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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi, I need to drive a power BJT with around 1-2 Amps to the base. The driver must be able to sink at least the same amount of current from the base during turn-off, for a few microseconds.
The thing is that I don't want to suck the whole drive current from the 12 volt supply for the driver, so a base drive transformer is necessary. The control IC is the TL494, with duty cycle from 0 to 50% and frequency from 20 to 50 kHz.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Hi Dr. DC
The traditional base drive method in TVs and early SMPS was a step-down transformer. It can be operated either in forward or flyback mode. I think flyback is preferred, for reasons that I've forgotten ever since I got hold of my first IGBT.
If the transformer supplies the turn-on current as a flyback converter, then it supplies the turn-off current as a forward converter and vice versa. So the core requires an air-gap for energy storage whichever way you do it.
I think a reasonable start would be to hook the primary between one of the TL494's open-collector outputs and the 12V rail, with a current limiting resistor in series. That would have it supplying the turn-on current in flyback mode: at 0% duty the transformer primary passes maximum current.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Thanks for the reply Steve The whole time I was trying to design the circuit to operate the transformer in forward mode. In flyback mode, the base current would decrease with higher duty cycle, or not?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
That is true, but on the other hand if you operate it in forward mode, you may not get enough turn-off current at low duty cycles, because the magnetising current doesn't get a chance to build up.
I'm going by some old book I read somewhere, that said that the transformer is normally used in flyback mode. I can't remember what book, or what reasons it gave, but I remember it because it seemed crazy.
Registered Member #816
Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156
While the transformer seems the most often preferred method. If you don’t need the isolation of a transformer, and you are operating in common emitter (perhaps I should say low side driver).
Another approach you could try is using a darlinton arrangement, or even possibly with the drive transistor in the darlington configuration replaced with a mosfet. I haven’t any schematic drawing software, to show what I mean but with the mosfet connected between the base and collector of the BJT.
You’d have something resembling an Igbt then, but you’ll need a pull down transistor between the base and emitter, or to a few volts –ve rail instead, otherwise the BJT will be really slow to turn off, at lower frequencies you may be able to simplify that to a large low value resistor like 10 ohms or less.
The only thing with the darlington configuration is the power device won’t be able to saturate, you can’t end up with less than the Vbe dropped across it, so the on state dissipation is going to be higher.
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Bipolar transistor base driver
