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How to avoid initial high current in switched capacitor?

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Member level 4
Jun 22, 2004
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initial high current

Hi all,
In switched capacitor dc-dc up converter, the initial current drawn is very high than the load current.How to avoid this high current. Anybody has circuit for the smooth start up of converter.

Re: initial high current

You can use "Inrush Current Soppressor" which when cold presents a high series resistance and limits current flow; as it warms the resistance reduces to allow normal current flow. For example device rated for Imax=2A will have resistance at 25degC approx 50ohm and at Imax this restance will drop to less then 1ohm..
Disadvantage: time constant which in many cases will be seconds.
But they are commanly used in switch mode power supplies, small electric motors etc..

Re: initial high current

what you probably have is a capacitor directlly across the power rails, so when it powers up you have a huge current surge.

use a LC filter , this will limit the high freq current surge.

something like

--!---L--!------ PWR
C1 C2

c1 small, C2 is the huge cap you already have.

there are other ways to do it, but they reduce the efficiency of the converter, hence whether you use em or not is up to you.

L is main compoentns here as cant have instant current through it.

Re: initial high current

how to do it in IC design.
the initial current from the voltage source during start up is arround 800mA where as, my load current at steady state is 100mA.

initial high current

no you want to use a soft start to avoid this kind of inrush. The question is: Do you regulate the output to a known value (12v out or x*FB) or do you simply flip caps to give VOUT=2VIN, etc?

You would use two different softstarts based on which style of circuit you have... If you don't want to post secret things, pm me.

Re: initial high current

hi electronrancher, did u get my pm

it is 1.2V to 3.3V up converter with regulation.

Re: initial high current


Maybe you can pay an attention on NTC( Negative Temperature Coefficient ) thermistor.
These devices are widely used in ac-dc power supplies for initial current limiting.

Best regards.

Re: initial high current

HI ,

you can use P-channel FET. You can control the FET gate to avoid inrush current.
Basically you need to add capacitor between Gate-Source.
It works definitely.

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