opamps coming off the rail

Are you saying the LM324 really is an op amp?

Mirror2.JPG

What kind of gate capacitance are you talking about? 1us settling implies a pretty high bandwidth to the circuit -- you're going to need a low output-impedance op-amp (and possibly a high-frequency high-power op- amp) to get any kind of bandwidth.

(I've done this once with IGBTs. 'taint easy when the power levels go up and the speed remains high).

--
Tim Wescott 
Control system and signal processing consulting 
www.wescottdesign.com

The IXYS part has something like 26 nF Cgs, astonishing. That seems to be the price you pay for a huge SOAR.

1us settling

For the initial application we could live with maybe a 5 us current risetime at

150 amps. But you're right, we'll need a hefty amount of gate drive. Something in the ballpark of 50 mA might do, which leads me back to LM8261 as a candidate. Something beefier would be nice for a standard-product pulser where bragging rights matter.

An opamp and a pair of complementary emitter followers would work, but again we'd need a negative supply. Maybe. Maybe not.

Microseconds aren't high speed!

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators

candidate.

It'll be interesting to see if there exist values which will make it both stable _and_ fast.

And the other issue I mentioned, gate charge plateau, remains ignored in this thread. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
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I love to cook with wine.     Sometimes I even put it in the food.

Current_Sources/

Big_Sink_1.JPG

It's not just raw current drive to fill the capacitance that you need at your desired bandwidth -- it's a low-enough impedance in the op-amp output so you don't have to cripple the loop keeping it stable against the FET gate capacitance.

(One of those amps designed for a capacitive load may help -- but it may just get really, really slow).

Your circuit, as shown, is going to have terrible crossover distortion, of a sort that's going to make for a high output impedance in the transition from charging to discharging the gate. That, in turn, will get you oscillation or a terribly slow loop.

You might be able to live with a resistor-loaded emitter follower, and just accept that you'll be burning up some watts to keep your output impedance and parts count low.

Controlling 150A at a 1MHz bandwidth is high speed.

--
Tim Wescott 
Control system and signal processing consulting 
www.wescottdesign.com

I've done this a zillion times, and it's not hard to get it to work. All I need is the right opamp.

One of my guys found the AD8565, which looks good. 250 mA peak output from an SC70!

Not really. Gate drive is managable. Cap ESL and fet source inductances don't matter at numbers like 10 to 100 amps/usec. Numbers in the amps per ns are "fast", where every nH makes volts of drop.

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation

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reminded me of this:

-D.pdf

describes using a sensefets kelvin connection to "bypass" the source inductance

-Lasse