Hi - I'm going to be using some N-FETs to turn on/off (from the high side) a large (10-30A), grounded load that is being powered by a ~55V power source. I would like to be able to switch them with 5V signals. Now, I am no genius, but I'm pretty sure I can't drive the gates with
5V when their drains are at 55V. So somehow I need to get a voltage say ~10-20V above the 55V power source that is being switched. I'm favoring a boost converter, just because that is the technique I'm most comfortable with. Specifically, it looks like I could use a Linear LT1171HVBut once I have this power source, how do I switch the gate, and how do I do it quickly? The hard thing is that I want to be able to switch this stuff with TTL level signals. So, right now I'm thinking about using an op-amp, say something like a TI OPA445
Now, to make things a bit more interesting: The power source is going to be fairly variable. It could be anywhere between about 10 and
55VDC. That means my DC/DC converter needs to be smart enough to have an output voltage that is ~10-20V higher than the input voltage. Or the gate switching circuit needs to be smart enough to bring the gate to about 10-20V higher than the input voltage. I tried working out a circuit to handle the feedback for the DC/DC converter to give it an output voltage of 20V higher than the input voltage, but, unless I messed something up, it looked like it'd require an analog multiplier and an op-amp, which is just getting complicated. So right now I'm thinking it'd be better to take care of the variable input voltage with the gate switching circuitry. I can't seem to come up with a circuit that will do this with just a single op-amp - everything I've thought of so far has needed two op-amps - but surely there's a way to do it. If not, oh well - it's not the end of the world, but I would like to use as few of these high voltage op-amps as possible as they seem to be pretty spendy.So - do you think this is the right direction for me to be going in? Or can anybody suggest a better method?
Thanks guys!
-Michael