Negative base current

Question

I am designing a flyback driver using a salvaged power transistor that was originally the driver transistor of a flyback in a ctr screen.

This power transistor is the 2SC5681

I want to drive this transistor with an IC, and the high base current required by this transitor (1-2A) cannot be directly supplied by the IC. So i need an intermediate stage to drive the transistor base.

First, i planned to put an npn transistor before the 2SC5681 in a darlington configuration to amplify the current gain.

but when I reviewed the 2SC5681 datasheet, i saw that this transistor needs a positive Ib (which is normal) AND a some negative current at the base for switch off (fast)

so my question is, can i use the darlington configuration to achieve the base drive despite this negative current? or what kind of simple base driver can i use for this transistor?

I had thought of using a push pull configuration like a mosfet gate drive system but is it adapted?

maxime

Answer 1

When transistors are used in Darlington configuration they are usually used with resistors R1, R2 to allow the driver to provide the reverse recovery current to Q2 and to prevent leakage (amplified by \$\beta^2\$) from turning on the transistors.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

To turn off a bipolar NPN transistor fast, it is useful to have a negative base bias (reverse the base current) to remove the stored base charge. The easiest way to accomplish this is to raise the emitter voltage slightly when the transistor is ON, so that a single-power-supply signal can sink base current.

^{simulate this circuit – Schematic created using CircuitLab}

This presumes that you can find resistor and C1 values suitable for the collector loads being driven by Q1.