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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: LTSpice model for a SiC MOSFET
Date: Mon, 19 May 2025 18:19:36 +1000
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On 19/05/2025 1:38 pm, john larkin wrote:
> On Mon, 19 May 2025 12:23:54 +1000, Bill Sloman <bill.sloman@ieee.org>
> wrote:
> 
>> On 19/05/2025 12:15 am, john larkin wrote:
>>> On Sun, 18 May 2025 18:11:58 +1000, Bill Sloman <bill.sloman@ieee.org>
>>> wrote:
>>>
>>>> I'm looking at a problem where somebody wants to step down a 1kV low
>>>> current source to 3.3V.
>>>>
>>>> The Baxandall class-D oscillator could do it, but it needs a pair 1.7kV
>>>> MOSFETs for the job. The Infineon SiC IMH170R450M1 would do it - though
>>>> it's a much higher current part (10A) than the job needs (about 1mA).
>>>>
>>>> I've dived into the Infineon rabbit-hole which promises LTSpice models,
>>>> but wasn't able to find one.
>>>>
>>>> Does anybody know of a similar - ideally cheaper and smaller - part for
>>>> which there is an LTSpice model?
>>>
>>> I use a Cree/Wolfspeed 1200v part, C2M0280120D, in my Pockels Cell
>>> driver.
>>>
>>> https://www.dropbox.com/scl/fi/5arhyamrp0bl3tgb2fasn/DSC02771.JPG?rlkey=3ttcc2yt6s9nrtdouuv3aneol&raw=1
>>>
>>> They do have an LT Spice model library that works.
>>>
>>> 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.
>>
>> 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.

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

> 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.

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

> 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
> 
> https://www.dropbox.com/scl/fi/57jecrzc894uvktv72wrg/P978_A18.jpg?rlkey=4095oct5enxqp556xf44oy491&raw=1
> 
> 
> 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.

The Baxandall configuration lends itself to simple drive circuits.

> 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.

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

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

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

-- 
Bill Sloman, Sydney