How to select a Super Capacitor?

super cap

Hi all,

I am designing an interface board to a GSM module and notice that many reference design make use of the Super Cap.

My question is, how do one choose the correct value of the Super Cap? As I have checked on most data sheet of Super Cap, only these parameters are specified, namely - Rated Voltage, Capacitance and ESR.

How do I know the rated current of the Super cap?
How do I know if the Super Cap withstand a momentary 2A of transmission burst?
What value of capacitance should I choose?

At this moment of time, i am connecting the Super cap directly to the output of my switching regulator. I am assuming when the transmission surge comes, it's the Super Cap that will provide, not the Switching regulator. I am wondering if this design is workable...

Thanks for any infor.

Best rgds,
Pang

Added after 8 minutes:

Hi all,

I have checked that the transmit burst is for a duration of 577us.

Thanks.

Pang

how to select capacitor

Capacitor is not working in conducted mode. So pay attention to rated voltage not current. It should be at least 2 times of the voltage apply to. Capacitor will lower down the voltage drop, but it would not provide the surge current needed.

The ESR should be less than 1 ohm normally. You could use tantalum or ceramic capacitor. 100uF could be a good choice. Or 470 uF.

top super cap

"..but it would not provide the surge current needed."

But actually providing the temporary surge current is what a super cap does. Which is why super cap are frequently used in camera flashes to cater for the momentary surge in current. As such i find a need to know the capability of the super cap in handling such a surge....

Thanks.

danish supercapacitor

2A for 588uS is a charge of 1.2mC. If you can tollerate a voltage drop of 0.1V,
you need C=I*t/U=2A*588uS/0.1V=11760uF. Bussman B1325-2R5106 is 10F/2.5V and have an internal resistance of 60mohm. They should be able to supply 2A. If you need more than 2.5V and want to connect them in series, be sure to add a charge equalization circuit. They will not survive overvoltages.