Sorry, the two BJT's are BD243B's and BD244B's which are NPN and PNP respectively and they have 180 ohm base resistors and are being switched by the SG3525 whose two output pins are 180° out of phase from each other. Meaning that the BJT's are push-pull through the gate drive transformers primary.I am unable to decipher your schematic.
Please download a Proper schematic drawing program, such as LTSpice, mark resistor values, Transistor numbers and make connections more clear.
Thanks. Is this why they are getting hot? What in the datasheet should I be looking at that says how slow BJT's are?Those BJT that you use are too slow. Use switching BJT
If transistors get hot, that implies excessive current through them with low voltage drop between emitter/collector, or low current and high voltage; either way, power dissipation.Thanks. Is this why they are getting hot? What in the datasheet should I be looking at that says how slow BJT's are?
Can you recommend some BJT's I should use? The main thing is to quickly switch the MOSFET gates.
Thanks! This helps. I do not have a scope but I will try what you said, I am switching them @ around 50khz.If transistors get hot, that implies excessive current through them with low voltage drop between emitter/collector, or low current and high voltage; either way, power dissipation.
I'm not 100% sure, because the schematic is tough to concentrate on the way it is drawn, but I would guess that the transistors are (npn/pnp pairs) on just long enough that they're shorting from npn's collector to pnp's emitter, pulling excessive current, and getting too hot to work properly before long.
Be sure that each transistor is getting a true square wave with no DC offset. DC offset will throw the whole operation off. If the SG chip is giving you 0 to +5v, that means the npn is happy but the pnp isn't as happy as it would be with 0 to -5v...I would check those waveforms.
I haven't looked into the transition frequencies or the capacitances of the transistors, but if the frequency is near 10-20 kHz, you should be O.K.
Also, on the transistors, for the purpose of testing, you might consider a few 1 ohm emitter ballast resistors. It's a quick method of ensuring that the output impedances of the transistors, in their fully-on state, have near uniform output impedances. And, they're a form of negative feedback. If the current begins to run away through the bjts, the voltage drop on those resistors rise, which limits the overall current that eventually gets through. You can also set up a protection circuit that monitors the voltage drop on these resistors and kicks the transistor base actuation signal upon a failure condition.
Hope this helps a bit.
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by Jake Hertz
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