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nobody

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How to reduce the time for overload recovery �or the current spike!!
Oct 08^th, 2010, 3:49am

Hello all

I am designing a ckt and the output node will be shorted to ground.
Therefore, the voltage at gate node should be clamped for the over-current protection.
At first, the over current will be limited around 1.0A. After the first over current protection, the output current should be limited around 200mA.
Figure A shows a good waveform which I want to design.
Figure B is my design. I used a clamping ckt to limit the output current and it appears that there is a current spike which is too large compared with Figure B.
I need suggestions to remove the current spike.
Thanks

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HdrChopper Community Fellow Offline Posts: 493	Re: How to reduce the time for overload recovery �or the current spike!! Reply #1 - Oct 12^th, 2010, 5:41pm Hi I'm assuming you are using a closed loop clamp circuit here (may be if you can post your schematic that will help). If that is correct it looks response B is a consequence of a rather slow / low gain loop. Try increasing your loop gain and / or ckt speed. Best Tosei
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nobody

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Re: How to reduce the time for overload recovery �or the current spike!!
Reply #2 - Oct 24^th, 2010, 6:45pm

A sensor ckt connected in parallel with the output transistor, senses the load current and converts the current signal into the voltage siganl through a resistor. The voltage signal will control the gate voltage of the output transistor. The time or the delay is partly due to slew rate and small signal BW. The overshoot or current spike is caused by large signal in my opinion and I would like to reduce that spike.
Thanks.

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HdrChopper

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Re: How to reduce the time for overload recovery �or the current spike!!
Reply #3 - Oct 31^st, 2010, 11:47am

I still insist that overshoot might be due to a not enough loop gain in your clamp circuit. I used a very similar approach several times and most of the cases I found overshoots were related to not enough loop gain (the problem will then be the stability associated to that loop).

When the current hit the clamp threshold the clamp ckt will enter into linear mode and will try to regulate such current flowing thru the output. Under such condition you get a certain gain and BW. If the loop gain is not large enough then the loop will not be able to �hard clamp�the current but instead will do it softly. That softness I think might be related to the overshoot you are trying to reduce.

Best
Tosei

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nobody

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Re: How to reduce the time for overload recovery �or the current spike!!
Reply #4 - Oct 31^st, 2010, 6:52pm

Hello, Tosei

Here is the circuit I used for over current protection. This loop is quick but the gain is probably not large enough. In my opinion, the reason that causes overshoot is pretty much like that causes slewing. This overshoot happens because the over current signal is too large and the loop is dead at that time . After some time, this protection works and performs in the linear fashion like an opamp.
Best

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