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Registered Member #816
Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156
What's also going to affect your switching times is the output capacitance of your mosfets in respect to your load impedance. If you have e.g 100pf combined drain gate/ drain source capacitance, and your load resistance is 1 Meg ohm, you'll have a low pass filter! With a cut off of 1.591 kHZ.
You could do something along the lines of a class 'E' amp and tune out the device's self capacitance by making it part of a resonant circuit. Then you'll be fixed to just one frequency. But when you start working things out at those sort of voltage levels. The sizes and values of the components may be unfeasible anyhow. Or make a half bridge out of two stacks, so high and low side, and accept the energy loss in switching. Smaller lower current fets will have less capacitance.
Have you tried finding 1700v parts. may save a few stages as opposed to 1000v ones?
As Ash has said, Tubes do have very low output capacitance, so are able to operate at RF frequency's with kilovolts. But I'm guessing that option is not what your looking for.
Had a tiring day today so hope I've not made any incorrect assumptions.
Registered Member #230
Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284
I have used tubes ie 4-125A's to fast switch 10kV And I have used Belkhe fast mosfet switches to switch into a capacitive load of 1200pf I had to put series resistors in to protect the switches from over current, 200 ohm in this case to safely switch from + 15kV to ground then -9kV then back to ground. My calculations showed around 10 amps flowing for around 200 nanoseconds Switching time around 0.25 us with steady state (no ringing left) after 2uS. To get minimal ringing I had to make my own 200 ohm coax to match the source impedance from the switches to the capacitive load
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Its been a long time since ive used tubes, and not really similar to this application.
im wondering if its possible to have a negative transformer, then a posistive transformer both attached to the element i need energized. then isolate them with diodes.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
johnf wrote ...
What is a positive transformer?? Have the laws of physics changed??
Maybe DC flybacks?....they can be configured to give positive or negative outputs.
Also, there are push-pull pulse transformers which use MetGlas cores which give a pulse, then then go through some 'normalising' phase before the next pulse, like this one (I'd need to re-read it to give a better explanation):
I'm still unsure of what Patrick actually meant, though
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
yes the lower side of the secondary would be attached to ground, then the two outputs would be anti-phased. its pretty common in USPS's, no special Higgs-boson's needed.
im trying to think of some clever way to prevent one flyback coil/diode for killing its counter part.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
well, i suppose, since i cant patent it.
im trying to get air to move from an undulating + then - field. if properly phased id like to see if it works or not. it would have to be primed with corona of course.
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High Voltage Swithching by Mosfet.
