Win's next 10kV project, a 1us ramp

Might work if you clamp it to 10kV (er, 13kV?) and use a high inductance charged to a chunky current, with F-all parasitic capacitance (that won't be hard, at least, with all the insulation required). Then add more C, and maybe some R and L, to tweak the slope and such. Point being, the

*unclamped* flyback pulse would be a decaying sinewave of a few hundred kV or so peak, so it's just faked by using some approximated slope. Could still be good enough though. You'll need more L for the slower slopes, since dI/dt = V/L means current drops off noticably for longer periods.

Tim

-- Deep Fryer: a very philosophical monk. Website:

Now you're *really* winging it. Crank the numbers.

--
 Thanks,
    - Win

Well what can you do in a transformer for a 1:12 1kV primary 10kVA rated secondary in the way of leakage, magnetizing inductances and secondary shunt capacitance? I still say it is a regulator function, not a servo function.

Well, that doesn't suprise me, but drawing it out, figuring 500pF gives 5A in the inductor, final capacitance energy will be 1/2 * 10kV^2 * 500pF =

50mJ so for 10% SWAG, the inductor needs to store around a half joule, rather chunky but not impossible. 0.5J at 5A is 40mH, pretty chunky for the required parasitic capacitance I suppose, but tigher requirements have been met in components..(?) Of course, most of half a joule has to be removed from the inductor, but a fast diode can dump that into a capacitor for recycling (or wasting, who cares). 40mH with 500pF is 7us quarter-wave, so 1us is certainly on the rising edge part. If the slope isn't smooth enough, you can always increase L, to swamp the dI/dt droop as I mentioned.

To use normal parts, you could wind a transformer, needing messy low leakage inductance of course, and use a MOSFET or IGBT to switch it (1kV at 50A is no problem, I've got a fifth of that (i.e., 200V) four feet away from me) and a diode to clamp the overshoot. Otherwise, 5A by 15kV isn't impossible for a suitable pentode.

I don't see anything patently wrong with it electrically, but the particulars are pretty strict, capacitance, leakage inductance (if using a transformer) and whatnot.

Or I could always ask you if you have any better ideas. But then, I could also ask why you posted this thread. But I won't. ;-)

Tim

-- Deep Fryer: a very philosophical monk. Website:

Well, I wouldn't say 32A at 800V is so great either...

I just won this little 5D21, a perky 20kV 15A baby for $6.50,

Having paid for his old tube, maybe I can go on and purchase the seller's "Genuine Broken Horn Rodeo Belt Buckle." :-)

--
 Thanks,
    - Win

100H? Nahhh ;-)

So, you'd want to increase C. Downside: more stored energy means way more inductor energy. So lesse, C = 5nF and L = 500H at 100mA... eh, okay, nevermind...

Is there a reasonable way you could switch to a follower for the slower sweep rates? Current demand is certainly lighter.

Tim

-- Deep Fryer: a very philosophical monk. Website:

Yes, I did similar sums last week, from a slightly different starting point, but producing similar numbers. The idea is just about ok for a 1uS ramp, but horrendous for 50uS.

My starting point was that a sinewave is a reasonable straight line from zero to about one third of Vpk. So you aim as if generating a 4.5uS quarter sine with 21KVpk, and clamp at 7KV, (with 35-off 200V TVS devices).

4.5uS quarter sine is a resonant frequency of 55KHZ. Taking your 500pF, that would need a secondary inductance of 18mH. 21KV across 500pF is 110mJ. That would require the 18mH secondary to have an Ipk of about 3.7A.

If the driving FETs were 1KV types clamped to 800V that would be a 1:8.75 turns ratio. Primary inductance would be 235uH, charged up to an Ipk of 32A.

Now do the same sums for a 50uS ramp. :(

--
Tony Williams.

I suppose, but it's better to simply reject the idea and move on, don't you think? I've sketched out five ways to do it, and have rejected three. The most appealing way is to use one 20kV tube. I won an old 5D21 on eBay (there are lots of them out there) for $6.50, and found a fellow who'll sell me a brand-new Eimac 4PR60C for $175.

One fellow reports on the net of getting these beasts to generate a 10kV 10A pulse with a 10ns risetime -- pretty impressive, huh?

--
 Thanks,
    - Win

The implied capacitance is what's impressive!

John

For a big 20kV tube that can do 20A, or more, it is impressive, only 8pF output and 37pF input capacitance. Knocks my socks off.

--
 Thanks,
    - Win

static induction thyristors?

