Nice drivers. I use something almost the same. I haven't yet figured out how to make the schematic large enough to read the part labels,
Thank you
. It's the "Raw" button on the right on top of the image, but here's a direct link:
https://github.com/vnevoa/DiffTrike/raw/MarkIV_RasPi_NJAY/Electronics/PowerBridge/hw/bridge-fet-drivers-schematics.pdfI just noticed the schottky diodes's symbols somehow ended up the same as PN ones, I'll have to change it when time permits. The github project under ".../PowerBridge/..." has a 25V 25A DC motor controller project with all design files for schematics (KiCAD but there are PDFs of all schematics), pcbs and fw for whoever wants to look.
There are a few "problems" with these drivers, but I also wanted to keep component count down, as much as desperately possible. For example, the low driver doesn't keep the FET off when the input is floating (in my application this isn't a problem because the high side ones are guaranteed off) and the top output transistor is always dropping 12V (and it needs a constant, although tiny, load, guaranteed by the FET's gate-source resistors).
Thanks for the suggestions below.
but see that first resistor, through which the input signal passes to the pull-down transistor's base? I add a Schottky diode in parallel with that. This accomplishes two things:
1) When connected to an MCU or other device which can sink current, and it switches to logic low, the Schottky conducts and bypasses the resistor. Allowing the full available sink current to drain the base charge, and improving transistor turn-off time. It does so even though there's an anti-saturation diode between that transistor's base and collector, they work great together.
I currently don't have any of those drivers with me, but I was pretty happy with their switching latency, basically "zero" (switching time in my app is decent too). I have some scope shots, including comparison with no anti-sat diode at all and with 1N4148 as anti-sat, but not here with me. When I get my hands on the drivers again I'll try that 2nd schottky to see what's the difference - as it is, the driver's output (loaded with the FET) starts reacting "instantly", but from your description I guess it goes up/down faster (it is actually irrelevant in my application, because the phase's di/dt is being limited by parasitic inductance at the FET's source (which is bad and good) - hey, I'm still learning!).
2) When it switches to logic high, the Schottky is reverse biased and does not conduct. Instead, it acts as a capacitor, in the neighborhood of 10pF. This is a significant portion of the the transistor's base capacitance. And so, by again bypassing the resistor, it improves turn-on time some too.
I see. The input's 0->1 transition, being fast, goes through the "schottky's capacitance" bypassing the resistor.