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!
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?