The LM2575 works a regulator only and don’t deliver any current to the load.
If the regulator switches “ON”, you find a voltage at the output of it. This makes T4 switching on also.
The collector of T4 now is nearly GND and also the Gate of FET T3. This FET switches on (it’s a P-FET!)
For T3 you can take everything what’s called Power P-FET with low RDSON, a IRF9540 is a good choice.
(The lower the higher the price 
The “invention” of this circuit is the Gate turn off of T3.
In ON State, The Gate of T3 has nearly GND potential, D9 conducts and T5 is off.
If now the Regulator switch it’s output OFF, then the Base of T4 is pulled low by R12 and no currents flow through T4.
Also the cathode of D9 is connected to Vcc via R13. The stored energy in the Gate capacity of T3 lets now pull T5′s emiter lower then the base. Whats happens if the Base of a NPN transistor has higher voltage than the emiter? It conducts ! Now, T3′s gate is directly connected to the source via T5 and the gate capacity can be discharged very fast.
This happens several times faster than if you only had R13 to discharge the gate capacity !
Read more Source:
http://www.aca-vogel.de/DCDC-Converter/
8AfromLM2575_en.html
Thank you.