Slew Rate Booster

something

Things like this have been done in the past and work fairly well so long as you keep them from crashing into the rails.

In the usual form of this, the circuit works as an inverting amplifier.

The slow amplifier has to be made such that it doesn't attempt to clamp its inverting input to ground. It also needs to have a high open loop output impedance.

The fast boosting section tends to make the output do what the input commands it to do, so the error signal on the slow amplifier's inverting input is greatly reduced.

If you are careful about preventing oscillations, you can use a couple of comparitors on the signal at the inverting node to develop the trigger signal for the booster. In your case, I think you could simply trip oneshots and a bit of interlock stuff from the comparitors.

slow

needs.

That looks OK. The gate drive could be some dirt-cheap ASDL transformers.

But how about this:

so the fets help when the current gets big enough to drop a few volts across R. Some sort of compound feedback would probably be needed, but that shouldn't be hard. As you suggest, current-limit the hv rails to protect the fets. This does provide your "adjustable amplitude."

For more interesting dynamics, R could be a true current limiter, like some depletion-mode Supertex fets.

The "wimpy hv opamp" could of course be based on my optocoupler trick, so the whole thing becomes about a dozen cheap parts.

John

Looks good.

You mean, some feedback around the HV amp only, and some more around the whole thing?

Yeah, but the wimpy amp now (as opposed to my original idea) needs to have the full 40V/Âs slew rate.

Actually I have the schematic of a commercial unit before me. They do use some optocoupler scheme for the positive drive, but I haven't fully understood it. They have the gate of a BUP37 IGBT riding at 14V on top of the output voltage, and a Darlington NPN between the IGBT's emitter and the output whose base is controlled by the OC.

--Daniel

Haude Daniel wrote: >

If you can't adapt the AoE Fig 3.75 ckt to fit your needs, then you don't know what you're doing. Take your question to SEB...

If it is that simple to adapt the circuit to do >40V/µsinto

Thanks for your help.

--Daniel

Not to mention that Fig. 3.75 doesn't really address, even with modifications, what Daniel specifically needs to do.

Daniel doesn't simply need a power booster.

It might be more accurately called a "reset".

...Jim Thompson

Something like that. The local fb would be fast, and the global fb a bit slower, especially if the load is capacitive.

Then ignore my suggestion! If tha driver amp isn't already this fast, the follower can't help, and you will have to go to the pulsed booster. That has interesting dynamics.

Sounds strange. Post to abse?

John

Fred, of course, doesn't want to help. He wants to demonstrate that we're stupid and he's smart. So far, it's not working.

John

The main thing he needs is voltage gain, and that 40v/us seems suspiciously close to a standard 35v/us interference level. It is best to just adapt the AoE circuit with suitable protection and current capacity.

But it's more fun to design stuff.

John

An edited portion of the ckt can be found here:

Note the line labeled "boost". This goes to a logic output in the wave generator section to help the high side driver to get its ass in gear on a rising slope. The negative side doesn't seem to need this.

I omitted the output current monitor / overcurrent shutoff portion. And I can't make up my mind if the circuit is ingenious or stupid. All I know is that the device has been working flawlessly for some

--Daniel

Actually I'm beginning to like this circut. I've figured out hoe the negative side works:

When U3's output is at or above 0V, R21 feeds 4.5 mA through D11 and D13, causing -490 V to appear at Q5's gate which is close to the -488 V at Q4's gate, so Q4 and Q5 are off.

Whan U3 goes negative, its output starts robbing about 1.1mA/V from this current, which makes Q5's gate move towards the negative rail with 4.2 V per volt change on U3's output. I need to look up the subthreshold characteristics of the IGBT, but at any rate this turns the Q5 and Q4 cascode-like combo, letting the output go negative. The R13/R28 divider provide a little negative feedback to Q3's base, probably for stability.

In the positive leg, Q1 and Q2 form a cascode driven by U1, riding on top of the output voltage. D4 and D5 provide fast turn-off of Q2 on a falling slope, and they make sure that Q2 can never turn on when Q4/Q5 are on.

At first I wondered why they would use IGBTs instead of MOSFETs. But it turns out that 1200V MOSFETs are rare (and may be even rarer back in

I think I'm gonna prototype this. But first I need to come up with some good SPICE models for optocouplers and IGBTs. Is Mr subthreshold, Win Hill, reading this?

--Daniel

Actually, fig 3.75 can be used to address the issue. Until last year we always made our own stick-slip piezo stepping drivers for our STMs, basically using the MOSFET totem-pole drive scheme shown in fig 3.75.

The problem with using a "reset" approach is it assumes you want to take the piezo all the way to the rail, which you may not want (although some commercial units do this). We wanted ours to be fully programmable.

What's needed is a high enough current capability and a fast enough slew rate. Fig 3.75 can easily operate at even 1A or above currents, because the pulldown and pullup devices are power MOSFETs. The Institute's AMP-10 circuit is an enhanced version of fig 3.75 with bipolar capability. An early version of the AMP-10A that I made in 1991 had a 40V/us slew rate, the AMP-10A-4 had 60V/us, and some later ones were faster yet, delivering high currents into the modern lower-voltage higher- capacitance piezo actuators.