How to find Power-on Surge Current

Why do you need to know this?

Normally, the power-on surge current of the capacitors on the board is much larger than power-on surge current of the IC's.

If you really do need to know, call the vendor's applications engineers or buy some chips and measure the inrush current.

--Mac

ICs generally do not have a turn on current surge.

When a turn on surge occurs, it is caused by the charging of the capacitors in a circuit.

To measure it, I put a 0.02 ohm resistor in series with the circuit and watch the surge with an oscilloscope. Then I calculate the surge current (Peak voltage / .02)

It is tricky when doing AC operated equipmnent. For them I have a 50A isolation transformer. A smaller one will work but this reduces te size of the surge because of the transformer's source impedance.

Dan

My board consists of various power supplies. +5V is the only input for my board. I am generating all the other supplies such as

+3.3v,+1.8v,+1.5v and +1v by buck convertor circuits in the board. I only designed the buck convertor ckt also.

My board contains four processors each will consume max of 4A while operating condn, So there may be a chance of inrush current to the processor IC's.In that case i need to design the buck convertor ckt in such a way that it withstand the surge. That's why i am asking this.

"Dan Fraser"

** Talk about moving the mountain to Mohammad.

You only need to isolate the voltage across a current sense resistor in the AC supply with a small transformer.

Or, better still, use a Hall Effect transducer if response to DC is wanted.

........ Phil

Where is this component inside each computer to cause such power on inrush? How big is that capacitor inside the processor? Inductor? Do transistors all turn on during power up and therefore create a power on short? Of course not.

But your design requires capacitors. Large electrolytic at the power connector and bypass capacitors distributed throughout the board. Therein lays your power on surge. Worst case estimate that power on transient by ignoring wire and PC board inductance. Assume all the capacitors are directly connected to the power supply.

Meanwhile processors have inrush current requirements - but not during power on. For example, the earliest Pentiums made demands on the adjacent power supply circuits. The processor might go from less than 1 amp to tens of amps ... in microseconds.

The real > My board consists of various power supplies. +5V is the only input for

On 6 Apr 2006 16:13:05 -0700, "w_tom" top posted:

--
You\'ve been posting here, through Google groups,  since December 8
of last year, so you must know how nearly all of us feel about top
posting, and yet you do nothing _but_ top post.

Even Google advises against it.

From:

http://groups.google.com/support/bin/answer.py?answer=12348&topic=250

"Summarize what you\'re following up.

When you click "Reply" under "show options" to follow up an existing
article, Google Groups includes the full article in quotes, with the
cursor at the top of the article. Tempting though it is to just
start
typing your message, please STOP and do two things first.  
Look at the quoted text and remove parts  that are irrelevant. 
Then, go to the BOTTOM of the article and start typing there.
Doing this makes it much easier for your readers to get through your
post.  They\'ll have a reminder of the relevant text before your 
comment, but won\'t have to re-read the entire article.  
And if your reply appears on a site before the original article
does, 
they\'ll get the gist of what you\'re talking about."

Why don\'t you get with the program and, when in Rome, do as the
Romans do ?

This sounds like a myth to me. I can't imagine why a chip would draw more current than the current stated in the absolute maximum ratings.

It is possible for a chip to draw more current than specified due to latch-up. If this is the case, there is probably an error in the design.

--
Reply to nico@nctdevpuntnl (punt=.)
Bedrijven en winkels vindt U op www.adresboekje.nl

Many years ago there were some bipolar chips (opamps I think) that drew quite a high supply current at a low supply voltage.... As I found out with a 100mA foldback power supply that just would not come up at power-on. :(

I don't know the reason.

--
Tony Williams.

Perhaps it was one with an internal cap (for frequency compensation) like the 741 and similar.

--
Paul Hovnanian     mailto:Paul@Hovnanian.com
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