I'm fairly new to the world of electronics, and transistors are still a bit confusing to me. I'm trying to build a flyback transformer driver based on the schematics at http://www.powerlabs.org/images/schematic1.gif and https://youtube.com/watch?v=0jWpQ8HEYgk . I'm using a 2N3055 transistor rated for up to 15 A on the collector with power dissipation up to 115 W.
At 4 V, the system draws about 2 A and I get several thousand volts out of the flyback, as expected. The transistor gets a little warm, but not too hot to touch. As I slowly increase the supply to 6 V, the current draw rises to 3 A and then the flyback suddenly stops outputting any voltage. When I examine the transistor, it shows only a few ohms of resistance between any pair of pins, which I assume means I killed it. Replacing the transistor fixes the circuit. Hooking the circuit up to the 12 V, nominally 2 A laptop supply that I hope to use once I'm done prototyping also causes the transistor to instantly blow.
My question is, what would cause the transistor to die at so much less than its rated specifications? After reading something about voltage spikes caused by inductance in the transformer, I added a diode from ground to the feedback coil as shown in the video above, but it doesn't seem to help. These transistors are about $3 apiece, so I'd prefer to go through as few of them as possible. Any ideas?