Avalanche mode pulser

[ceyhun]

Hello,
I am trying to design a pulser of 200V pulses with pulse width of ~5 ns. Rise and fall times should be relatively small. I thought of doing this with avalanche mode transistors but I don't have any for the moment. I have some power bjts (BD911) at hand. Is it possible to use any bjt in avalanche mode? If so, how can I find out avalanche breakdown voltage?

Thanks...

 

[DarwinNE]

Several general-purpose small signal transistors can be used very effectively as an avalanche pulse generator.
In the past, I used a few PNP such as 2n2907 (for smaller voltage pulses, such as 50 or 60V), but you can also use very common NPNs. The snapback characteristics of such transistors are not probably stated in the datasheets and are somewhat unpredictable, so to determine their avalanche threshold you may likely do some tests.
If you limit the power dissipated by the transistors, you can avoid destroying them.

If you need a few interesting advices about avalanche pulse generators based on small signal transistors you can have a look at this nice application note, from Jim Williams of Linear Technology:

http://cds.linear.com/docs/Application Note/an94f.pdf

BTW, if you need very short rise and fall times, the layout and the packages will become much critical and selecting the good transistor can be cumbersome.

Hope it helps.

 

[FvM]

Thanks for pointing to this nice application note. I didn't expect a profound avalanche pulse generator design from this side.

A delay line pulse generator, as utilized in this circuit, is an effective way to achieve rectangular pulses with an avalanche
switch. For 200 V output voltage, you most likely need cascaded transistors.
(See e.g. Benzel,D.M.,Pocha,M.D.,:1000-V,300-ps pulse-generation circuit using silicon avalanche devices.
Rev. Sci. Instrum. 56 (1985), S. 1456-1458)

It's basically possible to design an avalanche generator with high voltage transistors, using up to 400 - 500 V supply voltage,
but rise times aren't typically better than 2 ns then. Small signal transistors can achieve several 10 V pulse output with some 100 ps risetime.

As far as I'm aware of, Zetex is the only manufacturer offering specified avalanche transistors (at a respective high price)

 

[DarwinNE]

As far as I'm aware of, Zetex is the only manufacturer offering specified avalanche transistors (at a respective high price)

I confirm. Zetex has also some nice application notes too, for their transistors.

 

[dick_freebird]

At "only" 200V, you might be able to use GaN power FETs
(IR, EPC) although you still have the gate drive risetime
to deal with.

I have made transmission-line pulsers for ESD simulation
with rolls of coax cable and mercury relays. Once the arm
hits the mercury things are pretty quick and quiet.

 

[FvM]

It's good, that you mentioned mercury wetted relays. I forgot about these, because they aren't suited for applications
that require stable pulse triggering. But they are superior in simplicity and voltage rating.

I achieved these results with a Hamlin MDC-1 reed contact embedded in a 50 ohm transmission line:
max. pulse repetition frequency 300 Hz
Rise-/Fall-Time (10/90) 200 ps
max. voltage without risetime deterioration 1000 V
trigger delay ≈ 1ms
trigger jitter ≈ +/- 200 ns

For a 5 ns pulse, you need about 0.5m RG58 cable

 

[joker12]

Could I use avalanche mode for high voltage switching rather than pulsing? I want a solid state switch to switch about 1kV as fast as possible.

I see that some solid state relays have load current rise times of about 100us. I want the rise time to be closer to 1us or less.

 

[FvM]

Avalanche mode isn't actually suited for on-off switches. And 1 us is rather slow. You can achieve 100 ns and even less with cascaded
MOSFETs. Of course some effort for the isolated control has to be calculated. I guess, 1 us can work with "simple" transformer gate control.

 

[joker12]

I didn't think Avalanche would be an option for anything other than short pulses.

As for my switch, I want it to be as fast as possible, but cheap. What is transformer gate control?