Cheers Terry

The schematic of a high-voltage vacuum-tube solution indeed looks simple, until I add the issue of dealing with the changing grid voltage required to maintain a constant plate current, as the plate voltage drops from say 13kV to 3kV. :-(

So, given my familiarity with MOSFETs, I keep thinking about them.

A full 10kV ramp would mean that cascode FET stages are required. Here's an appealing simple two-stage cascode approach, capable of creating a -3kV ramp using three 2sk1412 1.5kV 150mA MOSFETs.

. -3kV . | . 1mA . -4.5kV ______ | out . | | +----------------------, . _|_ | | ___ | . --- 1mA | \\ _|_ . 2 Co | | | \\___-3kV --- Co . |____|_______||-' ramp | 75-100pF . | | ||-, Q1 | . _ | | 10V | gnd . __| |__ | '--|

Maybe use a toob for the cascode device (FET under cathode).

I've done lower voltage situations with a bipolar/op-amp under the cathode of a toob.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
      It\'s what you learn after you know it all that counts.

Dang, I was going to say that, but I was too busy eating.

Also fun: use a high-voltage rectifier tube, like a 1B3, and control the filament voltage to modulate its conductance. A bit slow for Win's app, I admit.

John

Winfield, i find a continuity error between my post and yours. I cannot find the quoted test in my post. Will you help me understand this?

--
JosephKK

Yes, of course, that's the way to do the tube solution. I realized that just after hitting SEND. I've often seen this done before, but my memory was slow to reveal the answer.

However, the tube approach is still rather complicated, because the "grounded grid" tube with its programmable current-sinking MOSFET cathode driver (500V, 1A - not too bad) and its 50W filament power, will all be sitting at -16kV. Hmm, that better all be inside a big Faraday cage with rounded corners. Can't have corona discharge off of our sharp-edged components! Maybe we can use a clear housing, or add some windows.

A 5D21 or 4PR60C tube needs to have 50W of filament heating removed (Eimac says by radiation and forced air, but there's no air chimney). Damm, I hate to think of all the dust that'll collect on the -16kV surfaces; maybe we can seal it, and use recirculating air with some type of heat-exchanger. We can monitor the temperature. Hmm.

We'll need a two-way optical-fiber communication link (for DAC and logic signals up, ADC diagnostic info back).

We'll need to make or buy a 75W, 20kV-insulation-rated power source for the cage, mostly for the filament. We can use a dc-dc converter inside the cage to get say 600V to 800V for the screen grid (the cathode will be 100 to 350V above the "grounded" grid). Hmm.

We might need a pullup resistor or current source for the cathode, up to the 600V supply, to allow for rapidly turning off the tube by charging the MOSFET's drain. Hmm, ahem. No, that probably won't be necessary, the tube's high current when running should do that fast enough, the cathode capacitance will be under 100pF. Hmm. That's only 200V/us at 20mA, maybe we'd better use one after all. The operating range of the pulser will be about 0.1mA to 1A.

Well, it's still less of a mess than the cascoded-MOSFET solutions I've sketched out so far... Hmm.

--
 Thanks,
    - Win

simple, until I add the issue of dealing with the changing grid voltage required to maintain a constant plate current, as the plate voltage drops from say 13kV to 3kV. :-(

Did you look at the curves for the 715-C at:

?

The curve looks quite pentode-like-- quite flat. Might only need a middling cathode resistor to flatten it out enough?

BTW these tubes are often on eBay for under $10. Of course, untested at 15kV!

Regards,

grg

Here's one approach to extending the circuit. You can see the complexity grows rapidly as cascode stages are added, because a new current sink is required for each gate-biasing capacitor, and these also have to be cascoded.

. -3kV . | . 1mA . | out . -4.5kV ______________ +--------------------, . | | | | . _|_ | | ___ -3.0 | . --- 0.5mA | \\ | . Co | | | \\ _|_ . -6.0kV ______ |____|_______||-' \\___-7.5kV --- Co . | | | | ||-, | . _|_ | | | 10V | -4.5kV ramp | . --- 0.3mA | '--|

Yeah, they're pretty flat, but we need it flatter, plus we need it programmable. That's not so hard, however, with a 500V power MOSFET in the cathode.

Right. I bought an old 5D21 on eBay for $6.50 this weekend, but a new production Eimac 4PR60C made 8 years ago set me back $175. Let me say I really appreciate your first suggesting the 715 -- that got me going in the right direction, and was a big help!

--
 Thanks,
    - Win