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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: LTSpice model for a SiC MOSFET
Date: Fri, 23 May 2025 15:17:00 +1000
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On 23/05/2025 4:46 am, piglet wrote:
> On 22/05/2025 2:41 pm, Bill Sloman wrote:
>> On 22/05/2025 8:43 pm, piglet wrote:
>>> Bill Sloman <bill.sloman@ieee.org> wrote:
>>>> On 22/05/2025 11:20 am, KevinJ93 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 Baxandall inverter looks as if it would work with sufficiently high
>>>> voltage MOSFET, which clearly exist, even if Infineon is being slow to
>>>> offer a Spice model to let me simulate it.
>>>>
>>>> It's a pretty simple circuit, even if the component parts look to be on
>>>> the expensive side - coping with even 1kV costs money.
>>>>
>>>
>>> Maybe a simple self oscillating two transistor half bridge as in CFL
>>> ballasts of a few decades ago?
>>
>> That's an exact (if incomplete) description  of the Baxandall Class-D 
>> oscillator.
>>
>> It has got two two transistors and it is self-oscillating, and Jim 
>> Williams did popularise it for driving CFL backlights. He never called 
>> it a Baxandall oscillator, but that's exactly what his Linear 
>> Technology application notes  AN45, AN49, AN51, AN55, AN61, and AN65 
>> talked about.
>>
> 
> There is a big difference in the voltage stress seen by the transistors.
> 
> The half bridge exposes each to 1kV max, the push-pull to nearly double 
> (I think you quoted 1.6kV).
> 
> Baxandall topology was great for tape recorder erase/bias oscillator 
> kinda stuff at low supply voltages but at HV not such a great choice.

If you've ever built a Royer inverter, you should be able to remember 
the horrible switching spikes you get when you switch from one side of 
the half-bridge to the other.

The Baxandall configuration does lumber you with a smooth peak at 1.67 
times the supply voltage - the peak of the half-sine wave train that has 
the same average amplitude as the supply voltage - but it does its 
switching at the lowest voltage points, and doesn't generate high 
frequency interference.

It's lot better at stepping up low voltages to high voltage than the 
Royer inverter, and Peter Baxandall seems to have invented it for a 
photomultiplier power supply, but really high voltage MOSFETs seem to 
make it a plausible choice for going the other way.

The transformer gets pretty horrible, but it should be practicable.

-- 
Bill Sloman, Sydney