Discrete FET gate driver for LED driver

[eem2am]

Hello,

The attached Buck converter (LED driver) has a P mosfet driven by a discrete BJT gate driver.
V(in) = 12V

The transistor Q1 is being driven into saturation, but still manages to switch the PFET off quickly (the turn OFF gate voltage plateau is only 82ns).

How can this be?, why is BJT Q1 not taking ages to turn off?, ...after all it was saturated

Also, the NPN Q2 suffers a periodic reverse voltage of -9V on its Vbe. Will this be OK?, ..since its only a spike, and not a persistent reverse voltage of -9V on the Vbe.

Here is datasheet for MMBT3904 showing max allowable Vbe on MMBT3904 is -6V
http://www.fairchildsemi.com/ds/2N/2N3904.pdf

(All waveforms below)
Schematic; PFET gate voltage; Q2 Vbe

 

[duke37]

There was a thread here studying the degradaton due to reverse e/b current. It is not a good thing. Spice does not seem to simulate this condition.

Try a diode across the e/b junction.

 

[(*steve*)]

I believe a diode across the BE junction will cause the switching speed to be reduced as the gate charge will get (at least partially) discharged through Q1 rather than Q3.

Perhaps a diode in series with the base or emitter would be preferable.

 

[KrisBlueNZ]

Q2 is seeing that negative base-emitter voltage because its emitter doesn't get pulled down towards 0V when its base is. This is because of the interaction of R6 and the MOSFET's gate capacitance. Why is R6 needed? You might be better to remove R6 and if necessary add a very small capacitance from BJT_bases to 0V for slew limiting to protect Q2 and Q3 from overcurrent. And/or use higher rated transistors for Q2 and Q3. Or connect Q2 and Q3 emitters together and use a series resistor from that node to the MOSFET gate.

I can't tell you why Q1 isn't turning off slowly in your simulation. I expect that LTSpice models charge storage.

I wonder why you're using a three-transistor buffer section AFTER a MOSFET gate driver! I just looked at the data sheet for the LTC1693-3 and it's an N-channel MOSFET driver for a grounded-source configuration only. You would be better to use the LTC1693-1 or the LTC1693-2 and use an N-channel MOSFET as your switching device. You could avoid all those transistors and actually make use of your driver IC!