Re: A double resonance solid state Tesla coil

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

Original poster: "jimmy hynes by way of Terry Fritz <teslalist-at-qwest-dot-net>" <chunkyboy86-at-yahoo-dot-com>

Justin,

Thanks for the thoughtful reply. I'll try to explain my 'double resonance' 
idea more clearly and address your comments in order. I've posted a 
schematic and some simulated waveforms at 
<http://www.hot-streamer-dot-com/temp/drsstc.doc>www.hot-streamer-dot-com/temp/drsstc.doc.

I think there's a clear advantages to having a resonant primary circuit in 
addition to the 'coupled' secondary resonance. The SSTC is efficient for 
'continuous wave' energy transfer, but to achieve the quick secondary 
voltage rise required for streamers, a very low primary inductance is 
required. This results in very large magnetizing currents with low power 
factor. Soft switching loss is low in most designs, but when you are trying 
to get quick rise times, there is so much magnetising current that you 
don't switch at a zero crossing anymore. The conduction loss is also large 
because of all the extra current, so most SSTC designs are stuck with a 
soft rise.

By driving the primary at its resonance, all the current is pulled from the 
DC supply in phase with the voltage, this is the key. We can dump more than 
25 joules into the secondary in 160 usecs using $30 worth of switches.

To acheive this fast rise without the second resonance, requires a couple 
thousand amps of magnetizing current and you lose the soft switching 
benefit because the current never crosses zero during the first dozen cycles!

Once we have transfered the energy from the primary LC to the secondary 
(about 10 cycles with a typical coupling ratio), ideally we'd open the 
switches to lock the energy into the secondary.  This isn't possible 
because the voltage induced in the primary would pump back through the 
diodes and into the DC power supply caps.  Instead, we close a path through 
the bridge and let some energy bounce back into the primary LC, like it 
would in a spark gap coil. Simulation shows that if the streamers load the 
secondary pretty well, there isn't much energy sucked back up by the 
primary. Spark gap coils work fine without 1st notch quenching, and the 
closed switch is equivalent.

Using the microcontroller, we can select our break rate without concern of 
overheating a spark gap or switches. I'm building a 6 inch secondary with a 
7x36 inch toroid during Christmas vacation. I expect there will be sparks 
or smoke flying soon.

Jimmy

  Tesla list <tesla-at-pupman-dot-com> wrote:
Original poster: "Justin Hays by way of Terry Fritz "

Hi Jimmy and All,

Merry Christmas!

(all snips from Jimmy)
 > None of the solid state tesla coils I've seen use a resonant
 > primary.

Normal continuous-wave SSTC's drive a resonant load, which is the
resonator. The resonant characteristics of the load are reflected
(inductively coupled) back to the drive circuit, therefore,
primary-side resonance is unecessary.

In other words, the primary is resonant without there being physical
components.

 > One of the downsides of the SSTC is the large magnetizing current,
 > especially when you try to get a fast voltage rise. This leads to
 > high losses in the power switches and makes it impractical to
 > generate 'normal' rise times and streamers.

There are very ve! ry little losses in the power switches in a properly
designed SSTC. The power switches change state (ON to OFF or vice
versa) when there is zero current flow...and since it takes voltage
AND current to dissipate power, there is no power loss. The vast
majority of SSTC's run about 90 percent efficient if not more.

 > Pspice simulation shows that the energy transfer is much more
 > efficient.

In switch-mode power supply lingo, SSTC's are resonant mode,
zero-current-switching forward converters. This type is one of the
most, if not THE most efficient topology known.

 > At this point, the switches are held closed emulate a spark
 > gap before it quenches. This prevents the diodes from sucking power

 > back from the secondary. After one or two beats, the switches are
 > left open as the secondary decays.

Keeping the power switches on as the secondary rings would not work.
Energy would be coupled out of t! he resonator through the primary, and
dissipated in resistan! ce in series with filter capacitors.

In the above snip, if you're referring to the freewheeling, or
"catch" diodes, they do not conduct when the SSTC is in tune. They
only conduct when there is current flowing through the power switches
when they switch (out of tune). The fast change in current (di/dt)
generates a voltage spike, which forward-biases the catch diodes,
finally resulting in a current spike through the diode(s).

 > Compared to a SSTC
 > 1) The fast rise allows real streamers instead of 'brush
discharge'
 > 2) There is no magnetizing current

I don't fully understand your idea, do you mean to turn the power
switches ON, let current flow, then turn them hard OFF in a
non-resonant fashion? Like a normal spark-gap type coil, just
replacing a spark gap with a big power device (solid-state spark
gap)?

Take care, have a good Christmas.

Justin Hays
KC5PNP
Email: justin-at-hvguy-dot-com
Website! : www.hvguy-dot-com

Jimmy