### Author Topic: How to calculate max inrush current for a SMPS?  (Read 1974 times)

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#### AxelDominatoR

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##### How to calculate max inrush current for a SMPS?
« on: July 10, 2016, 12:48:01 pm »
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?
Axel DominatoR ^^^ HC

#### T3sl4co1l

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##### Re: How to calculate max inrush current for a SMPS?
« Reply #1 on: July 10, 2016, 02:04:55 pm »
At instant of turn-on, caps are at zero and worst-case line is at peak 340V.  How much voltage drops across the NTC?  How much current does that draw?

There may be some finagling about whether actual-peak current matters, or RMS over a full cycle or quasi-peak or something like that.  You'd have to read the relevant standards to see how it's measured.

Tim
Seven Transistor Labs, LLC
Electronic Design, from Concept to Layout.
Need engineering assistance? Drop me a message!

#### AxelDominatoR

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##### Re: How to calculate max inrush current for a SMPS?
« Reply #2 on: July 10, 2016, 06:35:06 pm »
I found this online calculator, hopefully I'm on the right track: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capchg.html

By playing with it a little bit it seems like a 5 Ohm NTC will allow for something like 70A through the instant the supply turns on. It takes about 5ms (3RC) to get to a much more comfortable 3.something A.
So, if the above is correct, that means 5R is not the ideal choice, as I read here (http://sound.westhost.com/articles/inrush.htm):
Quote
Most manufacturers will use a negative temperature coefficient (NTC) thermistor to limit the inrush current, but it's not at all uncommon for them to get the value horribly wrong. One that I recently came across used a 4 ohm thermistor - completely useless, and I was able to measure 80A inrush peaks easily.

Now, what would be a better value? A 10R NTC would give me 37A: much better already. The 20R version brings it down to 18A, but it's less efficient afterwards as it will cause a higher voltage drop. There's an intermediate choice at 14R, too.

So, given the data above, should I go with the highest resistance that still allows me to be within specs? (for example, 20R?) Or should I choose a 10R which is more efficient when on a steady state?

I guess my question really is: how much inrush current should I be comfortable with, given the choices?
Axel DominatoR ^^^ HC

#### bktemp

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##### Re: How to calculate max inrush current for a SMPS?
« Reply #3 on: July 10, 2016, 07:01:33 pm »
Now, what would be a better value? A 10R NTC would give me 37A: much better already. The 20R version brings it down to 18A, but it's less efficient afterwards as it will cause a higher voltage drop. There's an intermediate choice at 14R, too.

So, given the data above, should I go with the highest resistance that still allows me to be within specs? (for example, 20R?) Or should I choose a 10R which is more efficient when on a steady state?

I guess my question really is: how much inrush current should I be comfortable with, given the choices?
There is no ideal choice: You can choose either a high inrush current + low steady state losses or lower inrush current + higher steady state losses. To get a high efficency many manufacturers use a rather low value for the NTC. You often see 20-40A inrush current even for a small SMPS.
In a good quality SMPS you often see a relay bypassing the NTC after a short time. That allows a high value NTC for a good soft start but eliminates the steady state losses.

#### AxelDominatoR

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##### Re: How to calculate max inrush current for a SMPS?
« Reply #4 on: July 10, 2016, 07:29:52 pm »

I read about a few different ways of achieving a better efficiency (in the sound.westhost.com article I linked). Given the SMPS is built on a simple single sided board and there's actually plenty of space for modifications, I wonder if I could implement at least a simple bypass system.
Axel DominatoR ^^^ HC

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