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

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"

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

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.

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.

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