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If I am charging a super capacitor with buck converter, that is a 1 amp, 1-18 volt output, and the capacitor is rated for 2.5 volts, how do I stop the buck converter from feeding it more than 2.5 volts? When I attach the capacitor to my digital power supply, it starts charging, and the voltage slowly increase to be more than 2.5 volts, at which point I turn it off. I was wondering will it draw more than 2.5 volts from my buck converter? If so how do I stop it?

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    \$\begingroup\$ Show the schematic of the buck converter. \$\endgroup\$ – Andy aka Jan 27 '16 at 17:58
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When I attach the capacitor to my digital power supply, it starts charging, and the voltage slowly increase to be more than 2.5 volts, at which point I turn it off.

You should not connect a (super) capacitor to any voltage higher than its rated voltage.

A minor over voltage may only reduce lifetime, but a higher voltage can cause gas generation and/or breakdown of dielectric material. That can result in explosion and/or chemistry leakage.

I was wondering will it draw more than 2.5 volts from my buck converter? If so how do I stop it?

It will "try" to charge up to whatever voltage you apply. But as I pointed out, at some point it will cause catastrophic failure.

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There are 2 ways you can achieve this. Which you choose will depend on how much control you have over the buck regulator and how important tight control of the output voltage is.

Method 1: Stable control loop

Method 2: Hysterical control.

With method 1 you vary the current that the buck regulator produces in order to maintain the output voltage at the desired level (2.5V). This requires control of the on-period of the regulator, this may be via a feedback terminal on the regulator chip, but it is not possible to be specific without a schematic. For this method to be successful you will need to make the loop stable to avoid oscillation, although this should not be too much of a problem with a large capacitor at the output.

With method 2 you only need to be able to turn the buck regulator on and off. You turn it off when the voltage across the super cap reaches the desired voltage e.g. 2.5V. You then turn it back on when the voltage falls to a slightly lower threshold e.g. 2.3V. This maintains the output voltage between the two thresholds.

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    \$\begingroup\$ I think you might mean Hysteretic control. \$\endgroup\$ – Andy aka Jan 27 '16 at 18:37

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