LTSpice model for a SiC MOSFET

I use a Cree/Wolfspeed 1200v part, C2M0280120D, in my Pockels Cell driver.

Gate drive for SiC parts is a bear. I did it myself, but I think there are chips for that now.

There are multi-kilovolt silicon mosfets too.

Baxandal looks to be inefficient and expensive as a low power converter. The drain swing is 2x the supply voltage, and it needs two fets and a difficult custom transformer.

RL

Ikv, sitting there all alone and bashfull. Not a common occurance. Is this the only power source in the vicinity? Tell us more.

RL

It isn't going to be inefficient. That configuration is famous efficient.

The drain swing is actually 1.67 times the supply voltage, but it does need two switching devices and a specially wound transformer (and we know how reluctant you are to design them or get them made).

It is probably going to be too expensive for the application, and we'd be grateful for your insights into a cheaper alternative. I can't think of one.

I'm not sure what the specs are, but I have a few ideas.

One could make a flyback converter with a high-ratio transformer. Coilcraft makes some, capacitor charging transformers and CCFLs. There must be crazy cheap Indian or Chinese CCFL transformers.

ST makes a 1400v NPN transistor for under a dollar.

It would be cool to put two drum core inductors next to one another, or on opposite sides of the board, to make a loosely coupled transformer, exactly what a forward converter needs.

I've finally finished by dummy load board... Gerber day is tomorrow. The paired Murata drum cores are spaced to tune the coupling factor to K=0.6

So, how to get the low duty cycle pulsed base drive? I'm thinking maybe an RC off the HV supply and a diac, a relaxation oscillator.

If the rig is a forward converter, we could make a non-saturating blocking oscillator, and share the transformer secondary to drive the base and rectify to 3.3v.

$6 or $7 might be a reasonable parts cost target in modest volume. Needs Spicing.

But you can't be bothered to post the part number.

You end up needing a lot more core material than the Baxandall configuration does. Been there, been pissed off by that.

The Baxandall configuration lends itself to simple drive circuits.

Of course you could, but you'd need to be mad to try.

Which is why I am looking for a Spice model of the 1.7kV transistor I know I can buy.

It's not my project. I just got asked about what a Baxandall down-converter for a 1kV 1mA source would look like.

Because it's not my project I'm not at liberty to talk about the power source. My first thought was an intensely radio-active source pushing out a lot of high energy beta rays (electrons) or alpha rays (helium nuclei), but it isn't.

You could put three (or even four) mains SMPS circuits in series across the 1kV, if you can force them to share the voltage fairly (using for example a string of zeners). The secondaries can be paralleled after the rectifiers without any need to synchronise them, (or you could figure out some way to synchronise them). You probably want to have single voltage reference and error amplifier giving feedback to all three primary stages, so that one doesn't try to supply the load all by itself. The transformers need to stand 1kV primary to secondary. Mains SMPS circuits must be cheap, probably three times cheaper than something fancy and custom.

It sort of limits your ability to help.

At 1W, you should be able to get away with murder, if efficiency isn't really an issue.

RL

I know what the power source is - in general terms. I haven't got any kind of okay to talk about it

Efficiency is an issue, if only on the sense that a 0.1W power source isn't up to much, so even 50% efficiency would be nice. Jim Williams got about 93% out of the Baxandall configuration, but 50% might be good enough.

Bill Sloman, Sydney

You are so smart that you obviously don't need my help.

I'd keep it simple and repurpose a backwards commodity CCFL ($0.50) transformer, in a low frequency (20-50KHz) buck regulator 'of sorts'.

A 1KV mosfet (why SIC?) would act as it's own TVS if the source was really self-limited. 2$

Trickle charge for start-up, with all control power on the LV side; an intermediate 8-20V rail.

From there it's a matter of your techy arranging 'who's on first' in a sensible manner.

RL

How about a piezoelectric transformer run in reverse? Neon tubes illuminating a solar cell? Capacitive divider using a spare core in the mains supply lead as one plate of the capacitor? (Depending on supply frequency and required output current.)

There was once a piezo-based isolator that could move some milliwatts.

I've used PV isolators to power floating mosfet gate drivers, but they top out well under 100 uA so can't power a lot of stuff.

Capacitive dividers are inherently lossy. Transformers are a PITA but are best for high-voltage-ratio power conversion.

I want to design something with one cheap drum core inductor on the top of a PCB coupling to another one on the bottom. Just don't have a use for it now.

I've seen scopes and such that have a "line" trigger, where the pickup was an insulated wire wrapped around the highside insulated AC power lead. Scopes don't always have line triggers much any more.

Some of our instruments have line trigger capability, but we ask the user to plug a transformer-type wart into the back for that.

Going from 1KV to 3.3v has interesting possibilities. The easy way is a resistor and a zener. Efficiency about 0.3%.

More efficient would be a resistor and a zener and then a buck switcher. 5% maybe.

I clearly need help here, but you don't seem to interested in being helpful.

Cold cathode fluorescent lamps are cranky beasts, and Jim Williams exploited the Baxandal configuration to get the voltages they need out of low voltage supplies, using bipolar transistor switches. I would not be all that optimistic about finding a CCFL transformer that I could repurpose.

Designing a transformer for the job isn't all that difficult and finding a local transformer shop to wind it wasn't all that hard when I did something similar in the Netherlands some twenty years ago.

The SIC part costs about $5.00. The guy I'm talking to came up with $200 regular silicon part with a higher on resistance and about ten times the internal capacitances. I'm not recommending SiC on principle - the limited search I did came up with a SiC part which looked as if might work, even if still too expensive to be attractive.

Managing start-up is part of the design process.

What you should have said is that you couldn't be bothered to use a search engine.

...much snippage

Why shoud he Bill? When all he gets from you is "you'd need to be mad to try".

I don't know of a specific part but a few seconds with a search engine found this: