Why Do Apparently Similar or Better HOTs Sometimes Run Hot and Blow?
It is often surprising that replacing a horizontal output transistor with one that has overall better specifications does not work out – it may run hot and fail. There is more to characterizing a transistor than just maximum voltage, current, and power dissipation. One important parameter is current gain: Too low a gain for a particular operating point may result in incomplete turn-on during scan resulting in high dissipation. You want the transistor to be in the fully saturated state. A larger HOT is more likely to have a lower current gain. If you read the app notes put out by the manufacturers like Motorola you will also find that fast turn off based drive (negative step) is actually not what you want since this traps excess carriers in the high resistivity collector region which leads to continued conduction and heating. The ideal waveform also provide adequate drive during scan but not excessive overdrive and is thus an increasing ramp to account for the increasing collector curre
