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Registered Member #610
Joined: Wed Mar 28 2007, 09:44PM
Location: Middletown, RI
Posts: 110
I am almost positive this question has been asked before, and I can't find the thread...
Does anyone have any experience on using a BJT to quickly remove charge from a MOSFET gate, if driven by a GDT?
I have a bunch of IRF450s that I'd like to use in a bridge, but I am concerned of the large gate capacitance. I'm planning on using boost diodes to reduce the high trr (650ns). I am aware that by using a floating supply (GDT output +/- 6 or 7.5V for 50% DT) I get more potential for removing gate charge during the off-cycle than if I used a chip-solution. I also get some dead-time out of it because the MOSFETs must be biased to >4V before they turn on.
If I add a BJT between the gate and source then the MOSFET turns off very fast, and I get more dead-time for the same frequency, which MAY translate into a higher maximum operating frequency. However, I don't see many people on this site using shut-off transistors (or chip drivers) and I was curious why. Is exceeding dv/dt as dangerous when going off as it is when going on?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
IF you use a GDT then to avoid saturation Integral(+Vgs.dt) = Integral(-Vgs.dt) For a typical 50:50 duty cycle this translates to |-Vgs| = |+Vgs| so the negative Vgs is used to turn off the transistor quickly, with or without a turn-off diode. Using a single GDT for upper and lower transistors helps prevent shoot-through. (guaranteed dead-time)
dVgs/dt is a limit because dVgs/dt x Cgs = Igate which has a limit.
Industrial bridge drivers commonly use a separate supply (hf transformer/rectifier etc) for each high-side transistor with varying complexities of gate drive switching dc-coupled to the gate, controlled by an opto-coupler. In this situation a turn-off (gate discharge) transistor is normal.
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Shutdown transistors for MOSFET gates
