I built this ZVS driver https://tesladownunder.com/Flyback_driverAndrineri.jpg and I use a 13.8 volt 7 amp power supply and was wondering if I could increase the output without increasing the input. Thanks 

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The only way to use the same input power for more output power you'd have to buy a more efficient power supply, there is nothing you can add to it that will increase the output power compared to input, there will always be conversion loss, voltage or current one always comes at the cost of the other, and there is always conversion loss. This is a law of nature that hasn't as yet been broken.