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I need an HV fast sqaure wave generator.

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Patrick

Tue Oct 18 2011, 08:21PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Pinky's Brain wrote ...

To reiterate, you need a half bridge ... and trying to build a half bridge out of avalanche transistors seems to me to be a bad idea.

its about 1.1pF, and what devicewould you use then ?

teravolt

Wed Oct 19 2011, 07:20PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

another option would be a reed pulser. If you could find a a high voltage reed relay and swtch it about 1hz with a pullup and some 50ohm cable you could get some fast rise times.

fets also have a transition of 85ns but i fear you probibly would want someting faster

Patrick

Thu Oct 20 2011, 01:13AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

teravolt wrote ...

another option would be a reed pulser. If you could find a a high voltage reed relay and swtch it about 1hz with a pullup and some 50ohm cable you could get some fast rise times.

fets also have a transition of 85ns but i fear you probibly would want someting faster

100 nS is fine im still trying to figure out why Pinkys brain says the 2n5551 transistors wont work?

Weston

Thu Oct 20 2011, 04:17AM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

Do you actually need a fast fall time? From what I have seen, most probe calibration is done on the rising edge of a waveform.

I built the circuit referenced to by Mattski. Works acceptably at 100 volts. Has some ringing at higher voltages. Here is a picture of the output waveform into a 50 ohm load.

Yellow is output current 0.5V/A blue is my p6015 1000x probe. I could most likely calibrate the probe with this signal at only 100 volts. If the waveform is not square enough you could possibly use a PFN.

Patrick

Thu Oct 20 2011, 04:38AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Weston wrote ...

Do you actually need a fast fall time? From what I have seen, most probe calibration is done on the rising edge of a waveform.

I built the circuit referenced to by Mattski. Works acceptably at 100 volts. Has some ringing at higher voltages. Here is a picture of the output waveform into a 50 ohm load.

this is exactly what i need! do you have a schematic? pics?

Weston

Thu Oct 20 2011, 05:11AM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

Its based on the schematic in the paper that Mattski linked to. "Stacking Power Mosfets for use in high speed instrumentation" You wont want to see pics because its a mess on a piece of hand isolation routed copper clad.

Patrick

Thu Oct 20 2011, 06:04AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Weston wrote ...

Its based on the schematic in the paper that Mattski linked to. "Stacking Power Mosfets for use in high speed instrumentation" You wont want to see pics because its a mess on a piece of hand isolation routed copper clad.

Ok let me read it again... Yes it looks like my top most schematic... ill build it then, any good fast mosfet HV transistors?

Mattski

Thu Oct 20 2011, 06:25AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

Patrick wrote ...

100 nS is fine im still trying to figure out why Pinkys brain says the 2n5551 transistors wont work?

What Pinky's Brain is saying is that if you simply take your circuit that you first posted and use the 2n5551 instead of the MOSFETS you had in there, you could get a fast fall time, but the rise time is still limited by the series resistance charging up the output capacitance of the transistor stack plus the probe capacitance plus whatever parasitic capacitance you have lying around. A half bridge gets around this by using a transistor (stack?) in place of a large value series resistor, so when it turns on it can quickly charge up all those capacitances. I'm not sure if you can use the avalanching trick though to make a half bridge, because it only applies to turning the transistor on quickly, not off quickly.

Then again maybe you only need a fast transistion for one half of your square wave to calibrate your probe, I'm not really sure. And of course you could try calibrating at a lower voltage like teravolt suggests.

teravolt

Thu Oct 20 2011, 01:47PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

are you interested on finding the harmonic response you may have to sweep it. if you had a hundred volt signal generator you can find what frequency it is moast sensitive. it probibly wont have a flat reponse. the advantage about using 100 to 200v is that you can use a standard calibrated probe and set up like a tek 3034 to be shure what you are looking at is real. so with a two chanel scope see if you can look at input vs output and change F and see what happens. I probly could make a fast square wave generator but a method of making shure that your input signal is truely what is at the tip of your probe. what do you think?

Patrick

Thu Oct 20 2011, 07:56PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Proud Mary wrote ...

If you Google 2N5551 avalanche pulse you'll at once find a number of papers detailing the use of this very inexpensive HV transistor in simple nanosecond and sub-nanosecond pulse generators with outputs up to 3kV from series strings.

Physical layout is the ultimate determinant of speed in the low nanosecond regime.

There is also a useful application note for the more expensive ZTX415 avalanche transistor giving other fast pulse generator circuits with PFNs which you could use with 2N5551:

Im thinking of using this circuit.

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