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Registered Member #2662
Joined: Fri Jan 29 2010, 10:14AM
Location:
Posts: 36
Thanks for those great papers, I wish i'd had them a few months ago!
I actually made an avalanche pulser, (not using ZTX415, but ZTX451's) it worked quite well.
I noticed two problems though, firstly, with a 50ohm load I could never get an output voltage above 2kV. I couldn't explain this, after a certain point adding more transistors didn't seem to have much effect on the output voltage (losses in switches, poor layout, insufficent stored energy?). I experimented with a Marx configuration too, this was not much better.
Second problem was triggering jitter, ~3ns, this is quite a lot when looking at a pulse FWHM of 10ns. I guess the fact that the ztx451 devices are not designed for avalanche operation and difference in avalanche breakdown voltage is likely to introduce jitter.
My interest in magnetic pulse compression comes from this experience. I can deliver a short 1kV pulse (I would use MOSFETs here not avalanche transostors, less jitter!)... what I want for my experiements is a >10kV shorter pulse!
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
iJim wrote ...
My interest in magnetic pulse compression comes from this experience. I can deliver a short 1kV pulse (I would use MOSFETs here not avalanche transostors, less jitter!)... what I want for my experiements is a >10kV shorter pulse!
Not sure it's possible!
There's lots of designs out there for 10kV fast pulse generators.
This one offers 39ns rise time:
DESIGN FOR A FET BASED 1 MHz, 10 kV PULSE GENERATOR M.J. Barnes
This one says it is a 600V version of a pulse gene originally devised for 50 - 100kV, and even has a colour picture of a funky little PCB with the diode pulse gizmo on it
NANOSECOND PULSE GENERATOR USING A FAST RECOVERY DIODE A. Kuthi, P. Gabrielsson, M. Behrend and M. Gundersen
Registered Member #2941
Joined: Fri Jun 25 2010, 08:08AM
Location:
Posts: 143
iJim wrote ...
Thanks for posting your results, they look interesting.
One question though, in your setup do you think that magnetic compression is responsible for shortening the pulse? Is the magnetic core actually saturating?
ok,I am not yet very good at that but in the set up the two trace are taken in the same time because I wanted to see the delay induce by the inductance,(the two coax to the scope have about the same size) I think the inductor let the voltage through only when it get saturated ,the small capacitance witch is after the first inductance filter the little bit of it witch pass through. in this set up ,just putting a coax for measurement at the begin of the magnetic pulse compression is enough to induce a drastic change of the pulse shape ,so I made a measurement before with just the pulser with out the MPC and an other at the out put of the MPC with out the cable in front ,and I do get higher voltage ,I guess it is due to the effect I don't see what else could do it.I will try more experiment tomorrow... ps:the scope is giving 500 mV per division but there is a 20 db attenuator on each cable ,so the peak voltage is about 15 volt.
Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615
You really want a square loop ferrite such as 3R1 for this sort of thing. They have a very abrupt change into saturation, and they stay saturated until a reverse field is applied. In other words, the remanent field is enough to maintain saturation as the inductor current drops to zero - like a switch in fact.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Proud Mary wrote ...
Alas, ZTX415 don't grow on trees! But before you leap wailing into the grave, consider using 2N5551 instead.
Very interesting Mary, thanks! I've been using the 2N2369 in avalanche generators at work, but it's a wimpy little thing that avalanches at 50V and makes a 10V pulse into 50 ohms. I'll have to try the 2N5551.
BTW: I had to get a batch of 2N2369s that avalanched for a small production run of the generators. First I tried the plastic TO92 ones and they didn't avalanche at all. Then I dug out some old metal-cased ones and they all worked great.
I ordered 100 of the metal ones (CDIL brand, made in India) from Farnell, and 99 of them avalanched.
Farnell also seem to sell 3R1 ferrite toroids.
Buy a pile and stack them up for pulse compression fun!
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Steve McConner wrote ...
Proud Mary wrote ...
Alas, ZTX415 don't grow on trees! But before you leap wailing into the grave, consider using 2N5551 instead.
Very interesting Mary, thanks! I've been using the 2N2369 in avalanche generators at work, but it's a wimpy little thing that avalanches at 50V and makes a 10V pulse into 50 ohms. I'll have to try the 2N5551.
BTW: I had to get a batch of 2N2369s that avalanched for a small production run of the generators. First I tried the plastic TO92 ones and they didn't avalanche at all. Then I dug out some old metal-cased ones and they all worked great.
I ordered 100 of the metal ones (CDIL brand, made in India) from Farnell, and 99 of them avalanched.
Time must be set aside for testing the 2N5551s one by one.
Writing of 2N5551, Liu Jinyuan, Shan Bing, and Chang Zenghu (above) "noted that the difference of breakdown voltage of this kind of transistor is much greater, and the breakdown voltage drifted during the test. This will result in a change of trigger delay and output swiching voltage. In order to resolve these problems, an avalanche transistors ‘‘burn-in’’ tester was used to ‘‘cook’’ a number of transistors for several months. We selected the transistors with the same breakdown voltage, the same trigger delay, the same rise time, and with small trigger jitter."
Afterthought added later:
I wonder if a magnetic bias winding could be added to the saturable reactors in the delay line, to enable a greater range of core materials to be used.
Registered Member #2662
Joined: Fri Jan 29 2010, 10:14AM
Location:
Posts: 36
Mary,
An additional winding around each core was what I had in mind! By individually controlling the magnetic bias of the ferrite in each stage of a multistage pulse compressor I hope to achieve some kind of control over the output pulse parameters!
Many thanks to 2Spoons for pointing out the 3R1 core material, I'm just about to place a Farnell order for a few 14mm toroids!
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Mary: My generator design ran the transistor a little below its natural breakdown voltage, and triggered it by pinging the base with a positive pulse coupled in through a tiny capacitor. This gave enough wiggle room that I didn't have to burn-in each transistor for months. The test procedure I wrote said to increase the supply voltage trimmer until the transistor avalanched by itself, then reduce it until it wouldn't fire even with the trigger, and finally set it halfway between those two points.
Well, at least, we shipped 20 generators last year and no customer has complained yet.
iJim: A bias winding is standard practice for these things because it doubles the available pulse energy. Also in the 4hv spirit, you should buy way too many of the largest toroids they sell
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Steve McConner wrote ...
Mary: My generator design ran the transistor a little below its natural breakdown voltage, and triggered it by pinging the base with a positive pulse coupled in through a tiny capacitor. This gave enough wiggle room that I didn't have to burn-in each transistor for months. The test procedure I wrote said to increase the supply voltage trimmer until the transistor avalanched by itself, then reduce it until it wouldn't fire even with the trigger, and finally set it halfway between those two points.
Well, at least, we shipped 20 generators last year and no customer has complained yet.
iJim: A bias winding is standard practice for these things because it doubles the available pulse energy. Also in the 4hv spirit, you should buy way too many of the largest toroids they sell
I have so far only been able to do the basic avalanche voltage test on a handful of my 2N5551 stock to verify that they function "off-label" as described. I'm waiting for a rainy afternoon with nothing better to do before getting stuck into testing the whole 500 pieces. I shan't be able to test trigger delay, rise time etc, as I have no equipment for making measurements in the nanosecond regime.
On a more general note, here is a pdf copy of Mali, P. Magnetic Amplifiers, Principles and Applications, Rider, New York, 1960, which has some chapters likely to be of interest to readers of this thread:
Lastly, a Lithuanian ebay seller whom I have dealt with in the past is offering ferrite toroids 125x80x12 mm, or 100mm diameter ones if you fancy something smaller.
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magnetic pulse compression
