Driving a power transistor with 2sc945

I'm designing a simple 15V to 5V DC voltage feadback mode non SMPS regulator.

I'm using E13007E2 power NPN switch mode transistor. And to drive that transistor I'm using C945 another transistor.

This is my schematic diagram of the circuit.

It gives 4.5V output when it's not loaded. But when it's loaded and after some time the C945 had gone. It's 2 junctions are open now. c945 is to shunt just only 6mA. Why it can't handle 6mA?

Any idea how could I calculate the operating points precisely?