If you have a half-bridge, or an H-bridge, or some other usual configuration; and you want to see the voltage you are getting on the gate, to check for any errors and shoot-through.
Do you do it like this;
CASE A
HIGH SIDE
$$ V(g) to V(s) $$
LOW SIDE
$$ V(g) to V(GND) $$
Or like this;
CASE B
HIGH SIDE
$$ V(g) to V(GND) $$
LOW SIDE
$$ V(g) to V(GND) $$
Because if I do the top one, I get (high side is RED, first pic RED is level shifted slightly);
This. Which is nice and square and uniform.
If I do the bottom one, I get this;
A bad wave with an equally terrible 5V constant at turn-off due to either Miller capacitance or magic.
So which way is correct, and why am I getting that constant 5V? It continues to ALWAYS be VCC – V(gth) of the high side MOSFET. So I guess it doesn’t do much harm since they’re all N-MOS and therefore it must be VCC + V(gth), not minus, to turn it on.
It’s just a really odd occurrence, and I think it’s because I’m measuring things stupidly, so how do I measure them correctly.
My schematic, for reference;
