fuse with the correct rating one, rebuilt a high voltage pcb track that didn't have enough clearance, etc). Now one of the last things on my to-do list is checking if the thermistor is correctly rated for this power supply as I suspect the manufacturer just chose the most commonly available part and stuck it in there instead of doing the proper research.
The thermistor currently in use is a "NTC 5D-15" from a chinese manufacturer. The data for this thermistor is 5R, 6A max steady current, 0.132R approx max resistance at max current, 20mW/C° power dissipation coefficient, 76 time constant.
Here's my research so far:
I calculated how many Joules my system will absorb at startup. The SMPS has two 250V, 680uF capacitors in series (that should result in 340uF) and I've got a result of 23.7J with a Vpeak of 373 (derived from 240Vrms + 10%).
That's one of the parameters required for choosing a NTC thermistor, so far so good.
The second parameter is the resistance. The formula I found is Vpeak / Max inrush current (A) and this is where I'm having trouble: how can I calculate this max inrush current? Is that some arbitrary amount I need to choose? Is that based on a reasonable amount that's low enough not to trip the circuit breaker?
I don't think I have the proper equipment to measure it, unfortunately, so I need to calculate it somehow (my oscilloscope is pretty much a toy and I would never use that at mains voltage levels).
Thank you so much for reading this!
TL:DR;
The SMPS has an output rating of 15A at 24V, powered using 240V. Two 680uF caps in series and the NTC thermistor is a 5D-15, with 5 Ohms and 6A max current. I want to check if this is the correct value and I need to understand the Max inrush current (A). How is it calculated?