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I need an HV fast sqaure wave generator.

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Patrick

Mon Oct 17 2011, 05:26AM

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

im needing a fast square wave generator, it will be used to calibrate my HV probes, and would be similar to the calibration point on the front on O-scopes for the compensating of the 10X probes.

I was thinking of 2kV at 100kHz with fast rise/fall time. so i have 1000v fast MOSFETs, but can they be serised to handle 3kV with just a simple resistive balancing circuit?

i have no idea what can be used for this purpose.

Is this even possible or is there a better way?

Mattski

Mon Oct 17 2011, 06:34AM

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

With a 10M series resistor there you'd have a pretty hard time getting a fast rise time on that node due to the RC time constant, though the fall time will be much faster.

I'm not familiar with MOSFET high voltage stacks, but

looks like it has some useful info.

Patrick

Mon Oct 17 2011, 06:45AM

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

Yeah i just threw some values out there, ill have to simulate this i guess but i dont know how to change the mosfet models in the libraries to refelct the properties of my real Mosfets, on MultiSIM 10.

Mattski

Mon Oct 17 2011, 08:13AM

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

The capacitance from the HV probe connected to this and possibly even interconnect wiring will probably also need to be added in addition to the MOSFET model to get a good idea of the rise/fall time.

Proud Mary

Mon Oct 17 2011, 10:23AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

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:

Pinky's Brain

Mon Oct 17 2011, 02:46PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

First of all you're going to need a half bridge since you are driving a capacitive load.

Second, as I said in the other thread ... if it just has to drive the HV divider then the load is essentially a couple 10s of pF, this limits the avalanche energy the MOSFETs (or parallel TVSs) might see, ignoring shoot through ... and half bridge drivers with programmable break before make delays are easy enough to find, so shoot through shouldn't be a problem either.

So I'd just use one of those ICs to drive GDTs on the MOSFET stacks on each leg and solder some TVSs across the MOSFETs just to be sure. Should work ... for this load.

Patrick

Mon Oct 17 2011, 03:51PM

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

excellent! i will investigate all your suggestions above.

im thinking i only need 100-200nS of rise and fall times too. So hopefully the layout lon the board wont be to bad.

Patrick

Tue Oct 18 2011, 03:26AM

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

Is this a good choice? 3.10 USD for 100 pcs.

I was thinking the TO-92 case type.

]2n5551.pdf[/file]

teravolt

Tue Oct 18 2011, 01:42PM

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

Patric you may not need 2kv to calibrate your probe are you looking for a fast rise or fall, because a ga301 has sub nano second turn on. they only do 100v max. you can series them and you could make a cable pulser wich could make very fast pulses.

Pinky's Brain

Tue Oct 18 2011, 03:07PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

Patrick wrote ...

Is this a good choice?

Have you considered what kind of pull up or down resistor you will need if you use your original circuit? (Whether with MOSFETs or avalanche transistors.) If you want a RC time of 100 ns with 10 pf capacitance you're already down to 10KOhm, that's going to burn 100s of Watts!

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.

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