[TCML] Re:Pros and cons

[uhvsystems@xxxxxxxx]

Well,it appears that for our purposes 3rd variant is the most adequate then.Having less complexity than DRSSTC but still offering enough of the control.Moreover,I checked files of the power electronics department to reveal some CM 600HB-90H modules are kept in stock.Rated 4500V,900A they are big IGBT bricks (maybe even too big).I'm not that much into power electronics to be sure if the module is suitable for tesla coil service I have on mind.Will investigate in their catalogue to find out more details.Generally such devices are quite robust,hot-spot and current crowding problems are nonexistent,and the need for external snubber circuits is minimal.If we manage to make it work in tesla coil service that would be great.With primary tank capacity 20 uF charged up to 4400V, and with tesla coil efficiency say about 80%, 155 joules could be supplied to tesla coil secondary circuit.The estimate  is sufficient to cover upper testing limits we are prepared to go to.
I must say few words about the efficiency and where is its importance for us.
The importance is mainly in the point that efficiency determines primary energy needed to charge secondary circuit to predetermined voltage.From the point of technical operation and reliability it is also important since nobody wants to overheat or blow up things due to increased dissipation.
>From the point of chasing some high efficiency figures for the sake of economical power utilization,well...that is totally irrelevant to us.

Regards,   

Fez Zaev

>
Dex Dexter wrote:
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As concerns control and overall regulation,there's no question about it
,SSTCs are superior.Regarding the issues peak power vs. average power,
there are essentially 3 different variants of SSTCs.
First,SSTC with continiouos wave excitatation.The peak power is close
to RMS input power,and you're not interested in that.
Second,DRSSTC which has much higher peak power than input power,and superb
possibilities of control.You're definitely interested in that.But DRSSTC are
the most complicated tesla coil topologies and more sensitive to tuning etc.
Third,is the version nicknamed OLTC,and here an ordinary spark gap is replaced
by IGBT.Mode of operation is the same as classical SGTC and peak power
just as well.Still you have great possibilty of control,and you're definitely
interested in that too.Since you are targeting frequencies around 25 khz,which
is rather low for tesla transformers,and this version benefits from lower
resonant frequencies ,I'd recommend  serious consideration of it.
Tesla  circuit has the topology of a series resonant converter,and as such
it has some advantages and some disadvantages.
>From the practical point the main disadvantage is a very low 
surge impendance of a primary circuit due to lower voltage IGBTs are rated for.
Regarding IGBT application,during turn off,the IGBT reacts relatively slow 
(becouse of the charge stored in its' junction),and if the voltage across IGBT
is still high the turn off loss will be high.To force the current to 0 before
turning off the device is the way to eliminate these loses.
Zero current switching which can be accomplished operating the circuit below the
resonance frequency,where the current through IGBT reverses before the branch is
turned off,requires the connection an antiparallel power diode to capture
reverse current.This way switching off losses are considerably reduced
(normally,in a real world there are no zero-loss power diodes and very action
of commutation dissipates some additional energy too).
On the other hand ,IGBT switching loss at turn on ,in the converter operated 
below resonant frequency,can't be reduced due to the fact that IGBT are hard
switched on.Fortunatelly,at switching frequency 100 Hz these losses are 
comparatively low.Thus,total silicon loss is somewhat higher than merely a
product of IGBT on state forward voltage drop and average current through it.
Conduction state IGBT losses are mostly the function of the average current
rather then rms current.High peak currents often found in tesla primary circuit
aren't so important.This fact makes IGBT an excellent device for tesla coil
applications.And as concerns losses in the winding of primary coil,primary
in condenser,and due to perhaps inductive coupling to surrounding objects,these
are hardly calculable.They are function of rms current,frequency,geometry
and differ from  case to case.Unfortunatelly they are significant ,and sometimes
even make dominant part of total dissipation..Overall dissipation can be 
determined only by measurements.So,"how efficient tesla coil can be?" is not 
easy question to answer to.People have measured transfer efficiency of 
energy from primary to secondary to be as high as 90% on some SGTC.
Wether this can be achieved by described IGBT coil,or even surpassed,largely
depends on engineering and constructing partameters.
I don't know if anybody ever measured something like that on such coil ,becouse
there are only few of them and they old what,10 years perhaps?
SGTC are over 100 years old.
Thinking of efficiency I would say there is general agreement that
DRSSTCs are the most efficient version among all tesla coil versions invented 
up-to-date.



Dex        
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