Path: news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail
From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: LTSpice model for a SiC MOSFET
Date: Wed, 28 May 2025 00:28:12 +1000
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On 27/05/2025 11:46 pm, john larkin wrote:
> On Tue, 27 May 2025 14:16:11 +0100, liz@poppyrecords.invalid.invalid
> (Liz Tuddenham) wrote:
> 
>> Bill Sloman <bill.sloman@ieee.org> wrote:
>>
>>> On 27/05/2025 9:09 am, john larkin wrote:
>>
>> [...]
>>>> 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.
>>
>> Good grief!   A 10-megohm quarter watt resistor with a zener diode is
>> the obvious answer.
> 
> A 3.3v real zener will probably leak more than 10uA. Low voltage
> zeners are awful.
> 
> And he wants 3 mA out... I think.

The guy I was talking to had 10uA at 1kV, and wanted a 3.3V output. 1mW 
at 3.3V is 3mA. The 3ma is the upper limit the circuit could supply.
>>
>> [...]
>>> Misunderstanding the constraints can lead people to propose
>>> inappropriate solutions.
>>
>> How can anyone misunderstand something they have never been told?

You managed it.

> Bill was the confused person.

John Larkin does like to think that.

> The problem gets much more interesting with 10 uA in and 99%
> efficiency requirement.

There never was an efficiency requirement. 50% would have been nice.

> A Baxandall isn't going pull microamps at 1 KV.

Keeping a resonant tank resonating at a couple of kV is going to use up 
power in the winding resistance. I think I can get 245H out of RM14 
core. If we can keep the parallel capacitance down to 25pF that would 
resonate at 2kHz and the reactive impedance would be 3 Mohm, giving a 
peak current 0f 1mA. The resistance of the winding is going to be about 
480R, so I^2.R is 500uW, or 0.5uA at 1kV.

A properly designed Baxandall might well pull less than 1uA if lightly 
loaded.

-- 
Bill Sloman, Sydney