How to quickly turn on/off N-FET switching high side of 55V?

Michael · 2007-12-12T14:18:02+00:00

Hi - I'm going to be using some N-FETs to turn on/off (from the highside) a large (10-30A), grounded load that is being powered by a ~55Vpower 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 with5V when their drains are at 55V. So somehow I need to get a voltagesay ~10-20V above the 55V power source that is being switched. I'mfavoring a boost converter, just because that is the technique I'mmost comfortable with. Specifically, it looks like I could use aLinear LT1171HV () to get that supply prettyeasily. I should mention that on this board I will probably also havesome sort of lower voltage supply, like maybe 12V or 5V or somethingalong those lines. Also, I should mention that on this one PCB I'llhave probably 3 of these switching circuits.But once I have this power source, how do I switch the gate, and howdo I do it quickly? The hard thing is that I want to be able to switchthis stuff with TTL level signals. So, right now I'm thinking aboutusing an op-amp, say something like a TI OPA445 ( though probably not that chip asit seems to have pretty limited availability right now. The OPA445's15V/us typical slew rate should probably be fast enough. I'm trying toavoid overheating the FET while it is switching from on to off andvice versa, hence the desire for fast switching speeds.Now, to make things a bit more interesting: The power source is goingto be fairly variable. It could be anywhere between about 10 and55VDC. That means my DC/DC converter needs to be smart enough to havean output voltage that is ~10-20V higher than the input voltage. Orthe gate switching circuit needs to be smart enough to bring the gateto about 10-20V higher than the input voltage. I tried working out acircuit to handle the feedback for the DC/DC converter to give it anoutput voltage of 20V higher than the input voltage, but, unless Imessed something up, it looked like it'd require an analog multiplierand an op-amp,...