Thank you erikl, but I forgot to specify that the cap. is inside a device. The voltage flip has to be done externally. Actually I implemented out a second cap. which is charged with opposite voltage. When the DPDT switches, the charges are shared, and half voltage(circa) drop appears on the internal cap. of the device. I was wondering how i can achieve a full voltage flip.
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Thank you erikl, but I forgot to say that the capacitor is inside a black box, than I should use external circuitry. Actually what I have done, is to use a second capacitor outside the black box charged with opposite voltage, and then using a DPDT I flip it. The problem is that no full flip is achieved.