I Recently started building a SEPIC converter using the TL494 and a BJT-based MOSFET gate drive circuit. When testing the gate drive circuit, I noticed something peculiar. I tested the gate drive circuit in this manner:
And this was the waveform at the gate of the MOSFET (yellow: Vgs; green: Vds):
Needless to say, its hideous. So I did some measurements and came to the conclusion (which I later found was incorrect) that the BJT Q10 was at fault. So I obviously replaced the transistor with what I thought was a 2n3904, but was actually a BS170. The waveform at the gate then became acceptable:
Once I realized that it was a BS170, I replaced it with another 2n3904, but the gate drive voltage was just as bad as it was the first time. I retested the circuit with another 2n3904, but I got the same result. The gate drive circuit worked perfectly with Q10 being the BS170, but not the 2n3904. I'd like to know why.
PS I removed the main MOSFET Q4 and tested the unloaded gate drive circuit with the 2n3904 again, and it spat out a beautiful square wave. When I loaded the driver with a MOSFET, the output collapsed again. It seems that this circuit only works with a small signal MOSFET as Q10 for some reason. Also, an LTspice simulation showed that the gate drive circuit works with the BJT as Q10, but it doesn't on the breadboard!