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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: LTSpice model for a SiC MOSFET
Date: Tue, 27 May 2025 14:37:58 +1000
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On 27/05/2025 9:09 am, john larkin wrote:
> On Wed, 21 May 2025 18:20:25 -0700, KevinJ93 <kevin_es@whitedigs.com>
> wrote:
> 
>> On 5/21/25 12:20 AM, Bill Sloman wrote:
>>> On 21/05/2025 3:47 am, KevinJ93 wrote:
>>>> On 5/20/25 1:46 AM, Bill Sloman wrote:
>>>>> On 20/05/2025 1:13 am, Liz Tuddenham wrote:
>>>>>> 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?
>>>>>>
>>>>>> How about a piezoelectric transformer run in reverse?
>>>>>
>>>>> The piezoelectric transformer is an interesting idea.
>>>>>
>>>>>> Neon tubes illuminating a solar cell?
>>>>>
>>>>> Neither is all that efficient.
>>>>>
>>>>>> 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.)
>>>>>
>>>>> I can't see how that could work. Charging up lots of capacitor is
>>>>> series, and discharging them in parallel is one mode of current
>>>>> multiplication, but about the only kind of switch that would work
>>>>> would be a reed relay, and they are slow and don't last long when
>>>>> cycled fast.
>>>>>
>>>>> Dry reeds are good for 10 million closures, mercury-wetted reeds for
>>>>> about 100 million, and neither is all that cheap or compact.
>>>>>
>>>>
>>>> The Art of Engineering #3 (I think) - describes a "Reverse Marx
>>>> Generator" that does exactly that (charging caps in series and
>>>> discharging in parallel). It uses diodes as the switching element.
>>>
>>> The forward diode drop is inconsequential at 1kV, but inconvenient at
>>> 3.3V. And you'd need 250 stages in this application.
>>>
>>> I've got AOE3. It's index doesn't point to any "reverse Marx generator".
>>> Google search throws up links, but nothing useful.
>>>
>>> The classic Marx generator uses spark gaps as its switches. I have used
>>> them myself (to start a xenon arc lamp), but they wouldn't be useful here.
>>>
>>
>> Sorry -- it is on page 440 of the X-chapters, not AOE3.
>>
>> The reverse Marx generator doesn't need to go all the way down to 3.3V
>> it could just increase the current and reduce voltage to the point where
>> a conventional converter (such as a flyback) can be used without
>> excessive voltage devices being used.
> 
> The specs, as far as I can tell, suggest 1KV at 1 ma in and 3.3v at 3
> ma out. The required efficiency is then 1%.

Actually 1kV at 10uA in. The one 1mA was a constraint on the switching 
transistor based on the current that might circulate in the resonant tank.

> So use a resistor and a zener to make 36 volts and dump it here:
> 
> https://www.digikey.com/en/products/detail/mornsun-america-llc/K78L03-1000R3/16571443
> 
> That will be about 3% efficient and cost under $2.

Misunderstanding the constraints can lead people to propose 
inappropriate solutions.

-- 
Bill Sloman, Sydney