Details of Super Capacitor

[ArunPS]

Hi, I am working on a super capacitor design where i need to provide about 750mA of current for 60 seconds. I need to provide 5 minutes for this duration. I would like to know the value of the super capacitor considering all these factors and including that we need to have a life of 8 years minimum. Is there any way to choose the cap other than considering initial and final voltages to calculate the super cap value. Thanks.

 

[treez]

its just literally your normal capacitor equations
Q = CV
I = dq/dt
i = cdv/dt

and work it from there.
If its a series string of super caps then you may need a charge equalisation circuit with it.

I am not sure what the voltage is that you have to provide the 750mA from?
That will influence things.

 

[FvM]

Some design parameters are missing (input and output voltage, cycles per hour/day/whatsoever for the lifetime calculation. Or do you mean 12 cycles/hour for 24 h/365 days?

In most supercap backup applications, you'll want constant output voltage. This can be best achieved with a buck, boost or buck-boost converter translating the varying capacitor voltage to constant output voltage and constant input voltage to varying capacitor voltage in return. Conversation of charge equations are replaced by conversation of energy law then, giving maximum energy efficiency.

 

[Warpspeed]

Is there any way to choose the cap other than considering initial and final voltages to calculate the super cap value. Thanks.

Voltages will be your most critical starting point parameters to design around.