These transistors sure do scream...

Playing with a switching circuit, yet I seem to have made the observation that these things are fantastic for linear.

Infineon SPA07N60C3, but everyone has their line of SuperJunction MOSFETs.

Circuit, for posterity:

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Intended load is high impedance, capacitive. It can easily source 5A peak though.

Since the load is capacitive, it operates in hard switching. The transistors start singing as soon as they get into the Miller plateau. Which in this circuit, I've intentionally exaggerated (27pF D-G), to help keep the transistor voltages matched.

A word about SuperJunction transistors: Coss tanks by two decades, over the

5 to 20V range. Very nonlinear, brutal. This is fantastic for switching converters, because it "cushions" the switching edge, doing a better job of snubbing than an external network ever could. By pushing all the Miller effect to the low voltage end, switching loss can be very low.

With stacked transistors, that works against me, because they'll probably be mismatched in the low-capacitance region. So the switching times, and voltages, probably won't be matched, forcing much more voltage across just one over-performing transistor.

So I increase Miller capacitance, so the rise is slower, and more linear.

And to protect against accidental turn-on or damage, due to opposite side hard-switching or output sparks, I put zener diodes on G-S. (Back-to-back pairs, since the drive is transformer coupled.)

I think between the zeners and the Miller cap, I've got a particularly nasty loop that makes a wonderful oscillator. In the 200 to 400MHz region, depending on which transistor you ask.

(Ferrite beads on the gate leads solves the oscillation, more or less.)

I'm definitely going to try an RF amplifier with these, soon. I can't do very much power, because of thermal limitations, and bandwidth won't be fantastic because of the high load resistance versus Coss (note that the load resistance has to be high, i.e. the supply voltage high, and because of power limits, the current relatively low, to stay in the low-Coss range). The useful frequency range seemingly should be worthy of vacuum tubes, though! Assuming lead parasitics don't trash it first, which is likely. :)

Tim

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Tim Williams
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Just checking ... is the transformer phasing different for top three stages vs lower three stages?

piglet

Reply to
piglet

It is better to be - if you do not want the whole string being a dead short from +1500 to ground.

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-TV
Reply to
Tauno Voipio

Tell us about your gate transformers. What risetime and falltime are you observing?

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    - Win
Reply to
Winfield Hill

What sort of switching speed do you need?

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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John Larkin

Pokey -- around a hundred nanoseconds.

Intended application: gate drive V_cm testing. A fairly large swing, at reasonably high dV/dt (a few kV/us), and essentially capacitive load.

It's doing about 7kV/us, which is nice, but I was hoping for >10. Maybe I'll drop the Miller caps to 10pF and see how she sings.

Tim

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Tim Williams

As Piglet noted, yes, the phasing is inverse for half of the windings. ;) The transformer impedance is pretty crappy (kind of intentionally), so the response is, uh, what was it... think the small-signal calculation was 3uH leakage per secondary, 33 ohms ESR and 2nF equivalent Cg. So whatever that works out to, as a LPF.

Drive is a pair of TC4420, so the drive resistance is lowish, but not scary low. LL + R_G dominates.

Gate waveform something like 200ns (full rise, about half being Miller plateau), output depends on supply voltage, but it's also about 200ns at the full 1500V supply (imagine that: constant drive current yields constant dV/dt? :) ).

Tim

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Tim Williams

Another way to get insane edge speeds is avalanche transistors. A modest stack of the Zetex SOT23 parts could switch a couple of KV in a couple of ns.

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John Larkin         Highland Technology, Inc 

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John Larkin

You should have two sets of drivers and gate transformers, one each for high and low sides. Then you can create an adjustable deadtime.

Another attractive idea: pos/neg pulsing to turn a set of MOSFETs on or off, with the gate capacitance storing the state. Combine fast on/off times with low switching frequencies.

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 Thanks, 
    - Win
Reply to
Winfield Hill

Why not simplify by using two 1.5kV MOSFETs? Or 1.7kV SiC parts, C2M1000170D, for $5 each. Skip the transformers, use one TI UCC21520.

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 Thanks, 
    - Win
Reply to
Winfield Hill

DEI/IXYS makes truly insane low-inductance-packaged HV mosfets and gate drivers. They also have some smaller, affordable gate drivers.

I like to use transmission-line transformers to step up fast edges, to avoid stacking fets.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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John Larkin

The old RadLab radar pulsers were cool. They used the stored energy in a transmission line to make rectangular pulses, with a single switch. I've made some very pretty pulses with a coax line and avalanche transistors: very small and simple circuit. I'm thinking one could merge the pulse storage line with the step-up-transformer function.

I asked about the RadLab books in a technical bookstore near the MIT campus. They'd never heard of them.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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John Larkin

Yes, out of print. I bought the complete set from a guy at Flea at MIT, and gave them to the Institute. One book specializes in high-power radar pulses.

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 Thanks, 
    - Win
Reply to
Winfield Hill

That's what a Blumlein pulse forming network does, sort-of. The load gets to see the full charging voltage, rather than half of it.

Glasoe & Lebacqz, "Pulse generators" has a nice chapter on the synthesis of PFNs.

Jeroen Belleman

Reply to
Jeroen Belleman

Nice! Did you draw this with Autodesk's Inventor?

Michael

Reply to
mrdarrett

It took me years to collect the full set, not cosmetically matched. Great stuff.

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John Larkin         Highland Technology, Inc 
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John Larkin

  1. Had 'em on hand.
  2. Hadn't done a stack before. Seems to work pretty well, even with the crappy drive!

Tim

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Tim Williams

Yup, though not too much current before life expectancy plummets. I can't imagine it's too easy connecting them in parallel either (yes, they can be triggered, with reasonable jitter, but that's a lot more bother).

I did a minor study of transistors I have laying around; few did not exhibit latching avalanche discharge, but many were finicky (the region of base resistance vs. collector current for pulsing behavior was small, and inconsistent between samples). None really seemed to be more powerful and faster than the usual tiny suspects (like 2N3904).

Avalanche also spreads out poorly -- so a very large transistor (like a 15A

1500V HOT) only 'ignites' in some random spot location, and becomes damaged at basically the same surge current as a 2N3904. (Afterwards, instead of collector leakage, there's C-E resistance, usually on the order of 40kohms. Characteristic of a microscopic burn hole.)

Tim

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Reply to
Tim Williams

GAH! Choke!

No, Altium. The real deal. ;-)

(You might recognize some shapes and default colors from Win's drawings as well; he uses an old copy of Protel, IIRC. The ground symbols haven't changed a pixel!)

Tim

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Tim Williams

What you can't see about the driver is, I've already done that. :) It's actually an astable driving a pair of monostables driving a pair of gate driver ICs (TC4420). So, one side pulses up, other side stays down; other side pulses up, one side stays down; etc. Pulse width is ~2.5us, repeat frequency low ~kHz.

Output is square-ish as long as it's dominant capacitive. At low frequencies, the 10M probe bleeds it down, of course, leaving a rounded wave.

Tim

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Tim Williams

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