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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: squeezing a field
Date: Mon, 28 Oct 2024 23:41:39 +1100
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On 28/10/2024 11:06 pm, piglet wrote:
> Bill Sloman <bill.sloman@ieee.org> wrote:
>> On 28/10/2024 3:43 am, legg wrote:
>>> On Sun, 27 Oct 2024 15:01:34 +1100, Bill Sloman <bill.sloman@ieee.org>
>>> wrote:
>>>
>>>> On 27/10/2024 2:20 am, legg wrote:
>>>>> On Fri, 25 Oct 2024 16:08:08 +1100, Bill Sloman <bill.sloman@ieee.org>
>>>>> wrote:
>>>>>
>>>>> <snip>
>>>>>> He was remarkably good, just not totally perfect.
>>>>>>
>>>>>> His footnote reference to "squegging" in the 1959 class-D oscillator
>>>>>> paper is another minor drop-off. He can't be blamed for it, but a
>>>>>> super-hero might have done better.
>>>>>
>>>>> What's wrong with 'squegging' ? It's a simple word that covers
>>>>> a host of faults that all give the same approximate symptom . .
>>>>
>>>> With the advantage of 65 years of hindsight, it looks as if what he was
>>>> seeing was gain in bipolar transistors running in the inverted mode.
>>>>
>>>> "Squegging" was mostly used for  weird oscillations in resonant circuits.
>>>>
>>>> Class-D oscillators built with MOSFet switches don't squeg. Class-D
>>>> oscillators built with bipolar transistors in LTSpice don't squeg either
>>>> - the Gummel-Poon transistor model doesn't model inverted mode behavior
>>>> all that well.
>>>
>>> Squegging in any oscillatory circuit, driven or otherwise,
>>> describes widely varying amplitudes that typically approach
>>> self-quenching and can otherwise approach unintentional
>>> overstess in the 'wobulating' cycle.
>>>
>>> Not what the doctor ordered, or the designer anticipated.
>>>
>>> Only blocking oscillators do it on purpose.
>>
>> What Baxandall was describing was a situation where you've built a
>> class-D oscillator and used a feed inductor which has an appreciably
>> higher inductance than the inverter transformer.
>>
>> If you simulate that in LTSpice, the voltage at the centre tap starts
>> off climbing up to about twice the steady-state peak and drops below the
>> rail during recovery, but this roller-coaster effect dies away. In real
>> life it doesn't (if you are using bipolar transistor for your switches).
>>
>> My guess is that you could stop it by adding the right zener diode
>> between the centre tap and ground - one that didn't ever conduct when
>> the circuit was running smoothly, but would start conducting if the
>> centre tap got much above the steady state peak. This stops the
>> centre-tap ever getting below the rail at the bottom of the start-up
>> roller coaster - or at least it does in LTSpice and would keep you away
>> from the mode of operation where the switching transistors were
>> operating in the inverted mode.
>>
>> Peter Baxandall invented the circuit before 1959, before Zener diodes
>> were widely available.
> 
> That topology is very critical around conduction overlap vs dead band and
> nano seconds matter.

If you drive the transistor bases with a centre tapped secondary (with 
many fewer turns) as Peter originally described, the nanoseconds look 
after themselves. Conduction overlap isn't a good idea but an handful of 
nanoseconds of underlap isn't a problem.

--
Bill Sloman Sydney