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Re: [TCML] Re: Solid state efficiency, was: mini Tesla coil specs

Hi Greg,
Thanks for the explanation of R=0.6 ohm for the rotary gap
of your coil (it makes sense).
But what do you mean by saying there is no IGBT switching loss?
Switching losses of IGBTs at typical tesla coil frequencies
are not so insignificant (althought generaly much lower than
in a spark of SGTC of same power).
The price paid for reduced conducting state voltage drop of IGBT,
compared with say that of MOSFET are increased switching times.
As it should be known IGBT turn-off transition exhibits 
phenomenon known as current tailing during which increased power
loss occur.Typical turn-off times of IGBTs are in the range
0.3...3 microsec.Assume ,for example,a typical coil frequency of
100 khz and coupling between primary & secondary k=0.2.
Assume that this is a high power coil and one decides to use a
typical commercialy available HV IGBT module rated 4500 V.
It has Toff~1.25 microsec.
Turning-off last for about 25% time of 1 semicycle of the coil's
operating frequency.

For the coil posted in the link before,that's not DRSSTC.
That's normal coil only with IGBT instead of a spark gap.
Characteristic surge impedance of the primary can be calculated
from 2 parameters given in the coil's specification:

Cp = 30 uF
F  = 66 kHz (w = 414 000 s^-1)

And so:
 Zch=1/(Cp*w)=0.081 ohm


--- lod@xxxxxxxxxxx wrote:

From: Greg Leyh <lod@xxxxxxxxxxx>
To: tesla@xxxxxxxxxx
Subject: [TCML] Re: Solid state efficiency, was: mini Tesla coil specs
Date: Tue, 17 Nov 2009 12:30:59 -0800

Hi Dex,

The total internal busbar inductance of one IGBT is about 33nH, which is 
negligible compared to the Lpri, at any frequency:


There are no switching losses, as the IGBT's always switch at zero 
voltage and current.

The 0.6ohm value was calculated using the RMS value of the voltage drop 
waveform.  The current waveform remained largely sinusoidal.

I'm not very familiar with DRSSTCs, but it appears that the unit 
featured in the URL runs about 1kV Vpri and 2.5kA Ipri.  This implies a 
Zpri around 0.4ohm.  Probably better to confirm this with its owner.

What is more important for underperformance?  I'd suggest unplugging the 
unit.  ;D   GL

> Greg,
> Couple of additional comments and Qs.
> I am aware an IGBT switch under given circumstances should
> be more  efficient than a spark gap switch.
> But it is not so simple.
> May I ask at what frequency IGBT resistance is 0.007 ohm?
> Certainly it not same at 60 Hz and 60 kHz.
> And then there is some IGBT switching loss involved at
> high frequencies,no?  
> Do you think that 0.6 ohm resistance of the spark is a constant
> value during the transfer of energy from primary to secondary
> (first notch primary ringdown)?
> If yes why,if not what 0.6 ohm represents then?
> I found an interesting design of IGBT coil here:
>  http://scopeboy.com/tesla/t4spec.html
> I calculated primary Zchar and it turns out to be only
>  about 0.09 ohm.
> This coil at 4 kW puts out 81 inches sparks.
> According to Freau empirical formula it should perform
> with sparks over 100 inches long.In your opinion what
> is more important for underperformance here :400 BPS or
> very low primary impedance and increased primary losses?
> Note that this coil also quenches at first notch.
> Dex 
> --- lod@xxxxxxxxxxx wrote:
> From: Greg Leyh <lod@xxxxxxxxxxx>
> To: tesla@xxxxxxxxxx
> Subject: [TCML] Re: Solid state efficiency, was: mini Tesla coil specs
> Date: Sun, 15 Nov 2009 12:11:43 -0800
> Hi Dex,
> 100BPS would probably produce longer arcs at constant wattage since Epri 
> would necessarily be higher.  However, the optimum BPS at constant power 
> would likely be somewhere 100 and 350BPS.
> At normal TC primary operating currents, the IGBTs I used have a much 
> lower R_effective than the 120L rotary gap.  The IGBTs exhibit about 
> 0.007Ohm where the SGTC is about 0.6Ohm.
> Expressed as a ratio against Zpri however, the difference is somewhat 
> less.  The ratio for the SS primary would be 0.75/0.007 = 107, where the 
> ratio for the SGTC system is 14/0.6 = 23.  So one could say that the SS 
> switch is about 4.5 times better than the SG switch.
> But again, the SS switch comes at a cost, both in terms of the IGBTs 
> themselves, the control circuitry, and the specialized coppersmithing 
> required for the primary circuit.   GL

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