Re: phase locked loop SSTC

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Kchdlh-at-aol-dot-com>

In a message dated 12/28/2001 11:15:44 AM Pacific Standard Time,
tesla-at-pupman-dot-com writes:


>
> Subj:Re: phase locked loop SSTC 
> Date:12/28/2001 11:15:44 AM Pacific Standard Time
> From:<mailto:tesla-at-pupman-dot-com>tesla-at-pupman-dot-com
> To:<mailto:tesla-at-pupman-dot-com>tesla-at-pupman-dot-com
> Sent from the Internet 
>
>
>
> Original poster: "Luc by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <ludev-at-videotron.ca>
>
> Hi list,
>
> Could someone here be enough generous to explain in simple term
> why can't you use a feedback loop like the grid loop in VTTC or
> the feedback loop used in simple flyback ( 2* 2N3055 circuit )
> for driving a half bridge mosfet.
>
> I ask this question in many occasion in the last 2 years and I
> never get an answer. Each time I try to rephrase the question
> because I know my english could be ambiguous sometime but whit
> out success.
>
> Cheers,
>
> Luc Benard
>
> Tesla list wrote:
> > 
> > Original poster: "Malcolm Watts by way of Terry Fritz
> <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>
> > 
> > Hi Paul,
> > 
> > On 27 Dec 2001, at 7:49, Tesla list wrote:
> > 
> > > Original poster: "Paul Nicholson by way of Terry Fritz
> > <twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>
> > >
> > > John Tomacic wrote:
> > >
> > > > use a current sense transformer in series with the secondary
> > > > driving the magnifier coil (placed in between the secondary
> > > > and ground).  The signal from the sense tx should be rectified,
> > > > filtered then fed to a comparator.  Since maximum current is
> > > > drawn through the coil at resonance, the frequency of the driver
> > > > can be adjusted by the comparator through a vco to always keep
> > > > the coil in resonance.
> > >
> > > I wrote:
> > >
> > > > John, can you help me understand just how the vco is tuned with
> > > > this arrangement?  I don't see how, with only amplitude info
> > > > from the sensor, the vco can 'know' which direction to tune, ie up
> > > > or down?
> > >
> > > Malcolm wrote:
> > >
> > > > A digital approach can pull a "direction" out of the signal (a D FF
> > > > is all it takes).
> > >
> > > Malcolm,
> > >
> > > Well, I don't understand this. John doesn't seem to be giving any
> > > hints, maybe you can explain?  How can you tune the VCO with just an
> > > amplitude feedback and a comparator?
> > 
> > If you apply one signal to the "D" input and the other to the clock
> > input, Q will be high or low depending on relative phasing of the two
> > signals. I realize that because the frequencies are different, the
> > phase will slide from lagging to leading (or vice-versa) but if the
> > difference is not too great, the direction indication should last for
> > long enough to enable locking. An XOR gate fed from the same signals
> > gives the degree of difference.
> >       Kind of handwaving stuff from the digital realm but I thought
> > it might be useful or applicable to the problem.
> > 
> > Regards,
> > Malcolm
>
> Luc

-

I haven't been following this discussion, but I see no reason to complicate
things:  My s.s. system uses simple positive feedback from the secondary's
return-end to excite its linear amplifier, MOSFET drivers and MOSFET
assemblies.  The key to doing that, I found, is to phase-shift the voltage
signal taken from the secondary by ~90 degrees, by connecting the secondary's
lead to ground via a low-Z capacitor--of 0.15 uF or so in my case.  The (small)
voltage across that capacitor then ends up in phase (or 180 out depending on
how the primary/secondary windings are phased) with the signal you need to
drive the MOSFETs, or tubes, or whatever.

A happy new year to you, & to all...

Ken Herrick