SSTC PLL Circuit Information

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

Original poster: "Daniel McCauley by way of Terry Fritz <twftesla-at-qwest-dot-net>" <dhmccauley-at-spacecatlighting-dot-com>

Here is a recent email i received from Richie Burnett regarding the use of
PLL circuits in Solid State Tesla Coils:


Richie writes:

I did do some work on this a few years ago when I did more Tesla Coil
stuff.

You certainly can use a Phase Locked Loop to make the drive oscillator
track the resonant frequency of the resonator,  but there can be many
problems.  The PLL has to be designed to minimise the phase error
between the applied voltage and the base current of the resonator, (not
the current in the primary winding.)  If the PLL is made to ensure that
resonator base-current is in phase with the drive waveform, then the
driver is always in tune and delivering maximum real power.

Note that most PLL circuits operate with a 90 degree phase lag between
the internal reference and the external feedback signal. This 90 degree
shift when locked is not acceptable for SSTC applications as it implies
zero real power throughput. The Phase Comparator must be chosen
appropriately to give 0 degrees of phase shift between the drive
voltage and the base current of the resonator.  This includes allowing
for any propagation delays in the logic and MOSFET drivers !
propagation delays in the

Noise is not so much of a problem, because the base current can be
monitored with a Current Transformer.  The waveshape is a sinewave
so it can be filtered to remove extraneous noise and transients.

The main problem is preventing the PLL from locking onto higher modes
of the resonator. This is particularly difficult to achieve under all
operating conditions. The load swings dramatically between no breakout,
corona into the air,  and a spark that connects to a grounded object.

If the PLL is not designed carefully, one of the higher spurious modes
of the resonator can move into the locking range when there is an arc
to ground.  This is quite nasty as it moves the voltage maxima on the
resonator to somewhere other than the top of the coil !

It is possible to use bandpass filters to clean-up the PLL feedback
signal from the CT.  However you have to be careful not to disturb the
phase of this reference, otherwise the PLL will not lock perfectly in
tune with the resonator any more.  This means you would not be
delivering maximum power, and the inverter must deal with driving into
a slightly reactive load.

-Richie Burnett,