Mosfet stack for higher breakdown voltage

[Jeg]

Hi to all
I have build a switch consist of 3 IRFP460 Mosfets capable for switching voltages up to 1500V.
It is based on this....

http://www.google.gr/url?sa=t&rct=j&q=&esrc=s&source=web&cd=6&ved=0CF4QFjAF&url=http%3A%2F%2Fcmosedu.com%2Fjbaker%2Fpapers%2F1992%2FRSI631992.pdf&ei=sqiAUuTXIaWK0AXi-4H4Bg&usg=AFQjCNFqStXAkoZnTDLSfnBlF4lZHTPwlQ&bvm=bv.56146854,d.d2k&cad=rja

After some tries and many destroyed mosfets i finally did it. The most difficult part was to calculate the exact parasitic capacitance values, so to open and close the stack the same time.

By the use of a weak output mosfet driver consisting of a 555 and a LM393 comparator for PWM, the stack opens but doesn't switch rail to rail. With 48V input to the mosfet from 4 X12V batteries, switches between 48V and 33V.
I suspect that it needs more current at the gate input so to fully charge the input capacitance and stay 'on' for a longer time period.

As for a start i would like to put a BD139 NPN transistor at the comparator output to drive the mosfet. I would use a totem pole but i don't have any readily PNP transistors. Do you think is enough for that job? Any drawbacks?

Thanks
Jeg

[TinselKoala]

One wonders why you didn't simply use the exact circuit given as Figure 2 in the pdf you linked. It switches 1500 V, uses the dirt-cheap IRF840 mosfets, and common 2n3904 and 2n3906 transistors to drive. Just about any signal, like from your 555+comparator PWM, could feed the transistors, I suppose.

[Jeg]

Hi Tinsel, tnks for the suggestion

My driver has an output of 12V peaks and that makes the input capacitance of IRFP460 as large as 41nF!. Perhaps I am wrong but i thought that i need Amber level peaks. I think i have some of them.. i'll try it tonight.

IRFP460 is my favorite one and i have many of them. It handles more current than IRF840 and this is very convenient to me.

[TinselKoala]

Hi Tinsel, tnks for the suggestion

My driver has an output of 12V peaks and that makes the input capacitance of IRFP460 as large as 41nF!. Perhaps I am wrong but i thought that i need Amber level peaks. I think i have some of them.. i'll try it tonight.

IRFP460 is my favorite one and i have many of them. It handles more current than IRF840 and this is very convenient to me.

I use the P460 a lot too; it is the mosfet I use in my little 4MHz SSTC, and in some other projects, but your system's current demand will be determined by the load. The 840 system in the schematic apparently provides the maximum current that a 50 ohm load can draw from a 1500 volt source, and the scopeshot shows an impressively short risetime for the pulse. I don't know what the max frequency could be with that schematic, but I'm already working on a solid state Tesla Coil based on it.

I'll be very interested to see what your final project looks like. I love HV, but recently I've been working with low-voltage high current mosfets like the IRF3205, which can handle 110 amps in a TO-220 package, with Rdss of 0.008 Ohm. Daisy-chaining a few of those to get to higher voltage handling might make a real "killer" SSTC primary driver!

[gyulasun]

Hi TinselKoala,

Would like to show you an oldy paper on a fast 8kV MOSFET switch, just some food for thought to daisy-chaining. The isolation between the low voltage input and the HV output side is partly solved by transmission line transformers, wound from RG316/U coax onto ferrite toroid cores.  You may wish to adopt the circuit to your high current MOSFETs, albeit the max 55V V_DS for a single IRF3205 sounds a bit low when it comes to chaining.  For a bit higher price there is the max 100V, 100A type with 0.0051 Ohm Rdss, like IPP100N10S3 ( http://www.digikey.com/product-detail/en/IPP100N10S3-05/IPP100N10S3-05-ND/2081166 ).  Of course there are some other types, with different prices, digikey is not the cheapest of course.  (IRFP4310ZPBF is a 120A, 100V with Rdss=0.006 Ohm max) or this: http://www.digikey.com/product-detail/en/PSMN8R5-100PSQ/568-10160-5-ND/3902745 )
I also understand if you have a different circuit topology for chaining MOSFET switches in series to increase breakdown voltage.

Here is the link and scroll down to the very bottom of the long page, the last line includes the pdf paper on the Fast 8kV MOSFET switch, just click on the letters PDF in the end of that line to get the file:
http://www.cchem.berkeley.edu/dmngrp/pubproject.htm 
♦ "Fast 8 kV MOSFET switch" R. E. Continetti, D. R. Cyr, and D. M. Neumark,  Rev. Sci. Instr.63, 1840 (1992)

Gyula