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In this case not recommend for use any analog (dissipative) regulator, because the dissipation about 35...40W! Use a simple switcher SMPS regulator from Nat*ional Semi*conductor with very low dissipation (LM2576-adj).
On NSC website You can find applications.
yes, think that means you would need to get 1.5 amps from the transformer, wich if you would use a switching regulator, you would need about 300-400A instead, so it would be better. if you were to use a LM317 regulator at full power it might be very possible to destroy it and certainli after a few minutes the termal protection will lower the voltage. i used it to power a 1.5Amp fan/6V from a 36 voltage suply and even with a large heatsink after a few minutes, 10-20, it stopped.
so the simple regualtor is out of the question
If you are using the IC in switching mode then ofcourse the circuit along with external transistor is going to be in switching mode too. And finally the output of a any switching regulator is in form of DC pulses which is fileterd and converted to pure DC using an external capacitor. S I don;t think in swithhing mode the external transistor is going to dissipate additional power.