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

Love it when you help, rather than just posting OT comments and requests.

You are a prince of a guy.

Both. We want a fairly linear 10kV ramp lasting 1us (or longer, that has to programmable). More important, the 10kV/us slewing has to be highly repeatable, because we'll be doing it repeatedly, signal-averaging the results into small time-slot bins each time.

That part's not critical, it can overshoot some, it can decay, ramp quickly down, whatever.

The unspecified upper part of your drawing will be a challenge.

I have a drawer full of 15kV and higher, glass vacuum relays that I've accumulated over the years. They can handle 10s of amps, and thus could conceivably be the switching basis for a passive circuit to create our 10kV 1us ramp. However, we need a repeatable ramp, hopefully without significant time jitter, since we'll be signal- averaging the data from thousands of scans. We will be digitizing the voltage ramp, so it's possible we could adjust the time bins after the fact. I'm not sure about contact bounce on my HV relays.

Yes, but I'll have a stack of say 15 FETs, and so I expect a large safety margin in the transient thermal resistance area.

Some years ago I purchased 200 IRFBG20 MOSFETs for $1.75 each (they're still available, now at $0.96/100). These are 1kV 1A TO-220 MOSFETs featuring moderately-low capacitance (most of the semi houses have moved away from 1A parts for their 1kV offerings, to 3A, etc., as the smallest ones in production). The IRFBG20's capacitance specs are Ciss=500pF, Coss=52pF, and Crss=17pF at 25V. See

The IRFBG20 is rated at 52W, and its single-pulse transient thermal impedance (fig 11) is 0.01C/W for 1us, or 0.03C/W for 10us, so, if we allow a 150C junction-temp jump, it can handle 15kW or 5kW, resp. Notice the sqrt-time factor at work there, as the heat spreads out. With 15 FETs equally sharing (another assumption), that's a 225kW or 75kW capability for 1us or 10us, which is far more than needed.

Getting the FETs to share equally may be a difficult. My IRFBG20 FETs have their avalanche breakdown at about 1200V, so they could theoretically handle an 5kW/1200 = 4.2A avalanche current for 10us. Hah, that's more current than they're up for anyway! Certainly if I'm switching anywhere from 0.5 to 2A, a short-lived FET avalanche isn't something to fear. But if we did fear this (or, ahem, if it failed to pass a know-nothing design review) I could parallel each FET with a stack of say five 200V 1.5kW TVS diodes (that'd be 75 TVS for the 15 FETs, sheesh!). TVS zener diodes feature two slugs of copper (one each side of the die) to absorb heat. The 1.5ke200 can handle 1.5kW for 1ms, or 20kW for 10us, so five in series can handle 100kW. This is 20x more than an IRFBG20 can handle, and it may be nearly 100x what we'll be needing for our 10kV ramp switch. Hmm, maybe I'll look into some smaller SMD-packaged TVS parts.

One other TVS issue. The capacitance of a 200V 1.5kW TVS is 80pF, measured at its breakdown voltage. Five in series would be 16pF, which compares to 52pF for the IRFBG20 (I know these values change with voltage, but this is just a quick look), so a TVS may not be too painful in that regard. If we consider 68pF/15 for an entire 15-stage 10kV FET-switch stack, that's only 4.5pF, which doesn't look bad either. ** Oops! ** ST's datasheet shows their 1.5ke200 to have 675pF of capacitance at 0V, which is 135pF for five. Ouch!

I've designed and made lots of fast high-voltage transformers over the years, and this is a possibility. But one serious problem is high-voltage transformers with good insulation tend to have high leakage inductance, which could kill my ability to create an accurate ramp waveform. So I'm not considering that approach seriously right now. But thanks for the suggestion.

Some kind of clever passive circuit?

Now, now. After we've struggled with it, Jim will probably tell us he did this back in 1963, or something. Sheesh!

Do you need continuous slew rate adjustment? A few relays could give you a number of steps.

John

I was being facetious. My HV experience is limited...

Tech in MHD lab while at MIT, a little ignitron, thyratron, spark gap stuff at 30KV, just on/off. Capacitor bank as big as a room ;-) Smear camera motor driver.

Ignition systems for automobiles... many conventional and CD designs.

...Jim Thompson

It's been a lo-o-o-ong time since I've lurked around a TV repair shop. Wonder how fast HV regulator tubes like 6BD4/6BK4 can be pushed?

...Jim Thompson

That could work.

Hmm.

I don't mind that part - my technician will do it. :-)

I like the constant-current into a capacitive load that a flyback coil / transformer can deliver. If the transformer has a 10:1 step-up ratio I could use 1kV MOSFETs to switch 5 to 10A on the primary... But I keep worrying about the transformer's leakage inductance. If the secondary + load was say 60pF total, then that'd have to be less than 17uH for a say 5MHz bandwidth. Hmm, 17uH doesn't look so hard.

How fast can they switch, can they do 10kV/us or more?

ROFLOLMGO!

Hi Win,

What about a triggered spark gap? It'll handle the volts & amps. Good fault tolerance. Dielectric is self healing. Good optical & acoustic entertainment side effects :)

Cheers, Ian Du Rieu The Leon Audio Company

Add .au to email address to reply.

OK, now we're talking a fet switch followed by an R-L-C ramp forming network with a few relay-switchable elements. The switch could be a stack of fets with transformer-coupled gate drive. That doesn't sound nearly as bad as trying to control the stacked fets in linear mode.

What's the actual breakdown voltage of RG-174 sized coax? I get it's way above 10KV. So a nice little transmission-line transformer would be easy to make.

I used to run surplus RG-59 at 15KV when I was a kid, playing with flashtubes and spark gaps and stuff.

John

Yes. Expect under a ns of jitter if you drive them right.

Hey, how about a krytron? Size of a walnut. They have a keep-alive and a radioactive thingie, so jitter is very low.

John

Where'd you get that? IIRC, it's about 1kV for extended use. And I've had RG-58 breakdown at 2kV. We use special grades of RG-9 at 10kV, but not with any of the usual connectors.

To make a nice 20us ramp the transformer needs a good low- frequency response, which implies a fair-sized core. It may not need transmission-line transformer HF bandwidth.

How long did it last? Most anything will last a few minutes or hours, even, before the breakdown paths start killing you.