Re: A belated s.s. realization

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

Original poster: "K. C. Herrick by way of Terry Fritz <teslalist-at-qwest-dot-net>" <kchdlh-at-juno-dot-com>

Yeah, I was suffering from a temporary anxiety attack.  As I now realize
I'd figured out long ago, my catch-diodes will just do their thing, and
when they're finished catching, then the transistors will take over.

Ken

On Wed, 05 Mar 2003 20:08:05 -0700 "Tesla list" <tesla-at-pupman-dot-com>
writes:
 > Original poster: "jimmy hynes by way of Terry Fritz
 > <teslalist-at-qwest-dot-net>" <chunkyboy86-at-yahoo-dot-com>
 >
 >
 > Hi Ken,
 >
 > This happens in h-bridge and half bridge circuits too, and is easier
 > to
 > visualize than in a  "daisy chain". Although it is more evident in
 > high
 > pulsed power SSTCs, CW SSTCs do this a little bit also. pspice
 > really makes
 > it easy to understand.
 >
 > With no load, the current wave form is triangular, and is 90 degrees
 > out of
 > phase with the voltage. During the first few cycles, the current is
 > almost
 > all positive, so the bottom MOSFET in the half bridge doesnt see
 > much
 > current. The current starts flowing through the fet near the end of
 > the
 > cycle.  With no magnetizing current, the current is a sinusoid, and
 > is in
 > phase with the voltage. At the time of switching, there is no
 > current
 > flowing, so the diodes dont do anything. In the real case, the catch
 > diodes
 > carry current for the first part of the cycle, then is carried by
 > the
 > MOSFETs. The circuit still functions normally; there is still a
 > square wave
 > voltage on the primary. the increased off center current just heats
 > up the
 > fets, diodes, and makes the whole thing more confusing. There is no
 > reason
 > to make any changes if it works :-)
 >
 > I hope this helps, but it is hard to explain something through only
 > words.
 > playing around with pspice really helps.
 >
 >   Tesla list <tesla-at-pupman-dot-com> wrote:
 > Original poster: "K. C. Herrick by way of Terry Fritz "
 >
 > Those of you who have paid attention to my "current-ring" s.s.
 > primary
 > design may want to read this. As you will recall, I employ two loops
 > of
 > primary conductor connected together with energy-storage
 > capacitors.
 > Each loop incorporates multiple switch-transistors and each set of
 > transistors acts alternately with the other set to pass current from
 > the
 > s unidirectionally through first one loop, then the other, thus
 > setting up the alternating magnetic field that excites the
 > secondary.
 >
 > Each transistor is protected against reverse--and excess
 > forward--voltage
 > by the "catch" diode that is connected across its opposite
 > transistor, by
 > coupling via the associated energy-storage capacitors. But there
 > lies
 > the rub. It appears that the circuit is a bit like t! he proverbial
 > bee:
 > unable to fly according to known aerodynamic principles, it flies
 > anyway
 > since it does not know that.
 >
 > When one of the sets of transistors is conducting, all is fine:
 > current
 > flows in a "daisy chain" through the transistors, the primary
 > conductors
 > and the storage capacitors, setting up that direction of
 > primary-current.
 > But then, when that set of transistors shuts off at the end of its
 > half-cycle and the other set becomes turned on, each transistor of
 > the
 > other set finds that it is paralleled by a catch-diode that is
 > mightily
 > conducting in its forward direction, catching the inductive
 > overshoot of
 > the opposite set of primary conductors. That forward direction of
 > diode
 > conduction places a low voltage of the wrong polarity across each
 > transistor that at that moment is supposed to begin conducting.
 > So--those transistors do not begin conducting and the circuit
 > doesn't
 > "fly".
 >
 > Except...it does fly; or at least,! is has been doing so. So those
 > of you
 > who may have been considering use of a similar configuration, take
 > heed.
 > While I take 5, likely a lot more than 5, trying to figure out a)
 > why it
 > has worked at all and b) how to change it so that it works
 > understandably.
 >
 > Ken Herrick
 >
 >
 >
 > Jimmy
 >
 >
 >
 >
 >