How to choose the expanding flow diode?

Good morning~all

I am working on a project which is about electronics. My professional knowledge is limited cause I am just an enthusiast.

I would like to make a circuit of Dc voltage by using L7815. This is the L7815's datasheet . However,the current is so low just about 20mA,(here it need to output 2A). Then I surfaced the internet to find the solutions, and here it said I just need to add a transistor.But I don’t know how to build the outside circuit about expanding flow diode?

Does anyone here can help me?

Thanks a lot!

@li ming

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