Hi all, I'm working on a project that requires charging and discharging a piezoactuator (150V, 1.5μF) extremely fast, IE total slew time allowed ~50μs (so somewhere around a 11A pulse). My background is in materials science, so after a lot of stumbling I've wound up with the following circuit based on what I have available for now:


I forgot to relabel X4-A and X5-A, they are STGP19NC60HD IGBTs, and Vcc/Vee for TL082 is +/- 18V.

Essentially a function generator provides a +/- 10V square wave, where at +10V optocoupler 1 (OC1) turns on and subsequently turns on IGBT X4-A, which allows the piezo to charge, as IGBT X5-A is off. When the square wave inverts to -10V OC1 and X4-A turn off, TL082 inverts the -10V, turns on OC2 and therefore turns on X5-A to discharge the piezo. While I know there are a lot of things with the circuit that probably don't make sense (there are some artifacts left over from a few other inspiration sources), this circuit slews the piezo at the rate set by the RC constant set by R7, as by (mediocre) design. The only problem is the piezo never reaches the voltage that is expected; 150V-voltage drop across IGBT X4-A (~few volts). Instead the voltage applied to the piezo is limited to and varies directly with the gate voltage.

I know that there is something very fundamental I'm not understanding about the IGBT's operation but I can't seem to figure it out or find a working circuit to modify. It would be great if there was some tweak to fix the voltage issue, but I wouldn't be surprised if this thing has to be completely scrapped and started from scratch.

I appreciate the help, and if there's a suggested search to make I'm all for it, I wasn't sure what to even search for. I'm not new to forums in general and if I had a nickel for every time I've posted "use the search function"...

 

I think I recall that at higher speeds, drive current must be rather high, as in more than 4N25 would provide. But please don't take my word for it.

 

Drive current to the piezo or to the IGBT?

 

To the IGBT gate, yes.

 

Gotcha, from memory they're voltage controlled and only take picocouloumbs to turn on, they seem to switch fast enough. I should add the waveforms I read off the scope.